The Facts On File

DICTIONARYof

EARTH SCIENCERevised Edition

The Facts On File

DICTIONARYof

EARTH SCIENCE

Edited byJacqueline Smith

Revised Edition

The Facts On File Dictionary of Earth ScienceRevised Edition

Copyright © 1976, 1979, 2000, 2006 by Market House Books Ltd

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PREFACE

This dictionary is one of a series designed for use in schools. It is intended for stu-dents of earth sciences, but we hope that it will also be helpful to other sciencestudents and to anyone interested in science. Facts On File also publishes dictionar-ies in a variety of disciplines, including astronomy, chemistry, biology, computerscience, physics, mathematics, forensic science, weather and climate, and marine sci-ence.

This book is based on an edition first published by the Macmillan Press in 1976 andit was last revised in 2000. This second Facts on File edition has been extensively re-vised and extended. The dictionary now contains over 3500 headwords covering theterminology of modern earth science. A totally new feature of this edition is the in-clusion of over 1,500 pronunciations for terms that are not in everyday use. An ap-pendix lists the chemical elements and their properties and includes a list of Web sitesand a bibliography. A guide to using the dictionary has also been added to this lat-est version of the book.

We would like to thank all the people who have cooperated in producing this book.A list of contributors is given on the acknowledgments page. We are also grateful tothe many people who have given additional help and advice.

v

ACKNOWLEDGMENTS

Editors (previous editions)

Stella Stiegeler B.Sc.John O. E. Clark B.Sc.

Contributors

B. M. Abbott B.Sc., Ph.D., F.G.S.Anna Clyde B.Sc.Andrew Hill B.Sc., Ph.D., F.G.S.I. P. Joliffe B.Sc., M.Sc, Ph.D., C.I.C.E., F.C.S.,

F.R.G.S.R. B. Lanwarne B.Sc.P. A. Smithson B.Sc., Ph.D., F.R.Met.S.T. J. Speechley B.Sc.F. A. Sultan B.Sc., M.Sc., D.I.C.S. D. Weaver B.Sc., Ph.D., F.G.S.A. C. Wornell B.A.

Pronunciations

William Gould B.A.

vi

CONTENTS

Preface v

Acknowledgments vi

Guide to Using the Dictionary viii

Pronunciation Key x

Entries A to Z 1

Appendixes

The Chemical Elements 383

Web Sites 385

Bibliography 387

vii

GUIDE TO USING THE DICTIONARY

The main features of dictionary entries are as follows.

HeadwordsThe main term being defined is in bold type:

abyssal rock A type of intrusive igneousrock formed deep within the Earth’s crust.

VariantsSometimes a word has a synonym or alternative spelling. This is placed in brackets afterthe headword, and is also in bold type:

bed (stratum) The smallest division ofstratified sedimentary rocks, consisting ofa single distinct sheetlike layer of sedimen-tary material ... .

Here, ‘stratum’ is another term for ‘bed’. Generally, the entry for the synonym consistsof a simple cross-reference:

stratum See bed.

AbbreviationsAbbreviations for terms are treated in the same way as variants:

Antarctic bottom water (AABW) Acold and extremely saline water mass, themost dense water of the global oceans, thatforms in the Southern Ocean along thecoast of Antarctica ... .

The entry for the abbreviation consists of a reference:

AABW See Antarctic bottom water.

Multiple definitionsSome terms have two or more distinct senses. These are numbered in bold type

aggregate 1. Mineral fragments and/orrock particles that combine to form a hardmass.2. (in soil science) A cluster of soil particlesthat have bonded together through aggre-gation.

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Cross-referencesThese are references within an entry to other entries that may give additional usefulinformation. Cross-references are indicated in two ways. When the word appears in thedefinition, it is printed in small capitals:

ablation cone A cone of FIRN, ice, orsnow covered in rock debris caused by dif-ferential ABLATION.

In this case there are cross-references to the entries for ‘firn’ and ‘ablation’.

Alternatively, a cross-reference may be indicated by ‘See’, ‘See also’, or ‘Compare’, usu-ally at the end of an entry:

alpine A type of climate found in moun-tainous areas above the timberline butbelow any permafrost level. Grass andother low-growing plants make up the typ-ical vegetation. See also alp.

Hidden entriesSometimes it is convenient to define one term within the entry for another term:

anemometer ... An instrument for mea-suring wind speed and often wind direc-tion. The most common type, the cupanemometer, has three cups mounted sym-metrically about a vertical rotating axis ....

Here, ‘cup anemometer’ is a hidden entry under ‘anemometer’, and is indicated by italictype. The individual entries consists of a simple cross-reference:

cup anemometer See anemometer.

PronunciationsWhere appropriate pronunciations are indicated immediately after the headword, en-closed in forward slashes:

abiotic /ay-bÿ-ot-ik/ Describing a non-living factor in an ECOSYSTEM.

Note that simple words in everyday language are not given pronunciations. Also head-words that are two-word phrases do not have pronunciations if the component words arepronounced elsewhere in the dictionary.

ix

/a/ as in back /bak/, active /ak-tiv//ă/ as in abduct /ăb-dukt/, gamma /gam-ă//ah/ as in palm /pahm/, father /fah-ther/,/air/ as in care /kair/, aerospace /air-ŏ-

spays//ar/ as in tar /tar/, starfish /star-fish/, heart

/hart//aw/ as in jaw /jaw/, gall /gawl/, taut /tawt//ay/ as in mania /may-niă/ ,grey /gray//b/ as in bed /bed//ch/ as in chin /chin//d/ as in day /day//e/ as in red /red//ĕ/ as in bowel /bow-ĕl//ee/ as in see /see/, haem /heem/, caffeine//kaf-een/,/ baby /bay-bee//eer/ as in fear /feer/, serum /seer-ŭm//er/ as in dermal /der-măl/, labour /lay-ber//ew/ as in dew /dew/, nucleus /new-klee-ŭs//ewr/ as in epidural /ep-i-dewr-ăl//f/ as in fat /fat/, phobia /foh-biă/, rough

/ruf//g/ as in gag /gag//h/ as in hip /hip//i/ as in fit /fit/, reduction /ri-duk-shăn//j/ as in jaw /jaw/, gene /jeen/, ridge /rij//k/ as in kidney /kid-nee/, chlorine /klor-

een/, crisis /krÿ-sis//ks/ as in toxic /toks-ik//kw/ as in quadrate /kwod-rayt//l/ as in liver /liv-er/, seal /seel//m/ as in milk /milk//n/ as in nit /nit/

/ng/ as in sing /sing//nk/ as in rank /rank/, bronchus /bronk-ŭs//o/ as in pot /pot//ô/ as in dog /dôg//o/ as in buttock /but-ŏk//oh/ as in home /hohm/, post /pohst//oi/ as in boil /boil//oo/ as in food /food/, croup /kroop/, fluke

/flook//oor/ as in pruritus /proor-ÿ-tis//or/ as in organ /or-găn/, wart /wort//ow/ as in powder /pow-der/, pouch

/powch//p/ as in pill /pil//r/ as in rib /rib//s/ as in skin /skin/, cell /sel//sh/ as in shock /shok/, action /ak-shŏn//t/ as in tone /tohn//th/ as in thin /thin/, stealth /stelth//th/ as in then /then/, bathe /bayth//u/ as in pulp /pulp/, blood /blud//ŭ/ as in typhus /tÿ-fŭs//û/ as in pull /pûl/, hook /hûk//v/ as in vein /vayn//w/ as in wind /wind//y/ as in yeast /yeest//ÿ/ as in bite /bÿt/, high /hÿ/, hyperfine /hÿ-

per-fÿn//yoo/ as in unit /yoo-nit/, formula /form-yoo-lă//yoor/ as in pure /pyoor/, ureter /yoor-ee-ter//ÿr/ as in fire /fÿr/

x

A consonant is sometimes doubled to prevent accidental mispronunciation of a syllableresembling a familiar word; for example, /ass-id/ /acid/,rather than /as-id/ and /ul-tră-/sonn-iks// /ultrasonics/, rather than /ul-tră-son-iks/. An apostrophe is used: (a) betweentwo consonants forming a syllable, as in /den-t’l/ /dental/,and (b) between two letters whenthe syllable might otherwise be mispronounced through resembling a familiar word, as in/th’e-ră-pee/ /therapy/ and /tal’k/ /talc/. The symbols used are:

PRONUNCIATION KEY

aa /ah-ah/ A LAVA with an extremelyrough spinose surface. Compare blocklava; pahoehoe.

AABW See Antarctic bottom water.

Aalenian /ah-lee-nee-ăn/ The earliest partof the Middle JURASSIC Period.

abandoned channel The channel cutby a stream that has now ceased to flowthrough it. This may result from the streamforming a new channel; e.g. an oxbow (seeoxbow lake) or cutoff, in which the curveof a meander becomes cut off when thestream breaks through to form a new chan-nel.

abandoned cliff See cliff.

abiotic /ay-bÿ-ot-ik/ Describing a non-living factor in an ECOSYSTEM. Light, rain-fall, soil type, and temperature are allabiotic factors. Chemical residues from ar-tificial fertilizers and pollution constituteharmful abiotic factors. See also biotic.

ablation /ab-lay-shŏn/ The disappear-ance of snow and ice by melting and evap-oration. This can refer to ice crystals orsnow flakes in the atmosphere but is mostcommonly used for glacier ice and surfacesnow cover, when it can also includeavalanching. The rate of ablation is pri-marily controlled by air temperature butsunshine, rainfall, humidity, and windspeed also affect the process.

ablation cone A cone of FIRN, ice, orsnow covered in rock debris caused by dif-ferential ABLATION.

ablation moraine Debris of rock frag-

ments left at the side of a glacier followingABLATION.

ablation till TILL material formerly pre-sent on the surface of glaciers and ice sheetsand subsequently deposited as a result ofthe melting of the ice beneath. These de-posits tend to contain relatively little finematerial because this may be removed bymeltwater prior to deposition. Comparelodgment till.

Abney level /ab-nee/ A simple surveyinginstrument for measuring slope angles. Thelevel is held by hand, objects being viewedthrough a sighting tube. The observer maystand at the top or bottom of a slope, butmust sight onto a point at a height equiva-lent to his or her eye-level. Once an objectis sighted, adjustment is made to a tiltingspirit level, an image of the bubble beingvisible in the sighting tube. When the bub-ble image coincides with the object sightthe angle of slope can be read off an at-tached scale, which is provided with avernier to permit reading down to ten min-utes of arc. The instrument is not very ac-curate, but is small and quick to use. Theinstrument is named for the English engi-neer and photographic chemist Sir Williamde Wiveleslie Abney (1843–1921).

abnormal erosion (accelerated erosion)Erosion acting faster than normal as a re-sult of the removal of the vegetation coverby human agencies. It is abnormal in thesense that it is superimposed upon naturalprocesses. See also blowout; human influ-ence on geomorphology.

abrasion /ă-bray-zhŏn/ The wearing awayof rocks by an agent of transportationcharged with a load of already eroded ma-

1

A

abrasion platform

2

terial, which acts as a tool for cutting,grinding, scratching, and polishing. All themajor transportation agents (runningwater, wind, moving ice, and sea wavesand currents) can abrade so long as theycarry debris. Abrasion by water and icecharacteristically produces rounded forms,and abrasion by ice also produces stria-tions, while the sand-blasting effect ofwind abrasion is most effective in a narrowzone just above ground level, resulting inundercut features. As abrasion takes place,the corresponding reduction in size of theinitial load debris is known as ATTRITION. Adistinction is sometimes made betweenabrasion and CORRASION, which refers tothe erosional processes that result inabraded rock surfaces.

abrasion platform See wave-cut plat-form.

absolute age The age of a fossil, rockformation, or individual rock, usuallystated in years. Such ages are usually deter-mined by DENDROCHRONOLOGY or RADIO-METRIC DATING. See also relative dating.

absolute drought In British climatol-ogy, a prolonged period (usually at least 15days) during which no more than 0.25 mmof rain falls on any day.

absolute humidity The mass of watervapor in a unit volume of air (usuallystated in units of g m–3 or kg m–3). It de-pends on the temperature and pressure ofthe air. See humidity.

absolute instability See instability.

absolute plate motion The motion of aLITHOSPHERIC PLATE with respect to a fixedframe of reference, such as a hot spot orpole of rotation. Compare relative platemotion. See also plate tectonics.

absolute stability See stability.

absolute temperature A temperaturescale based on the coldest temperature thatis physically possible. This absolute zero oftemperature is –273.l5°C, but for meteoro-

logical purposes the absolute temperatureis taken as the Celsius temperature plus273°, so that the freezing point of water is273° and boiling point 373°. Formerlymeasured in degrees absolute (°A), it isnow measured in KELVINS.

absolute vorticity See vorticity.

absorption 1. The process by which en-ergy is converted into another form of en-ergy when a waveform is transmittedthrough a medium (e.g. the loss of energyby a seismic wave in the form of heat).2. (in meterology) The conversion of short-or long-wave radiation to a different formof energy by gases in the atmosphere. Ab-sorption is highly selective in terms ofwavelength; some wavelengths are entirelyabsorbed and others are totally unaffected.The main atmospheric gases, oxygen andnitrogen, are not very important as ab-sorbers of radiation, but minor gases suchas carbon dioxide, water vapor, ozone, andnitrous oxide have a significant effect. Seealso atmospheric window.

abstraction 1. The fraction of PRECIPI-TATION that does not immediately run off.It is absorbed, evaporated, stored, or tran-spired.2. The union of two streams resulting fromerosion of the land (DIVIDE) between them.It generally occurs in gullies and ravines.

ABW See Arctic bottom water.

abyss /a-biss/ See deep.

abyssal hill /a-biss-ăl/ A large dome-shaped submarine hill on the ABYSSAL

PLAIN. Such hills rise to heights of 1000 min up to 6000 m of water and may be sev-eral kilometers wide. There are many alongthe MID-ATLANTIC RIDGE and in the PacificOcean.

abyssal plain An extremely smoothportion of the deep-sea floor. The gradi-ents across abyssal plains fall within therange of 1:1000 to 1:10 000, which meansthat variations in depth amount to only afew meters. This remarkable degree of flat-

ness has come to light because of deep-seaphotographs and high-precision soundingtechniques. In the Atlantic Ocean, abyssalplains range between 200 and 400 km inwidth, but they can be several hundredkilometers wide. They tail upwards intothe continental rise and frequently, in aseaward direction, merge into ABYSSAL

HILLS. Their sediments are varied; whilemost are thinly veneered with pelagic sedi-ment, perhaps globigerina ooze and redclay, they also display sediments and plantand animal remains that normally charac-terize shallow-water environments. Thereason for this may be the operation ofTURBIDITY CURRENTS.

abyssal rock A type of intrusive igneousrock formed deep within the Earth’s crust.

abyssal zone A zone of greatest oceandepth, lying seaward of and deeper thanthe BATHYAL ZONE of the continental slope,that is below a depth of some 1000 m, andincluding the deeper parts of the oceansand the deep-sea trenches. Lying betweenthe abyssal and bathyal zones is the CONTI-NENTAL RISE, which is often bordered on itsseaward flank by abyssal plains andabyssal hills. The pelagic–abyssal environ-ment (see pelagic (def. 1)), which is notreached for at least several hundred kilo-meters from the coast, globally representsan area of 250 × 106 sq km, i.e. roughlyhalf the area of the Earth. Few organismslive in these depths, where pressure is highand light does not reach, and deposition ofsediment is very slow.

Acadian orogeny A phase of mountainbuilding that affected the North Ap-palachian region of North America, ex-tending from what is present-day NewYork to the Gaspé region of Canada. Ittook place during the late Devonian andearly Mississippian Periods, spanningabout 50 million years, and resulted fromthe westward movement of the Avalon ter-rane onto the continental landmass of Lau-rentia, closing a part of the proto-Atlantic(Iapetus) Ocean.

acanthite See argentite.

Acanthodii A class of extinct fishes,sometimes known as the spiny sharks, thatincluded the earliest-known jawed verte-brates. The Acanthodii appeared duringthe Upper Silurian Period, some 400 mil-lion years ago, and lasted through to thePermian Period.

accelerated erosion See abnormal ero-sion.

acceleration of free fall (acceleration dueto gravity) The acceleration (g) of a bodyfalling freely in a vacuum in the Earth’sgravitational field. The standard value is9.806 65 m s–2, although actual values de-pend on the distance from the Earth’s cen-ter of mass.

accessory mineral A mineral that ispresent in small quantities in a rock anddoes not affect the overall character of thatrock for classification purposes. Compareessential mineral.

acclimatization The process by whichhumans become adapted to living in amarkedly unfamiliar climatic regime. Thisnormally refers to a change to hot andhumid or especially cold climatic condi-tions. If acclimatization has not takenplace, severe physiological stress may re-sult. The full process may take up to tenyears.

accordant /ă-kor-dănt/ (conformable)Describing a drainage pattern that is con-trolled by the structures over which itflows. Actual patterns vary greatly, de-pending on the nature of the structures orlithologies. Compare discordant (def. 2).

accordion folding /ă-kor-dee-ăn/ A typeof folding in rocks, in which the beds of thehinge area are markedly thickened andsharply folded, while on the limbs the bedsare straight and of uniform thickness.

accretion /ă-kree-shŏn/ 1. (in meteorol-ogy) The process of ice crystal growth bycollision with water droplets in clouds.

3

accretion

This is one of the mechanisms by whichminute cloud droplets achieve sufficientsize to give rainfall.2. See nucleation.

accretionary wedge (accretionary prism)A collection of sediments that accumulatesat a PLATE BOUNDARY, on the landward sideof an ocean TRENCH. It consists of intenselydeformed sediments that have beendragged down on the subducting oceanicplate then scraped off onto the overridingcontinental plate to form the wedge. Seriesof wedges ride up, one upon the other, withrepeated thrust faults. See also plate tec-tonics; subduction zone.

accumulated temperature A methodof indicating the excess or deficit of tem-perature with reference to a specified valuefor a specified period. Two temperaturebases that have been frequently used are6°C for the commencement of plantgrowth and 16°C for heating requirements.The accumulated temperature is calculatedby taking the number of hours in a specificperiod the temperature was above orbelow the set value and multiplying by themean temperature during this time to givethe number of degree hours. More com-monly, daily or monthly values of accumu-lated temperature are obtained from dailyor mean values rather than hourly ones.

accumulation 1. The total amount ofPRECIPITATION that gathers on a snowfieldor glacier.2. The overall result of all processes thatadd mass to a snowfield, glacier, or ice floe,including snow from avalanches, snowfall,and windblown snow.

achnelichs /ak-nĕ-liks/ Fragments ofglassy smooth PYROCLASTIC ROCK derivedfrom basaltic LAVA that has sprayed in theair while molten and solidified. Pele’s tearsare achnelichs.

achondrite /ay-kon-drÿt/ A stony METE-ORITE that does not contain CHONDRULES.Compare chondrite.

acicular /ă-sik-yŭ-ler/ Describing a crys-tal having a needle-like habit. See crystalhabit.

acid brown soil A type of soil found inthe BROWN EARTH zone on base-deficientparent materials. Such soils are stronglyacid, with a moder humus, and althoughthe B horizon is more clearly differentiatedby color than in the true brown earth thereis no appreciable eluviation of clay or ironoxides. With time it seems likely that thesesoils would become podzolic brown earths.

acid lava Slow-moving viscous LAVA

containing a high proportion of silica. Pro-duced by so-called acidic volcanoes, it gen-erally solidifies very quickly. See also basiclava.

acid rain (acid precipitation) Any PRE-CIPITATION (including fog, rain, sleet, andsnow) that is acidic due to the presence ofsulfur dioxide and nitrogen oxides in theatmosphere. Most of these pollutants de-rive from the burning of FOSSIL FUELS. Acidrain kills trees, poisons rivers and lakes,and corrodes buildings. Runoff of acid rainaffects the mineral composition of the soil.

acid rock The dominant chemical con-stituent of igneous rocks is silica, SiO2,which typically ranges from 35–75% (byweight). Arbitrary divisions are made asfollows: acid – SiO2 >66%, intermediate –SiO2 55–66%, basic – SiO2 45–55%, andultrabasic – SiO2 %t;45%. These strict di-visions have been largely abandoned butthe general descriptive terms acid, basic,etc., remain. In current usage, an acid rockcontains in excess of about 10% freeQUARTZ, e.g.: granite, granodiorite, rhyo-lite.

acid soil A soil with a pH value of lessthan 7.0. Soil acidity is measured on thepH scale, which is related to the concentra-tion of hydrogen ions in the soil (a neutralsoil is given a pH value of 7.0; if the valueis greater than this it is termed an alkalinesoil). Thus there is an inverse relationshipbetween pH value and the concentration ofhydrogen ions. Acid soils develop where,

accretionary wedge

4

for some reason, there is a lack of the ex-changeable bases in the soil, such as cal-cium and sodium. The bases have beenlargely replaced by the two cations hydro-gen and aluminum, which control soilacidity. Possible acid-forming factors areleaching, organic matter containing fewbases, and an acid parent material. Acidsoils are therefore particularly common inthe humid tropics and the humid temperatelands. Examples of typical acid soils arepodzols, brown earths, and latosols. Fromthe agricultural viewpoint most cultivatedcrops will thrive on mildly acid soils.

acmite /ak-mÿt/ See aegirine.

acre A unit of area equal to 4840 yd2

(equivalent to 4046.86 m2 or 0.4047hectares).

acre foot A unit of volume equal to anarea of 1 acre to a depth of 1 foot (equiva-lent to 43 560 ft3 or 1233.5 m3). It is usedfor expressing the volumes of lakes andreservoirs.

acritarchs A group of organic-walledmicrofossils of various forms in the sizerange of 5–200 µm across (most are20–150 µm across). It is not known whatthese organisms were although they are be-lieved to be derived from phytoplankton(the name acritarch means of uncertain ori-gin). Acritarchs range from Precambrian toHolocene and are especially useful in thebiostratigraphy of the Proterozoic Eon andPaleozoic Era.

actinolite A monoclinic AMPHIBOLE.

actinomycetes /ak-tă-noh-mÿ-seets/ Aer-obic bacteria that have a filamentous habitas do fungi. Optimum conditions are amoist well-aerated soil with a pH between6.0 and 7.5. They are more prolific thanother bacteria and fungi in drier areas andare important also in that they can decom-pose the more resistant soil organic matter.

Actinopterygii /ak-tă-nop-tĕ-rij-ee-ÿ/Ray-finned bony fish in which the pairedfins are not fleshy (compare Crossoptery-

gii) but have narrow bases and are sup-ported by thin fin rays. The Actinopterygiialso have a single dorsal fin. The group in-cludes the modern TELEOSTEI. See also Os-teichthyes.

active continental margin (active mar-gin) A margin of a continent that co-incides with the margin of a LITHOSPHERIC

PLATE where earthquakes, volcanic activity,and mountain building are taking place,and that is often bordered by an oceanictrench offshore. Compare passive conti-nental margin.

active layer See permafrost.

actual evapotranspiration (AE) Thetrue amount of water vapor returned to theatmosphere through evaporation from aground surface and transpiration from itsplant cover where the water supply is lim-ited. Compare potential evapotranspira-tion.

actualism /ak-chû-ă-liz-ăm/ See unifor-mitarianism.

actuopaleontology /ak-chû-oh-pay-lee-on-tol-ŏ-jee, -pal-ee-/ The branch of paleon-tology in which investigations into modernorganisms, including their effects and re-mains in modern environments, are di-rected toward the understanding of fossilanalogs. Based upon uniformitarian princi-ples (see uniformitarianism), actuopaleon-tology provides a means of relating variousTRACE FOSSILS to the particular animalsfrom which they are derived and of under-standing the changes that an organism un-dergoes between death and fossilization(see taphonomy; thanatocoenosis). BIO-STRATONOMY is one aspect of actuopaleon-tology.

adamantine /ad-ă-man-tin, -teen, -tÿn/Describing a mineral that has a brilliant di-amondlike luster.

adamellite /ad-ă-mel-ÿt/ A variety ofgranite consisting of about equal propor-tions of potassium feldspar and sodic pla-

5

adamellite

gioclase feldspar together with one or moreferromagnesian minerals.

adiabatic /ad-ee-ă-bat-ik/ Denoting anatmospheric process in which there is noexchange of heat between the system andits environment. In the more rapid ex-changes, such as thermals rising from theground surface, this is a reasonable ap-proximation. In these circumstances, thechange in temperature of rising air is deter-mined by the physical properties of the airand the external pressure. As air pressuredecreases with height above the groundsurface, rising air expands, and exerts me-chanical work on its environment. This ne-cessitates a loss of heat energy from therising air and its temperature falls. The rateof fall of temperature in an adiabaticprocess is constant for our atmosphericcomposition, being 0.98°C/100 m, and isknown as the DRY ADIABATIC LAPSE RATE.See also saturated adiabatic lapse rate.

adit /ad-it/ A horizontal or slightly up-ward-sloping closed-ended passage into amine, usually constructed to intersect aseam of coal or vein of mineral. The slopeensures that any water drains out of theadit.

adjacent sea See marginal sea.

adobe /ă-doh-bee/ A mixture of silt andclay, common in Mexico and the south-western USA. It has long been used formaking bricks because it dries to a hardweatherproof mass. See also loess.

adret /ad-ret/ The sunny or, in the Nhemisphere, south-facing slope of a valley.It is the side favored for farming, as in theEuropean Alps.

adsorption (in soil science) The processby which compounds, molecules, or ionsare taken up and attached to the surface ofa particle (typically clay or humus) bychemical or physical means.

adularia /aj-ŭ-lair-ee-ă/ A variety of al-kali FELDSPAR.

advection /ad-vek-shŏn/ The horizontalcomponent in the transfer of air properties.For example, the heat and water vaporcontent of the air at the Earth’s surfacevaries appreciably and by the wind systemsthese properties are transferred to otherareas. With winds from tropical latitudesthere is advection of warm air, and frompolar latitudes, advection of cold air.

advection fog Fog formed by the hori-zontal transfer of moist air over a cold sur-face, which sufficiently cools the lowerlayers of the atmosphere to give saturationand condensation. In summer it occursover cool seas, such as the North Sea, theLabrador area, and off the coast of Cali-fornia, frequently affecting the adjacentcoasts. In winter the advection of warmmoist air over a cold snow-covered groundcan also produce this type of fog.

AE See actual evapotranspiration.

aegirine /ee-jeer-ÿn, -een, -in/ (acmite) Asodic clinopyroxene. See pyroxene.

aegirine-augite See pyroxene.

aeolian erosion See eolian erosion.

aeolian form See eolian form.

aeolian transport See eolian transport.

aerial photography Photography ofthe Earth’s surface from airborne plat-forms, such as satellites, spacecraft, andaircraft; it is the oldest method of REMOTE

SENSING. The photographs may be verticalor oblique (high-oblique or low-oblique)and are frequently taken in runs of over-lapping prints, which can be assembledinto mosaics (print lay-down). Three-dimensional images can be viewed whenpairs of photographs taken with a stereo-scopic camera are viewed through a stereo-scopic viewer (stereoscope). Aerialphotography has widespread applicationswithin the earth sciences, e.g. in map-making and the revision of existing maps,and the production of three-dimensionalmodels of the land surface; in land-use

adiabatic

6

planning and mapping; and in providingdata on topography, geology, hydrology,and vegetation and soils. Color infraredfilm, which is sensitive to near-infraredwavelengths and visible wavelengths, isused, for example, in vegetation and land-use studies.

aerodynamic roughness An index ofthe nature of airflow near the ground sur-face. A surface is aerodynamically smoothif there is a layer of air immediately aboveit that has laminar flow. However, in me-teorological terms, nearly all surfaces areaerodynamically rough, producing turbu-lent flow down to the ground surface, evenfor the lightest winds.

aerolite /air-ŏ-lÿt/ A stony METEORITE

made up of silicate minerals.

aeromagnetic survey Measurement ofvariations in the Earth’s magnetic field car-ried out using an instrument known as amagnetometer that is attached to an air-craft (airplane or helicopter). The aircraftmay be flown at a fixed height above sealevel or at a constant height above theground surface along flight lines that are afixed distance apart. The magnetometerusually records the total intensity of themagnetic field along the flight lines. Differ-ences between the measurements obtainedcompared to theoretical values representchanges in the type of rock, or the thick-ness of the rock, below the ground surface.

aeronautical chart A form of map pro-duced essentially for air navigation or pi-lotage. In addition to showing the relevanttopographical features, such as contours,vegetation, roads, and cultural detail, themap shows supplementary information forspecialized use, such as detailed verticalobstruction information, flight areas, aircorridors, etc.

aeronomy /air-on-ŏ-mee/ The science ofthe upper atmosphere of the Earth, oratmosphere of other planetary bodies. Itincludes study of the chemical composi-tion, physical properties, motions, andeffects of radiation. Dissociation and ion-

ization of gas molecules takes place; thelower limit for these processes to occur isabout 30 km. As the relationships betweenthe upper and lower atmosphere are as yetlittle understood, there has been consider-able research recently into the possible ef-fects of these upper levels in troposphericweather.

aerosol A particle of matter that islarger than a molecule but small enough toremain suspended in the atmosphere.Aerosols may be solid or liquid and play animportant part in many atmosphericprocesses, such as precipitation formation,atmospheric electrification, radiation bal-ances, and visibility. The origins ofaerosols are diverse. Over sea areas, seaspray provides large salt nuclei and overland, weathering dusts of clay particles areprobably the major source.

afforestation /ă-fô-rĕ-stay-shŏn/ Thelarge-scale planting or replanting of trees.They may be planted to prevent soil ERO-SION, to act as WINDBREAKS, or as a sourceof timber (usually for wood pulp).

aftershocks A series of minor shocks orvibrations that follow the main series ofEARTHQUAKE shocks. In general they origi-nate at or near the focus of the main earth-quake as a result of the readjustment ofrocks that have over-reacted during themain period of movement. Dependingupon the size of the earthquake they cancontinue for a few days or months.

agate /ag-it, -ayt/ An extremely fine-grained type of QUARTZ consisting of alter-nate bands of colored CHALCEDONY. It maybe white, gray, red, brown, or black, andcan be dyed other colors. It generally oc-curs in rock cavities. It is used for makingornaments and as a semiprecious gem-stone. See also moss agate; silica minerals.

age An interval of geologic time in theChronomeric Standard scale of chronos-tratigraphic classification (see chronos-tratigraphy). The equivalent StratomericStandard term, indicating the body of rockformed during this time, is the STAGE. Ages

7

age

may be grouped together to form EPOCHS

and may themselves be comprised of sev-eral CHRONS.

ageostrophic wind /ay-gee-ŏ-stroff-ik/The vector difference between the actualwind and the geostrophic wind. Withoutan ageostrophic component there would beno changes in pressure systems and atmos-pheric circulation would be a perfect bal-ance between the pressure gradient and theCORIOLIS EFFECT.

agglomerate /ă-glom-ĕ-rayt vb., <-glom-ĕ-rit, -rayt n./ A mixture of coarse angularfragments of rock and finer-grained ma-terial formed during a volcanic explosion.See pyroclastic rock.

agglutinate See pyroclastic rock.

aggradation /ag-ră-day-shŏn/ The rais-ing of the level of the land surface throughdeposition, e.g. as a result of fluvial, ma-rine, eolian, or slope processes. The causeof aggradation in a river is incompetence(see competence) or incapacity (see capac-ity) of the river to transport its load, lead-ing to deposition. This may arise throughan increase in the volume of load suppliedto the river, a loss of speed or volume offlow, or most commonly a rise in baselevel. Another common cause of aggrada-tion is the damming of a stream, artificiallycreating a new base level for the upperreaches of a river. Originally graded to sealevel, the upper reaches will aggrade to thenew base level, the height of the dam’sspillway. See also degradation.

aggregate 1. Mineral fragments and/orrock particles that combine to form a hardmass.2. (in soil science) A cluster of soil particlesthat have bonded together through aggre-gation.

aggregation 1. (in meteorology) Theprocess by which ice crystals grow to formsnowflakes through collision and clumpingtogether.2. (in soil science) The process by whichsoil particles bond together to form soil ag-gregates. The presence of organic matteraids the process.

agmatite /ag-mă-tÿt/ A MIGMATITE inwhich melanocratic material occurs as an-gular inclusions in a leucocratic granitichost, giving an overall appearance resem-bling a breccia.

Agnatha /ag-ă-th’ă/ Primitive jawlessfish, from which all more advanced verte-brate types have presumably evolved.Modern species are few and representgroups unknown as fossils. The diverse ex-tinct forms can be grouped together as theOSTRACODERMI, which were abundant inthe Silurian and Devonian and are used instratigraphic CORRELATION. CompareGnathostomata.

agonic line /ă-gon-ik, ay-/ A line joiningall points on the surface of the Earth wherethe ANGLE OF DECLINATION is zero. Alongsuch a line, magnetic north and true northcoincide. See also isogon.

Agulhas Current A warm surface cur-rent that flows in the S Indian Ocean fromthe South Equatorial Current (see equator-ial current) and is additionally fed by theMozambique Current. It flows south-west-ward off the E coast of southern Africa tojoin the eastward flow between Africa andAustralia. It is one of the fastest flowingocean currents with a maximum flow ve-locity of some 5 knots off the SE coast ofSouth Africa.

A horizon The uppermost layer of soil,or topsoil. It consists of fine soil particles

ageostrophic wind

8

long profileof stream

height of spillway =new base level

aggradation inthis stretch

erosiondownstream

SEA(ultimate base level)

Aggradation in a dammed stream

and HUMUS. Some soluble material willhave been dissolved out and passed down-ward to the B HORIZON beneath. See alsohorizon (def. 1).

air The mixture of gases in the atmos-phere. Its composition is almost uniformthroughout the troposphere and is shownin the table below. Only water vapor andcarbon dioxide vary appreciably, the for-mer in relation to evaporation and precipi-tation and the latter through plantphotosynthesis.

air-fall deposit A sediment composedof fallout from a cloud of airborne materialfrom an erupting volcano, as opposed toLAVA (which generally flows from a vol-cano).

air mass An area of the lower atmos-phere with similar properties of tempera-ture and moisture in the horizontal field.At the margins of the air mass, temperaturegradients become steep at a transition zoneknown as a FRONT. The uniformity ofproperties is achieved by prolonged con-tact with the underlying surface and littlehorizontal or vertical mixing. These re-quirements are experienced in areas of highpressure or anticyclones, which are the

main source areas for air masses. Awayfrom their source areas, air masses undergomodification by coming in contact withdifferent surfaces with the result that in ashort period of time they can become in-distinguishable. As most parts of the worldrepresent modification zones rather thansource areas, air mass terminology is lessfrequently used than formerly. There havebeen many attempts to classify air masses,but the most frequently quoted is that bythe Swedish meteorologist Tor Bergeron(1891–1977). Two basic air masses areidentified on thermal properties – polar (P)and tropical (T), and two by moisture cat-egories – maritime (m) and continental (c).The temperature of the air mass relative tothe surface over which it is passing is in-cluded as warm (w) or cold (k) to give awide range of combinations – mPw, cTk,etc. The American meteorologist Hurd C.Willett (1903–92) modified this systemslightly to add stable (s) or unstable (u) toindicate the likelihood of precipitation inthe system. Other classifications distin-guish equatorial, monsoon, and Arctic (orAntarctic) but difficulties can arise in iden-tification.

air pollution Substances (e.g. gases,aerosols, and particles) released into the at-mosphere, by natural or human activity,that can cause harm to animals and plants.The combustion of fossil fuel is a majorsource of air pollutants: exhaust fromgasoline and diesel engines contributecarbon monoxide, volatile organic com-pounds, and nitrogen oxides; electricity-generating plants and industrial plantsburn fossil fuels releasing sulfur oxides;ground-level ozone is formed in a photo-chemical reaction when sunlight acts on ni-trogen dioxides and volatile organiccompounds close to the ground. Particu-late matter also contributes to air pollu-tion, e.g. emissions of soot particles fromthe smokestacks of power plants and parti-cles released during industrial processesand fires. The presence of aerosols andgaseous pollutants above an urban or in-dustrial area may occur in sufficient quan-tities to form SMOG.

9

air pollution

COMPOSITION OF AIR(% by volume)

nitrogen (N2) 78.08oxygen (O2) 20.95carbon dioxide (CO2) 0.03argon (Ar) 0.93neon (Ne) 1.82 × 10–3

helium (He) 5.24 × 10–4

methane (CH4) 1.5 × 10–4

krypton (Kr) 1.14 × 10–4

xenon (Xe) 8.7 × 10–5

ozone (O3) 1 × 10–5

nitrous oxide (N2O) 3 × 10–5

water (H2O) variable, up to 1.00

hydrogen (H2) 5 × 10–5

air stream A flow of air that is not nec-essarily homogeneous but has a distinctiveorigin. Air streams are therefore distin-guished by their direction of approachrather than assuming any specific thermalor stability properties. The mid-latitudewesterlies can be regarded as being a mix-ture of slightly baroclinic air streamsbounded by sharp frontal zones.

Airy’s hypothesis of isostasy /air-eez/The English astronomer George BiddellAiry (1801–92) proposed that in order forisostatic equilibrium to exist, mountainranges must have roots proportional totheir height, i.e. the highest mountainshave the deepest roots. These roots arecomposed of sialic material and displace anequivalent volume of sima, thereby causingthe gravity anomalies present near moun-tain chains.

Aitken nucleus /ayt-kĕn/ See nucleus.

Aitoff’s equal-area projection /ÿ-toff/A CYLINDRICAL PROJECTION of a hemi-sphere. The major axis, the Equator, istwice the length of the minor, centralmeridian, axis. The projection is boundedby an ellipse. The main characteristics ofthis projection are that it is an equal-areaprojection and landmasses near the centerof the area covered are of quite good shape,although the distortion increases towardsthe east and west limits of the projection. Itis named for the French cartographerDavid Aitoff (1854–1933). See also mapprojection.

AIW See Antarctic intermediate water.

alabaster A fine-grained white, some-times translucent, variety of GYPSUM, usedfor making ornaments.

Alaska Current A comparatively warmsurface oceanic current that flows counter-clockwise in the Gulf of Alaska. It resultsfrom a northward extension of part of theNorth Pacific Current where this is di-verted on meeting the North Americancontinent.

albedo /al-bee-doh/ An index of reflec-tion comprising the ratio of reflected radi-ation to the total incident radiation.Usually this value is expressed in a percent-age form for visible wavelengths. Typicalvalues for surface albedo are: forest5–10%, wet soil 10%, sand 20–30%, grass25%, old snow 55%, concrete 17–27%,fresh snow 80%. Water surfaces vary fromabout 5% with high sun and calm seas to70% at low elevation and rough seas. Theplanetary albedo of the Earth measuredfrom artificial satellites is approximately34%, which means that over one third ofthe Sun’s radiation is returned to spacewithout a change of wavelength.

Alberta clipper (Alberta low) A fast-moving low-pressure system that forms orredevelops, generally in winter, to the lee ofthe Canadian Rocky Mountains, in or nearAlberta. The depressions track eastward tobring cold snowy weather and strong gustywinds around the Canadian–US border,into the plains states of the N USA andacross the Great Lakes. Sometimes the de-pressions travel as far as the MaritimeProvinces of Canada and the NE USAwhere they may intensify on meeting rela-tively warm moist Atlantic air off the eastcoast and bring heavy snow over land.

albite /al-bÿt/ A sodic plagioclase FELD-SPAR.

Aleutian Low /ă-loo-shăn/ The meanlow pressure center in the N Pacific Ocean.It represents a statistical average of pres-sure value and location, which in turn aredetermined by the tracks of the depressionsand the point at which they reach theirlowest pressure. It is most marked in thewinter.

alexandrite /al-ig-zan-drÿt/ A transpar-ent emerald-green type of CHRYSOBERYL,used as a semiprecious gemstone. It has theunusual property of turning red in artificiallight.

alfisol /al-fă-sôl/ One of the twelve soilorders of the US SOIL TAXONOMY, coveringpedalfers that are equivalent to the gray-

air stream

10

brown podzolic, gray-wooded, gray-forest,sol lessivé, degraded chernozem, andplanosol soils of the old American classifi-cation. They are found in the humid re-gions of the world under deciduouswoodland or grassland vegetation. Thedominant soil-forming process is leaching,which is more intense in these soils than inthe inceptisols but less than in the spo-dosols. They are productive soils and favorthe more common agricultural crops.

algae /al-jee/ A group of largely aquaticorganisms formerly classified as plants. Itincludes both microscopic forms, such asthe DIATOMS, as well as the multicellularseaweeds, which may grow to a large size.Algae are subdivided into different phylathat are now usually placed in the kingdomProtista (or Protoctista). The only geologi-cally significant algae are those havinghard parts, which may form bioherms, ei-ther by trapping sediment or by secretingmassive laminated structures of calciumcarbonate. Such structures commonly con-stitute lower Paleozoic REEFS. Comparedwith bacteria and fungi, algae are relativelyunimportant in soils, but they are often pi-oneers in colonizing new ground and maynumber as many as 100 000 per gram ofdry surface soil.

algal bloom /al-găl/ A sudden increasein the numbers of ALGAE in a body of watersuch as a sea, lake, or river. It may becaused by an increase in the amount of ni-trates, phosphates, and other nutrients.Forecasting systems to provide advancewarning of harmful algal blooms havebeen established in some locations (e.g.Karenia brevis blooms in the Gulf of Mex-ico) using sources that include satellite im-agery, field observations, and data frombuoys. See eutrophication.

alidade /al-ă-dayd/ A surveying instru-ment used for sighting onto objects of de-tail and for defining the rays to be drawn tothem in PLANE TABLING. The alidade is basi-cally a ruler of metal or wood with a verti-cal slit sight at the observer’s end and avertical stretched wire sight at the other. (Atelescope is fixed parallel to the ruler in

more sophisticated types of alidade.) Theruler edge is placed against the pointmarked on the table over which the appa-ratus is standing, and detail to be fixed issighted onto. A ray is then drawn on theplane table sheet along the ruler edge. Al-though either side of the ruler may be usedinitially, once one ray has been drawn, onlythat side may be used until that sheet iscompleted.

alkali /al-kă-lÿ/ (alkaline) (in petrologyand mineralogy) Denoting igneous rocksand minerals that have high contents of thealkali metal oxides, Na2O and K2O. For agiven silica content such rocks are rela-tively richer in sodium and potassium andpoorer in calcium compared with calc-al-kaline rocks. See alkali basalt; alkali gab-bro; granite; ijolite; nephelinite.

alkali basalt A basic undersaturatedvolcanic rock that is the fine-grained equiv-alent of ALKALI GABBRO. The essential min-erals of all basalts are plagioclase feldsparof labradorite-bytownite composition andpyroxene. In alkali basalts the pyroxene isaugite or titanaugite and olivine is presentin abundance. Olivine is frequentlyrimmed or pseudomorphed by the alter-ation products iddingsite and/or serpen-tine. Small amounts of alkali feldsparand/or feldspathoid (nepheline or analcite)may be present. Alkali basalts are typicallyporphyritic. Basalts containing large pla-gioclase phenocrysts are referred to asfeldsparphyric or big feldspar basalts.Those rich in olivine and augite are termedpicrite basalts. Oceanite and ankaramiteare varieties in which olivine and augite re-spectively have become concentrated.

Alkali basalts are usually holocrys-talline and have ophitic or intergranulartextures. Nodules of gabbro and peridotiteare often found in alkali basalt lavas. Withan increase in the amount of nepheline togreater than 10%, alkali basalts pass intobasanites (olivine-bearing) and tephrites(olivine-free). In some rocks, the place ofnepheline is taken by analcite or leucite andsuch terms as analcite-basanite and leucite-tephrite are appropriate. See also basalt.

11

alkali basalt

alkali feldspar A member of a series ofminerals with composition varying be-tween the two end-members albite(NaAlSi3O8) and orthoclase (KAlSi3O8).See feldspar.

alkali gabbro Alkali gabbros andsyenogabbros are basic plutonic rocks con-taining, in addition to the normal gabbromineralogy, alkali feldspar and/or felds-pathoids. Syenogabbros contain approxi-mately equal amounts of alkali andplagioclase feldspar with TITANAUGITE,ANALCIME, and/or NEPHELINE plus or minusolivine. Having more sodium and potas-sium than gabbros, syenogabbros are, asthe name implies, related to SYENITES.

Essexite consists of labradorite, ti-tanaugite, and olivine together with smallamounts of nepheline and/or analcime. Al-kali feldspar, apatite, and ilmenite mayalso be present. Teschenite and crinaniteare analcime-bearing varieties from whichnepheline is excluded. Theralite is anepheline-bearing gabbro containing noanalcime. Kentallenite is a saturated rockcontaining augite, olivine, biotite, la-bradorite, and orthoclase and is equivalentto olivine-monzonite.

The names of the plutonic rocks are ap-plied to the medium-grained varieties of doleritic aspect but with the prefix‘micro-’. The volcanic equivalents includebasanites, tephrites, and trachybasalts.Teschenite and theralite are found in dif-ferentiated sills and dikes.

alkaline soil A soil with a pH valuegreater than 7.0. Compare acid soil.

allanite /al-ă-nÿt/ (orthite) One of theEPIDOTE group of minerals.

Allerød /al-ĕ-rohd/ A phase of warmingof about 1000 years during the period ofdeglaciation after the Würm/Weichsel/Wis-consin ice age. In many parts of NW Eu-rope this was followed by a sudden coolingfrom 8800 to 8300 BC. The type-site fromwhich the period takes its name is in Den-mark.

allivalite /al-ă-vă-lÿt/ A gabbro consist-ing of olivine and plagioclase feldspar ofbytownite–anorthite composition.

allochem /al-ŏ-kem/ A discrete calcare-ous particle that has usually been trans-ported at some stage, including FOSSILS,ooliths (see oolite), INTRACLASTS, and PEL-LETS, found in LIMESTONES; the terminologyused in the petrographic description andclassification of limestones is based uponthese constituents and the matrix in whichthey are set (see micrite; sparite). Thus alimestone composed of fossil fragments setin a micrite matrix is a biomicrite; a pelletlimestone with a sparite matrix is apelsparite.

allochthonous /ă-lok-thŏ-nŭs/ Denot-ing an isolated mass of rock displaced overa considerable distance from its originalsource by tectonic processes such as low-angle thrusting. The mass of rock is knownas an allochthon. Compare autochtho-nous.

allogenic /al-ŏ-jen-il/ (allothigenous) De-scribing rock constituents that wereformed at some distance from their presentlocation, particularly minerals or rockfragments derived from existing rocks.Compare authigenic.

allotriomorphic /al-ŏ-trÿ-mor-fik/ De-scribing a rock in which the majority ofcrystals are euhedral. Compare hypid-iomorphic; idiomorphic.

allotrope A form of a chemical elementthat differs (usually in its crystal structure)from another form of the element.Graphite, diamond, and buckminster-fullerene, for example, are allotropic formsof carbon.

alluvial cone A steep alluvial depositthat forms where a fast-flowing stream de-bouches onto a lowland plain. Apart fromits steepness, it in other respects resemblesan ALLUVIAL FAN.

alluvial fan A fan of material depositedby a stream where it debouches from a

alkali feldspar

12

mountain front onto a plain, with the apexof the fan at the point of emergence fromthe mountains. In its mountain tract, thestream will have been confined into a sin-gle flow in a rock-cut gorge, but on reach-ing the plain this control is lost, and itbreaks up into a number of distributaries.This increases the wetted perimeter, orarea of contact between water and land,and hence friction increases; with more ofits energy used in overcoming friction, lessis available for sediment transport, so de-position occurs, in the form of a fan. Thisis the direct landborne analogy to DELTA

formation, and alluvial fans are character-istic landforms of arid and semiarid envi-ronments.

alluvium The products of SEDIMENTA-TION by rivers, sometimes including de-posits in estuaries, lakes, and other bodiesof fresh water. Alluvium includes materialof a wide range of PARTICLE SIZES, usuallyrestricted to the silt size fraction of0.006–0.02 mm.

There is a marked decrease in the size ofalluvial material down-valley, with finermaterial in the lower reaches. This may bedue to SORTING, which leaves coarse ma-terial behind and carries finer materialdown-valley, or it may be due to progres-sive wear of the material as it travels down-stream. Another cause may be that smallervalley-side slopes downstream supply afiner caliber of material to the stream.

Within the FLOODPLAIN, alluvium (in thelooser sense) varies in size from the finestclay and silt-sized material through sandsto coarse angular gravels. The gravels form

the basal portions of alluvial valleys, andmay have originated at the end of the PLEIS-TOCENE period under PERIGLACIAL condi-tions; in some rivers they are attributed todeposition within the channel in conditionsin which severe scour removed the finematerial. Silts, sands, and gravels are moreimportant, constituting 75% of the Missis-sippi alluvium. Sands constitute the pointbars formed on the inside of meanderbends, and are an important constituent oflevées. Silts and clays are deposited in thelee of point bars or as overbank deposits intimes of flood, covering the far reaches ofthe floodplains with backswamp deposits.

almandine /al-măn-deen, -din/ A lightred to red-brown member of the GARNET

group of minerals, Fe3Al2(SiO4)3. Deep redcrystals are valued as semiprecious gem-stones.

alnoite /al-noh-ÿt/ A basic or ultrabasicdike rock composed largely of melilite andbiotite with subordinate pyroxene, calcite,and olivine. Alnoite is found in ijolite-car-bonatite complexes.

alp A region of grassland on a highmountainside. Above the timberline andcovered with snow in the winter, alpinegrassland may be used as pasture for graz-ing animals in summer. It is named afterthe Alps in west-central Europe.

alpine A type of climate found in moun-tainous areas above the timberline butbelow any PERMAFROST level. Grass andother low-growing plants make up the typ-ical vegetation. See also alp.

Alpine folding A recent type of large-scale FOLDING that created such mountainranges as the Alps, Andes, Himalayas, andRockies.

alpine glow (alpenglow) A sequence ofcolors seen on snow-covered mountaintops when the Sun nears the horizon atsunset. The mountain tops are first yellow-orange, then pinkish, and finally purple.The series of colors, reversed, also occurs

13

alpine glow

MOUNTAIN

FRONT

PLAIN stream

distributaries

contours of fan

Alluvial fan

at sunrise on mountain tops facing the ris-ing Sun but is less common.

Alpine–Himalayan chain A predomi-nantly east–west trending orogenic belt,extending from Spain through Europe to SAsia. It was formed mainly in theOligocene and Miocene Epochs by the clo-sure of the former Tethys Sea, mainly as aresult of the northerly drift of the Africancontinental plate. It consists of a series ofigneous, metamorphic, and deformed sedi-mentary rocks, which now form the Alpsand Himalayan Mountains.

altiplanation /al-ti-plă-nay-shŏn/ A pro-cess associated with a PERIGLACIAL environ-ment in which terraces or benches are cutin solid rock in hillside or summit loca-tions. The weathering and erosion of therocks is achieved by a combination ofFROST shattering, SOLIFLUCTION, and CON-GELITURBATION. Terrace formation is prob-ably initiated by the production of aNIVATION HOLLOW in hard rock, the back-wall of which then retreats, causing an en-largement of the flat or gently slopingterrace in front. Altiplanation terraces mayhave a cover of debris, which can exhibitPATTERNED GROUND.

altiplano /ahl-ti-plah-noh/ A high plateauthat is surrounded by mountains. A typicalexample is the South American Altiplanobetween two extensions of the Andes,mainly in Bolivia and southwestern Peru. Itcontains volcanic material and eroded sed-iments, with evidence of ancient lakes inthe form of salt basins.

altithermal See hypsithermal.

altitude A measure of height, usuallytaken to be the height above mean sealevel. Sometimes the altitude of a hill ormountain is given in terms of its verticalheight from base to summit.

altocumulus /al-toh-kyoo-myŭ-lŭs/ Atype of cloud that indicates some form ofvertical motion at medium levels in the at-mosphere. It includes a wide range of cloudorigins from genuine convection to billow

and orographic clouds where the atmos-phere is essentially stable, but the uplift isforced by mountains. Altocumulus cloudsgenerally occur as globular masses inbands across the sky.

altostratus /al-toh-strat-ŭs/ A grayishcloud sheet normally composed of a mix-ture of ice crystals and water droplets. It isdistinguished from cirrostratus by lying atlower levels, being somewhat thicker, andnot exhibiting halo phenomena. The Sunmay be seen through the thinner parts ofthe cloud. It is frequently followed by rainbecause the approach of a warm front isheralded by this cloud type.

alum 1. In general terms, any double saltconsisting of the sulfates of a trivalentmetal and a monovalent metal, with 24molecules of water of crystallization; itsgeneral formula is A2(SO4)3.B2SO4.24H2O,where A is the trivalent metal and B is themonovalent metal.2. Specifically, aluminum potassium sul-fate, Al2(SO4)3.K2SO4.24H2O, also knownas potash alum. It occurs naturally as themineral kalinite.

alumina The mineral form of aluminumoxide, Al2O3.

aluminum silicates There are three alu-minum silicates with the compositionAl2SiO5: the polymorphs ANDALUSITE,KYANITE, and SILLIMANITE. They are foundin metamorphic rocks, mostly of argilla-ceous composition. Each has a different yetclosely related crystal structure and is sta-ble over a different range of pressures andtemperatures.

Andalusite is orthorhombic and usuallypink or white in color. The variety chiasto-lite shows a regular arrangement of impu-rities in the form of a cross. Andalusite ischaracteristic of the low-pressure and high-temperature conditions associated withcontact metamorphism around igneous in-trusions. Kyanite is triclinic and often blue-green to white in color. It is stable at higherpressures than andalusite and is found inintermediate- to high-grade regionallymetamorphosed rocks. Sillimanite is or-

Alpine–Himalayan chain

14

thorhombic, usually white in color, andcommonly occurs as acicular crystals,hence the alternative name fibrolite. It isstable at higher temperatures than kyaniteor andalusite and is found in the highestgrades of thermally and regionally meta-morphosed rocks. The orthorhombic alu-minum silicate mullite has a compositionAl6Si2O13. It is found in argillaceous xeno-liths (buchites) in basic igneous rocks.

alum shale A claylike rock containingpotash ALUM, formerly much used as asource of alum for industry. It forms fromshales when sulfides they contain decom-pose to produce sulfuric acid, which reactswith mica to form aluminum sulfate.

amazonite /am-ă-zŏ-nÿt/ (amazonstone)A type of microcline FELDSPAR, blue-greenor green in color, sometimes used as asemiprecious gemstone.

amber Yellow translucent fossilizedresin once exuded by trees and often en-closing insects that have been trapped priorto hardening.

amblygonite /am-blig-ŏ-nÿt/ A greenishor white lithium-containing mineral,(Li,Na)Al(PO4)(F,OH). It crystallizes inthe triclinic system, occurs in PEGMATITES,and is used as a source of lithium.

amethyst A transparent purple varietyof QUARTZ, used as a semiprecious gem-stone. The color is due to impurities of ironoxide. See silica minerals.

ammonite /am-ŏ-nÿt/ One of the moreadvanced mollusks of the subclass AM-MONOIDEA, whose shells had extremelyconvoluted crinkled suture lines. They areknown from the Permian, are valuable asMesozoic zone fossils, and became extinctat the end of the Cretaceous Period.

Ammonoidea /am-ŏ-noi-dee-ă/ An ex-tinct subclass of marine mollusks of theclass CEPHALOPODA. Ammonoids had exter-nal shells, usually coiled in a plane spiraland divided by septa into chambers. Theshells ranged from about 25 mm in diame-

ter to over 2 m in some species. The animalinhabited the terminal, and most recentlyformed, of these. The chambers were con-nected by a sometimes discontinuous cal-careous tube known as a siphuncle. Thesepta met the inner wall of the shell at in-tersections called suture lines, and thecharacter of these in fossil shells is used in taxonomic classification. The Am-monoidea had relatively complex suturelines compared with those of the NAU-TILOIDEA; early ammonoids had angularfolded suture lines (see goniatite) but in thelater AMMONITES the suture lines werehighly convoluted and crinkled. Am-monoids probably evolved from the nau-tiloids and the earliest are known fromrocks of Silurian age. They reached theirpeak of development in the Mesozoic, forwhich they are important ZONE FOSSILS,and became extinct at the end of this era.

amorphous Having no regular atomicstructure; noncrystalline.

Amphibia /am-fib-ee-ă/ The first classof vertebrates to colonize land, evolvingfrom crossopterygian fishes in the late De-vonian. These fish had a bony skeleton thatcould provide support out of water; theyalso had lungs and their fleshy paired finsevolved into the more substantial limbs ofthe Amphibia. Amphibian eggs remain un-protected and must be laid in water, wherethe animals, too, spend their early life astadpole larvae before undergoing meta-morphosis into the terrestrial adults. TheAmphibia were abundant in Carboniferousswamps, some reaching the size of moderncrocodiles, but the class was in decline bythe end of the Paleozoic. Modern represen-tatives include newts, salamanders, toads,and frogs. Amphibians were the ancestorsof the reptiles.

amphibole /am-fă-bohl/ Any member ofa group of rock-forming minerals that have(Si,Al)O4 tetrahedra linked to form a double chain. PYROXENES are a similargroup but with a single chain structure.The general amphibole formula isX23Y5Z8O22(OH)2, where X = Ca, Na, K,Mg, or Fe2+, Y = Mg, Fe2+, Fe3+, Al, Ti, or

15

amphibole

Mn, and Z = Si or Al. The hydroxyl ionsmay be replaced by F, Cl, or O. Most am-phiboles are monoclinic but anthophylliteand gedrite are orthorhombic. Subgroupsare based on the dominant cation occupy-ing the X position. 1. anthophyllite-cummingtonite subgroup:

anthophyllite(Mg,Fe2+)7(Si8O22)(OH,F)2gedrite(Mg,Fe2+)6Al((Si,Al)8O22)(OH,F)2cummingtonite(Mg,Fe2+)

7(Si8O22)(OH)2grunerite(Fe2+)4(Fe2+,Mg)3(Si8O22)(OH)2

2. hornblende subgroup:tremolite Ca2Mg5(Si8O22)(OH,F)2actinoliteCa2(Mg,Fe2+)5(Si8O22)(OH,F)2hornblendeNaCa2(Mg,Fe2+,Fe3+,Al)5((Si,Al)8O22)

(OH,F)2edeniteNaCa2(Mg,Fe2+)5(Si7AlO22)(OH,F)2hastingsiteNaCa2(Fe2+,Mg,Al,Fe3+)5(Si6Al2O22)

(OH,F)2kaersutiteCa2(Na,K)(Mg,Fe2+,Fe3+)4Ti(Si6Al2O22)

(O,OH,F)23. alkali amphibole subgroup:

glaucophaneNa2,Mg3,Al2(Si8O22)(OH,F)2riebeckite Na2,Fe3

2+Fe23+ (Si8O22)(OH,F)2

richteriteNa2Ca(Mg,Fe2+,Mn,Fe3+,Al)5(Si8O22)

(OH,F)2katophoriteNa2CaFe4

2+(Fe3+,Al)(Si7AlO22)(OH,F)2The angle between the prismatic cleav-

ages of amphiboles is 124°, the corre-sponding angle for pyroxenes being 87°.Some amphiboles occur in fibrous forms;asbestos is a fibrous form of actinolite.Members of the anthophyllite-cumming-tonite subgroup together with tremolite,actinolite, and hornblende occur in meta-morphic rocks. Hornblende is also com-mon in igneous rocks. The alkaliamphiboles occur in alkali igneous rockswith the exception of glaucophane, which

is an index mineral of the glaucophaneschist facies.

amphibolite /am-fib-ŏ-lÿt/ A metamor-phic rock consisting predominantly of am-phibole. See also metamorphic facies.

amphidromic system /am-fă-drom-ik/ A type of tide or tidal system in whichthe high water rotates around a centralpoint (the amphidromic point). It is oneconsequence of the modifying influencethat the Earth’s rotation has on a standingoscillation. The range of the tide is nil, orvery small, at the amphidromic point itselfbut increases outward from the point. Thetimes of low and high water progress in acounterclockwise or clockwise directionaround the amphidromic point. In the Nhemisphere, high water rotates counter-clockwise round the central point.

AMS See annual maximum series.

amygdale /ă-mig-dayl/ A spheroidal orellipsoidal VESICLE within a lava, filled withdeuteric or secondary minerals often in azonal arrangement. Typical amygdaloidalminerals include calcite, zeolites, andquartz.

anabatic wind /an-ă-bat-ik/ An upslopebreeze often developing when mountainslopes are heated by the Sun during calmconditions. As turbulence is greater duringthe day it is more often suppressed than thenight-time equivalent, the KATABATIC WIND.

anafront /an-ă-frunt/ Any frontal sur-face at which the warm air is rising. As aircools on rising, condensation and precipi-tation are more extensive with this type offront than on a KATAFRONT where air is de-scending.

analcime /an-al-sim, -sÿm/ (analcite) Seefeldspathoid.

analogs Similar patterns of the surfaceatmospheric pressure field that occur atdifferent times or different places. Thebasic assumption in analog forecasting isthat if two pressure situations are identical

amphibolite

16

then the weather sequences that followedthe first occasion will also follow the sec-ond. However, since analogs are neveridentical in all important features, onlygeneral trends can be deduced.

anatase /an-ă-tayss/ A brown to blacktetragonal polymorph of titanium dioxide,TiO2, found in vein deposits and peg-matites. See also brookite; rutile.

anatexis /an-ă-teks-iss/ The partial melt-ing of rock, which can then change compo-sition by being contaminated, by mixingwith other rocks, or by MAGMATIC DIFFER-ENTIATION. See also assimilation.

anauxite /an-awks-ÿt/ A claylike alumi-nosilicate mineral, Al2(SiO)7(OH)4. It re-sembles kaolinite in composition (see clayminerals).

andalusite /an-dă-loo-sÿt/ A hard pink,gray, or brown mineral form of aluminumsilicate, Al2SiO5. It crystallizes in the or-thorhombic system and occurs in meta-morphic rocks such as gneiss and schist. Itis used as a refractory and as a semi-precious gemstone. See also aluminum sili-cates.

andesine /an-dĕ-zin/ A variety of plagio-clase FELDSPAR.

andesite /an-dĕ-zÿt/ The fine-grainedvolcanic equivalent of DIORITE, oversatu-rated or saturated in composition (seesilica saturation). These rocks are charac-terized by the presence of plagioclase in therange oligoclase-andesine, often occurringas strongly zoned phenocrysts. The maficminerals, augite, hypersthene, hornblende,biotite, and olivine, occur both as phe-nocrysts and in the groundmass. Smallamounts of quartz and alkali feldspar mayalso be present. With an increase in theamount of quartz, andesites pass intoDACITES and with an increase in the pro-portions of alkali feldspar, into LATITES

(trachyandesites). Most andesites are por-phyritic and have pilotaxitic or trachytictextures. Andesites are the intermediatemembers of the calc-alkaline volcanic suite

and are associated with basalts and rhyo-lites in island arcs and orogenic regions.

Members of the alkaline basalt–trachytesuite containing dominant oligoclase andandesine are called mugearite and hawaiiterespectively. These rocks may be distin-guished from andesites by their basic gen-erally undersaturated compositions, higherolivine contents, and differing field associ-ations. Classification of volcanic rocks isoften very difficult in petrographic termsbecause of the fine grain size, and divisionsare made on a chemical basis.

Andesite line A boundary that delimitsthose parts of the Earth’s surface that areof true oceanic structure as compared withtrue continental structure. The line can bedrawn throughout the Pacific Ocean: onthe ocean side of it are volcanic rocks thatare entirely basic in character, but no in-situ volcanic rocks of the continental type.In the case of the W coast of North andSouth America, the Andesite line runs par-allel to and comparatively close to thecoast.

andisol /an-dŏ-sôl/ A soil that has devel-oped recently on base-rich volcanic materi-als; one of the twelve soil orders of the US

SOIL TAXONOMY. The upper A horizon ofthese soils is dark owing to the presence oforganic matter that is well mixed by earth-worm activity. This is a low-density friablelayer and it overlies a more compact Bhorizon in which there has been little ap-preciable change in the content of clay.With increased duration of soil formationthese soils become deeper and profile dif-ferentiation becomes more apparent. Seealso inceptisol.

andradite /an-dră-dÿt/ A yellow, green,or brown type of GARNET, Ca3Fe2(SiO4)3. Itoccurs in metamorphosed limestones andis used as a semiprecious gemstone.

anemometer /an-ĕ-mom-ĕ-ter/ An in-strument for measuring wind speed andoften wind direction. The most commontype, the cup anemometer, has three cupsmounted symmetrically about a vertical ro-tating axis, so that the rate of rotation is

17

anemometer

proportional to the wind speed. Wind canalso be measured by the pressure it exertson a plate or in a tube, or by the degree ofcooling of a hot wire.

aneroid barometer /an-ĕ-roid/ Themost commonly used instrument for mea-suring air pressure. In its simplest form itconsists of a corrugated metal box fromwhich the air is evacuated. The top andbottom of the box are separated by aspring and changes of atmospheric pres-sure produce movements in the distanceapart of the faces. The changes are thenamplified by a series of levers and can berecorded on a chart. See also barometer.

angiosperms /an-jee-oh-spermz/ Theflowering plants. They differ from theGYMNOSPERMS in that their seeds are pro-tected by an outer casing known as acarpel. Angiosperms are thought to havearisen in the Triassic; they became com-mon during the Cretaceous and by the endof this period had replaced the gym-nosperms as the dominant land plants, aposition they still hold today. They occupya great variety of terrestrial and freshwaterenvironments and range in size from smallherbaceous plants to giant trees. An-giosperms form an important source offood and habitats for both birds and mam-mals, reflected in the fact that these groups,too, began their main evolutionary radia-tion during the Cretaceous. The evolutionand spread of the grasses, probably duringthe Miocene, had a particularly importanteffect on the environment, and thereforeon animal life.

There are two main divisions of an-giosperms, the classes Monocotyledoneaeand Dicotyledoneae.

angle of declination (magnetic declina-tion) The angle between geographicalnorth and the direction of the magneticmeridian.

angle of incidence The angle between alight ray arriving at a surface and the nor-mal to that surface at that point.

angle of magnetic inclination See dip(def. 2).

angle of reflection The angle between alight ray reflected from a surface and thenormal to that surface at the point of re-flection. Other types of waves may also bereflected.

angle of refraction The angle betweena refracted light ray and the normal fromthe surface at which the ray is refracted atthe point of refraction. Other types ofwaves may also be refracted.

angle of rest (angle of repose) The char-acteristic maximum angle of slope at whicha pile of unconsolidated material is stable,the angle depending on the particle size ofthe material. Above that angle, the slopewill be unstable and subject to slides andflow, which will reduce the angle of slopeuntil it is again stable.

At rest, the particles interdigitate witheach other, so that the weight of each oneis balanced against the one below. As theangle becomes too steep, the particles up-slope tend to lack support at the base andslide over the particles in front, movingdownslope until the debris pile is again bal-anced. Larger material has a higher angleof rest than fine material; in most rocksand sands the angle is 30–35°. Where thematerial forming a slope is unconsolidated,

aneroid barometer

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Angle of rest

its angle of rest determines the angle ofslope; such slopes are called talus slopes,constant slopes, or colluvial slopes (see col-luvium).

anglesite /ang-glĕ-sÿt/ A white mineralform of lead sulfate, PbSO4. Generallyformed by the oxidation of GALENA, it crys-tallizes in the orthorhombic system. It is acommon secondary mineral, used as asource of lead.

angular momentum The product ofthe linear velocity of a point on the Earth’ssurface, as it moves from west to eastowing to the Earth’s rotation, and the per-pendicular distance of that point from theEarth’s axis of rotation, a line through thenorth and south poles. Its value is greatestat the Equator, where the perpendiculardistance is at a maximum, and zero at thepoles.

angular unconformity A plane of ero-sion, generally related to earth movements,marking the boundary between two rockunits of different age. The beds above andbelow the plane of erosion have differentangles of dip, the lower beds generally dip-ping more steeply. See also unconformity.Compare disconformity.

angular velocity The rate of turning ofa rotating body, expressed in degrees, radi-ans, or revolutions per unit of time. In me-teorology, it can be used to indicate thehorizontal rate of rotation around an areaof low pressure or, on a large scale, the ro-tation of the Earth. As the Earth makes onecomplete rotation in a day, its angular ve-locity is 15° longitude per hour, or 7.29 ×10–5 radians per second.

angulate drainage /ang-gyŭ-lit, -layt/ Adrainage pattern in which streams are de-veloped equally in two directions, like theRECTANGULAR DRAINAGE pattern, but in thiscase the streams meet at an acute anglerather than right angles. The pattern re-sults from structural guidance by two setsof joints or faults in an area with homoge-neous strata.

anhedral /an-hee-drăl/ Describing crys-tals having irregular boundaries, and hav-ing no crystal faces. Compare euhedral;subhedral.

anhydrite /an-hÿ-drÿt/ An orthorhom-bic mineral form of calcium carbonate,CaSO4. It is usually white or colorless, oc-curring in massive or fibrous forms inevaporite deposits. See also gypsum.

anisotropic /an-ÿ-sŏ-trop-ik/ Having dif-ferent physical properties when measuredin different directions. Crystals belongingto systems other than cubic are opticallyanisotropic and light is transmitted at dif-fering velocities along different directionswithin each crystal. When a ray of light en-ters an anisotropic crystal it is split intotwo rays, the ordinary and extraordinaryrays, which travel in different directionsand are polarized in perpendicular planes.Such crystals are said to exhibit double re-fraction or BIREFRINGENCE.

ankaramite /ank-a-ră-mÿt/ A basalticrock containing a high proportion ofaugite phenocrysts. Compare oceanite.

ankerite /ank-ĕ-rÿt/ A gray to brown va-riety of the mineral DOLOMITE that containsiron, Ca(Fe,Mg,Mn)(CO3)2. See also car-bonate minerals.

Annelida /a-nel-ă-dă/ The phylum of an-imals containing the true worms, such asthe common earthworm, whose bodies aredivided into a number of segments. An-nelids of one group secrete calcareoustubes, and fossil specimens of these exist,but most annelids are soft bodied and notoften preserved after death. Certain smallsiliceous or chitinous remains known asscolecodonts, occurring in rocks of Or-dovician age onwards, are thought to beannelid jaw parts. The effects of worms areoften seen in rocks as TRACE FOSSILS, trails,burrows, or other characteristic distur-bances of the sediment, features presenteven in some Precambrian rocks.

annual maximum series (AMS) (in hy-drology) A series of data that consists of

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annual maximum series

the event with the largest observed magni-tude (e.g., the largest observed flow of ariver) that occurred in each year.

anomaly 1. (in climatology) The depar-ture of any climatic element from the long-period average for that location. Thespatial distribution of an anomaly is an in-teresting feature because it helps provide agreater understanding of the interactionbetween the atmosphere and the surface. Agood example of a temperature anomaly isthe area of NW Europe where mean annualtemperatures are much higher than the lat-itudinal average by as much as 12°C inparts of coastal Norway.2. (in geophysics) The difference betweenobserved and computed geophysical val-ues, especially with regard to the thermal,gravitational, and magnetic properties ofthe Earth. See gravity anomaly.

anorthite /an-or-th’ÿt/ A calcic plagio-clase FELDSPAR.

anorthoclase /an-or-th’ŏ-klayss/ A vari-ety of alkali FELDSPAR.

anorthosite /an-or-th’ŏ-sÿt/ A type ofcoarse-grained leucocratic igneous rockthat occurs as vast intrusive bodies or asmembers of layered basic complexes. Pla-gioclase feldspar constitutes over 90% ofanorthosite and is generally of labradoritecomposition but anorthosites composed ofplagioclase ranging from andesine to anor-thite are known. Minor mafic minerals in-clude augite and hypersthene and with anincrease in the amount of mafic minerals,anorthosites pass into gabbros.

Antarctic bottom water (AABW) Acold and extremely saline water mass, themost dense water of the global oceans, thatforms in the Southern Ocean along thecoast of Antarctica, especially in the Wed-dell and Ross Seas, during deep winter con-vection. Cooling and freezing of thesurface sea water to form pure ice releasessalt into the water beneath the ice. Thiscold dense water sinks, carrying oxygenand nutrients down to the bottom of theAntarctic Ocean, and flows northward

along the sea floor, below other watermasses. See bottom water. See also Antarc-tic intermediate water; Arctic bottomwater.

Antarctic Circle The line of latitude66°30′S. In December there is continuousdaylight for 24 hours and in June continu-ous darkness for 24 hours along this line.

Antarctic high See polar high.

Antarctic intermediate water (AIW) Acold and moderately saline water mass thatforms around Antarctica between 45° and55°S and flows north to the Antarctic con-vergence zone where it sinks below the lesssaline Subantarctic surface water. It can betraced at depths of between 700 and 1000m in the North Atlantic.

antecedence /an-tĕ-see-dĕns/ A type ofDISCORDANT DRAINAGE, originating when apreexisting drainage pattern cuts downinto a rising fold, or a series of rising folds,in its path. The cause of the folding will betectonic, and in the best developed exam-ples, the rate of uplift of the folds isequaled by the rate of downcutting of thestreams, eventually leaving them in deepCANYONS. As an explanation of discor-dance, it is less commonly used than SUPER-IMPOSITION, but is often used to explain theorigin of streams cutting through moun-tain ranges in deep gorges, e.g. the streamsthat cut through the Himalayas.

anthophyllite /an-th’off-ŏ-lÿt/ An ortho-rhombic mineral of the AMPHIBOLE group.

Anthozoa /an-thŏ-zoh-ă/ A class of thephylum CNIDARIA that includes the seaanemones and CORALS. The corals are geo-logically the most important anthozoansbecause many of them possess hard partsthat may be fossilized. Their evolution maybe used in BIOSTRATIGRAPHY and the colo-nial forms are important reef-builders.Typically, corals secrete a cup (corallum),from whose wall (theca) supporting plate-like septa project radially toward the cen-ter. The Tabulata (tabulate corals) areknown from the Ordovician Period to the

anomaly

20

Jurassic, although the group had greatlydeclined by the end of the Paleozoic. Theywere colonial forms having only rudimen-tary septa or none at all. The walls were re-inforced by tabulae, calcareous platesextending across each coralite. The Ru-gosa, or Tetracoralla, (rugose corals) arefirst known from the Ordovician and areconfined to the Paleozoic. Their septaoccur in multiples of four and are strength-ened by many small plates known as dis-sepiments. The group included bothsolitary and colonial species. The Sclerac-tinia, or Hexacoralla, includes the modernreef-building corals, whose septa are inmultiples of six. They range from the Tri-assic Period onwards.

anthracite /an-thră-sÿt/ Coal that has avery high fixed carbon content and a lowamount of volatiles.

anthraxolite /an-thraks-ŏ-lÿt/ A hardtype of BITUMEN that occurs in sedimentaryrocks. It is often found associated with OIL

SHALES.

anthropomorphic soil /an-thrŏ-pŏ-mor-fik/ An intrazonal soil that has beenformed as a direct result of human activity.Farming practices through the ages have,in some cases, produced distinctive soils ordistinctive surface horizons. An importantanthropomorphic soil is the paddy soil.This is similar to a gley and its featureshave been formed by the process of alter-nate wetting and drying, which has beencontrolled by farmers. Many soils have an-thropic EPIPEDONS (surface horizons) orplaggen epipedons. The former have a highphosphate content and the latter a high or-ganic matter content.

anticline /an-tee-klÿn/ An arch-shapedfold into which rock strata have been com-pressed, the oldest rocks occurring in thecore. See fold.

anticlinorium /an-tee-klÿ-nor-ee-ŭm,-noh-ree/ A large-scale regional feature, manykilometers in diameter, consisting of an an-ticlinal structure with several minor foldson its limbs.

anticyclone /an-tee-sÿ-klohn/ (high) Anarea in the air of higher pressure than thesurrounding air, with a closed isobar of ap-proximately circular form at its center.Winds circulate in a clockwise manneraround the high pressure center in the Nhemisphere, and counterclockwise in the Shemisphere, but are generally light. Onceestablished, the anticyclone moves onlyslowly and normally is a significant featureof surface pressure charts for a muchlonger period than cyclones. Anticyclonescan be subdivided into two categories, coldand warm. Cold anticyclones are shallowfeatures produced by strong radiationalcooling at the surface or in the cold air be-hind a depression. The tropopause tends tobe low above a cold anticyclone. Far moreimportant are the warm anticyclones,which are characterized by warmer tem-peratures throughout a deep troposphere,with a cold stratosphere above. Their mostfrequent locations are in the subtropicalhigh-pressure belts, but they do move pole-ward and block the normal westerly cir-culation producing temperature andprecipitation anomalies in these areas. De-scending motion is characteristic of anticy-clones. They represent the parts of theatmosphere in which air slowly subsidesfrom higher levels, warming and drying asit does so. This has the effect of stabilizingthe atmosphere within an anticyclone sothat rain is infrequent. In the surface layers,divergence of airflow takes place to main-tain the continuity of subsidence. Comparecyclone.

anticyclonic gloom /an-tee-sÿ-klon-ik/ Acondition of low visibility; it occurs underanticyclonic conditions when the layer ofair near the ground undergoes little descentand, if moist, can easily become saturatedto give a uniform layer of cloud. This is astable situation with the cloud slowly gain-ing in thickness through radiational cool-ing of the layer aloft. Above this level theair is much warmer due to descent; a tem-perature INVERSION and stability are main-tained. At the surface, little light penetratesthe cloud and pollution can be trapped togive poor illumination. If prolonged, thesecircumstances can give high pollution con-

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anticyclonic gloom

centrations, which may reach dangerouslevels.

antiform /an-tee-form/ An upward-clos-ing structure, the precise stratigraphic rela-tionships of whose strata are not known,i.e. the core need not be the oldest rocks. Itresults from complex folding in orogenicareas. See fold.

antigorite /an-tig-ŏ-rÿt/ A mineral formof hydrated iron-containing magnesiumsilicate, (Mg,Fe)3Si2O5(OH)4, a variety ofSERPENTINE. It occurs as fibers or undulat-ing plates.

antiperthite /an-tee-per-th’ÿt/ An inter-growth of orthoclase and albite feldspar inwhich the orthoclase occurs as patches inthe albite host. See also exsolution;perthite.

antithetic fault A minor normal faultassociated with a major fault but whoseplanes dip into the main fault plane.

antitrades /an-tee-traydz/ Originally con-sidered to be the returning flow in theupper atmosphere for the surface TRADE

WINDS, antitrades now more generally indi-cate the airflow above the NE or SE tradewinds, blowing in a general westerly direc-tion at heights of about 1800 m.

antitriptic wind /an-tee-trip-tik/ A windin which the forces controlling air move-ment are the pressure gradient force andfriction. The CORIOLIS EFFECT and cen-tripetal acceleration are neglected eitherbecause of the small scale of the system,such as a sea breeze, or because the Corio-lis effect is negligible, as it is near the Equa-tor.

anvil A CUMULONIMBUS cloud in whichthe top reaches the tropopause where, as aresult of a temperature inversion, or strongwind shear, the main cloud updrafts are de-flected horizontally causing the ice crystalsat that level to spread out. From theground, the whole cloud has the appear-ance of a blacksmith’s anvil.

apatite /ap-ă-tÿt/ A hexagonal phos-phate mineral of composition Ca5(PO4)3-(OH,F,Cl), the commonest phosphorusmineral. It may be formed as an accessorymineral in igneous rocks of most composi-tions. Apatite also occurs as a detrital min-eral in sedimentary rocks and inmetamorphic rocks.

aphanitic /af-ă-nit-ik/ Denoting an ig-neous rock that is so fine-grained that indi-vidual crystals cannot be resolved with thenaked eye. Compare phanerocrystalline.

aphelion /ă-fee-lee-ŏn/ The point on theorbit of a planet that is farthest from thecentral axis of rotation. Compare perihe-lion.

aphotic zone /ay-foh-tik/ Ocean depthsbelow the maximum depth at which pho-tosynthesis takes place because of lack oflight, stretching from 200 m below sealevel to the sea floor. See also disphoticzone; euphotic zone.

aphyric /ă-fi-rik, -fÿ-/ Denoting an ig-neous rock that is not PORPHYRITIC.

aplites /ap-lÿts/ Leucocratic acid igneousrocks of medium to fine grain size occur-ring as thin veins and dikes around graniteintrusions. Aplites consist largely of quartzand alkali feldspar and have micrographicor SACCHAROIDAL textures. They arethought to have crystallized from residualliquids lower in volatiles than pegmatites.

apophyllite /ă-poff-ă-lÿt, ap-ŏ-fil-ÿt/ Arare mineral of composition KFCa4(Si8O20).8H2O found in AMYGDALES in basalts.

apparent dip The angle between astructural surface, e.g. a bedding plane,and the horizontal, measured vertically inany direction except that which is at 90° tothe strike of the surface. This results in avalue of dip that is less than that of true DIP.

appinite /ap-ă-nÿt/ A melanocratic vari-ety of DIORITE.

antiform

22

applanation /ap-lă-nay-shŏn/ The pro-cesses that reduce the heights of land fea-tures and cause an area to become morelike a plain. It results from the erosion ofhigh areas and the deposition of sedimentsin low areas.

apron A deposit of unconsolidated frag-ments that forms a broad extension infront of a glacier or at the base of a moun-tain.

apron reef See reef.

aquamarine A transparent blue-greenvariety of BERYL that occurs, sometimes asvery large crystals, in pegmatite. It is val-ued as a semiprecious gemstone.

aquatic Describing any organism that isfound in water. In an aquatic environmenttemperatures generally vary little, dehydra-tion is virtually impossible, and the waterprovides physical support for plants andinvertebrate animals.

aquifer /ak-wă-fer/ Any water-saturatedrock horizon that has sufficient porosityand permeability to yield economic sup-plies of groundwater, either as springs or inwells.

aragonite /a-ră-gŏ-nÿt/ A white or graymineral form of calcium carbonate,CaCO3. It crystallizes in the orthorhombicsystem and occurs near the surface in sedi-mentary rocks and in deposits in caves andfrom hot mineral springs. It also occurs inPEARLS and in the shells of some inverte-brate marine animals. See also carbonateminerals.

Archaeopteryx /ar-kee-op-tĕ-riks/ Theearliest known bird, whose fossils comefrom late Jurassic strata. Good fossilpreservation reveals that Archaeopteryxhad feathers but in its skeletal structure itresembled the archosaur reptiles fromwhich it had evolved. The wings wereprimitive and the three separate fingers hadclaws. Other reptilian features includedteeth, a long tail, and solid bones (compareAves). Its flying ability was probably poor.

Archean /ar-kee-ăn/ See Precambrian.

archipelagic apron /ar-kă-pe-laj-il/ Themoatlike rock filling that may surround agroup of volcanic islands or seamounts.The moat filling is usually gently slopingand smooth, the upper surface being calledan archipelagic plain. These groups of is-lands or seamounts probably depress theEarth’s crust to the extent that a moat ordepression is developed, but the depressionmay be concealed by a sedimentary over-lay. Part of the Hawaiian Islands display amoatlike form surrounding them.

archipelago A group of islands in fairlyclose proximity. The term is sometimesused to describe sea areas that contain nu-merous scattered islands.

Archosauria /ar-kŏ-sor-ee-ă/ A subclassof the Reptilia consisting principally of theextinct DINOSAURS and PTEROSAURIA and themodern crocodiles. Primarily carnivorous,with simple pointed teeth, they show a ten-dency to bipedalism involving develop-ment of the hind limbs, a correspondingreduction of the fore limbs, and a well-de-veloped tail for balancing the body. Ar-chosaurs became the dominant form ofterrestrial life during the Jurassic and Cre-taceous periods; some became secondarilyquadrupedal and vegetarian.

Arctic bottom water (ABW) A deepwater mass in the Arctic Ocean. It origi-nates partly from the Arctic shelf regions;the freezing of sea water to form sea ice re-leases salt to the water below. The in-creased density of this water resulting fromthe salinity causes it to sink to the seafloor.It mixes with deep water that forms duringwinter in the Greenland Sea when coolingof surface water leads to vertical convec-tion. Submarine topographic barriers, suchas the Bering Sill, restrict the water mainlyto the bottom of the Arctic Ocean but somedoes intermittently flow through narrowchannels to enter the Atlantic from aroundGreenland. See bottom water. See alsoAntarctic bottom water.

23

Arctic bottom water

Arctic Circle The line of latitude66°30′N. In June there is continuous day-light for 24 hours and in December contin-uous darkness for 24 hours along this line.

Arctic high See polar high.

Arctic oscillation Periodic fluctuationsin atmospheric pressure that occur in themid-latitudes and Arctic above about45°N. In a ‘positive phase’ of the oscilla-tion there is a strengthening of the counter-clockwise polar vortex extending from thelower stratosphere to the surface. The Arc-tic surface air pressure decreases whilethere is higher pressure at mid-latitudes.The North Atlantic storm tracks bring rainand mild temperatures to N Europe whileconditions are drier in the W USA andMediterranean. In the ‘negative phase’ thestrength of the vortex decreases and thereis comparatively high pressure over theArctic region and low pressure in the mid-latitudes. Cold air from the Arctic extendsdown into the USA and Europe and Asia.During the1980s and 1990s the positivephase was dominant. The Arctic oscillationis closely related to the NORTH ATLANTIC

OSCILLATION.

Arctic sea smoke A type of fog formedby evaporation from a relatively warm seasurface into cold air aloft. Condensationtakes place in the cold air, but unless an in-version develops, further mixing preventsthe extensive development of fog. In theAleutian Islands, fog depths up to 1500 mmay occur under suitable conditions.

Arctic warming The marked warmingof the Arctic area that took place betweenthe l920s and 1950s. The ice-free period in-creased and mean annual temperaturesrose by about 4–10°C. It was primarily theresult of the more northerly tracks of theAtlantic and Pacific depressions carryingmoist warm air toward the poles. Althoughthe amount of warming has decreased con-siderably, Arctic temperatures are stillhigher than pre-1920 and may be risingagain.

arc-trench gap At a destructive PLATE

BOUNDARY, a region 50–400 km wide be-tween an ocean trench and a volcanic arc,often associated with an ISLAND ARC. Gen-erally the wider the gap, the faster is therate of convergence between the two tec-tonic plates involved.

arcuate delta /ar-kyû-it,-ayt/ A DELTA

with a rounded convex margin.

arenaceous /a-rĕ-nay-shŭs/ Describing aCLASTIC sedimentary deposit or rock inwhich the constituent fragments are ofsand grade in size. This includes particlesfrom 0.06 mm to 2 mm in diameter. Are-naceous rocks are also referred to as SAND-STONES. The grains are often fragments ofcrystals, although lithic sandstones areknown (see graywacke).

arenite /a-rĕ-nÿt/ 1. A type of SAND-STONE that has little or no matrix materialbinding the grains together.2. A sedimentary rock that consists ofsand-sized grains (up to 2 mm across). Thegrains need not be silica or silicates, andmay include fragments of carbonate rocks.

arête /a-ret/ A sharp ridge bounded bysteep slopes and found in glaciated or for-merly glaciated areas. These forms seldomdevelop in isolation and headwall erosionof two adjacent cirques causes the forma-tion of an arête. Continued headwall reces-sion will result in the destruction of thearête and the cirques will merge.

argentite /ar-jĕn-tÿt/ A mineral form ofsilver sulfide, Ag2S. It crystallizes in thecubic system but is stable only at high tem-peratures; below 180°C it changes to themonoclinic mineral acanthite. It is the mostimportant ore of silver.

argillaceous /ar-jă-lay-shŭs/ Describinga CLASTIC sedimentary deposit or rock inwhich the constituent fragments are of SILT

or CLAY grade in size. This includes parti-cles smaller than 0.06 mm in diameter.These consist of finely ground rock as wellas the various clay minerals that have beenproduced in the course of weathering of the

Arctic Circle

24

parent rock. SILTSTONES and MUDSTONES

are rocks formed of sediment in this sizerange.

argillite /ar-jă-lÿt/ A slatelike sedimen-tary rock that is formed from shale or mud-stone by the effects of pressure andcementation. Unlike slate, however, it hasno distinct cleavage planes.

argon An inert gas comprising about1% of the atmosphere by volume. It has nometeorological significance.

arid climate A climate that experiencesa moisture deficiency sufficient to inhibitbut not prevent natural vegetative growthand a mean annual rainfall usually taken asbelow 250 mm. Attempts to produce a pre-cise definition of aridity involve an assess-ment of the efficiency of precipitation. Aridclimates are found in areas of semiperma-nent anticyclones, where cold ocean cur-rents stabilize the lower atmosphere,within extensive mountain ranges, and inlocations at enormous distances from thesea. See also desert.

aridisol /ă-rid-ă-sôl/ One of the twelvesoil orders of the US SOIL TAXONOMY, in-cluding saline and alkaline mineral soils ofdesert areas. They are characterized by loworganic matter contents and a horizon ofcalcium or sodium accumulation within 1m of the surface. They are infertile becauseof lack of moisture, coarse particle size,and their susceptibility to erosion due tolack of vegetation. In some profiles,groundwater may concentrate sodium totoxic proportions, forming a salic horizonin which sodium salts exceed 2% of themineral matter. The aridisols include truedesert soils, sierozem, solonchak, andsolonetz.

aridity index /a-rid-ă-tee/ An assess-ment of the degree of dryness of a climate.There are a variety of such indices, the bestknown being devised by the American cli-matologist–geographer C. Warren Thorn-thwaite (1899–1963) in 1948. Mostinvolve the relationships between tempera-ture, total rainfall, and humidity.

arkose /ar-kohss/ An ARENACEOUS sedi-mentary rock that includes more than 25%FELDSPAR in its composition.

Armorican orogeny /ar-mor-ă-kăn/ Aperiod of mountain-building during theUpper Paleozoic affecting W Europe,named after Armorica (Brittany). It is partof the more complex VARISCAN orogeny butcharacterized by a roughly northwest-southeast trend.

arrival time The first recording of aseismic disturbance. Close to an earth-quake, there is little difference in the arrivaltimes of primary (P) waves and secondary(S) waves. Farther away, the faster P wavesarrive first. The distance to the earthquakecan be calculated from this difference in ar-rival times. See primary wave; secondarywave.

arroyo /ă-roi-yoh/ An ephemeral streamof the semiarid USA and Latin America.Arroyos originate as discontinuous gullieson a hillside where the vegetation has beenlocally weakened by trampling, grazing, orfire. Headward erosion during periodswhen the gullies contain water leads to co-alescence, and the formation of a continu-ous gully, or arroyo. See also abnormalerosion; gully erosion.

arsenopyrite /ar-sĕ-noh-pÿ-rÿt/ An or-thorhombic arsenic mineral of composi-tion FeAsS, found in hydrothermal veins.

artesian /ar-tee-zhăn/ Describing waterthat has moved underground from its orig-inal source. This may occur by percolationupward along a sloping AQUIFER with theresult that the artesian water is above thelevel of the water table.

artesian basin A SYNCLINE that has alayer of permeable rock between two im-permeable layers. Water can be obtainedby drilling boreholes into the permeablelayer. If this layer is below the water tablein nearby hills, the water may flow up theborehole under hydrostatic pressure, like afountain, forming an artesian well.

25

artesian basin

Arthropoda /ar-throp-ŏ-dă/ The phy-lum of animals having jointed limbs and asegmented body protected by a chitinousexoskeleton. The Arthropoda is the largestphylum in the animal kingdom and its sixmost important classes are the CRUSTACEA,Arachnida (spiders and scorpions), Myri-apoda (centipedes and millipedes), Mero-stomata (king crabs and the extincteurypterids), INSECTA, and the extinctTRILOBITA. Fossils of arthropods representmainly aquatic benthonic forms, whichlived in the most favorable environmentsfor preservation; such arthropods arethought to have been recognized in theCambrian, perhaps even the Precambrian.

artificial satellite See satellite.

Artiodactyla /ar-tee-oh-dak-tă-lă/ Theorder of herbivorous hoofed mammalscomprising those with an even number oftoes, such as antelopes, cattle, deer, andpigs. Originating in the Eocene and at firstoutnumbered by early perissodactyls, theyare now the more successful group. Themost primitive members are the pigs,which still possess four toes. In the rumi-nants the limbs are developed for fast run-ning with the loss of all but two of thedigits, and the teeth are modified to dealwith a coarse vegetable diet. ComparePerissodactyla.

asbestos A fibrous variety of AMPHI-BOLE, usually tremolite or actinolite. TheSERPENTINE mineral chrysotile is used in themanufacture of commercial asbestos.

aseismic plate /ay-sÿz-mik/ Any of thelarge areas of the Earth’s crust above theasthenosphere, within which there are rel-atively few earthquakes.

aseismic ridge A ridge on the seabedthat, unlike a MID-OCEAN RIDGE, has no vol-canic activity along it.

ash See pyroclastic rock.

ash flow deposit A volcanic deposit re-sulting from a NUÉE ARDENTE, in which anavalanche of glowing ash flows rapidly

down the side of a volcano. The resultingsediment may be partly blocky, but alsocontains tuffs where shards of glassy lava,carried by the flow, have become weldedtogether.

aspect The direction in which a slope orsurface faces, expressed usually in compassdirections such as degrees from north in aclockwise direction.

asphalt /as-fawlt, -falt/ A type of brownor black BITUMEN consisting mainly of car-bon disulfide (CS2) and hydrogen. It variesfrom a thick viscous liquid to a tarry solid.It occurs in oil-bearing rocks and probablyrepresents an early stage in the formationof PETROLEUM from buried marine organ-isms. A similar substance remains after thedistillation of crude oil.

assimilation /ă-sim-ă-lay-shŏn/ The in-corporation of rock into a body of magma.During intrusion, magma forces its wayalong joints and cracks in the country orwall rock so that large blocks (XENOLITHS)may become detached and sink into themagma. This process, known as stoping,may occur on a large scale when large vol-umes of magma are emplaced. The com-plex interactions taking place between themagma and solid rock are known by theterm syntexis. Melting of the rock andchemical reaction with the magma mayresult in the complete digestion or assimi-lation of the incorporated material. Xeno-liths survive only when assimilation isincomplete. Contaminated magma that hasassimilated large quantities of country rockis said to be hybrid. Hybridization also oc-curs when two different magmas aremixed. That such a process has taken placemay be indicated by the occurrence of cor-roded XENOCRYSTS within an igneous rock.

asterism /ass-ter-iz-ăm/ An optical phe-nomenon displayed by some crystals thatproduce starlike flashes of light. Caused bytiny needle-shaped inclusions, it may occurin transmitted light (as with some forms ofmica) or in reflected light (as with star sap-phire).

Arthropoda

26

asthenosphere /ăs-th’en-ŏ-sfeer/ A zonewithin the Earth’s upper MANTLE in whichthe velocity of seismic waves is consider-ably reduced. Movement between theEarth’s outer lithosphere and inner meso-sphere is thought to take place along thiszone, which is capable of prolonged plasticdeformation. It is thought to be composedof partly molten peridotite, with a liquidfraction having the composition of basalt.It is developed between 50 and 240 km be-neath the Earth’s surface.

astrobleme /ass-trŏ-bleem/ An ancientcrater on the Earth’s surface resulting fromthe impact of an extraterrestrial body.

astrogeology /ass-troh-jee-ol-ŏ-jee/ Theapplication of terrestrial geology to study-ing the origin and history of extraterres-trial objects in the Solar System.

asymmetric fold /ay-să-met-rik/ A foldin which the axial plane is not vertical,with the result that the two limbs have dif-ferent angles of dip. See diagram at FOLD.

Atlantic period The period from 5000to 3000 BC, within the present interglacial,when the climate of much of NW Europewas warm and moist, achieving the highestmean annual temperatures since the lastglaciation. It is also known as a CLIMATIC

OPTIMUM.

atmophile /at-mŏ-fÿl/ An element thatoccurs naturally as a gas, such as helium insome uranium ores and the various gases inthe ATMOSPHERE. See also chalcophile;lithophile; siderophile.

atmosphere The mixture of gases sur-rounding the Earth, which we breathe andwhich provides our weather. The atmos-phere can be divided into a number of lay-ers on thermal and lapse-rate properties.The troposphere is the lowest, extendingup to approximately 10 to 15 km.Throughout this layer, there is a decreaseof temperature with height, and it is thesource of all precipitation and most of ourweather phenomena. Above this layer,temperatures gradually increase through

the stratosphere to about 50 km, decreasethrough the mesosphere reaching values ofabout –90°C between 80 and 90 km, andfinally there is the thermosphere or ionos-phere where absorption of ultraviolet radi-ation of shorter wavelengths than thatabsorbed by ozone causes a rise of temper-ature to values of 1500 to 2000°C at be-tween 300 and 400 km. Gases becomeionized at these high levels, which is of vitalimportance in radio communication. See il-lustration overleaf.

The distribution of water vapor in theatmosphere is very variable. It dependsupon the amount of evaporation from thesurface and the amount of moisture ad-vected from elsewhere. In continental inte-riors the value is very low. It is importantas an absorber of long-wave radiation, sothat moist air reduces the rate at which theground surface cools at night by increasingcounter-radiation. Moisture is also re-quired in the atmosphere for precipitation,although the absolute amount is less im-portant than other factors favoring precip-itation formation.

The atmosphere acts as a protection tothe ground surface by burning up most cos-mic particles, which can be seen as meteorsin the night sky. See also air; general circu-lation of the atmosphere.

atmospheric pressure The pressure atthe Earth’s surface that results from theweight of the ATMOSPHERE. At sea level,standard atmospheric pressure is 760 mmof mercury, 101.3 kilopascals, or 1013.25millibars. It decreases with increasing alti-tude, and is measured using a BAROMETER.

atmospheric window The region in theradiation spectrum containing wavelengthsbetween 8.5 and 11 µm, which are not ab-sorbed to any great extent by atmosphericgases. In the absence of cloud, terrestrialradiation of this wavelength is lost tospace, enabling the cooling of the Earth totake place. Other narrower wavebandsalso do not absorb long-wave radiation,but these are of less importance in this con-text. See also absorption.

27

atmospheric window

28

400 –

300 –

200 –

100 –

50 – – 30 – – 10 – – – – – –

mesopause

mesosphere

stratopause

troposphere

stratosphere

tropopause

–100° 0° +100° +200°

temperature/°Cpressure/mb

(millibars)

ionosphere

0.000001

0.01

55

0.78

265

1013sea level

hei

gh

t/km

temperature °C

Atmosphere

atoll A ring-shaped reef, island, or is-lands that surround or nearly surround alagoonal area of water, in which detritalmaterial may collect. The surrounding rimmay itself lie in shallow water, in whichcase the central area of water is ratherdeeper, sometimes very deep (the averagedepth according to one authority is 45 m).Not all oceanic atolls are formed entirely ofcoral, and certain calcareous algae mayconstitute the bulk of the reef material.Atolls are based on some kind of platform,often an eroded platform that subsided atsome stage and on which coral growth orother organic growth later occurred. Anatollon is a small atoll lying on the flank ofa larger atoll. See also reef.

attenuation The loss of energy of elec-tromagnetic waves as they pass throughthe atmosphere. It is caused by ABSORPTION

and SCATTERING by the molecules and par-ticles of the atmosphere. In the ionosphere,free electrons absorb kinetic energy fromthe radiation and lose it in subsequent col-lisions.

attrition The reduction in particle sizesof sediment by rubbing and grinding action(see abrasion) during transport.

augen /aw-jĕn/ Large eye-shaped crys-tals, commonly of feldspar, that have sur-vived the intense shearing of schist andgneiss taking place during the formation ofa MYLONITE.

augite /aw-jÿt/ A monoclinic PYROXENE.

aulacogen /aw-lak-ŏ-jĕn/ A long depres-sion on the edge of a continent bounded byFAULTS, which did not develop into an ac-tive rift as two continental plates splitapart. See also rift valley.

aurora A transient optical phenomenonusually seen in the polar skies at the time ofsolar flare activity. The displays of the au-rora can vary from shimmering rays to col-ored corona effects occupying large partsof the sky. In the N hemisphere it is calledthe aurora borealis (or northern lights) andin the S hemisphere the aurora australis (or

southern lights). The displays are most fre-quent around the geomagnetic poles, witha daily occurrence maximum about mid-night. There is some evidence of a seasonalmaximum about the time of the equinoxes,but the major maximum follows the 11.2year solar activity cycle.

The aurora is caused by the interactionof the Earth’s tenuous upper atmosphereand charged particles streaming from theSun. The solar particles are deflected by thegeomagnetic field and so only occur in lim-ited parts of the upper atmosphere, at aheight of around 100 km.

autecology /aw-tĕ-kol-ŏ-jee/ A branchof ECOLOGY that studies the interaction be-tween a single species and its environment.

authigenic /aw-th’ă-jen-ik/ (authigenous)Describing rock constituents that wereformed in situ, coming into existence dur-ing or after the formation of the rock inwhich they lie. Compare allogenic.

autochthonous /aw-tok-th’ŏ-nŭs/ De-scribing rocks that are still in their place offormation and have not been displaced bythrusting, e.g. a folded sequence of rockswhose roots are still connected. Compareallochthonous.

autolith /aw-toh-lith/ An igneous INCLU-SION in an igneous rock. It consists of ma-terial that has crystallized from magmaand is thus genetically related to the sur-rounding rock. See also xenolith.

automatic weather station A meteor-ological station that is equipped with in-struments to automatically make andrecord observations (e.g. air temperature,relative humidity, solar radiation, baro-metric pressure, wind speed, and wind di-rection) without the necessity for anobserver to be present. The data may bestored at the station until retrieved by a vis-iting observer, transmitted at intervals, ortransmitted continuously to a base station.

autotrophic /aw-tŏ-troff-ik/ Describingan organism that produces its own ‘food’directly from inorganic substances.

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autotrophic

Chlorophyll-containing plants and algaeare typical autotrophs; they produce or-ganic compounds from carbon dioxide andwater during photosynthesis, using the en-ergy of sunlight. Some types of BACTERIA

are also autotrophs.

autumnal equinox See equinox.

auxiliary mineral Any relatively rarelight-colored mineral that occurs in an ig-neous rock. Such minerals include APATITE,CORUNDUM, FLUORITE, and MUSCOVITE.

avalanche A rapid movement of snowen masse down steep slopes, which mustusually have an angle greater than 22°. Itmay consist of fresh powdery snow, slidingover the contact surface of compactedolder snow; of slabs of consolidated snow,which roll down the slope; or of the wholeice and snow cover of a slope, in whichcase much rock material is also carriedwith the fall. If avalanches are a frequentoccurrence in any one location, they can beimportant from the geomorphologicalpoint of view, because they maintain an ex-posed bedrock surface, which is thereforesusceptible to continued weathering. Seealso mass movement.

aventurine /ă-ven-chŭ-rin/ A spangledtranslucent form of quartz or feldspar,used as a semiprecious stone. Its appear-ance is caused by inclusions of particles ofhematite, mica, or other mineral.

Aves /ah-veez/ The birds: a class of ver-tebrates distinguished from the REPTILIA bythe presence of feathers. They probablyarose from primitive archosaur reptiles (seeArchaeopteryx) and most of their evolu-tion is connected with adaptation forflight. The wings are formed from thewhole forearm and three fused fingers(compare Pterosauria). They are warm-blooded and have short tails, a large breastbone for the attachment of flight muscles,and light hollow bones. All modern birdsare toothless but teeth were present in theirMesozoic ancestors. Because of the fragilenature of the skeleton, fossils are few.There have been a number of secondarily

flightless forms – the ostrich, emu, etc. –often of large size.

axial modulus (in geophysics) The ratioof stress to strain, when in the presence oflaterally confining forces.

axial plane A plane that passes throughthe successive hinge lines of the beds in aFOLD. Different types of fold are character-ized by different angles of inclination fromthis plane.

axial plane cleavage Cleavage planesthat are parallel to the axial plane of aFOLD. Generally the cleavage is related tominor fold axes, but occasionally it may bemore closely related to the region’s foldtrend.

axial rift zone See median valley.

axial trace The intersection of a FOLD

AXIS with the Earth’s surface, reflecting thetrend of the fold. (See diagram at FOLD.)

axinite /aks-ă-bÿt/ A lilac-brown tri-clinic mineral with the compositionCa2(Mn,Fe2+)Al2BO3(Si4O12)OH, pro-duced during the boron metasomatism ofcalcium-rich sediments and igneous rocks.

axis of the Earth A line that joins theNorth and South Poles, about which theEarth rotates every 24 hours.

azimuth /az-ă-mŭth/ 1. (in surveying) Ahorizontal angle measured clockwise fromtrue north (true azimuth) or magneticnorth (magnetic azimuth) to another point.2. (in astronomy) The angle between theplane of the meridian of the observer andthe vertical plane passing through a heav-enly body.

azimuthal equal-area projection /ă-zim-ŭ-th’ăl/ A MAP PROJECTION differing fromthe other azimuthal projections in that thespacing of the parallels decreases with in-creasing distance from the center of theprojection, producing the equal-area prop-erty.

autumnal equinox

30

azimuthal equidistant projection AMAP PROJECTION that is the same as the az-imuthal projection in that the straight linesradiating from the center of the projectionall have their true bearings, but it has theadditional property that the distancesalong these lines are true to scale. It is notan equal-area projection, nor is it confor-mal.

azimuthal projection (zenithal projec-tion) A MAP PROJECTION constructed asthough a plane were placed at a tangent tothe Earth’s surface and the portion of theEarth covered by it were projected onto theplane. The result is that all points havetheir true compass bearings. The tangentplane is not always drawn at the pole; itcan be constructed anywhere on the sur-face of the globe, the point where the tan-gent touches the Earth being the center ofthe map.

Azoic /ă-zoh-ik/ Designating PRECAM-BRIAN rocks that were deposited before theorigin of life. In practice, however, it is dif-ficult to distinguish such rocks and theterm is little used. It is never used for un-fossiliferous PHANEROZOIC strata.

azonal soil /ay-zoh-năl/ Soil lacking a Bhorizon owing to insufficient time for com-plete pedogenesis. Profiles of azonal soilstherefore mainly reflect the influence ofparent material and show an A horizonslightly darker than the C horizon becauseof additions of organic matter. The threemain groups are lithosols, regosols, and al-luvial soils. In the US SOIL TAXONOMY

azonal soils are classified as ENTISOLS.

Azores high (Azores anticyclone) Thesemipermanent anticyclone centered in theAtlantic near the Azores Islands. It is partof the subtropical high-pressure systems ofthe N hemisphere. Its mean latitudinal po-sition oscillates, being farther south in win-ter and farther north in summer, withhigh-pressure cells occasionally driftingnortheastward from the main center toform blocking anticyclones within thewesterly circulation.

azurite /azh-û-rÿt/ A deep blue mineralform of basic copper carbonate,Cu3(CO3)2(OH)2. It crystallizes in themonoclinic system, and occurs in oxidixedcopper deposits, often associated withmalachite. It is a copper ore and also asemiprecious gemstone.

31

azurite

back-bent occlusion See occlusion.

backing In meteorology, a counter-clockwise change in wind direction, suchas from easterly to northeasterly. See alsoveering.

backshore The part of a beach that liesabove the level of normal high spring tides.Only when exceptionally high spring tidesoccur, or severe storms take place, doesthis zone come under the influence of waveaction. Cliffs or sand dunes behind beachesare usually included as part of the back-shore.

backsight See leveling.

backwash The return of water down abeach, under the influence of gravity, fol-lowing the breaking of a wave and theassociated SWASH. The difference in load-carrying capacity between the swash andbackwash of prevalent waves determineswhether a beach will aggrade or degrade.Whereas flat waves tend to produce astrong swash, steep waves, which breakvertically onto the beach, result in a pow-erful backwash and cause a net seawardmovement of beach materials.

bacteria Single-celled microorganismsthat are fundamental to soil productivity.Where oxygen is lacking they account formost of the biochemical changes in the soil.Environmental controls such as moisture,aeration, temperature, acidity, and organicmatter result in a constantly fluctuatingbacterial population in the soil. It has beenestimated there may be as many as 4000million per gram of soil.

They are commonly classified into twobroad groups in relation to energy supply:

autotrophic bacteria, which obtain theirenergy from the oxidation of mineral sub-stances and their carbon mostly from car-bon dioxide; heterotrophic bacteria, whichobtain their energy and carbon directlyfrom the soil organic matter. Certain en-zymic transformations in the soil are dom-inated by bacteria. Autotrophic bacteriaare much less abundant than heterotrophicbut because they include the bacteria re-sponsible for nitrification and sulfur oxida-tion they are of paramount importance.See nitrification; nitrogen fixation; sulfuroxidation.

By their action of breaking down or-ganic matter, bacteria are probably instru-mental in the formation of oil. They arealso involved in inorganic reactions, suchas those leading to the origin of iron ore de-posits. There is evidence indicating thatbacteria were present in PRECAMBRIAN

times: structures called STROMATOLITES,which are formed by the action of bacteria,notably blue-green bacteria (cyanobac-teria; formerly called blue-green algae), arepresent in Precambrian rocks.

badlands An eroded furrowed land-scape in a dry region, such as parts of Ne-braska and South Dakota in the AmericanWest. There is little or no vegetation and soany rainwater runs off quickly along theshort steep slopes, further eroding any ex-posed or soft rocks.

bahada /bah-hah-dă, -th</ See bajada.

Bai-u season /bÿ-oo/ A period in earlyJune when the summer circulation ofChina and Japan commences, bringingwith it a marked rainfall maximum. Therains are the result of weak nonfrontal dis-turbances within the southwesterly flow,

32

B

which rapidly moves northward at thistime as the upper westerlies move to thenorth of the Himalayas.

bajada /bah-hah-dă, -th</ (bahada) Thegently sloping surface leading from amountain range down to an INLAND BASIN

in arid and semiarid areas. The term hasbeen used both for coalesced alluvial fansoccurring on the rock-cut PEDIMENT at thefoot of the mountain front and for a gentlysloping concave surface (composed of in-creasingly fine-grained particles) leadingdown from the pediment to the flat basinor playa. When the term is applied to theformer case, this sloping surface, whichstill exists, is known as a peripediment. Ineither case, the composing material is de-rived from the mountains and is broughtdown by surface runoff, which follows theinfrequent but heavy downpours.

bald-headed Denoting an anticlinalfold whose upper beds have been erodedaway, exposing the older rocks of its core.

ball-and-pillow structure A structurein sedimentary rocks that occurs wheresandstones are interbedded with mudstonein which globular masses and pillows ofsandstone occur within the mudstone. It isproduced by unconsolidated sand sinkinginto fluid mud below.

ball clay A very plastic fine-texturedgray or buff-colored clay that often con-tains some organic matter. Also called pipeclay, it is used to make ceramics.

ball lightning A rare form of lightningthat is not, as yet, adequately understood.Its very existence has been debated becausemost reported cases follow brilliant light-ning flashes and so could be physiologicalin origin, an afterimage being produced inthe eye. Reports of the lightning suggest abrilliant sphere with a diameter of 1 cm to1 m, which can drift almost randomly at afew meters per second.

banded agate A type of AGATE that hasalternate bands of colors. The bands maybe blended into each other or be sharply

defined; they may be concentric or take theform of wavy lines. See also onyx.

banded iron formation (BIF) SEDIMEN-TARY ROCKS in the form of beds or layersthat consist of CHALCEDONY, CHERT, JASPER,or QUARTZ and at least 25% iron. The ironis usually in the form of hematite, mag-netite, or other oxides and may be used aslow-grade ore.

bank 1. Either side of a river channel,best marked where the river has consider-able powers of vertical erosion. At BANK-FULL no banks are exposed.2. A mass of sand, mud, shells, etc., usuallybelow the water surface, and often ellipti-cal in shape. Banks of unbedded limestoneare distinguished from reefs by their lack ofrigid framework formed by corals or othercalcareous organisms.

bankfull The state of flow of a river im-mediately before flooding when the river’schannel is full with water to its brim. Instreams of very different sizes, and in dif-ferent environments, it has a fairly con-stant recurrence interval of about onceevery 1.5 years.

banner cloud A stationary cloud thatappears to be attached to an isolatedmountain peak. It extends downwind fromthe peak for some distance and so resem-bles a flag or banner. The Matterhorn isprobably the best known summit with afrequent occurrence of this type of cloud.The origin of the cloud is due to the liftingof air in the eddy of the leeward side of thepeak, although an aerodynamic pressurereduction may play some part in its forma-tion.

bar 1. An elongated body of sediment,such as sand or shingle, occurring in the seamore or less parallel with the coastline andsometimes attached to it. It may be perma-nently submerged (see submarine bar) orbe submerged for part of the tide, for in-stance at the mouth of a river or near theentrance to a harbor. Some such bars aremuch exposed at low tide. See also barrier

33

bar

beach; bay bar; longshore bar; offshorebar; spit; tombolo.2. A unit of pressure. See millibar.

barbed drainage A drainage pattern inwhich a stream suddenly bends back on it-self and flows in an almost reverse direc-tion, often due to glacial blocking, rivercapture, or local tectonic movement in itspath.

barchan /bar-kahn/ A crescentic dunecharacterized by an oval shape upwind,with a slip face and two wings spreadingout downwind. Barchans occur in desertareas with a unidirectional wind regime.The slip face is formed by erosion of ma-terial from the upwind side, and its trans-port to the summit where it builds up untilunstable, at which point it slumps forwardto its angle of rest. The wings develop be-

cause the rate of advance of the dune is in-versely proportional to its height, so thelower sides extend downwind faster thanthe center; at their tips they end in a dwin-dling of the sand pile to nil, which isreached at the point where the slip face up-wind ceases to shelter the wings, and theybecome subject to wind erosion. Barchanstypically occur in belts up to 300 km longand over 12 km wide.

barite /bair-ÿt/ See barytes.

baroclinic /ba-rŏ-klin-ik/ 1. (in meteo-rology) Describing a state of the atmos-phere where the surfaces of constantpressure (isobars) intersect into surfaces ofconstant density (isopycnics). This state isthe result of large horizontal temperaturegradients and is believed to be responsiblein part for the formation of mid-latitudedepressions.2. (in oceanography) Describing a state ofthe ocean where the surfaces of constantpressure intersect into surfaces of constantdensity. Compare barotropic.

barometer An instrument for measur-ing pressure, invented in 1643 by the Ital-ian scientist Evangelista Torricelli(1608–47). The standard instrument, themercurial barometer (Kew pattern orFortin barometer), measures the height towhich a column of pure mercury can besupported by the atmospheric pressure ex-erted upon it. Corrections have to be madeto this value to allow for variations in grav-ity, the purity of the mercury, and the am-bient temperature. See also aneroidbarometer.

barotropic /ba-rŏ-trop-ik/ Describingthe state of the atmosphere or ocean whenthe surfaces of constant pressure are paral-lel to the surfaces of constant density. Al-though this is a useful concept intheoretical studies, such an ideal staterarely, if ever, occurs in the atmosphere.Compare baroclinic.

barrage A large engineered structure ofconcrete or soil and rocks, constructedacross a river. It dams the flow to create a

barbed drainage

34

blockage

stream

Barbed drainage

unidirectionalwind

summit 30 m (max.)

slipface(at angle of rest of sand)

_

400

m (

max

.)40

0 m

(m

ax.)

400

m (

max

.)w

ing

s

_

_ _400 m (max.)

Barchan

lake, generally for use in the irrigation ofadjacent farmland. A barrage across an es-tuary also prevents exceptionally high tidesfrom causing flooding.

barrier beach An elongated accumula-tion of sand, shingle, or in-situ rock lyingroughly parallel to the coast but separatedfrom it by a channel, lagoon, or otherwater area. The distance offshore may varyfrom a few meters to several kilometers.Some lie across bay mouths or estuaries.Off low coasts, barriers are often in theform of barrier islands, the lagoonal areasbehind being of the mangrove type inwarm sea regions and of the salt-marshtype or even devoid of vegetation in thecase of temperate or cold sea regions.Many barriers, such as those off the E coastof the USA and the Gulf of Mexico coast,carry dunes. Most sand and shingle barri-ers are destined to overwash, even breach-ing on occasions during severe storms. Themore common sandy barriers are formedby the continued activity of CONSTRUCTIVE

WAVES, which can progressively build upmaterial above water level. Shingle barriersare formed by storm waves, the main inputof material resulting from longshore move-ments.

barrier island See barrier beach.

barrier reef An elongate accumulationof coral, extending upward from the seafloor to about low-tide level, parallel withthe coast but separated from it by a lagoon.A reef may lie between several meters andseveral kilometers offshore. They are be-lieved to have been formed as a result ofthe submergence of a flat surface as thepostglacial sea level rose. The growth ofcoral organisms kept pace with the risingwater so that great thicknesses could de-velop. The best-known example is theGREAT BARRIER REEF.

Barrow zone See zone (def. 1).

barysphere /ba-ră-sfeer/ All of theEarth’s interior beneath the lithosphere,i.e. the asthenosphere, most of the uppermantle, and the outer and inner core.

barytes /bă-rÿ-teez/ (barite) An or-thorhombic mineral form of barium sul-fate, BaSO4. It is usually white but impurevarieties are colored. Barytes has a highdensity, which distinguishes it from calcite.A continuous chemical series exists be-tween barytes and celestite (SrSO4). Bothminerals are found in hydrothermal veinswith galena, sphalerite, fluorite, and calciteas well as in cavities in limestone.

basal conglomerate A CONGLOMERATE

at the base of a sequence of sedimentaryrocks. It consists of a mixture of well-sorted coarse particles, usually depositedas a thin layer by an encroaching sea.

basal sapping Any of a range ofprocesses that act to remove debris fromthe foot of a slope. Basal sapping is impor-tant in the creation and maintenance ofFREE FACES, leading to near vertical or steepslopes that are often unstable, hence caus-ing MASS MOVEMENTS. For this reason itconstitutes the major explanation of PAR-ALLEL RETREAT of slopes, because the debrisat the foot is constantly removed and thesteepness of the slope is maintained, thuspreventing slope decline. Types of basalsapping include wave action at the foot ofa sea cliff, lateral erosion by a river at thefoot of a bluff, headward erosion ofstreams issuing at the foot of a slope, andmass movements produced by heterogen-eity of rock strata.

basalt /bă-sawlt, bass-awlt/ A basic vol-canic rock, the fine-grained equivalent ofGABBRO. Its essential constituents are calcicplagioclase, generally in the rangelabradorite-bytownite, and pyroxene.Olivine may not always be present butmagnetite and apatite are invariable acces-sories. Basalts range from undersaturatedto oversaturated compositions (see silicasaturation) but two extremes are recog-nized, named ALKALI BASALT and tholeiite.

Alkali basalts are undersaturated andcontain abundant olivine both as phe-nocrysts and in the groundmass. The py-roxenes of alkali basalt are calcium-richaugite or titanaugite. Tholeiites are satu-rated or slightly oversaturated in composi-

35

basalt

tion. They contain sufficient silica to partlyor completely convert olivine to orthopy-roxene and are characterized by this reac-tion relationship, which is indicated by themantling of olivine by calcium-poor pyrox-ene (hypersthene or pigeonite). When pre-sent, olivine occurs only as phenocrystsand is absent from the groundmass.Tholeiites typically have intersertal tex-tures and contain silica minerals in thegroundmass.

A third type of basalt is now widely rec-ognized, namely high-alumina basalt (orcalc-alkali basalt) but mineralogical cri-teria alone are insufficient to distinguishthe different basalt types and chemical cri-teria are applied. Even so, the three basalttypes, alkali basalt, high-alumina basalt,and tholeiite, are gradational in composi-tion. High-alumina basalt is distinguishedby an Al2O3 content in excess of 17% inaphyric rocks. The Na2O + K2O contentlies between those of the other two typesbut mineralogically, high-alumina basalthas a strong affinity with tholeiite. Threevolcanic associations corresponding to thethree basalt types may be considered. 1. alkali olivine–basalt–mugearite–

trachyte/phonolite 2. high-alumina basalt–andesite–dacite–

rhyolite (calc-alkaline suite) 3. tholeiite–tholeiitic andesite–rhyolite

Members of the alkali basalt associa-tion are found on oceanic islands and con-tinents. Calc-alkaline volcanic rocks arecharacteristic of island arcs and orogenicbelts. Tholeiites occur on the continents,especially along continental margins. Mostof the plateau or flood basalts of the worldare of tholeiitic composition. The basaltsof the ocean floor generated at the MID-OCEAN RIDGES are tholeiites with extremelylow K2O and TiO2 contents.

basanite /bass-8abreve;-nÿt/ A type ofolivine BASALT containing augite, plagio-clase, and a FELDSPATHOID mineral.

base (in soil science) The neutral or al-kaline constituent in the soil, notably cal-cium, magnesium, sodium, and potassium.Each soil has a theoretical maximum con-tent of bases and the actual content is ex-

pressed as a percentage of this theoreticalfigure to express the degree of base satura-tion. A high base saturation, especiallywith calcium, generally promotes a neutralsoil, with good structure, aeration, and fer-tility. A low base saturation represents anacidic soil. Different bases dominate in dif-ferent environments: calcium is common-est in temperate soils, sodium in saline andalkaline soils of the arid environments.

base construction line The line drawnat right angles to the CENTRAL MERIDIAN ofa map projection, from which the othermeridians of the map are established.

base level An imaginary line runningfrom SEA LEVEL under the landmass, risingslightly above the horizontal. It constitutesthe controlling level down to which, butnot below which, a river can cut its valley.

Local base levels may replace sea levelas the controlling lower limit of erosion forcertain parts of streams, or in certain re-gions; for example, a dam across a stream,a hard rock band, or a lake will constitutethe local base levels for the upstream sec-tion of a river. In inland drainage basins,the base level will be the lake to which thestreams flow; the Lake Eyre basin in Aus-tralia has a local level 14 m below sea level.No river ever actually reaches base level,except at its mouth, because it must retainsome gradient in order to flow.

Sea level change, tectonic movement, orthe removal of ice pressure from a formerlyglaciated area can all change the positionof sea level (the ultimate base level) relativeto the land. Base level, along with climateand geology, is one of the independentvariables that control the processes in aparticular cycle of erosion. Changes in baselevel therefore produce fundamentalchanges in the geomorphological system,to the extent that a major base level changeinitiates a new cycle of erosion.

If the land is raised relative to the sea,base level movement is negative; if viceversa, it is positive. The former tends toincrease erosional activity, the latter to ini-tiate deposition. See also eustasy; rejuvena-tion.

basanite

36

baseline A line that is measured very ac-curately as part of a TRIANGULATION

scheme. Great accuracy is required becausethe location of all the points within the tri-angulation area is based upon this onemeasured line, although the largestschemes may use two or three baselines.They are traditionally measured using longtapes or wires, supported above theground. Several corrections must be ap-plied to the taped lengths to allow for suchfactors as slope, thermal conditions, andsag. The use of electronic instruments insurveying allow the measurements of longbaselines without the need for tapes: boththe GEODIMETER and TELLUROMETER pro-vide a rapid means of measuring a baselinevery accurately.

base map A map or chart used as a baseto which different types of information canbe added or overprinted. It usually con-tains basic information, e.g. major politicalboundaries, drainage, and coastlines. Thebase is used when several maps of an areaare required, each covering a differenttopic, such as geology, vegetation, popula-tion distributions, etc.

basement The level below which sedi-mentary rocks do not occur. Rocks belowthis level are generally igneous or meta-morphic.

basic lava A type of dark-colored LAVA

containing basic ferromagnesian mineralsand less than 50% silica. It flows freelyfrom a volcano’s crater or fissures, quicklyspreading across the terrain. It becomesBASALT when it solidifies. Weathered basiclava forms a rich soil, much used for farm-ing. See also acid lava.

basic rock An igneous rock with a silicacontent of 45–55% (by weight). Suchrocks consist largely of relatively silica-poor minerals such as olivine, pyroxene,and calcic plagioclase feldspar. Typicalbasic rocks are basalt, dolerite, and gab-bro. See acid rock; intermediate rock; ul-trabasic rock.

basin 1. A large sediment-filled depres-sion often present in cratonic areas; it maybe circular or elliptical.2. A synclinal structure having a plungeddepression, i.e. two directions of plunge at180° to each other but plunging towardeach other.3. See drainage basin.

bastnaesite /bast-nă-sÿt/ A yellow tored-brown mineral, (Ce,La)CO3(F,OH),which occurs in alkaline igneous rockssuch as CARBONATITE. It is used as a sourceof rare-earth elements.

batholith /bath-ŏ-lith/ A large body ofintrusive igneous rock consisting of severalplutons joined at depth and occupyingmany thousands of square kilometers.They are generally composed of granitematerial and are associated with mountainbelts.

bathyal zone /bath-ee-ăl/ The CONTI-NENTAL SLOPE zone within the oceans. Itembraces depths of some 200 m out todepths of about 1000 m and lies betweenthe relatively shallow NERITIC zone, withinwhich there is greater sediment deposition,and the deep ABYSSAL ZONE. Its upper limitcorresponds approximately with the SHELF-EDGE zone and its lower limit is quite arbi-trary, because the depth of 1000 m doesnot mark an abrupt change in topographyor other factors. Light reaches only to thetop of this zone but animal life is quiteabundant and varied. Globally the bathyalenvironment covers approximately 40 mil-lion sq km.

bathyorographical /bath-ee-ô-rŏ-graf-ă-kăl/ Describing a map showing theheight of the land and depth of the sea,usually by layer-coloring.

bathyscaphe /bath-ă-skayff/ A sphericalor sausage-shaped vessel designed to en-able investigators to descend into the seaand to observe, through portholes or otherobservation windows, the marine environ-ment. The first of such devices to achieveuseful underwater work were the Ameri-can biologist William Beebe’s (1877–1912)

37

bathyscaphe

bathysphere and the American engineerOtis Barton’s benthoscope. By about 1953,improved bathyscaphes were reachingdepths in excess of 3000 m during experi-mental trials. During 1960, the bathy-scaphe Trieste landed on the floor of theMariana Trench (10 911 m). Bathyscaphescan be equipped with grab-sampling, cor-ing, and other equipment.

bauxite /bawks-ÿt/ An aluminum oreformed by the weathering of aluminum-rich, relatively iron- and silicon-poor rocks(mainly syenites) under tropical condi-tions. The breakdown of aluminosilicateminerals and the removal of silica by leach-ing leaves a residue composed mainly ofboehmite (AlO(OH)), diaspore (HAlO2),and gibbsite (Al(OH)3).

bay An inlet or indentation in the shoreof a lake or the sea. Bays are commonlyformed by DIFFERENTIAL EROSION, in whichsofter rocks are worn away faster than theharder rocks surrounding them.

bay bar A barrier of sand or shingle thatextends across the entrance to a bay, effec-tively straightening the coastline. The mostlikely causes of a bay bar are an enlarge-ment of a single SPIT in one direction or theindependent growth, and subsequent coa-lescence, of two spits that increase theirlengths toward each other. Once closed offby a bay bar, the bay will gradually fill upwith sediment.

bayou /bÿ-oo/ A marshy area of an estu-ary or where a lake outflows, as occurs typ-ically in the southeastern states of the USA.The water flows only sluggishly or may bestagnant.

beach An accumulation of unconsoli-dated materials found in the zone of inter-section between land and sea. For mostpractical purposes the beach can be consid-ered to extend from the highest point of se-vere storm-wave activity (see storm beach)down to the point at which waves ap-proaching the coast first start to cause ex-tensive movement of seabed materials.Beach materials are located on a founda-

tion of eroded solid rock (see wave-cutplatform); they usually form a smooth pro-file that is gently convex and is sometimesinterrupted by BEACH RIDGES. The responseof a beach to the activity of waves andother agents of erosion and depositionlargely depends upon the nature and thick-ness of the deposits, sand and shingle be-having very differently.

beach cusp See cusp.

beach ridge An upstanding linear accu-mulation found on shingle beaches (an ac-cumulation developed in sand is known asa BERM). Whenever CONSTRUCTIVE WAVES

are active on a shingle beach, a small ridgewill be produced at the limit of SWASH foreach high tide. As maximum tide level fallsfrom spring to neap level, a series of ridgeswill be produced; these will be destroyedagain on tidal rise to spring level, when justone ridge remains at high-water springlevel. Much larger shingle ridges can beproduced in connection with severestorms. These develop far above the swashlimits of normal waves (see storm beach).

beach rock Consolidated and erosion-resistant sand-forms, often reeflike, thatdevelop along some intertidal shores intropical and subtropical regions. Beachrock has also been found in Portugal,Hawaii, Morocco, and certain temperateregions. Its origin has provoked a greatdeal of argument and speculation. In gen-eral, it appears to develop on account of ce-mentation or lithification processes, thecement being of a calcareous type (oftencalcite or aragonite). Hence, geologistsclassify beach rock as a calcarenite. Itsrapid consolidation may be aided by thepresence of microorganisms such as bacte-ria and unicellular algae, and certainly byhigh temperatures. Beach rock, whichtends to form as small outcrops, is not al-ways found on all beaches in the same lo-cality. Once formed, however, it tends toresist the action of waves and may act as abarrier to sediment movement.

beaded esker An ESKER comprising al-ternate wide and narrow segments, which

bauxite

38

reflect differential input rates of flu-vioglacial sediments. The narrow sectionswere formed during colder periods, whenthere was less meltwater and hence lesssediment, whereas the wider parts were de-posited in warmer periods. These segmentsmay mark successive winter and summeraccumulations as an ice front (or glaciersnout) retreated. Beaded eskers are com-paratively rare.

bearing A horizontal angle measuredclockwise from a known direction, usuallynorth, to another point. If the initial direc-tion is true north then the angles are truebearings. Angles measured with referenceto magnetic north, such as those obtainedwith a prismatic compass, are known asmagnetic or compass bearings.

Beaufort scale /boh-fert/ A scale ofwind speed based on easily observable in-dications such as tree movement andsmoke. The scale ranges from 0 to 17(numbers 13 to 17 were added by the USWeather Bureau in 1955) with the num-bers and indicators as shown in the table.The scale is named for the British hydrog-rapher Sir Francis Beaufort (1774–1857).

bed (stratum) The smallest division ofstratified sedimentary rocks, consisting ofa single distinct sheetlike layer of sedimen-tary material, separated from the bedsabove and below by relatively well-definedplanar surfaces called BEDDING PLANES,which mark a break in sedimentation. Abed may be part of a MEMBER (see alsomarker bed). See lithostratigraphy; stratig-raphy.

39

bed

BEAUFORT SCALE

Speed/knots (km/hr)0 Calm Smoke rises vertically <1 1 Light air Smoke or leaves indicates move-

ment, otherwise almost calm 1–3 (1–5)2 Light breeze Wind felt on face, leaves rustle, etc. 4–6 (6–11)3 Gentle breeze Flag extended; leaves and twigs in

constant motion 7–10 (12–19)4 Moderate breeze Small branches moved; dust and

litter raised 11–16 (20–28)5 Fresh breeze Small trees begin to sway 17–21 (29–38)6 Strong breeze Large branches in motion; whis-

tling in telephone wires 22–27 (39–49)7 Near gale Whole trees in motion; inconveni-

ence experienced in walking 28–33 (50–61)8 Fresh gale (gale) Twigs broken off; walking impeded 34–40 (62–74)9 Strong gale Slight structural damage

experienced 41–47 (75–88)10 Whole gale (storm) Widespread damage to trees and

buildings 48–55 (89–102)11 Storm (violent storm) Very rarely experienced inland,

severe damage results 56–63 (103–114)12–17 Hurricane Very rarely experienced inland,

severe damage results >64 (>117)

bedding The parallel layering of SEDI-MENTARY ROCKS. The different layers mayhave different compositions or structures.

bedding plane Any of the planes thatseparate each of the layers or BEDS (strata)in a sedimentary rock formation. There aregenerally differences of color, composition,or structure on each side of such a beddingplane.

bedding-plane slip The displacementof one bed of rock over another, takingplace along the bedding plane between ad-jacent beds. It is usually associated withfolding, where in extreme cases it can leadto DÉCOLLEMENT.

bed load (bottom load; traction load)The material that is carried along the bedof a stream or the sea by moving water, oralong the ground by the wind. It consists ofparticles that are too large to be trans-ported in suspension (see suspended load).

bedrock The solid unweathered rockthat lies below soil or unconsolidated ma-terial, or is exposed at the surface.

beef Thin beds and veins of fibrous CAL-CITE. See carbonate minerals.

belemnite /bel-ĕm-nÿt/ Any mollusk ofthe extinct subclass BELEMNOIDEA. Belem-nites are important fossils from rocks ofthe Mesozoic Era.

Belemnoidea /bel-ĕm-noi-dee-ă/ An ex-tinct subclass of marine mollusks of theclass CEPHALOPODA. With squids and octo-puses, they are classified as Coleoidea (orDibranchiata). They had an internal bullet-shaped shell with simple septa dividing itinto chambers; this shell is usually the onlypart of the animal to be fossilized. Belem-nites are known from Lower Carbonifer-ous rocks and extend to the beginning ofthe Paleogene, but they reached their max-imum development in the Jurassic and Cre-taceous Periods.

bench mark A mark indicating a pointof known position and height, which has

been surveyed extremely accurately by thenational surveying body of the countryconcerned.

Benguela Current /ben-gwel-ă/ A seacurrent that flows northward off the coastof southwest Africa. Part of the region inwhich it flows is important because of theexistence of an UPWELLING of cold waterand a zone of marked divergence. The up-welling water comes from subsurfacewater perhaps only two or three hundredmeters deep. The most marked flow in theBenguela Current, which can be up toabout 16 million cubic meters per second,occurs between the Cape of Good Hopeand latitude 18°S.

Benioff zone /ben-ee-off/ The inclinedseismic zone within the Earth’s lithosphereextending down at an angle of usuallyaround 45° from the base of an oceantrench to the asthenosphere. It is typical ofdestructive PLATE BOUNDARIES where oneplate overrides another as a result of sea-floor spreading. These are zones whereocean floor is consumed. The shallowestearthquake foci along the zone occur nearthe base of the trench, the foci becomingprogressively deeper with distance alongthe Benioff zone from the trench. The zoneis named for the American seismologistHugo Benioff (1899–1968).

benitoite /be-nee-toh-ÿt/ A silicate min-eral of barium and titanium, noted for itsstrongly dichroic nature. It varies in colorfrom deep sapphire blue to colorless, de-pending on whether it is viewed by re-flected or transmitted light.

benmoreite /ben-mor-ÿt/ A basic ig-neous rock composed of an alkaliFELDSPAR, olivine, and pyroxene. See tra-chybasalt.

benthic /ben-th’ik/ Describing an organ-ism that lives on the floor of the sea or alake. See benthos.

benthos /ben-th’os/ Plant and animalmarine life, both small and large, livingwithin the waters of the sea or on the sea

bedding

40

bed (see also pelagic (def. 2)). They are afundamental part of the complex foodchains that operate within marine environ-ments. Benthos exist in numerous forms,the greatest variety occurring within theshallow-water environments associatedwith continental shelves. In the photic(light-penetrating) zone, the benthos feedon both phytoplankton and zooplankton;they are especially well developed in plank-ton-rich areas and often poorly developedin less fertile areas. The benthos help tomake available to larger marine creaturesthe tiny food particles that they consume orproduce. Vagrant benthos are capable ofactive movement on or within the sedi-ment; sessile benthos remain attached tothe sea floor.

bentonite /ben-tŏ-nÿt/ Clay formed bythe alteration and weathering of tuffs andvolcanic ash. See clay minerals.

Bergeron–Findeisen theory /bair-zhĕ-rawn fin-dÿ-zĕn/ (Bergeron theory) Atheory of precipitation formation proposedby the Norwegian meteorologist Tor Bergeron (1891–1977) in 1933 and sub-sequently modified by the German meteor-ologist Walter Findeisen (1909–45). Itsuggested that ice crystals and supercooledwater droplets could exist together atcertain levels in the clouds where the tem-perature was between –15° and –30°C. Be-cause the saturation vapor pressure for iceis lower than that for a water surface, therewould be preferential deposition of mois-ture on the ice crystals, causing them togrow sufficiently for snowflakes or rain-drops to result.

bergschrund /berk-shrûnt/ A deep nar-row chasm frequently found in CIRQUES inwhich the upper part of the CIRQUE GLACIER

has become separated from the rock head-wall. They were formerly believed to be themost favorable sites for FREEZE-THAW activ-ity, which causes headwall retreat. How-ever, more recent work has shown thatfluctuations of temperature within berg-schrunds are infrequently of sufficientmagnitude or rapidity to cause any rockbreakdown. Only in the upper parts of

open bergschrunds, where cold air can cir-culate freely, will there be any appreciableattack on the rock of the headwall.

berm /berm/ An accumulation of ma-terial found a little way above the meanhigh-water mark on sand beaches. Theberm is usually flat (although it may slopegently landward), is of variable width, andis characterized by a marked break of slopeat the seaward edge. Berms are created bythe action of CONSTRUCTIVE WAVES but theirgrowth is not a very rapid process. A beachof coarse sand, having a steeper gradientthan one of finer material, will develop aberm more rapidly, since wave action is re-stricted to a narrower width of beach. Seealso beach ridge.

beryl A hexagonal mineral of composi-tion Be3Al2Si6O18 found in granitic rocksand PEGMATITES. Beryl is white to palegreen in color. The semiprecious gem vari-ety aquamarine is bluish green. The brightgreen variety is EMERALD, found mainly inmetamorphic rocks.

B horizon The second highest layer ofsoil, or upper subsoil, immediately belowthe A HORIZON. It has less humus and con-tains less weathered material, but may con-tain chemicals washed down from above.See also horizon (def. 1).

BIF See banded iron formation.

bifurcation ratio The quantitative rela-tionship between STREAM ORDERS, ex-pressed as the number of streams of oneorder divided by the number of streams ofthe next highest order. Values are com-monly between 2.5 and 3.5, but are higherin long thin drainage basins dominated byone master stream.

billow cloud Cloud found in a series ofregular bands with clear areas between,usually of similar width to that of thecloud. They occur most frequently at aheight of about 6–8 km, and appear to bedue to a strong increase of wind speed withheight when the airflow is stable, but theprecise mechanism is still in doubt.

41

billow cloud

bimineralic /bÿ-min-ĕ-ral-ik/ Describ-ing a rock consisting of only two kinds ofminerals, e.g. websterite.

bioclastic rock /bÿ-oh-klas-tik/ A sedi-mentary rock that contains the fragmen-tary remains of once-living organisms, suchas the shell fragments that make up somekinds of limestone. See also biogenic rock.

biocoenosis /bÿ-oh-see-noh-sis/ (life as-semblage) An assemblage of fossil organ-isms associated and occurring in the sameposition as they occupied in life. Some bio-herms and reefs approximate to this condi-tion but such assemblages are usually rare,because most organisms suffer damage andtransport after death. Compare thanato-coenosis. See also taphonomy.

biogenic rock /bÿ-ŏ-jen-ik/ A type ofrock directly created by living organisms,their remains, or activities. Examples in-clude coal, coral reefs, and limestones com-posed of the shells of mollusks. See alsobioclastic rock.

biogeochemical cycle The pathwaysand methods by which chemical elementsare moved and circulated within the eco-sphere from living organisms to the physi-cal environment and back again. Thechemicals may be in gaseous, liquid, orsolid form and form different chemicalcompounds at various stages in the cycle.Biogeochemical cycles have been docu-mented for many elements, e.g. carbon,oxygen, nitrogen, phosphorous, sulfur, andwater. A biogeochemical cycle in which theelements are trace elements essential to lifeis known as a NUTRIENT CYCLE. See alsocarbon cycle; nitrogen cycle.

biogeography /bÿ-oh-jee-og-ră-fee/ Thescientific study of the distribution of plantsand animals around the world. It includesstudies of soil (pedology), climate, ecology,and ecosystems.

bioherm /bÿ-oh-herm/ A REEF of un-stratified limestone formed by organicprocesses and usually having the shape of adome rather than a linear feature. Most re-

cent bioherms are produced by CORALS, butin the past important reef-building organ-isms have included sponges (see Porifera)and certain algae. A bioherm may includea hard skeletal structure secreted by the or-ganism itself, the remains of animals thatlive in the environment it produces, andtrapped sediment. They are mainly marine.

biological weathering A mechanism inwhich rocks are broken down by the actionof plants and animals. For example, plantroots can enter cracks in rocks and breakthe rocks as the roots grow and expand.Bacteria, worms, mollusks, and other in-vertebrates can also split rocks or wearthem away. See also weathering.

biomass /bÿ-oh-mass/ The total organicmatter – plant and animal – in a particulararea. It is usually stated as the dried weightper unit area, such as kg m–2. Most of thebiomass is derived from plants. Biomassvaries widely in different regions of theworld, from less than 0.02 kg m–2 in polarregions to as much as 45 kg m–2 in tropicalrainforests.

biome /bÿ-ohm/ A major ecologicalcommunity of plants and animals thatshare the same climate and vegetation. It isthe largest such community recognized byecologists and may include several HABI-TATS. Examples of biomes include desert,grassland, savanna, taiga, temperate rain-forest, tropical rainforest, and tundra.

biosome /bÿ-oh-sohm/ An accumulationof sediment that has been deposited underconstant biological conditions.

biosphere /bÿ-ŏ-sfeer/ The region of theEarth’s crust and atmosphere in which lifeexists; it contains a number of HABITATS.The biosphere extends from about 3 mbelow ground to 30 m above it. It also in-cludes aquatic zones extending to about200 m deep.

biostratigraphy /bÿ-oh-stră-tig-ră-fee/The branch of stratigraphy that utilizes in-formation from fossils in the CALIBRATION

of stratigraphic sequences of rock in which

bimineralic

42

they occur. The evolution of organisms isbelieved to be a continuous unidirectionalprocess and it therefore provides an impor-tant means of correlating and comparingseparate rock sections. The fundamentalbiostratigraphic division is the ZONE. Somegeologists make no distinction betweenbiostratigraphy and CHRONOSTRATIGRAPHY

and include the biostratigraphic zonewithin the chronostratigraphic hierarchyof terms. Others consider biostratigraphyto be simply one method of calibrating theessentially separate chronostratigraphicscale (see chronozone).

biostratonomy /bÿ-oh-stră-tonn-ŏ-mee/The branch of paleontology concernedwith the processes by which the remains oforganisms become embedded in rock. Thisis important in interpreting the significanceof fossil assemblages, many of which areTHANATOCOENOSES. See actuopaleontol-ogy; taphonomy.

biostrome /bÿ-ŏ-strohm/ A structure sim-ilar to a BIOHERM but not swelling into amound or lenslike body. It more closely re-sembles a broad sheet of sediment consist-ing of a large quantity of organic remains.

biotic Describing the living factors in anECOSYSTEM. They result from the activitiesof animals and plants, and include compe-tition and feeding. See also abiotic.

biotite /bÿ-ŏ-tÿt/ One of the major formsof MICA which forms dark brown or blackshiny slabs that split easily into transparentflakes. It is found widely in many kinds ofigneous, metamorphic, and even sedimen-tary rocks.

bioturbation /bÿ-oh-ter-bay-ʃhobreve;n/The reworking and further degradation ofsediment by the action of organisms mov-ing through it and feeding on it.

birds See Aves.

bird’s-foot delta An elongated form ofDELTA, with sediment deposited in a finger-like pattern following the courses of thevarious distributary streams.

birefringence /bÿ-ri-frin-jens/ The dou-ble refraction of light entering an aniso-tropic crystal, when it is split into twoplane-polarized rays, the ordinary and ex-traordinary rays. The two rays vibrate inmutually perpendicular planes and travelthrough the crystal in different directionsand with different velocities (there is a fastray and a slow ray) and thus have differentrefractive indices. This may be illustratedby the well-known experiment of viewing asingle spot on a piece of paper through apiece of Iceland spar. Two images of thespot are seen representing the ordinary andextraordinary rays.

When an anisotropic crystal is viewedunder a petrological (polarizing) micro-scope, bright polarization colors are seenand the crystal is said to exhibit birefrin-gence. The two rays produced by the min-eral are recombined in the microscope andtheir optical interference produces the po-larization colors. These colors are of greatuse in the identification of minerals.

bittern The brine that remains whenuseful more soluble minerals (such as car-nallite and polyhalite) have been removedfrom sea water by evaporation. Bittern canitself be used as a source of bromides andiodides.

bitumen A semisolid or solid flammablesubstance that consists mainly of hydrocar-bons, particularly those occurring in PE-TROLEUM. See also asphalt.

bituminous coal A black or darkbrown COAL with a high carbon content,the type most commonly used as fuel. Itburns with a yellowish smoky flame and isused for making degassed coal (smokelessfuel) and coke. See also anthracite; lignite.

Bivalvia /bÿ-val-vee-ă/ (Lamellibranchia;Pelecypoda) A class of the phylum MOL-LUSCA whose members are characterized bya two-valved shell protecting the body. Thevalves are secreted by the mantle and meetat a hinge line, where they are joined by anelastic ligament. The hinge is aligned andstrengthened by a system of interlockingteeth and sockets, which are used in the

43

Bivalvia

taxonomic classification of the group. Theshell has growth lines and often ornamen-tation. The animals are bilaterally symmet-rical, with the plane of symmetry passingbetween the valves, unlike the BRACHIO-PODA, which are symmetrical across thevalves. Some bivalves have secondarily lostthis symmetry and the valves are unequal.

Bivalves are aquatic (mostly marine),occupying a great variety of habitats, andinclude free-swimming, sessile, and bur-rowing forms. They range from the Cam-brian Period to the present day and havebeen used biostratigraphically as ZONE FOS-SILS, particularly in the Upper Carbonifer-ous and Paleogene.

black alkali soil See solonetz.

blackband ironstone A SEDIMENTARY

ROCK, containing iron, that occurs in asso-ciation with coal deposits. It contains ironcarbonate (SIDERITE) and is used as an ironore.

black-body radiation A perfect blackbody is one that absorbs all radiationfalling upon it and emits, for any tempera-ture, the maximum amount of radiant en-ergy. No real substance achieves thisdegree of physical perfection, but the insideof a sphere coated with carbon black ap-proaches it. Some substances can act asblack bodies for certain wavelengths only.An extreme example of this is snow, whichhas a high albedo for visible light, but ef-fectively acts as a black body for wave-lengths greater than 1.5 µm.

black earth See chernozem.

black ice (glazed frost) A smooth trans-parent thin coating of ice formed on as-phalted road or other surfaces, having ablack almost invisible appearance. It isformed when drizzle or light rain falls ontoa surface that is at a temperature below0°C.

black jack See sphalerite.

black sand A type of sand found onbeaches or in alluvial deposits that con-

tains dark heavy minerals such as ILMENITE

and MAGNETITE (containing iron) and RU-TILE (containing titanium).

black smoker See hydrothermal vent.

blanket bog See organic soil.

bleaching See coral bleaching.

blende /blend/ Any sulfide mineral, es-pecially one used as an ore. The bestknown is zinc blende (see sphalerite).

blizzard Heavy snow falling or driftingduring strong winds.

block-faulting A series of normal faultsthat separate an area into a series of HORSTS

and GRABENS. See fault.

blocking The situation arising whenanomalous pressure patterns develop in thezone of the mid-latitude westerlies. It morecommonly refers to anticyclone formationsthat act as a block to the normal depressiontracks, which then move toward the north-east and southeast around the anticycloneto give spells of quiet and dry weather nearthe blocking high. The two favored sitesfor the location of blocking highs are overNW Europe and the NW Pacific. The pre-cise reasons for these positions are not un-derstood but they are believed to be due tothe interaction between the westerly circu-lation and the north–south oriented moun-tain ranges such as the Rockies.

block lava A LAVA the surface of whichis broken into large retangular blocks.Compare aa; pahoehoe.

block mountain A steep-sided moun-tain or plateau that has been uplifted be-tween a pair of parallel FAULTS.

block stream A pile of rock debris thathas accumulated at the head of a ravine,probably as a result of glacial action. It isalso called a rock stream.

blowhole A vertical fissure in the roofof a sea cave or cliff through which sea

black alkali soil

44

water is forced as a jet at high tide. It isformed when wave action causes erosionalong a joint in the rock.

blowout A break in the crest of a sanddune, caused by vegetation destruction orgullying. It leads to increased sand mobilityand wind speed at the break point, with thesand being blown downwind as a PARA-BOLIC DUNE bowing out from the transverseline. By the time stage 3 is reached, unlesspractical measures are taken to break windspeed and reinstate the vegetation, theblowout becomes progressively enlargedby advancing downwind and widens itselfthrough undercutting of the vegetation oneither side.

blue ground See kimberlite.

blue john See fluorite.

bluff (river cliff) A steeply sloped riverbank on the outside of a MEANDER. It iscaused by erosion by the faster-flowingwater on the outside of the bend. It may beas much as 100 m in height. Any otherheadland or inland cliff may also be calleda bluff.

body wave A seismic wave, generatedeither by an earthquake or explosion,which reaches a recording station after

having traveled through the Earth’s inte-rior. Body waves include primary (P or lon-gitudinal) and secondary (S or shear)waves.

bog A waterlogged area of land, result-ing from poor drainage, in which vegeta-tion becomes partly decomposed. Theground is spongy and wet, and eventuallythe vegetation forms an acid PEAT. See alsoorganic soil.

bog soil See organic soil.

bole Thin red weathered horizonsformed between basalt lava flows. Thepresence of bole indicates subaerial extru-sion and a tropical climate. See laterite.

bolson /bohl-sŏn, bohl-sohn/ See inlandbasin.

bomb (volcanic bomb) See pyroclasticrock.

Bonne’s projection /bon-ĕ/ A type ofconical MAP PROJECTION, with the centralmeridian straight and truly divided, theparallels all concentric circles, and theother meridians composite curves joiningdivisions on the correctly divided parallels.The scale along the parallels is kept true.The shapes of landmasses are distortedwith distance from the central meridianbut the projection is equal-area. The pro-jection is named for the French hydrogra-

45

Bonne’s projection

SEA

WIND gully ordestroyedvegetation

progressiveenlargement

blowout(parabolicdune)

transversedune

Fig. 1 Fig. 2

Fig. 3 Fig. 4

advance

Blowout formation

•center of curvaturepole

80°

60°

40°

20°

Bonne’s projection

pher and cartographer Rigobert Bonne(c.1727–c.95). See conical projection.

bora /bor-ă, boh-ră/ A cold, dry, andgusty northeasterly wind that sweepsthrough the mountain gaps of the WBalkan Peninsula to the Adriatic coast. It isstrongest in winter, with mean speeds ofabout 27 knots (50 km/hr), and is the resultof high pressure over central Europe and adeep depression to the southwest. Thepresence of the mountains helps the fun-neling of the wind and its rapid descentprevents adiabatic warming. See also mis-tral.

borax /bor-aks, boh-raks/ A mineralform of hydrated sodium borate, Na2B4O7.10H2O. It occurs as deposits and on thesurface of the ground in dry regions,formed by the evaporation of alkalinelakes or hot springs. The transparent crys-tals can lose water of crystallization to be-come tincalconite, a chalky white mineral.It is used as a source of boron.

bore A tidal phenomenon that occurs incertain rivers or estuaries. The tidal rangeis usually fairly large and may lead to atidal flood wave with a high sharply devel-oped front. The frontal area is often char-acterized by subsidiary waves or furrows.With the initial flooding of the tide, a surg-ing inflow may develop and travel rapidlysome kilometers upstream. Well-knownbores occur in the Amazon, the QiantangRiver in China, the Hooghly River in India,and the Bay of Fundy. The large bore thattravels up the Qiantang River has a fast-moving abrupt front up to 4 m in height,necessitating the prior removal of boatsfrom the river.

Boreal climate /bor-ee-ăl, boh-ree/ A cli-matic type in Köppen’s classificationsystem (see climatic classification) charac-terized by cold snowy winters and warmsummers, giving a large annual range oftemperature. Precipitation totals are small.It is found over the American, European,and Asian continents between latitudes40°N and 65°N with the S boundary beingfarther north on the warmer W coasts and

trending WNW–ESE toward the coldercontinental interiors.

Boreal forest See taiga.

Boreal period The period from 7500 BC

to 5000 BC, which was characterized by animproving climate following the deglacia-tion of the last ice age. It was named Borealbecause the vegetation at this time in NWEurope, where the first investigations wereconducted, was predominantly coniferousas in the Boreal areas at present.

borehole logging A method of record-ing the physical characteristics of materialsbelow the ground surface by drilling wellsor boreholes. Observations may be madeby examining the cuttings of material re-moved during the drilling process, by low-ering instruments to measure differentphysical properties into the resultant wellor borehole, or by studying the core that isextracted. Borehole logging is used, for ex-ample, in geophysics in the identificationof ores and fossil fuels, in stratigraphy, andin hydrology to study groundwater flow.See also electrical logging.

borer See marine borer.

bornhardt /born-hart/ A dome-shapedmound of rock; a large INSELBERG.

bornite /born-ÿt/ (peacock ore) A red-brown mineral form of copper iron sulfide,Cu5FeS4, with a characteristic purplish iri-descent tarnish. It occurs associated withother copper ores.

borolonite /bŏ-rol-ŏ-nÿt/ A nepheline-containing type of SYENITE.

boss A roughly circular igneous intru-sion that has a diameter of less than 25 km.Compare stock.

botryoidal /bot-ree-oi-dăl/ Describing amineral consisting of spheroidal aggregatesresembling a bunch of grapes. Comparereniform.

bora

46

bottom current A current that flowsclose to the sea floor or within the lower-most layers of water in deep-sea areas. Theterm has also been used to describe the cur-rents over shelf areas that arise owing todensity-driven flow (see density current),tidal flow, or wave-induced flow. In thecase of deep-water bottom currents, flowvelocities have been calculated theoreti-cally, others have been measured using cur-rent meters or special floats. Bottom flowin part of the Antarctic bottom current canattain 3 cm/sec but is usually much less.The speed of the bottom current on the Wflank of the Atlantic Ocean is, theoreticallyat least, between 0.5 to 2 cm/sec. Bottomtopography may significantly hinder theflow of bottom water, for example thetransverse ridges in the South Atlantic,which the Antarctic bottom current has tonegotiate. Deep gashes or gaps throughsuch features are important in channelingbottom flow. Some sub-Arctic bottomwater has its progress southward off theLabrador coast partly blocked by theLabrador submarine rise.

bottom load See bed load.

bottomset bed See delta deposit.

bottom water The lowermost layer ofwater in the sea, particularly the watermasses that move slowly in deep oceanareas. Bottom water is relatively cold anddense. Most of the North Atlantic bottomwater has a temperature around 1–2°C.The Polar ANTARCTIC BOTTOM WATER origi-nates in the surface layers of the Antarcticand later flows as a deep-water layer. Dur-ing the fall and early winter, the very coldshelf water sinks owing to convection, andslides down the continental slope to theabyssal depths. This happens especially inthe Weddell Sea area. The deep water thenspreads outward, especially eastward intothe Southern Indian Ocean, although influ-enced by bottom topography. In contrast,the bulk of North Atlantic bottom wateroriginates outside the Arctic. See also Arc-tic bottom water.

boudinage /boo-dăn-ahzh/ Pillow-likestructures formed in rock beds. When acompetent bed of rock is enclosed by in-competent material and is then stretched orsqueezed, the competent bed breaks in a se-ries of short sections, because it cannot de-form plastically to the same extent as theincompetent material. As the competentlayer deforms, it thins locally and PINCH

AND SWELL structures develop. On furtherstretching this thinning continues untilnecks are formed; finally the layer breaks.They were originally described as boudinsby French geologists, to whom they resem-bled black sausages.

Bouguer anomaly /boo-gay/ A gravityanomaly that takes into account the effectof topography but not isostatic compensa-tion, first observed by the French physicistand mathematician Pierre Bouguer (1698–1758) in 1735.

boulder clay See till.

boundary current A fairly fast-movingcurrent that flows near the edge of anocean, usually along its W flank. Theoreti-cal models of oceanic flow, particularlythat of the American oceanographer HenryStommel (1920–92), reveal that thereshould be a fairly localized current flowingat depth, in a southward direction, alongthe W flank of the Atlantic. The necessityfor such currents arises from a considera-tion of pressure gradients within theoceans and the forces associated withEarth rotation. Stommel’s theory suggeststhat there should also be a current alongthe W flank of the Pacific Ocean, althoughnot in this case everywhere directed south-ward. Field measurements have demon-strated that in the case of the Atlantic,there is a south-flowing current under-neath the Gulf Stream.

boundary layer The layer of the atmos-phere in which movement is closely deter-mined by the presence of the groundsurface. In practice, this layer is subdividedinto the surface boundary layer, extendingto approximately 100 m, and the planetaryboundary layer, with an upper limit of

47

boundary layer

about 600 m above the ground. In the for-mer, the influence of the surface is para-mount, whereas in the latter layer, itremains significant but not dominant.

boundary wave See internal wave.

bourne A stream, especially one in chalkdistricts such as S England. When the watertable falls in summer, many bournes dryup.

Bowen ratio /boh-ĕn/ The ratio of thetransfer of sensible heat from the groundsurface to that of latent heat effected by theprocesses of turbulence and conduction.Over oceans, the value of this ratio can beas low as 0.1 indicating that 90% of theheat flux from the surface is in the form oflatent heat. Desert areas represent the con-verse, with no latent heat component be-cause all heat transfer is in the form ofsensible heat. The ratio is named for theAmerican astronomer Ira Sprague Bowen(1898–1973).

boxwork The reticulated outward ap-pearance of the ceiling of a cave, resultingfrom the deposition of hydrated iron ox-ides in fractures and cavities (from whichmaterial has been eroded away).

Brachiopoda /brak-ee-op-ŏ-dă/ A phy-lum of invertebrate animals having a bodyenclosed by two valves of a chitinous orcalcareous material. Brachiopods are bilat-erally symmetrical, but unlike the BIVALVIA

the plane of symmetry passes across thevalves, not between them. The valves arethus dissimilar but each is symmetricalabout its left and right halves. The phylumis divided into two classes, the Inarticulataand the Articulata. In the latter the shellsare connected by teeth and sockets along ahinge line. The shells of the Inarticulata areheld together only by muscles. Brachio-pods are marine benthonic animals at-tached to the substrate by a pedicle, whichcommonly protrudes through an openingin the larger valve (the pedicle valve). Theother valve is called the brachial valve. Ithas an internal brachidium, which sup-ports the structure responsible for main-

taining a current of water for respiratoryand feeding purposes. Brachiopods showvariations in shell morphology and orna-mentation that are valuable in taxonomicclassification.

The earliest fossil brachiopods areknown from Lower Cambrian rocks; theyflourished throughout the Paleozoic Eraand were especially abundant and diversein the Silurian and Devonian Periods.Many became extinct at the end of the Pa-leozoic. There was a revival of a fewgroups in the Mesozoic but brachiopodsare relatively insignificant today. Theyhave been used in the stratigraphic classifi-cation of the rocks in which they occur.

brackish water Water that is partlyfresh and partly saline. Areas of such wateroccur, for instance, behind coastal barriersthat partly inhibit tidal penetration, hencethe incursion of saline water, but still allowthe incursion of fresh water from streamsand rivers.

braided stream A stream (or river) thatdivides into two or more smaller channels,which further branch and separate beforerejoining farther downstream. The result isa network of islands, sandbars, and chan-nels. It occurs when the volume of flow ofthe stream varies widely from time to time(as in dry regions) and there is plenty of BED

LOAD. It also occurs in front of a meltingglacier. See also braiding.

braiding The division of a stream into acomplex pattern of several small channelsdivided by mid-channel bars. After straightand meandering channels, braiding is thethird major channel pattern a river canhave. It is often due to lack of COMPETENCE

or CAPACITY of the river to carry the loadsupplied to it. As a result the excess load isdumped in mid-channel to form the char-acteristic mid-channel bars that divide thechannel into many separate subchannels.In this form, it is often seen in theperiglacial environment (see river terrace).Easily erodible banks and variable dis-charge are also said to favor braiding. Me-andering patterns may change to braided

boundary wave

48

patterns if the stream gradient or dischargeis increased.

breaker A mass of turbulent water mov-ing landward, as a result of the breaking ofa wave on approaching the coast andreaching shallower water. The decrease inwater depth causes a reduction in velocityof the wave, an increase in height, and a de-crease in length. The orbital paths of waterparticles within the wave form also change,from open circles to open ellipses, until thewater under the crest is moving faster thanthe wave form, at which point the wavebreaks. Whether a breaker results in net ac-cumulation or erosion of beach materialsdepends largely upon the nature of thebreaking. The crest of a plunging breakerfalls vertically into the trough, producinglittle SWASH, but considerable BACKWASH.Spilling breakers, however, break forwardand the associated stronger swash aidsconstructive activity. See also constructivewave; destructive wave.

breaker zone See surf zone.

breccia /brech-ee-ă, bresh-/ A RUDACEOUS

sedimentary rock in which the constituentclasts or fragments are angular. Compareconglomerate.

brickearth Fine-grained unconsolidateddeposits (sand, silt, and clay) associatedwith periglacial conditions in the Pleis-tocene. There are two types of brickearth.Aeolian loess consists of very well sortedmaterial of predominantly silt size, pickedup from glacial outwash, frost-shattereddebris, or the floodplains of rivers, andtransported by cold dry winds in steppelikeconditions. The deposit is largely quartz,but with a mineralogy reflecting the rocksof origin, and forms a sheet over the preex-isting landscape when deposited in a wedgethat thins away from the source. It is com-monest in E Europe, but thin veneers reachE and SE England.

Overbank material deposited on flood-plains by Pleistocene rivers is also termedbrickearth. This type has a lower silt con-tent than the wind-lain type, and is lesswell sorted.

brine A solution of common salt(sodium chloride, NaCl) as occurs in seawater and in salt lakes. Naturally occur-ring brines are important sources of saltsand other chemicals.

brittle failure The breaking of a rockwhile still within the elastic range.

brittle strength The stress applied to arock at the point at which it breaks withinthe elastic range.

broad A small shallow lake in the EastAnglia district of E England, formed on thesite of peat extraction several centuriesago. Broads are prone to silting up and pol-lution.

bronzite /bron-zÿt/ An orthorhombic PY-ROXENE.

bronzitite /bron-ză-tÿt/ A monominer-alic ultramafic rock consisting wholly ofbronzite.

brookite /brûk-ÿt/ A brown to black or-thorhombic polymorph of titanium diox-ide, TiO2, found in hydrothermal veindeposits. See also anatase; rutile.

brown calcareous soil A type of calci-morphic soil, typically found in the humidtemperate regions of Europe and Americaunder base-rich vegetation. Soil develop-ment is more advanced than in a rendzina,producing a soil with an A (B) C system ofhorizons. The soil is deeper, though rarelymore than 70 cm, owing to its developmentover a parent material with a greater in-soluble residue. The upper part of the soilhas a mull humus and is dark red/brown incolor with a neutral or slightly acid reac-tion. It has a crumb structure and is rich inorganic matter. With depth the A horizonmerges into an ill-defined B horizon, whichis often lighter in color and contains manyfragments of parent material. They aregood agricultural soils and fall into theMOLLISOL order of the US SOIL TAXONOMY.

brown clay See red clay.

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brown clay

brown coal See lignite.

brown earth A type of soil rangingfrom a true brown earth, which exhibitsonly mild leaching, to a PODZOL, which ex-hibits extreme leaching. Brown earths arefound to the south of the pozdol zonewhere precipitation is in excess of evapora-tion. True brown earths lack the distinctivehorizonation of a podzol. Organic matteris rapidly formed owing to the deciduousforest vegetation being decomposed by themore abundant soil fauna. There is a thor-ough mixing of the mull humus into the Ahorizon. Carbonates are completelyleached from the soil but there is no move-ment of the sesquioxides, as is shown bythe constant silica:sesquioxide ratio downthe profile. Consequently there is a weaklydeveloped B horizon. These soils coverlarge areas of the middle latitudes, oftenforming on deposits of the Pleistoceneglaciations. With their good crumb struc-ture, mild acidity, and free drainage theyare important agriculturally. See also acidbrown soil; brown podzolic soil; sollessivé.

brown podzolic soil /pod-zol-ik/ A soilthat is transitional in location and proper-ties between BROWN EARTHS and PODZOLS.An Ea (eluvial) horizon is still lacking butthey are more acid in nature and have alower base status than a brown earth. Anincrease in free iron oxide results in a loosecrumb structured B horizon. These soilsare typical of the northeastern USA wherethey are found to the south of the true pod-zol zone. They fall into the SPODOSOL orderof the US SOIL TAXONOMY.

brucite /brooss-ÿt/ A white or greenishmineral with a layered structure consistingof magnesium hydroxide, Mg(OH)2. Itcommonly occurs as a hydration productof periclase in thermally metamorphoseddolomites.

Brückner cycle /brûk-ner/ A climaticcycle of about 35 years, which was de-duced by the German geographer and cli-matologist Eduard Brückner (1862–1927)in 1890 on the basis of non-instrumental

data, such as harvests. He distinguished al-ternating warm and dry spells and cold andwet spells, but the cycle is not well definedand has too small an amplitude to have anyforecasting value.

brunizem /broo-nă-zem, broo-nă-zyom/See prairie soil.

Bryozoa /brÿ-ŏ-zoh-ă/ (Ectoprocta) Aphylum of small aquatic colonial inverte-brate animals including the moss animalsand sea mats. A bryozoan colony, knownas a zooarium, is made up of numerouspolyps, each of which inhabits a small tubeof calcareous or chitinous material. Thecolonies show great diversity in shape:many resemble seaweeds and are attachedto the sea floor; others are encrusted on thehard parts of other animals or rocks. Fos-sils of the Bryozoa are known from rocksof the Cambrian Period onward and thegroup is still flourishing today. Bryozoanshave a potential use in micropaleontologi-cal stratigraphy and correlation.

Buchan spell /buk-ăn/ A period of theyear when temperatures were anomalouslycool or warm as deduced by the Scottishmeteorologist Alexander Buchan (1829–1907), working on 50 years of recordsprior to 1867 for SE Scotland. The periodsof below-average temperature were Feb.7–14, April 11–14, May 9–14, June29–July 4, Aug. 6–11, and Nov. 6–13, andof above-average temperatures July 12–15,Aug. 12–15, and Dec. 3–14. Althoughthere is some evidence of such periodicitiesfrom year to year they are not sufficientlypersistent or clearly defined to specificdates and so Buchan’s spells are largely ofclimatological curiosity only.

Buchan zone See zone (def. 1).

buchite See contact metamorphism.

bulk modulus The ratio of compressivestress in rocks to the resulting change involume.

buoyancy (in meteorology) The temper-ature difference between parcels of warm

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50

air and the surrounding cooler environ-ment. Because air density is inversely pro-portional to air temperature, parcels orthermals of warm air have an upward ac-celeration through the cooler surroundingsjust as a cork rises through water becauseof its lower density. Where parts of theEarth’s surface are heated differentially,the warmer parts will heat the air in con-tact, which will then possess buoyancy rel-ative to its surroundings. This is the basisof natural convection within the atmos-phere.

Burgess shale /bur-jĕss/ A bed of blackMiddle-Cambrian shale in British Colum-bia, Canada. It is the site of significant dis-coveries of the FOSSILS of invertebrateanimals, which occur as films of carbon be-tween the BEDDING PLANES of the rock.

buried topography A preexisting land-scape that has been subsequently buried byyounger strata.

bush An area of scrub in an isolated wildlandscape, as occurs in parts of Australia,New Zealand, southern Africa, and theUSA.

butte A small upstanding mass of rock,usually consisting of resistant capping ma-terial overlying some softer though pro-tected rock type. It has steep sides and isproduced as a result of long-continuedback-wearing of a MESA.

Buys Ballot’s law A law stating that ifan observer stands with his or her back tothe wind, then pressure will be lower onthe left-hand side than to the right in the Nhemisphere. For the S hemisphere the con-verse is true. This means that winds willblow in a counterclockwise manneraround a depression in the N hemisphere,and vice versa in the S hemisphere. The lawis named for the Dutch meteorologistChristoph Hendrik Diederik Buys Ballot(1817–90).

bysmalith /biz-mă-lith/ A steep-sidedvertical igneous body, roughly cylindricalin form and following a steeply inclinedfault. It arches up the overlying countryrock or becomes exposed at the surface.

bytownite /bÿ-town-ÿt/ A calcic plagio-clase FELDSPAR.

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bytownite

caatinga /kah-ting-gă/ A type of thornyscrub that occurs in northeastern Brazil.The plants include acacias, cacti, and otherdrought-resistant vegetation (there is verylittle rainfall in winter).

cadastral map /kă-dass-trăl/ Usually alarge-scale map, showing the boundaries ofsubdivisions of land. It is used for record-ing ownership of land.

Cainozoic /see-nŏ-zoh-ik, sen-ŏ-/ SeeCenozoic.

calamine 1. See hemimorphite.2. In the UK, another name for SMITH-SONITE.

calc-alkaline /kal’k-al-kă-lÿn, -lin/ De-scribing igneous rocks that have relativelylower sodium and potassium contents andhigher calcium contents for a given silicapercentage than do ALKALI rocks. The calc-alkaline volcanic suite is represented by theassociation of basalt, andesite, and rhyolitein orogenic regions and island arcs.

calcarenite /kal-ka-rĕ-nÿt/ A clasticLIMESTONE of average grain size betweenthat of CALCISILTITE and CALCIRUDITE. It iscomposed of particles between 0.06 mmand 2.0 mm in diameter. Calcarenites aresubdivided into: very fine, between 0.06and 0.12 mm; fine, from 0.12 mm to 0.25mm; medium, from 0.25 mm to 0.5 mm;coarse, from 0.5 mm to 1.0 mm; verycoarse, having diameters from 1.0 to 2.0mm.

calcareous /kal-kair-ee-ŭs/ Describingrocks or soils that contain CALCIUM CAR-BONATE. For example, limestone and chalkare calcareous rocks.

calcicole /kal-să-kohl/ (calciphile) A plantthat grows best on chalky (i.e. alkaline)soils. See also calcifuge.

calcification (in soils) The redistribu-tion and accumulation of calcium carbon-ate in soils. The process is especiallycharacteristic of arid and semi-arid areaswhere the natural vegetation is grasslandand the soil parent material contains cal-cium carbonate derived from limestone.Carbonic acid reacts with the calcium car-bonate and the products remain dissolvedin solution as ions. This solution may becarried down to the B horizon where it isprecipitated, or carried upward as a resultof capillary action from water below. Theaccumulation of calcium carbonate mayform a calcic horizon.

calcifuge /kal-să-fyooj/ (acidophile) Aplant that grows best on acid (i.e. lime-free)soils. See also calcicole.

calcilutite /kal-sil-yŭ-tÿt/ A clastic lime-stone of average grain size less than that ofCALCISILTITES, i.e. less than 0.004 mm in di-ameter.

calcimorphic soil /kal-să-mor-fik/ Analkaline soil occurring within broad cli-matic zones where parent material is thedominant factor in soil formation. Theparent material is calcareous and its ex-treme nature resists any development to-ward acidity. Such soils are usually dark,organic-rich, and abundant in soil fauna.Two main soil types can be recognized be-longing to this group: the RENDZINA andthe BROWN CALCAREOUS SOIL.

calcirudite /kal-sir-yŭ-dÿt/ A clastic lime-stone of average grain size larger than that

52

C

of CALCARENITES, i.e. greater than 2 mm indiameter.

calcisiltite /kal-siss-ăl-tÿt/ A clastic lime-stone of a grain size intermediate betweenthat of CALCILUTITES and CALCARENITES. It isformed of particles between 0.004 mm and0.06 mm in diameter. Those of grades upto 0.03 mm are fine calcisiltites; thoseabove this are coarse calcisiltites.

calcite /kal-sÿt/ The principal and moststable mineral form of calcium carbonate,CaCO3. It is the chief component of lime-stone and marble, and occurs in the shellsof mollusks. It forms colorless, white, orgray hexagonal crystals, sometimes col-ored by impurities. See also carbonate min-erals.

calcium carbonate A white solid(CaCO3) that occurs naturall in two forms:CALCITE and ARAGONITE. It is largely insol-uble in water but is soluble in water thatcontains dissolved carbon dioxide. See alsocarbonate minerals.

calcrete /kal-kreet/ (caliche) Crusts andnodules of LIMESTONE precipitated at ornear the surface of the ground in semiaridregions. It results from evaporation ofmoisture from the soil and may be associ-ated with deposits of gravel and sand.

caldera /kal-deer-yă, -dair-/ A volcaniccrater whose diameter exceeds 1 km andcan reach up to 20 km, generally resultingfrom the collapse or explosive removal ofthe top of a volcano. See also supervol-cano.

Caledonian orogeny /kal-ĕ-doh-nee-ăn/The Lower Paleozoic orogeny in which theCaledonian mountains, extending fromIreland via Scotland to Scandinavia, wereformed. It resulted from the closure of theProto-Atlantic Ocean, between the Balticand Canadian Shields.

calibration (in stratigraphy) The deter-mination of a rock sequence in relation toan independent timescale. This scale maybe determined radiometrically or by corre-

lation with another continuously operatingundirectional non-reversible process suchas EVOLUTION, using the fossils contained inthe rocks.

caliche /kah-lee-chay/ See calcrete.

caliper log A subsurface logging tech-nique, which records the variations withdepth in the diameter of an uncased borehole.

calorie A unit of heat now supersededby the joule. It was based on the amount ofheat required to raise the temperature of 1gram of water by 1°C. Because this amountdepended on the initial water temperature,the standard calorie was taken as that at15°C, raising the water temperature from14.5°C to 15.5°C. 1 cal (15°C) = 4.1855joules.

calving The production of icebergs bythe splitting off of large slabs of ice from aglacier or ice sheet at the edge of the sea.

cambering An apparently increased diptoward valley bottoms found within hori-zontally bedded solid rocks located aboveweaker unconsolidated rocks, such asclays. No clear explanation has yet beendiscovered, for this phenomenon.

Cambrian /kam-bree-ăn/ The earliestperiod of PHANEROZOIC time and of the PA-LEOZOIC Era. Rocks laid down during thisperiod are the first to show an abundanceof fossils, which consist of primitive repre-sentatives of most of the invertebrate ani-mal phyla known today. The Cambrianbegan about 570 million years ago, follow-ing the PRECAMBRIAN, and was succeededabout 505 million years ago by the OR-DOVICIAN. The Cambrian System is oftendivided into Early, Middle, and Late. Thename Cambrian is derived from the ancientname for Wales (Cambria), where rockscontaining the earliest fossils were firststudied. Cambrian rocks occur across allcontinents, the most complete being inNorth America and Siberia, and are pre-dominantly sedimentary in origin. ManyCambrian rocks show evidence of deposi-

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Cambrian

tion in or near shallow areas during marinetransgressions into continental areas, andoceans extended over most of what is nowNorth America during the period.

Cambrian fossils represent the animalsthat lived in the seas of this period. Trilo-bites were especially abundant and areused in the stratigraphic subdivision of thesystem. Other important groups includethe brachiopods (inarticulate species beingdominant), gastropods, primitive echino-derms, and ostracods and there is evidencefrom TRACE FOSSILS of a variety of worms.Bivalves and graptolites had appeared bythe end of the period.

Campbell–Stokes recorder See sun-shine recorder.

camptonite /kamp-tŏ-nÿt/ An alkalineLAMPROPHYRE.

Canadian Shield A large area of Pre-cambrian rock that occupies 5 million sqkm of Canada. Most of the shield consistsof GRANITE and banded GNEISS; the remain-der includes volcanic rock and some sedi-mentary deposits. There are extensivedeposits of metal-bearing minerals, includ-ing those of copper, gold, iron, nickel, andsilver.

cancrinite /kan-kră-nÿy/ See feldspath-oids.

cannel coal /kan-ĕl/ A fine-grained lus-trous bituminous coal which burns with abright smoky flame; the common fuel coal.Geologically, it is an example of a car-bonaceous rock. See also coal.

canyon A deep steep-sided section of ariver valley, the depth of which consider-ably exceeds its width, normally found inarid or semiarid regions. Canyons are usu-ally produced where rivers have beendeeply incised owing to ANTECEDENCE, oras a result of downcutting by rivers whosesources are located in areas of greater pre-cipitation than those in which the canyonsare formed. This continuous externalsource of water enables efficient down-ward erosion to proceed, while the local

lack of precipitation hinders weathering ofthe sides and hence their degradation.Many canyons are formed by erosion ofhorizontally bedded rock alternations,which produce a stepped valley cross-pro-file, owing to their differential resistancesto erosion. Others are said to have beenformed by cavern collapse or by the frozenground and blocked valley phenomena ofthe Pleistocene. See also submarinecanyon.

capacity 1. (stream capacity) (in geol-ogy) The maximum amount of sediment ofa certain size that a stream can carry as bedload. This decreases as the grain size of thesediment becomes larger, but increases asstream gradient becomes steeper or dis-charge becomes greater. Since the trans-porting surface is the stream bed, the widerit is the more it can transport; to cancel thisout capacity is usually expressed as aweight of sediment per unit width of bed.Even so, capacity is still very much a func-tion of bed width, since for a given slopeand discharge, the velocity at the bed isgreater in a wide shallow stream than anarrow deep one. If the grain sizes beingtransported are a fair mixture of large andsmall, capacity varies with the third powerof the velocity, with a lower power if thematerial is mostly coarse, and with a higherpower if it is mostly fine.2. (carrying capacity) (in ecology) Themaximum BIOMASS a region can support, orthe maximum number of animals it cansustain during the harshest time of theyear. If the capacity is exceeded, there willbe insufficient resources, such as food, forthe population. The number of animalsmust fall, through starvation, failure to re-produce, or migration.

capillarity /kap-ă-la-ră-tee/ (in soil sci-ence) The mechanism whereby capillarywater moves vertically up the soil profilefrom the groundwater table or moist sub-soil; it is a process typical of arid and semi-arid zones where evaporation of waterexceeds precipitation. It is more effectiveon claylike than sandy soils, with a maxi-mum height of rise of 2.5 m in clays, 0.7 min sands. As the water rises it brings with it

Campbell–Stokes recorder

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dissolved salts and these are precipitated atthe point where the capillary ‘current’ fi-nally dries out, which may be at the soilsurface or within the profile, forming a saltaccumulation, e.g. in SOLONETZ and SOLON-CHAK soils. In arid zones subject to irriga-tion, excessive watering artificiallyaccelerates the capillary currents, and for-merly fertile areas can be ruined by the vastsalt deposits that result.

capillary water /kap-ă-lair-ee/ (in soilscience) Water held in the small poreswithin the soil, existing as a film aroundsoil particles. It is this water that is mostlytaken up by plant roots for plant growth,as opposed to the gravitational water thatrapidly flows through the soil and removesplant nutrients during wet periods, and hy-groscopic water, which is the water that re-mains in the soil even after air drying andis not available to plants.

capillary wave A water wave whoselength is less than 2.5 cm. Its speed of prop-agation is largely determined by the mea-sure of surface tension that exists in thewater, a force that tends to restore thewater surface to a horizontal position. Aswith all sea waves, the generation of capil-lary waves requires the transference of en-ergy from wind flow to the surface water inthe sea. Capillary waves may develop, forinstance, from the action of dropping a tinystone into a pond.

cap rock 1. A layer of SHALE or other im-pervious rock that overlies porous rockscontaining deposits of oil or natural gas.2. A layer of anhydrite, gypsum, calcite,and sulfur that forms a hard covering ontop of a SALT DOME. In the Gulf Coast re-gion of the USA, cap rock is a major sourceof sulfur.

carbonaceous chondrite /kar-bŏ-nay-shŭs/ A stony METEORITE that containsCHONDRULES in a claylike matrix of sili-cates. These meteorites get their name be-cause of their carbon content – up to 3%mainly in the form of hydrocarbons. Someexperts believe that they resemble the orig-

inal material from which planets wereformed.

carbonate minerals A group of miner-als in which the anion (CO3)2– is the fun-damental unit. The common rock-formingcarbonate minerals fall into three sub-groups as follows: 1. calcite – CaCO3, magnesite – MgCO3,

siderite – FeCO3, rhodocrosite –MnCO3.

2. dolomite – CaMg(CO3)2, ankerite –CaFe(CO3)2.

3. aragonite – CaCO3, strontianite –SrCO3, witherite – BaCO3.The calcite and dolomite minerals have

trigonal symmetry. Dolomite results fromthe substitution of calcium ions in calciteby the divalent cations Mg2+ and Fe2+, andthere is continuous replacement of Mg2+

by Fe2+ between dolomite and ankerite.Calcite and aragonite are polymorphs ofcalcium carbonate, the latter being or-thorhombic and the higher-pressure form.Calcite is usually colorless or white with ahardness 3 (see Mohs’ scale of hardness)and cleaves into perfect rhombs. Its ex-treme double refraction (see birefringence)is apparent in crystals of the transparentvariety Iceland spar. Calcite effervescesstrongly in cold dilute hydrochloric acid,other carbonates reacting weakly unlessthe acid is warmed. Most limestones con-sist largely of calcite, which may be a pri-mary precipitate or in the form of fossilshells. Calcite also occurs as a secondarycementing material in sediments. Veins andbeds of fibrous calcite are called beef. Dur-ing metamorphism, a pure limestone re-crystallizes to form marble but if impurecalcite reacts it produces such minerals asdiopside, wollastonite, and grossular gar-net. Calcite occurs in hydrothermal veins,in amygdales, and as a primary magmaticmineral in carbonatites.

Magnesite is usually white or colorlessand occurs as an alteration product ofmagnesium-rich rocks under conditions inwhich carbon dioxide is available.Rhodocrosite is pink and is found in meta-somatic veins and pegmatites. Siderite isusually brown and is a major constituent of

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carbonate minerals

bedded ironstones, an important iron ore.It is also found in hydrothermal veins.

Dolomite is colorless, white, or grayand may form as a primary precipitate.Dolomitization of calcite and aragonitetakes place by reaction with magnesium-bearing sea water or by the permeation ofmagnesium-rich solutions along cracks andjoints in limestones. During metamor-phism, dolomite breaks down to producepericlase and brucite. Yellow-brownankerite is found with dolomite in hy-drothermal veins and carbonatites.

Aragonite is colorless or white and ismetastable at normal temperatures andpressures. Many shells are formed of arag-onite, which in time undergoes recrystal-lization to calcite. Aragonite occurs as aprimary precipitate and in amygdales.Strontianite and witherite are usually col-orless, white, or yellow, occurring in hy-drothermal veins and carbonatites.

The basic copper carbonates, malachite(Cu2(OH)2CO3) and azurite (Cu3(CO3)2-(OH)2), are bright green and bright bluerespectively. Malachite has a banded bo-tryoidal form and occurs with crystals ofazurite in the oxidized zones of copper de-posits.

carbonation /kar-bŏ-nay-shŭn/ A pro-cess of chemical weathering that involvesthe dissolving of soluble rocks and miner-als by weak carbonic acid, formed by thecombination of water and carbon dioxidederived from the atmosphere or from soils.Carbonation is most effective on lime-stones, in which calcium carbonate is con-verted to calcium bicarbonate and isremoved in solution.

carbonatite /kar-bon-ă-tÿt/ An igneousrock composed chiefly of the carbonateminerals calcite, dolomite, and ankerite.Apatite and magnetite are common acces-sories together with the silicate minerals,alkali feldspar, nepheline, melilite, biotite,melanite, and sodic pyroxene. Certain ele-ments, particularly niobium, barium,strontium, and the rare earths, are concen-trated in carbonatites, which are often ofeconomic importance. Carbonatites occuras central intrusive masses, dikes, and cone

sheets in ijolite complexes and are oftensurrounded by zones of FENITIZATION. Suchalkaline complexes are confined to stablecontinental regions, particularly those thathave been subject to rifting.

carbon cycle The series of chemical re-actions that circulate carbon through theglobal ECOSYSTEM. Plants take up carbondioxide during PHOTOSYNTHESIS, convert-ing it to carbohydrates and releasing oxy-gen. The plants use the carbohydrates as‘food’ (or animals eat the plants), oxidizingthem during respiration to release carbondioxide. The burning of FOSSIL FUELS andforest trees also releases carbon dioxideinto the atmosphere, contributing to theGREENHOUSE EFFECT. See also carbon sink.

carbon-14 dating See radiocarbon dat-ing.

carbon dioxide A gas occupying only asmall proportion of the atmosphere(0.05% by weight) but having very impor-tant consequences. It has the property ofabsorbing radiation in wavelengths similarto those emitted by the Earth, thus it pre-vents the loss of much terrestrial radiationand maintains, together with water vaporand some other trace gases, a higher tem-perature for the Earth than would other-wise occur (see greenhouse gas). It is vitallyimportant in plant growth for the processof PHOTOSYNTHESIS. The combustion of fos-sil fuel, such as coal and oil, releases car-bon dioxide and the amount in theatmosphere is now some 30% higher thanbefore the Industrial Revolution. It is be-lieved that this could have the effect of in-creasing radiation absorption and so raisethe Earth’s mean temperature. However,the oceans and the terrestrial biosphere(plants and soil) are a great store of carbondioxide and absorb part of the increase (seecarbon sink). See greenhouse effect.

Carboniferous /kar-bŏ-niff-ĕ-rŭs/ Theperiod of the PALEOZOIC Era that followedthe DEVONIAN and preceded the PERMIAN. Itbegan about 360 million years ago andlasted until about 286 million years ago.North American geologists divide this unit

carbonation

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of time into two periods, the MISSISSIPPIAN

and PENNSYLVANIAN, corresponding ap-proximately to the time represented by theLower and Upper Carboniferous respec-tively. The lower part of the CarboniferousSystem, sometimes referred to as the Di-nantian, is formed of two divisions, theTournasian and Viséan Stages. The upperpart, the Silesian, consists of the Na-murian, Westphalian, and StephanianStages.

Evidence suggests that most of the con-tinents of today formed two superconti-nents in the Carboniferous Period:Gondwanaland (present-day Africa, SouthAmerica, India, Middle East, Australia,and Antarctica) was located in the S hemi-sphere and Laurasia (present-day NorthAmerica, Greenland, and N Europe) in theN hemisphere. By the end of the periodthese continental plates had collided, clos-ing the Tethys Sea; the mountain-buildingepisodes of the Variscan and Alleghenyorogenies are attributed to these move-ments. Large tropical swamps, from whichthe coal deposits of the later Carboniferousoriginated, extended across what is nowNorth America, Europe, and Siberia. Therocks of the Lower Carboniferous or Mis-sissippian are characterized by limestonesformed in shallow seas of the continentalshelves, although deeper-water facies ofshales and sandstones were also formed.(In Britain limestone is so characteristic ofthis division that it is often called the Car-boniferous Limestone.) The shallow seassupported a diverse fauna of Foraminifera,corals, Bryozoa, brachiopods, crinoids,blastoids, and other invertebrates, some ofwhich contributed to reef building. TheUpper Carboniferous saw the alternatingtransgression and regression of seas overcoastal swamps and facies mark a return tomore terrestrial and freshwater conditions,being composed of cyclothems of deltaicsandstones and shales with coal seams. Thecoal is especially prominent in the upperpart of the division, being formed fromvast forests of primitive land plants, suchas ferns and horsetails. (In Britain theUpper Carboniferous is often divided intotwo units: the Millstone Grit and the CoalMeasures.) The Upper Carboniferous con-

tains fossils of freshwater bivalves, whichare used stratigraphically. Fish remainedabundant and amphibians became morecommon, some of them evolving into rep-tiles at the end of the period.

Carboniferous Limestone The lowestlithological division of the CARBONIFEROUS

System in Britain. It corresponds to theLower Carboniferous (or Dinantian).

carbonization /kar-bŏ-ni-zay-shŭn/ Theprocess that leads to the preservation ofFOSSILS as thin films of carbon in sedimen-tary rock. As sediments are laid downunder water, organic matter decomposeswith the release of hydrogen, oxygen, andnitrogen, leaving only the carbon. Plants,arthropods, and other invertebrates, andfish have all been preserved in this manner.See also Burgess shale.

carbon sink A pool or reservoir that canabsorb or take up carbon from anotherpart of the CARBON CYCLE. The atmosphere,terrestrial biosphere (plants and soils),oceans, and sediments can be regarded ascarbon sinks.

cardinal points The four main points ordirections of the compass: north, south,east, and west.

carnallite /kar-nă-lÿt/ A white mineralsalt, a double chloride of potassium andmagnesium, KMgCl3.6H2O. It occurs inunderground salt deposits derived from seawater, and is used in fertilizers as a sourceof potassium.

carnegieite /kar-nay-gee-ÿt/ See feld-spathoid.

carnelian /kar-neel-yăn/ (cornelian) Atranslucent red to brown type of CHAL-CEDONY, which gets its color from inclu-sions of hematite. It is used as asemiprecious gemstone.

Carnivora /kar-niv-ŏ-ră/ The order ofthe class MAMMALIA that includes cats,dogs, bears, weasels, etc. Carnivores haveteeth modified for eating flesh and a skele-

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Carnivora

tal structure suited to rapid locomotionand the capture of prey. They evolved fromthe INSECTIVORA during the PaleoceneEpoch. Early Cenozoic forms, classified asthe Creodonta and now extinct, includedthe largest carnivores known, some havingskulls up to one meter in length. Moderncarnivore groups evolved during the lateEocene and Oligocene.

carnotite /kar-nŏ-tÿt/ A yellow mineralcontaining uranium and vanadium,K2(UO2)2(VO4)2.3H2O. It occurs in sand-stone or near petrified wood, is highly ra-dioactive, and used as a source of uranium.

carrying capacity See capacity (def. 2).

cartography The technique and scienceof representing spatial relationships bymeans of maps (including geographicmaps, plans, charts, and globes). This in-cludes working from original surveys, pho-tography, and other COMPILED MAPS,covering every aspect of map productionfrom carrying out surveys to final printingof the completed copies, including suchproblems as MAP PROJECTIONS and researchinto all available information. Digital car-tography, the use of computer-based infor-mation systems in the production and useof maps, is replacing many of the more tra-ditional methods.

cartometric testing /kar-tŏ-met-rik/ Atechnique enabling a cartographer to assessthe accuracy value of a map provided themap concerned is gridded. Accepted math-ematical formulas are used to test the hor-izontal and vertical accuracy of the mapagainst a control. The resultant answergives the average deviation of the distancesor elevations on the map from the true po-sition or elevation on the ground.

cartouche /kar-toosh/ A panel on a mapgiving its title, scale, etc. The cartographersof the Dutch Renaissance and the Eliza-bethans specialized in highly decorativecartouches.

cascade fold One of a series of minor

folds developed on the limb of a majorfold, as a result of gravity collapse.

cassiterite /ka-sit-ĕ-rÿt/ A brown toblack mineral form of tin dioxide, SnO2,found in hydrothermal veins and metaso-matic deposits associated with acid igneousrocks and in alluvial deposits.

cast A type of fossil consisting of aPSEUDOMORPH in which the skeletal parts ofthe organism have been dissolved and theresulting space replaced by a secondarymaterial, producing a replica of the origi-nal form. In some cases a cast may preservefeatures of only the outer or inner surfacesof a structure such as a shell. These areknown respectively as external and inter-nal casts. Compare mold.

castle kopje /kop-yay/ A type of INSEL-BERG developed by DEEP WEATHERING andEXHUMATION in a rock divided by evenlyspaced vertical and horizontal joints.Stacks of these fairly regular blocks give acastlelike stepped appearance. The termhas also been applied to heaps of joint-blocks in haphazard arrangements, whichhave resulted from the collapse of otherlandforms or from the exhumation of iso-lated CORESTONES. Inselberg domes maydegenerate into similar features followingundercutting and collapse.

CAT See clear-air turbulence.

cataclasis /kă-tak-lă siss/ The deforma-tion of rocks by the mechanical process ofshearing and granulation. Such cataclasticrocks are said to have undergone disloca-tion metamorphism and range fromcoarsely broken breccias to intensely de-formed mylonites.

catastrophism /kă-tass-trŏ-fiz-ăm/ Thetheory, now generally thought to be false,that past geologic changes have occurredas a result of a number of sudden catastro-phes (compare uniformitarianism). A suc-cession of catastrophes was also invoked toexplain the extinction of organisms, linkedto special creations to account for the ap-pearance of new forms.

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58

catchment area The area from which ariver and its tributaries obtain their water.It may cover a few square meters or covera large part of a continent (such as theAmazon basin). Rainwater may run off thesurface into the river or seep into the soiland flow to the river as GROUNDWATER. Seealso drainage basin.

catena /kă-tee-nă/ The regular repetitionof a characteristic sequence of soil profilesassociated with a particular topography inthe tropics. Relief and drainage are thedominant controlling factors of formation.G. Milne first used the term when mappingthe soils of East Africa in the 1930s. Therehas been confusion as to whether it refersto soils on a uniform lithology or whetherit can be used where another soil-formingfactor besides relief and drainage varies(the term toposequence is used in the latterinstance).

cation exchange /kat-ÿ-ŏn/ (in soil sci-ence) An internal chemical phenomenon insoils whereby a positively charged ion(cation) held on the surface of a colloid (ei-ther a clay or humus particle) can be ex-changed for another cation present in thesurrounding electrolyte. Cations includethe soil bases, notably calcium, potassium,sodium, and magnesium, and the acidity-inducing cations, notably hydrogen andaluminum. A typical example is for a cal-cium ion, held by its positive charge to thenegatively charged surface of the colloid,to be replaced by hydrogen from the sur-rounding fluid. This makes the colloidmore acid, the displaced calcium possiblybeing leached away. The cation exchangecapacity of the soil is expressed in milli-equivalents per 100 grams of clay. Thisvaries with different colloids: in the sim-plest most highly weathered clays (kaolin)it is lowest, perhaps 3–15 m.e./100 g,reaching 80–100 in the 2:1 lattice clays(e.g. montmorillonite), and then highest ofall, perhaps 200+ m.e./100 g, in humus col-loids.

cat’s eye Any of various semipreciousgemstones that in reflected light display abright band, like the pupil of a cat’s eye.

The best-known example is a green type ofCHRYSOBERYL.

cauldron Any collapse structure of vol-canic origin, as where a block of countryrock sinks into the underlying magma.There are no limitations as to the size of thestructure or the extent of subsequent ero-sion.

cave A hollow that is formed in rock by erosion, particularly by the action ofwater on LIMESTONE. Erosion starts by en-larging joints in BEDDING PLANES and atother points of weakness. Undergroundstreams can carve subterranean caverns,and wave action at the coast can formcaves in cliffs. There the erosive effect maybe augmented by CORRASION. Water satu-rated with carbonates may drip into cavesand form STALACTITES AND STALAGMITES.

cavitation One of the processes of flu-vial erosion that characterizes the high-ve-locity parts of streams, such as waterfallsand rapids. It is relatively rare comparedwith the common processes of CORROSION

and ABRASION, but very effective in terms oferosive power where it does occur, beinglargely responsible for the rapid destruc-tion of waterfalls and rapids. Constrictionof flow raises the velocity and kinetic en-ergy of a stream, which is compensated bya decrease in the pressure of the water. Thismay lead to the formation of bubbles in thestream, which subsequently collapse, giv-ing off severe shock waves when velocitydecreases as the channel widens again andwater pressure can rise. The stress pro-duced by the collapsing bubbles exerts a se-vere stress on the channel walls, speedingup erosion.

cay /kee, kay/ (key) A small sea island. Itis generally low-lying, possibly having a lit-tle vegetation, and usually formed in sandor coral. The term as applied for exampleto tiny islands or islets off the SouthFlorida coast refers to quite small coralshoals.

celestite /sell-ĕ-stÿt/ (celestine) A white,blue, or red mineral form of strontium sul-

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celestite

fate, SrSO4. It crystallizes in the or-thorhombic system, and generally occursassociated with halite and gypsum, in nod-ules in limestone, or with sulfur (as inSicily). It is used as a source of strontiumand its compounds. See also barytes.

celsian /sell-see-ăn/ A barium-contain-ing type of FELDSPAR, BaAl2Si2O8. It oftenoccurs associated with manganese ores.

Celsius scale /sell-see-ŭs/ A scale of tem-perature named for the Swedish astron-omer Anders Celsius (1701–44), who wasthe first to divide the interval between thefreezing point and boiling point of waterinto 100 parts. Although it was formerlyknown as the centigrade scale, an interna-tional agreement on units now favors theterm Celsius.

cementation /see-men-tay-shŏn/ A rock-forming process in which fragments arebound together by a cement precipitatedbetween the grains, making a solid sedi-mentary rock. These natural cements in-clude calcite, carbonates, iron oxides, andsilica (the commonest).

Cenozoic /see-nŏ-zoh-ik, sen-ŏ-/ (Caino-zoic; Kainozoic) The era of geologic timefollowing the MESOZOIC and beginningabout 65 million years ago. It includes thePALEOGENE and NEOGENE Periods. Formerlyit was divided into the TERTIARY and QUA-TERNARY Periods but Tertiary is no longerrecommended, according to the Interna-tional Commission on Stratigraphy. andproposals have been made for the Quater-nary to be formally recognized as a sub-eraof the Cenozoic. The Cenozoic has some-times been incorrectly referred to as beingsynonymous with the Tertiary. Followingthe extinction of most of the reptiles, whichwere the dominant animals of the Meso-zoic, the previously insignificant mammalsunderwent evolutionary radiation into anabundant, diverse, and dominant group.For this reason the Cenozoic is oftenknown as the age of mammals. Birds andflowering plants also flourished and the in-vertebrate types characteristic of the Meso-zoic were replaced by essentially modern

forms. The Alpine episode of orogenic ac-tivity extends into this era. See also geo-logic timescale.

centigrade scale /sen-tă-grayd/ See Cel-sius scale.

central meridian The line of longitudethat forms the axis of a map projection.Usually the central meridian is in the mid-dle of the projection, as in theSANSON–FLAMSTEED PROJECTION.

central-vent volcano A volcano thathas one central vent, around which there isa tendency for volcanic debris to accumu-late into a roughly symmetrical cone.

centrifugal force /sen-trif-ŭ-găl, -yŭ-găl/The apparent force acting outward fromthe axis of rotation in a rotating system. Itsmagnitude is v2/r where v is the linear ve-locity and r is the radius of curvature of thebody’s path, or ω2r where ω is the angularvelocity of the body. This force is equal andopposite to the centripetal force, whichtends to maintain the body in its curvedpath. In the atmosphere, these two forcesoperate when air is moving in a curvedpath relative to the ground surface (i.e.around a depression). It is also a part of theobserved force of gravity, which is the re-sultant of the true gravitational attractiontoward the Earth’s axis and the centrifugalforce resulting from the Earth’s rotation.However, the centrifugal acceleration isless than 1% of the acceleration of free fall.

centripetal acceleration /sen-trip-ĕ-t’l/The acceleration of a body traveling in acurved path. It is equal and opposite to theCENTRIFUGAL FORCE per unit mass.

centroclinal /sen-trŏ-klÿ-năl/ Describ-ing a basin in which the rock strata all diptoward a central low point, characteristicof cratonic areas. See also pericline.

Cephalopoda /sef-ă-lop-ŏ-dă/ A class ofthe phylum MOLLUSCA whose membersusually have an internal or external shelldivided into chambers and a foot modifiedinto a set of tentacles surrounding the

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60

mouth. All cephalopods are marine. Theyare usually subdivided into three groups:the NAUTILOIDEA, the extinct AMMONOIDEA

and the Coleoidea (or Dibranchiata),which includes the modern squid and octo-pus and the extinct BELEMNOIDEA. Fossils ofcephalopods are known from the Cam-brian onward and have been used as zonefossils.

cerussite /seer-ŭ-sÿt/ A mineral form oflead carbonate, PbCO3. It crystallizes inthe orthorhombic system, and is a sec-ondary lead mineral, often formed by theoxidation of GALENA, and is used as asource of lead.

chabazite /chab-ă-zÿt/ A colorless,white, or pink hydrated calcium aluminumsilicate, with some sodium and potassium,(Ca,Na2)Al2Si4O12.6H2O. It crystallizes inthe hexagonal system and is a member ofthe ZEOLITE group.

chain The main equipment used inCHAIN SURVEYING, consisting of links ofthick steel wire each connected to the nextby three rings. Most chains now in usehave an overall length of 20 m, each indi-vidual link being 200 mm long from itscenter to the center of the middle connect-ing ring. Markers designate each wholemeter, while those at five-meter intervalshave a different color.

chain surveying (chaining) A surveyingmethod used for areas containing concen-trated detail, such as urban areas, wherehigh accuracy is required. The CHAIN is runout along a convenient line, the direction ofwhich is obtained using a prismatic com-pass or theodolite. Points of detail are thenfixed by measuring the perpendicular dis-tance from the chain to the feature using atape, these measurements being known asoffsets. The distance along the chain andthe length of the offset, for every detailpoint, are noted on the booking sheet. Asan alternative to offsets, a point may befixed by measuring two lines, known asties, from different parts of the chain,thereby forming a triangle.

chalcanthite /kal-kan-th’ÿt/ A brightblue mineral form of hydrated copper sul-fate, CuSO4.5H2O. It crystallizes in the tri-clinic system and occurs in oxidizedregions of copper deposits.

chalcedony /kal-sed-ŏ-nee, kal-sĕ-doh-nee/ A very fine-grained lustrous semi-transparent type of silica, SiO2, which maybe colored by impurities (see silica miner-als). It generally occurs as a deposit in cav-ities in lava and sedimentary rocks. It isused as a semiprecious gemstone.

chalcocite /kal-kŏ-sÿt/ (chalcosine) Adark gray or black lustrous mineral form ofcopper sulfide, Cu2S. It crystallizes in theorthorhombic system but is more oftenfound as masses. It is an important sourceof copper.

chalcophile /kal-kŏ-fÿl/ An element thatgenerally occurs in association with sulfur,especially as a sulfide. Chalcophile ele-ments include arsenic, copper, lead, mer-cury, silver, sulfur, and zinc. See alsoatmophile; lithophile; siderophile.

chalcopyrite /kal-kŏ-pÿ-rÿt/ (copperpyrites) A brass-yellow copper iron sul-fide, CuFeS2, found in hydrothermal andmetasomatic veins and associated with theupper portions of acid igneous intrusions.It is the most important source of copper.

chalcosine See chalcocite.

chalk A very fine-grained pure whiteLIMESTONE (calcium carbonate, CaCO3)formed predominantly from COCCOLITHS,but including other invertebrate skeletalfragments. It is the characteristic rock ofmuch of the upper CRETACEOUS system inW Europe; the White Cliffs of Dover, SEEngland, are a famous example of exposedchalk deposits. Extensive deposits alsooccur in the USA.

chalybite /kal-ă-bÿt/ See siderite.

chamosite /sham-ŏ-zÿt/ A gray-greeniron-containing mineral, a member of theCHLORITE group.

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chamosite

Chandler wobble /chan-dler, chahn-/The small measurable wobble of the Earthwith respect to its axis of rotation, discov-ered in 1891 and found to have a period of14 months and an amplitude of 0°5′. Itsorigin is unknown but it is assumed to beconnected with movement of materials in-side the Earth. It is named for the Americanastronomer Seth Carlo Chandler (1846–1913).

channel Any waterway with free-flow-ing water, such as a natural stream or river,a connection between two lakes or seas, oran artificial canal.

chaparral /shap-ă-al/ A type of lowdense scrub with evergreen vegetation,such as that in California and other partsof the USA. See also maquis.

charnockite /char-nŏ-kÿt/ A rock that ismineralogically similar to leucocraticGRANULITES and contains hypersthene,quartz, and feldspar with or without gar-net. Unlike granulites, the quartz does notoccur in a platy flattened form. Basic vari-eties rich in plagioclase feldspar may betermed enderbite. The formation ofcharnockites is controversial but it is likelythat they originate either by the crystalliza-tion of granite magma under high-grademetamorphic conditions or by the high-grade metamorphism of preexisting ig-neous rocks.

chart A specialized form of map used fornavigation and other specific purposes, e.g.synoptic (weather) charts, star charts. Themost common chart is the hydrographicchart.

chatter mark 1. A crescentic fractureseen in hard rock subjected at some time tothe passage of GLACIER ice. Although theconvexity of these marks can point in ei-ther direction, a study of the fracture planefrom which rock material has been re-moved will indicate the actual direction offormer ice movement. The marks are be-lieved to be created by the impact androlling of boulders held loosely within thebase of the glacier.

2. The impact mark frequently seen on abeach cobble (see percussion mark).

chelation /ki-lay-shŏn/ (in soil science)The combination of sesquioxides with or-ganic acids. Water passing through leaf lit-ter acquires an extract from the organicmaterial that is capable of combining withthe metallic cations in the soil, notably ironand aluminum, to form a chelate of organicand metal constituents. Experiments sug-gest that leaf litter is a more powerfulchelating agent than peat or humus, andthat the process involved is one of reduc-tion and solution that renders the metalmore mobile. The chelating agent in the lit-ter is easily extracted by water passing overit, but its strength varies between differenttypes of litter, pine needles being amongthe most powerful. This is generallythought to be an important process in pod-zolization.

cheluviation /ki-loo-vee-ay-shŏn/ Thecombination of CHELATION and ELUVIA-TION. Water with organic extracts forms achelate with sesquioxides in the soil andthen moves down through the profile car-rying the sesquioxides in solution. It ismost effective in moving iron and alu-minum.

chemical weathering The breakdownof solid rock through a number of chemicalreactions, which may cause the removal ofcements, resulting in weakness, or the for-mation of secondary minerals less resistantto erosion than those of the fresh rock. Thedifferent types of chemical action that areimportant in weathering are CARBONATION,HYDRATION, HYDROLYSIS, LIMESTONE SOLU-TION, OXIDATION, and REDUCTION.

chenier /shin-ĕ-ree/ A BEACH RIDGE on amudflat, particularly in an estuary, consist-ing of sand or shell fragments. As mudflatschange form through water action, thecoarser material in the sediments tends toconcentrate in such ridges.

chernozem /cher-nŏ-zem, cher-nŏ-zyom/(black earth) A pedocal similar to aprairie soil. It is typically found in conti-

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nental interiors, such as the Russiansteppes, where there is an annual precipita-tion of approximately 500 mm with aslight rainfall maximum in summer. Theprocess of calcification is dominant result-ing in a soil with an A Cca C system ofhorizons (a B horizon is absent from truechernozems). The dark base-rich A hori-zon, has mull humus resulting from the de-composition of the natural vegetation ofgrassland, incorporated to a considerabledepth by rich faunal activity. This horizonmay extend down to 100 cm before passinginto the Cca horizon of calcium carbonateconcretions. The parent material often re-sembles loess. These soils with their neutralto slightly acid reaction and their excellentcrumb structure are agriculturally some ofthe most important in the world. They fallinto the MOLLISOL order of the US SOIL TAX-ONOMY.

chert An extremely hard, white, gray, orblack type of CHALCEDONY that occurs asmasses or layers in limestone. The nodularform is known as flint. See also silica min-erals.

chestnut soil A pedocal typical in theUkraine and the Great Plains of NorthAmerica. Conditions here are more aridthan in the chernozem belt and conse-quently the natural vegetation consists of atussocky grass cover. These soils are muchshallower than chernozems and horizona-tion is not so distinct. The A horizon ischestnut brown in color and has a platy orprismatic rather than a crumb structure.This merges at about 25 cm into the B hori-zon, which is lighter in color owing to thepresence of calcium carbonate and oftengypsum. The calcium carbonate may forma distinct concretionary horizon (Cca) atonly 50 cm depth. This lies above the par-ent material, which is often loess. Thesesoils in the American West are subject tosevere wind erosion and are used mainlyfor grazing. They fall into the MOLLISOL

order of the US SOIL TAXONOMY.

chevron fold An accordian fold whoselimbs are of equal length.

chiastolite /kÿ-ass-tŏ-lÿt/ A variety ofANDALUSITE in which impurities arearranged in the form of a cross. See alu-minum silicates.

Chile saltpeter /chil-ee/ A mineral formof sodium nitrate, NaNO3, that occurs assurface deposits in Chile. It is an importantsource of nitrates for making fertilizers andexplosives.

chimney See smoker.

china clay See kaolin.

chinook /chă-nook, -nûk/ A type of föhnwind that occurs on the E slopes of theRocky Mountains. It is named for a NativeAmerican people that lived near the mouth of the Columbia River and origi-nally referred to a warm moist SW windthat blew across this area. Later it was usedfor a warm dry wind on the E slopes blow-ing from the Chinook region. The term isnow only used for the dry wind bringingsudden increases of temperature and verylow humidities with rapid thaw conditionswhen snow is on the ground.

chlorite /klor-ÿt, kloh-rÿt/ The group ofminerals having a general composition(Mg,Al,Fe)12 (Si,Al)8O20(OH)16. They arestructurally similar to the micas and arecomposed of alternating layers of talc,Y6Z8O20(OH)4, and brucite, Y6(OH)12,where Y = Mg,Al,Fe and Z = Si,Al. Chlo-rites are monoclinic and typically green orwhite in color. They are common in low-grade metamorphic rocks, particularlythose of the greenschist facies. Chloritesalso occur as secondary minerals in ig-neous rocks owing to the hydrothermal al-teration of ferromagnesian minerals suchas pyriboles. Oxidized chlorites such aschamosite, which have a high content ofiron, are found in argillaceous sediments asauthigenic and detrital grains.

chloritoid /klor-ră-toid, kloh-ră-/ A mem-ber of a group of dark green minerals withthe composition (Fe2+,Mg)2Al(OH)4Al3-O2(SiO4)2, having a structure made up oflayers (Fe2+,Mg)4Al2O4(OH)8 and Al6O16

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chloritoid

linked by SiO4 tetrahedra. Chloritoid min-erals are found in aluminum- and iron-richlow-grade metamorphosed argillaceoussediments.

Chondrichthyes /kon-drik-th’ÿ-eez/ Car-tilaginous jawed fish, including modernsharks and rays. Because of the absence ofbone, good fossils are rare, although teethare often preserved. The Chondrichthyesprobably evolved from the PLACODERMI

and are almost exclusively marine. Theyfirst appeared in the late Devonian andcontinued to be important through theMesozoic and Cenozoic to the present day.Compare Osteichthyes.

chondrite /kon-drÿt/ A stony METEORITE

that contains chondrules. Compare achon-drite.

chondrule /kon-drool/ A small roundedbody usually of olivine or enstatite foundin stony METEORITES.

chop The sea surface under the influenceof a fairly moderate breeze, with waveshaving pointed crests and steep slopes,quickly developing as the breeze mounts.

Chordata /kor-day-tă/ The phylum ofanimals possessing, at least primitively, aninternal skeletal rod, the notochord, run-ning the length of the body. Members ofthis phylum include the VERTEBRATA, whichcontribute significantly to the fossil record.Other subphyla consist predominantly ofsmall soft-bodied sessile marine creatures,which are rarely preserved as fossils. Someof these have an active larval stage, and theevolution of vertebrates may have begunwith the retention of the characteristics ofthe larvae into the adult state.

C horizon The material from which soilis formed, the lower subsoil lying betweenthe upper subsoil (B HORIZON) and thebedrock. It contains no humus and will notsupport plant life. See also horizon (def. 1).

chorochromatic map /kor-oh-krŏ-mat-ik, koh-roh-/ A map in which nonquanti-

tative spatial distributions are shown bycolor tinting (e.g. of coalfields in an area).

chorographic map /kor-ŏ-graf-ik, koh-rŏ-/ A map on which regions are delin-eated, usually large regions, e.g. countriesor continents. Atlases and small-scale wallmaps, particularly those showing politicaldivisions, come into this category.

choropleth map /kor-ŏ-pleth, koh-rŏ-/ Amap showing quantitative spatial distribu-tions (e.g. of population) calculated fromaverage values per unit area.

chromite /kroh-mÿt/ A brownish-blackmineral form of iron and chromium oxide,FeCr2O4, a member of the SPINEL group. Itcrystallizes in the cubic system and occursin basic igneous rocks. It is the main sourceof chromium.

chron /kron/ The smallest interval of ge-ologic time in the hierarchy of theChronomeric Standard terms used inCHRONOSTRATIGRAPHY. The equivalentStratomeric Standard term, indicating thebody of rock formed during this time, is theCHRONOZONE. Chrons may be grouped to-gether to form an AGE.

chronomere /kron-ŏ-meer/ (in chrono-stratigraphy) Any interval of geologic time.Chronomeres are not of standard uniformduration.

chronostratigraphy /kron-oh-stră-tig-ră-fee/ The branch of STRATIGRAPHY linkedto the concept of time, rather than beinglimited only to considerations of lithologyand spatial distribution (see lithostratigra-phy). There are two parallel hierarchies offormal terms. The Chronomeric Standardterms are applied to intervals of geologictime; the Stratomeric Standard terms areapplied to the bodies of rock laid downduring these time intervals. The Chrono-meric Standard hierarchy is as follows:EON, ERA, PERIOD, EPOCH, AGE, and CHRON;for the major divisions of geologic time, seethe table at GEOLOGIC TIMESCALE. TheStratomeric Standard hierarchy of termshas no equivalents to the eon and era; it

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consists of the SYSTEM, SERIES, STAGE, andCHRONOZONE. The individual time in-tervals are not of standard duration and,similarly, the rock divisions are not of uni-form magnitude. Some geologists regardchronostratigraphy as essentially identicalto BIOSTRATIGRAPHY whereas others insiston a formal separation of the conceptsinvolved, regarding biostratigraphy as sim-ply one method of calibrating chronostrati-graphic scales.

chronotaxis /kron-ŏ-taks-iss/ The oc-currence of units of rocks of equivalent agein separate successions. These rock unitsare described as chronotaxial. Chronotaxiscan rarely be demonstrated with certainty.Compare homotaxis. See also correlation.

chronozone /kron-ŏ-zohn/ The smallestdivision of rock in the Standard Strato-meric scheme of stratigraphic classifica-tion (see chronostratigraphy). It indicatesthe body of rock that has formed duringone CHRON. Chronozones are defined at aparticular TYPE SECTION and should benamed after geographical localities. At theTYPE LOCALITY a chronozone may corre-spond to a biostratigraphical ZONE butother criteria are often used in calibrationand in correlation with other areas.

chrysoberyl /kriss-ŏ-be-răl/ A usuallyyellow to green mineral form of berylliumaluminate, BeAl2O4. It crystallizes in theorthorhombic system, often as star-shapedcrystals. Some types, such as alexandrite,are used as semiprecious gemstones. Seealso cat’s eye.

chrysocolla /kriss-ŏ-kol-ă/ A bright bluemineral, a hydrated silicate of copper,Cu2H2Si2O5(OH)4. It occurs in thin seamsor as incrustations, usually in oxidizedcopper or copper sulfide deposits.

chrysotile /kriss-ŏ-tÿl/ A white, gray, orgreen fibrous SERPENTINE mineral, once thechief source of commercial ASBESTOS.

cinder A dark-colored porous volcanicfragment, generally consisting of BASALT orANDESITE. Up to 30 mm across, cinders are

semifluid when ejected from a volcano butare solid by the time they fall to theground. See also pyroclastic rock.

cinder cone (scoria cone) A small vol-canic cone formed from accumulations oftephra (cinders or scoriae) that have beenejected under high pressure from a narrowvent as frothy magma, which has thencooled rapidly. The tephra builds uparound the vent to form a steep conical hillwith a central crater. Cinder cones may bea high as several hundred meters. See pyro-clastic rock; volcano.

cinnabar /sin-ă-bar/ A bright red mer-cury sulfide mineral, HgS, found in veinsand impregnations associated with vol-canic rocks. It is the main source of mer-cury.

CIPW classification See norm.

circulation index A measure of thestrength of the atmospheric circulation:used for any wind system, it commonlyrefers to the zonal component of the west-erlies or the trade winds. See also generalcirculation of the atmosphere.

Circum-Pacific Belt A narrow beltbordering the Pacific Ocean, in which 75%of the present earthquakes occur. It con-sists of igneous, metamorphic, and tecton-ically deformed sedimentary rocks.

cirque /serk/ A rounded rock basin,often containing a lake or CIRQUE GLACIER,enclosed by high headwalls and sidewalls,which are steep and frequently frost-shat-tered. In Scotland cirques are called cor-ries; in Wales, cwms. The floor is usually ofsmoothed striated rock and frequently hasa deepened basin form, with an associatedconvex rock lip, which may be moraine-covered. Though varying considerably insize, many cirques possess roughly thesame proportions having a length to heightratio of about 3:1.

It is believed that cirques can developfrom any hollow in which snow can accu-mulate, but in many cases they have devel-oped from preexisting water-eroded

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features. The processes responsible fortheir creation include: FREEZE-THAW actionon the headwall, which enlarges the cirqueoverall; abrasion by rotationally slippingice, which deepens the rock basin; joint-block removal, which can act over thewhole surface area. Cirques constitutesome of the most characteristic features ofglacial erosion.

cirque glacier A small glacier found in avalley head or depression on a mountainslope. Being small, they can form quickly(within 100 years) and are usually the firstglaciers to form and the last to disappearduring a period of glaciation. The ice oc-curs in distinct bands reflecting successiveyears’ accumulation of FIRN. The ice tendsto move downward in the upper part (firnzone), parallel with the glacier surface inthe central area, and upward in the lowestzone, i.e. a rotational movement. This isbecause most accumulation occurs in theupper zone, beneath the headwall, whereasmost melting is from the lower part. Thisresults in over-steepening of the glacier sur-face, and in order to return to a more bal-anced shape, rotation occurs when theimbalance is sufficient to overcome frictionbetween the ice and the bedrock. See alsocirque.

cirrocumulus /si-roh-kyoo-myŭ-lŭs/ Acloud type indicating convection in theupper levels of the troposphere. Composedof ice crystals, the cloud has a mottled ap-pearance consisting of small convective el-ements, often regularly arranged, withclear spaces between.

cirrostratus /si-roh-stray-tŭs/ A thinhigh-level layer cloud composed of icecrystals. It is transparent and gives the im-pression of a veil spreading across the sky.During the day, cirrostratus clouds pro-duce a halo effect, which can be seen quiteclearly around the Sun. They are usuallythe precursor of precipitation, heraldingthe approach of depressions.

cirrus /si-rŭs/ A detached cloud in theform of a wispy streamer or filament of icecrystals. They occur at the higher levels ofthe troposphere, above 6000 m, and withtemperatures below –25°C. When theyhave a hooked appearance, they are knownas mare’s tails and signify rain.

citrine /ait-rÿn/ A yellow or yellow-brown type of QUARTZ, used as a semi-precious gemstone. Dark samples aresometimes called false TOPAZ. See also sil-ica minerals.

clapotis /kla-po-tee/ A STANDING WAVE

that is generally formed when the incidentsea waves meet waves being reflected froma vertical barrier and where the depth ofwater in front of the barrier exceeds thewavelength of the incident waves. An in-terference pattern is set up, in which thereis no horizontal travel of the standing wavecrests. The incident waves generally ap-proach the barrier, perhaps a seawall,breakwater, or vertical cliff-face, more orless head on, i.e. their crests are roughlyparallel to the barrier. Occasionally, thesame phenomenon occurs when waves ap-proach a very steep beach, though in thiscase the amount of reflection is less.

class A group in the taxonomic classifi-cation of organisms. Several related classesform a PHYLUM, and a class itself is com-posed of one or more ORDERS grouped to-gether. For example, the Mammalia andReptilia are two classes of the phylumChordata. See taxonomy.

clast A small piece of rock that has beenremoved from a larger mass by some frag-mentation process. CLASTIC rocks and sedi-ments are composed of clasts.

cirque glacier

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headwall

lipice

deepened rock basindeepened rock basindeepened rock basin

rotational slip

Cirque (cross-section)

clastic /klass-tik/ Describing a sedimentor a sedimentary rock consisting of frag-ments of broken rock (clasts) that havebeen eroded, transported (usually bywater), and deposited. Clastic sedimentsinclude a wide range of sediment particlesizes from large boulders to fine-grainedsediments, such as silt. They are character-istic of coastal and shelf areas, and espe-cially the littoral zone. The particle sizesmay display wide and abrupt variations,for rarely are littoral or shelf sedimentsneatly sorted and graded, at least over largedistances. Detrital sediments are often de-scribed as having a clastic structure. Clas-tic rocks (fragmental rocks), consolidatedclastic sediments, include BRECCIA, CON-GLOMERATE, MUDSTONE, SANDSTONE, andSHALE.

clay 1. Particles of a size less than 1/256mm in diameter. They are usually smallflaky CLAY MINERALS formed during theweathering of other rocks. The grade ofparticles immediately larger than clay isSILT. Together they form the ARGILLACEOUS

division of clastic sediments.2. (in soil science) In the classification usedby the US Department of Agriculture, a soilseparate in which the mineral particles areless than 0.002 mm in diameter.

clay minerals Hydrous silicates, mainlyof aluminum and magnesium, occurring asplaty or fibrous crystals that have a layeredstructure and the ability to take up and losewater. They are the main constituents ofargillaceous rocks and are responsible forthe plastic properties of clay. Four maingroups may be considered: 1. Kaolinite group: kaolinite, Al(Si4O10)-(OH)8, is produced during the weatheringand hydrothermal alteration of feldsparsand feldspathoids under acid conditions.The large-scale production of kaolinite bythe alteration of granite gives rise toKAOLIN (china clay) deposits. 2. Illite group: illite and hydromicas are thedominant clay minerals in shales and mud-stones and are derived from the alterationof micas and feldspars under alkaline con-ditions.

3. Montmorillonite-smectite group: theseminerals are produced during the alter-ation of basic rocks under alkaline condi-tions and are the principal components ofbentonite and fuller’s earth. 4. Vermiculite: this mineral is producedduring the hydrothermal alteration of bi-otite and also occurs at contacts where acidmagma has intruded basic rock.

clear-air turbulence (CAT) Sudden se-vere turbulence occasionally encounteredby aircraft flying at high levels of the at-mosphere, which is unconnected with thevertical turbulence of convection clouds. Itappears to be the result of large verticalwind shears associated with high static sta-bility. These conditions produce the effectof two unmixed layers and on the bound-ary between them this wavelike motion de-velops, producing billowing. Clear-airturbulence can be detected by high-pow-ered radar. See also Kelvin–Helmholtz in-stability.

cleat A joint or system of joints devel-oped within a coal seam. Generally twosets occur at right angles to each other,along which the coal fractures preferen-tially.

cleavage 1. The tendency of a rock tobreak into closely spaced planar structuresor fractures as a result of deformation ormetamorphism. See also fan cleavage; flowcleavage; fracture cleavage; slaty cleavage.2. In minerals, the tendency to split alongplanes of weakness in the molecular frame-work.

cleavage plane The plane of fracture ora series of planar structures developed in adeformed or metamorphosed rock, alongwhich it splits preferentially. Cleavageplanes also occur in mineral cleavage.

cliff A high steep rock face. Coastal cliffsare attacked at their base by wave actionand above by subaerial weathering anderosional processes. The steepest cliffs arethose formed in resistant massive rocktypes, such as granite, whereas softerrocks, being more susceptible to erosion by

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both waves and subaerial processes, retreatand degrade comparatively rapidly. Ascliffs retreat a WAVE-CUT PLATFORM is fre-quently left projecting out to sea.

Abandoned cliffs were eroded at theirbase by waves at a time when sea level wassome way above that of the present, as onthe melting of the Pleistocene ice sheets, sothat they now stand some distance fromthe sea.

climate The synthesis of day-to-dayWEATHER variations in a locality. The ‘nor-mal’ climatic conditions prevailing can besummarized, but since it is impossible toinclude the wide range of values in a singlenumber and climate is not an unchangingfeature, this method on its own is not avery satisfactory indicator of actual condi-tions.

Climate is usually taken to include thefollowing weather elements: temperature,precipitation, humidity, sunshine, andwind velocity. Minor aspects include cloudamount, fog, snow, thunder, and gale fre-quencies. The major factors are summa-rized in terms of arithmetic averages forspecific periods, usually based on at least30 years of records. To indicate the rangeof values within the instrumental records,extremes are often quoted. More recently,greater emphasis has been made on the fac-tors determining climate.

On a large scale, the climate is deter-mined by latitude, by altitude, and by loca-tion relative to the continental margins andthe main circulation belts of the Earth. Ona smaller scale, more local factors can beimportant, such as aspect and degree of ex-posure to prevailing winds.

climate model See general circulationmodel.

climatic change Climate is not staticbut constantly changes in response to vari-ations in the factors that control it, primar-ily the nature of the main circulation belts.If records of temperature had been main-tained for the past million years, it wouldbe evident that throughout this periodthere were oscillations between warmer pe-riods and cooler periods on a wide range of

timescales. On a large scale there would bethe swings between the ICE AGES and thewarmer INTERGLACIAL periods, continuingthrough the whole time spectrum to indi-vidual years. Most of the evidence for thesechanges comes from the nature of sedi-ments deposited and the fauna and florathey contain, because it is obviously onlyrelatively recently that documentary evi-dence and finally meteorological instru-ments give us more precise indicators of theprevailing climate. The diagram illustratesknown changes of temperature in NW Eu-

climate

68

0° 5° 10° 15° 20°| | | |1000

2000 –

3000 –

4000 –5000 –

7000 –

–10 000 –

–

–

––––––

–

8000 –

100 000 –

–

–

––––––

1 000 0000° 5° 10° 15° 20°

| | | |

year

sb

efo

rep

rese

nt

Flandrian

Devensian

Ipswichian

Wolstonian

Hoxnian

AnglianCromerian

Beestonian

Paztonian

Baventian

Antian

Changes in July mean temperature in lowlandEngland

rope (England) in this period. However,much smaller changes became apparent,which would not have been preserved bygeologic or biological evidence, so thecurve is not based on consistent evidence.

The causes of climatic changes are notwell understood and likely to prove verycomplex. The shorter-period fluctuationswill probably have a different origin fromthe factors producing an ice age, but al-though mathematical models can showchanges that could be produced by varyingclimatic controls, it is almost impossible toprove that any cause or combination ofcauses was the real reason for past climaticchanges. Among probable factors influenc-ing the major changes are variations insolar output, the MILANKOVITCH CYCLES,the movement of continental positions toaffect the oceanic circulation, mountainbuilding, volcanic eruptions, and the in-crease in the carbon dioxide content of theatmosphere (see greenhouse effect).

climatic classification The climate ofany individual site on the Earth’s surface isunique, but there are great similarities be-tween many records. To rationalize thelarge amount of data from observingpoints, many attempts have been made toclassify climates into a small number ofcategories based on specific factors of sim-ilarity. There are two main bases for classi-fication, empirical and genetic, the formerbeing based on analysis of the observationsand the latter on the atmospheric circula-tions from which the climate results. Mostsystems have used the empirical approach,with the earlier ones linking this with veg-etation boundaries.

The most popular classification, theKöppen climate classification, which is stillused in a modified form, was proposed bythe German meteorologist Vladimir Köp-pen (1846–1940) in 1918. He adopted50°F (10°C) and 64°F (18°C) as criticalvalues to produce five major climatic cate-gories incorporating precipitation regionstoo. These are (A) tropical rainy climates,with an average temperature for everymonth above 18°C, no cool season, andabundant precipitation that exceeds evapo-ration; (B) dry climates, where evaporation

exceeds precipitation on average through-out the year; (C) humid mesothermal cli-mates, where the coldest month has anaverage temperature less than 18°C butabove –3°C, with at least one month hav-ing an average temperature above 10°C;(D) humid microthermal, where thewarmest month is above 10°C but thecoldest below –3°C; (E) ice climates, wherethe mean temperature of the warmestmonth is less than 10°C. These are furthersubdivided into a large number of climatictypes based upon seasonal temperature dif-ferences and variations in amount and dis-tribution of precipitation. The simplicity ofthe system and its relationships with vege-tation types have made this classificationvery popular although it does have limita-tions due to its arbitrary temperatureboundaries.

A more detailed classification (seeThornthwaite classification) system wasdevised initially in 1933 and modified in1948. This was based on two climatic fac-tors, a moisture index and a thermal effi-ciency index, which are independent of anyother geographical factor. However, thevalues were more difficult to calculate and,although frequently quoted as an example,the system is rarely used in practice. Indi-vidual aspects of the system, such as poten-tial evapotranspiration, have been morefully developed.

Other empirical classifications havebeen proposed, some being based onhuman physiological responses to climatein terms of wind speed, temperature, andhumidity combinations, others on the rela-tionship between the actual climate of asite and a stated ideal climate.

Genetic classifications are based on theatmospheric circulations that determineclimate. While being meteorologicallysound, in terms of the resulting tempera-ture and precipitation, quite variedweather conditions can be found withinthe same circulation belt and so such clas-sifications have received little popular ap-peal.

climatic geomorphology The branchof geomorphology concerning the role ofclimate in landscape evolution. Landscapes

69

climatic geomorphology

are a product of processes that are condi-tioned by climate, geology, and base level:of these three, climate is said to be the mostimportant, such that by its control, areas ofdiffering climate tend also to have differentlandforms. The concept is particularlypopular among European geomorpholo-gists: the French geomorphologists JeanTricart and André Cailleux elaborated it asan alternative to the Davisian CYCLE OF

EROSION concept, the difference being thatthe latter states that stage of evolution isthe major factor determining the appear-ance of a landscape, whereas climatic geo-morphology emphasizes that it is theprocesses operating, as conditioned by cli-mate, that dictate the form of an area. Therole of past climatic fluctuations in influ-encing the appearance of current land-scapes is also involved.

climatic optimum The period duringwhich temperatures reached their highestlevels within an interglacial stage of thePleistocene period. The climatic optimumof the present interglacial occurred about5000 years ago and is known as the AT-LANTIC PERIOD.

climatic region An area of the Earth’ssurface having relatively uniform climaticproperties and usually determined througha climatic classification system.

climatic zone The present distributionof climates on the Earth’s surface exhibitsa measure of zonal symmetry with theboundaries running approximately alonglines of latitude. Because of this, eight prin-cipal climatic zones have been distin-guished within which there is a certainamount of homogeneity. These zones are:the tropical rainy climate near the Equator,the two steppe and desert areas north andsouth of the Equator, the humid temperatezones, a Boreal climate in the N hemi-sphere, and the two polar zones.

climatograph /klÿ-mat-ŏ-graf -grahf/ Acircular diagram showing the seasonaltemperature variations of an area in graph-ical form.

climatology /klÿ-mă-tol-ŏ-jee/ The studyof CLIMATE. The emphasis on particular as-pects of climatology has produced manysubdivisions, such as applied, regional,physical, and synoptic climatology. Seealso meteorology.

climograph /klÿ-mŏ-graf, -grahf/ (climo-gram) A diagram in the form of a graphin which climatic features at any one placeare plotted against each other, for examplewet-bulb temperatures against relative hu-midities.

clinometer /klÿ-nom-ĕ-ter/ An instru-ment that measures angles of slope or incli-nation, such as DIP.

clinopyroxene /klÿ-noh-pÿ-roks-een/ Amonoclinic type of PYROXENE.

clinozoisite /klÿ-noh-zoh-ă-sÿt/ A typeof EPIDOTE that differs from ZOISITE in crys-tallizing in the monoclinic system.

clint See limestone pavement.

clintonite /klin-tŏ-nÿt/ A member of theMICA group of minerals that differs opti-cally from XANTHOPHYLLITE.

closed fold A fold that has been com-pressed so much that both limbs are paral-lel.

closure The distance between the high-

climatic optimum

70

JulyAugust

September

OctoberApril

November

DecemberJanuary

February

March

May

June 30°C30°C30°C

20°C20°C20°C

10°C10°C10°C

0°C0°C0°C

Climatograph: mean monthly maximum tem-peratures at Kew (London)

est point of a structure of folded rocks orstructural traps and the lowest contourthat totally encloses the structure. The clo-sure determines the maximum area inwhich the structure can contain a fluid, e.g.oil.

cloud A mixture of minute waterdroplets and/or ice crystals suspended inthe atmosphere and produced by conden-sation or freezing of water vapor. They re-main suspended within the cloud becauseof their small size and grow only when con-ditions are favorable for precipitation.Lower clouds are usually composed almostentirely of supercooled water droplets, butwith cooler temperatures ice crystals be-come more frequent until at temperaturesbelow –40°C the cloud is entirely ice crys-tals. Almost all clouds are found within thetroposphere and are the result of cooling.This can be brought about by convection,uplift over mountains, or large-scale ascentin depressions. See also cloud classifica-tion.

cloud base The point at which the fre-quency of water droplets in the atmospherebecomes sufficient to reduce visibility tothat characteristic of cloud. From theground surface this appears to be a veryrapid transition but in reality it representsa zone of transition. Cloud base indicatesthe level at which rising air reaches satura-tion and condensation results.

cloud classification Various classifica-tions of cloud have been proposed, but theone most commonly used is based on cloudappearance and height. The major groupsare divided into ten genera following planttaxonomic classification, comprising CIR-RUS (Ci), CIRROSTRATUS (Cs), CIRROCUMU-LUS (Cc), ALTOSTRATUS (As), ALTOCUMULUS

(Ac), STRATUS (St), STRATOCUMULUS (Sc),CUMULUS (Cu), CUMULONIMBUS (Cb), andNIMBOSTRATUS (Ns). The determination ofcloud species is based on the appearance ofthe cloud, on its structure, and if possibleon the physical processes involved in itsformation. 14 species names are commonlyused and can be referred to more than onecloud genus: fibratus – applied to cirrus

and cirrostratus clouds in the form of fila-ments but without tufts or hooks; spissatus– denoting dense opaque cirrus cloud,often originating from the cumulonimbusanvil; uncinus – denoting cirrus clouds thatare hooked or comma-shaped and fre-quently parallel; stratiformis – spread outinto an extensive horizontal layer, appliedto altocumulus and stratocumulus clouds;nebulosus – exhibiting a nebulous veil withno distinct details, usually cirrostratus oraltostratus; lenticularis – lens-shaped, withsharp margins (most commonly applied toaltocumulus cloud of orographic origin);castellanus – having a castellated or crenel-lated appearance connected to a commoncloud base (altocumulus show this charac-teristic most frequently); floccus – having aragged base and a small tuft or protuber-ance with cumuliform character above (cir-rus, cirrocumulus, and altocumulus);fractus – denoting stratus or cumulusclouds with a broken and ragged appear-ance; humilis – denoting fair-weather cu-mulus clouds, showing small verticaldevelopment and sometimes flattened;mediocris – denoting cumulus clouds of in-termediate vertical growth; congestus – de-noting active and developing cumulusclouds, often with the appearance of a cau-liflower; calvus – denoting an intermediatestage between cumulus and cumulonimbusin which no cirriform part can be distin-guished; capillatus – denoting a cumu-lonimbus with distinct cirriformappearance frequently in the form of ananvil. If any further distinctive propertiesof arrangement or transparency are appar-ent then these are distinguished as vari-eties. In this way, three names could beused to classify a particular cloud. Cirri-form clouds are normally above 5000 m,alto-clouds from 2000–5000 m, and strat-iform (St, Sc, Ns) at lower levels. Cumulus,cumulonimbus, and to some extent nimbo-stratus develop vertically and so do not fitthis height classification very well.

cloud condensation nucleus See con-densation.

cloud forest A type of forest that growson tropical mountains at high altitudes.

71

cloud forest

The trees obtain most of their moisture aswater that condenses out of the air asclouds.

cloud patterns From satellite images itis evident that clouds exhibit a degree oflarge-scale organization that produces dis-tinctive patterns. In the analysis of theseimages, patterns such as vortices, spirals,comma-shaped masses, wave clouds, lines,and striations may be distinguished.

cloud seeding A method of trying toinduce clouds to give precipitation. It at-tempts to simulate the natural precipita-tion mechanism either by adding dry ice(frozen carbon dioxide), which causesspontaneous freezing of water droplets andincreases their rate of growth, or by addingsilver iodide, which has a similar crystallattice to ice and so acts as ice nuclei to as-sist the natural processes.

Although some success has been appar-ent, it is difficult to prove that seeding canincrease rainfall because the clouds thatwere seeded may have given rain naturallyand the observed increases are not suffi-cient to disprove this possibility.

Cnidaria /nÿ-dair-ee-ăn/ A phylum ofsimple multicellular animals that includesthe jellyfish and CORALS. They are aquatic,mainly marine, and may be colonial or soli-tary, attached or free-swimming. There aretwo types of individual: the polyp, which iscylindrical and sedentary, and the medusa,which is disk-shaped and free-living. TheCnidaria contains three classes: the HYDRO-ZOA, which have both polyps and medusae;the SCYPHOZOA (jellyfish), which have onlymedusae; and the ANTHOZOA (e.g. corals),which contain only polyps. The corals aregeologically the most important cnidariansbecause many of them are colonial, form-ing reefs, and possess hard parts, which canbe preserved.

coal A carbonaceous deposit formedfrom fossil plant remains. Coalificationproceeds from partially decomposed veg-etable matter such as PEAT, through LIG-NITE, subbituminous coal, BITUMINOUS COAL,semibituminous coal, semianthracite, to

ANTHRACITE. During this process the per-centage of carbon increases and volatilesand moisture are gradually eliminated.These are all woody or humic coals. An-other group are termed sapropelic coals,and these are derived from algae, spores,and finely divided plant material. See alsocannel coal.

coalescence See collision–coalescenceprocess.

Coal Measures The uppermost of thethree lithological divisions of the CAR-BONIFEROUS System in Britain. It corre-sponds to the upper half of the UpperCarboniferous.

coastal deposit All the deposits foundat any particular coastal location are de-rived from one or more of the followingfour sources: (1) the cliffs along the coast inthat vicinity; (2) inland, transported byrivers or the wind; (3) offshore, movedlandward by wave action; (4) farther alongthe coast, transported by LONGSHORE DRIFT

(this material must at one time have beenderived from one of the other threesources). The nature of the materials andtheir quantities will depend upon the rocktypes found along the coast and inland andupon their relative contributions. A wholerange of deposits, from massive bouldersthrough shingle and sand to fine clays, canbe found in different coastal sites.

coastal plain A gently sloping plainleading from the foot of inland uplandareas down to the coast, and largely con-tinuous with the continental shelf underthe sea. Such plains are formed by the con-tinuous processes of erosion in the inlandareas, transportation of material seaward,and its deposition either on land, near thecoast, or out to sea. Some coastal plainshave resulted from the comparatively re-cent emergence of parts of a flat continen-tal shelf due to a relative fall in sea level.

coastline The landward limit of thebeach, the boundary between the coast andthe shore, or the line that forms the bound-ary between the land and the water. Per-

cloud patterns

72

haps the simplest application of the term isto take the coastline as the extreme upperlimit of direct wave action, which is thelimit of storm wave swash during equinoc-tial spring tides.

coastline of emergence A shorelinethat reveals evidence that the land has risenwith respect to the sea. Typical features in-clude wave-cut platforms with overhang-ing cliffs, and raised beach terraces.

coastline of submergence A shorelinethat reveals evidence that the land has sunkwith respect to the sea. Typical features in-clude many bays and continuing erosion ofcliffs and headlands by wave action.

cobble A rounded fragment of rock thatis larger than a pebble but smaller than aboulder. The accepted size range is be-tween 65 mm and 255 mm across.

coccoliths /kok-ŏ-liths/ Minute roundcalcareous plates that formed part of theprotective covering of a group of unicellu-lar algae of the phylum Haptomonada (orHaptophyta). They have been reportedfrom the Upper Cambrian but were morecommon in the Mesozoic, especially theCretaceous, when they contributed largelyto the formation of chalk.

coesite /koh-ĕ-sÿt/ A very dense form ofQUARTZ, SiO2, stable at high pressures andoccurring in and around craters caused bymeteorite impact. See also silica minerals.

cohesion (in geology) In a rock or soil,the property by which particles or surfacesadhere together. It is the part of the mate-rial’s shear strength that does not dependon friction between the particles or sur-faces.

col 1. A pattern of isobars resembling ageographical col or saddle, i.e. a shallowdip at a high level. A col that is elongatedalong the high-pressure axis is known as ananticyclonic col, and if along the low-pres-sure axis is called a cyclonic col. Winds arelight in this type of pressure system but thepattern is rarely long-lasting.

2. A depression forming a pass over amountain ridge or other high ground, com-monly formed by back-to-back cirque de-velopment or from the beheading of adip-slope valley in a cuesta formation byscarp retreat.

cold front A concentrated thermal gra-dient whose movement is such that warmair is replaced by cold air. Such fronts areusually found to the rear of a depressionand are associated with a sudden veering ofthe wind, often with a marked increase inspeed, a fall of temperature, a belt of cloud,and sometimes heavy precipitation. Thefront slopes upward into the atmosphere atan angle of about 1 in 50. All these charac-teristics refer to the ideal front, andwhereas most have somewhat similarproperties, each individual will have slightdifferences. See also warm front.

cold glacier (polar glacier) A glacierwithin which the temperature remains solow that pressure-melting does not occur.In contrast to temperate glaciers these con-tain very little or no meltwater to act as alubricant for movement. The glacier mayremain static, being frozen to the bedrock,and requires great shear stress to inducemovement, which will be very slow. Theseglaciers contain very little ENGLACIAL ma-

73

cold glacier

low100810081008101210121012

hig

h

highCOL

102010201020

1012

1012

1012

100810081008

100410041004 100010001000

996996996992992992

100810081008

COL

low

low

high

102410241024

Fig. 1: Anticyclonic colFig. 1: Anticyclonic colFig. 1: Anticyclonic col

Fig. 2: Cyclonic colFig. 2: Cyclonic colFig. 2: Cyclonic col

1016101610161020 mb1020 mb1020 mb

101610161016

101210121012

100010001000 1004

10041004

Pressure patterns in a col

terial. However, cold glaciers have beenfound with large bed loads, and becausethese can only become accumulated as a re-sult of basal freezing and thawing (seeglacial plucking) it is believed that the ero-sion responsible may have taken place dur-ing a period of warmer climate.

cold occlusion See occlusion.

cold pole That location in each hemi-sphere where the lowest air temperatureshave been measured. In the N hemispherethis is the area of NE Siberia where at bothVerkhoyansk and Oymyakon, –68°C(–90°F) has been recorded. The mean Jan-uary temperature at the former station is–51°C (–60°F), which represents the cold-est spot on average. In the S hemisphere,the high-altitude research stations on theAntarctic Ice Plateau have the record witha temperature of –89.2°C (–128.6°F) atVostok. Both cold poles are in areas whereradiational cooling is extreme under clearskies and low relative humidity.

cold pool An area of cold air within theatmosphere surrounded by air of highertemperature at that level. Because this usu-ally leads to a steepening of the LAPSE RATE,it may lead to instability and the develop-ment of showers.

colemanite /kohl-măn-ÿt/ A white or col-orless mineral form of hydrated calciumborate, Ca2B6O11.5H2O. It crystallizes inthe monoclinic system and occurs as mas-sive crystals in clay nodules originating inCenozoic lakes. It is an important source ofboron.

collision–coalescence process A the-ory for the process by which raindrops arebelieved to grow in warm clouds, based onthe mechanisms of collision, coalescence,and rear capture. Cloud droplets form bycondensation of water vapor onto cloudcondensation nuclei (see condensation).The larger droplets, with terminal veloci-ties increasing in proportion to their diam-eters, begin to fall faster than the smallerdroplets and, through collision with thesmaller droplets and coalescence in which

the droplets merge together, they increasein size to form raindrops. Those smallerdrops that are swept into the rear of largerdrops may also be absorbed, the mecha-nism being known as rear capture. A rain-drop becomes unstable once it reachesaround 5 mm and breaks up into smallerdrops. The collision–coalescence processoccurs in tropical convectional clouds, butis also applicable to clouds in the mid-latitudes. See also Bergeron–Findeisenprocess.

collision zone The region where tectonicmovements have caused two continents tocollide, with the closure of the ocean be-tween them. See also plate tectonics.

colluvium /kŏ-loo-vee-ŭm/ (talus) Theweathered debris (scree, mud, etc.) accu-mulated at the foot of a slope, which hasoriginated from the erosion of the rockfaceabove.

colony An association of animals of thesame species having structural and physio-logical connections between the individu-als. Examples of colonial animals are theBRYOZOA and the reef-forming corals (seeAnthozoa).

color index An index of the relativeproportions of light and dark minerals inigneous rocks. The following divisions aremade according to the percentage of darkminerals:

leucocratic 0–30%mesocratic 30–60%melanocratic 60–90%hypermelanic 90–100%

The terms felsic and mafic are synonymouswith light and dark minerals respectively.In nearly all cases dark minerals are ferro-magnesian minerals.

columbite /kŏ-lum-bÿt/ A mineral thatconsists of a mixed niobate and tantalate ofiron and manganese, (Fe,Mn)(Nb,Ta)2O6,in which there is more niobium than tanta-lum. It is so called because niobium wasonce known as columbium, and is found ingranites and pegmatites. See also tantalite.

cold occlusion

74

columnar joint One of a series ofhexagonal or pentagonal joints resulting incolumns of igneous rock, especially basaltlava. A good example is at Fingal’s Cave,Staffa, Scotland.

comagmatic /koh-mag-mat-ik/ Describ-ing a series of igneous rocks that are de-rived from the same original magmathrough the various processes of differenti-ation. See magmatic differentiation.

comber /koh-mer/ A wave traveling indeep water, which has a steep and high-breaking crest. The crests, much largerthan whitecaps, are blown forward underpowerful wind action. The term has alsobeen applied to certain types of spillingBREAKERS.

comfort index A measure of climaticcomfort experienced by humans. It is as-sumed that the ideal comfort state is be-tween 40 and 70% relative humidity andair temperature between 60°F (15.5°C)and 80°F (27°C). The index is subjective inthat individuals may prefer lower or highertemperatures depending upon their metab-olism, but it does give an assessment of cli-mate or weather conditions relative to anideal state.

community (in ecology) Any group ofplants and animals that occupy the samehabitat and interact in various ways. Usu-ally the dominant feature, such as a typicaltype of plant, gives its name to the commu-nity (e.g. beechwood).

compaction The compression of a sedi-ment with the result that it occupies lessvolume (i.e., the layer becomes thinner). Itis generally caused by the weight of morerecent material being deposited above. Itmay also be caused by earth movements or,on soil, by human activity. Compacted soilis less porous, so that puddles and muddypatches may form after rainfall.

compass traverse See traversing.

compensation level A hypotheticallevel within the Earth above which all

columns of rock material having a unitcross-sectional area must have the samemass. See Pratt’s hypothesis.

competence 1. (in hydrology) A mea-sure of the largest size of grain that astream can move as bed load. It is deter-mined mostly by the stream velocity at thebed, which is controlled in turn by severalfactors, especially stream gradient, and theratio between width and depth: bed veloc-ity is higher, with other things equal, in awide shallow river than a deep narrow one.With the larger grain sizes (coarse sand andgravel) there is a sixth-power relationshipbetween velocity and competence. In anyone river, competence varies with flooddischarge over time; over space it varieswith turbulence in the form of eddies risingfrom obstructions on the bed, which exertlift, momentarily increasing the maximumsize of liftable debris.2. (in geology) The degree to which beds ofrock can be folded without flowage orchange in thickness. Such strata respond todeformation by fracture rather than flow.

compiled map A map that is producedfrom other large-scale mapping rather thanoriginal survey work. Compiled maps areusually reductions of other maps to whichselected amendments have been made be-fore reduction.

complex A complicated body of rockthat cannot be designated by any of theother formal terms used in LITHOSTRATIG-RAPHY. A complex is usually composed of avariety of rock types and it may have dis-tinctive structural characteristics. The termmay also be used informally to refer to anycomplicated association of rocks.

composite log The interpreted geologiccolumn encountered in a well, recon-structed from the evidence of geologic sam-pling and electrical investigations carriedout in the borehole.

composite map A map, usually com-piled, that brings together for ease of com-parison data usually portrayed on two or

75

composite map

more maps, e.g. land use and populationdistribution.

composite volcano (stratovolcano) Avolcano constructed of alternating layersof lava and pyroclastic deposits, generallydeveloped by volcanoes with fairly viscouslavas, for example, andesites.

concave slope A hill slope that issteeper at the top than at the bottom. On amap of the feature, contour lines are closertogether at the top of the slope. See alsoconvex slope.

concentric dike (ring dike) A roughlycircular vertical dike developing when thecircular fractures produced by the upwardpressure of magma in a chamber are in-truded by magma. The related structuresknown as cone sheets are developed by asimilar stress distribution. They surround apoint source (the intruding magma) anddip inward toward it, having the form of aseries of concentric conical sheets infilledby magma.

conchoidal /kon-koi-dăl/ Describing atype of curving fracture marked by concen-tric arcuate ridges. It is characteristic ofminerals such as quartz and glassy andaphanitic rocks, particularly obsidian andphonolite.

concordant coast (longitudinal coast) Acoastline that parallels the hills and otherfeatures just inland of it. If the land sinkswith respect to the sea (a coastline of sub-mergence), the sea floods the valleys be-tween the hills, which become a chain ofislands.

concretion A roughly spherical body ofmaterial precipitated from a solution.

condensation The physical process oftransformation from the vapor to the liq-uid state. In the atmosphere, condensationrefers to the formation of liquid water fromwater vapor as a result of the atmospherebecoming saturated; for this to take placecooling must occur. If the atmosphere wereabsolutely pure, no condensation would

take place until extreme supersaturationexisted, but throughout the tropospherethere are abundant nuclei on which con-densation can take place on saturation.These cloud condensation nuclei consist oftiny crystals of sea salts, industrial and nat-ural smoke particles, as well as volcanic,soil, or desert dusts. The condensation nu-clei are around 0.1 µm to 1 µm in diame-ter. On condensation, latent heat isreleased equivalent to about 2500 joulesper gram of water, which reduces the rateof cooling.

condensation level The level in the at-mosphere at which CONDENSATION takesplace. It is normally used for calculatingCLOUD BASE on a thermodynamic diagram,by simulating the lifting of air of knownproperties of temperature and humidity.

conditional instability The state of apart of the atmosphere in which the ENVI-RONMENTAL LAPSE RATE lies between thedry adiabatic and saturated adiabatic lapserates. This means that if a parcel of air risesfrom the ground it will cool at the dry adi-abatic lapse rate (DALR) and so remaincooler than the environment; under theseconditions the atmosphere is stable. How-ever, if the parcel reaches saturation it willthen cool at a slower rate and could be-come warmer than the environment on fur-ther uplift, and so become unstable.

conduction The mechanism by whichheat is transferred through a substance bymolecular motion without overall motionof the substance itself. Air is a very poorconductor of heat and this method oftransfer is relevant only when the air is ab-solutely calm. This may occur very close tothe ground surface, but in most studies ofthe atmosphere, conduction can be ne-glected.

conduit See vent.

cone of depression A conical dip thatoccurs in the water table around a well aswater is removed from the well. As a result,the water level in the well gets lower andlower.

composite volcano

76

cone sheet See concentric dike.

confidence limit A statistical conceptused to assess the degree of confidence thatcan be attached to estimates of true meansor standard deviations when based on onlya sample of the data. For example, if theannual rainfall totals for a particular loca-tion are only known for 15 years, then themean of this sample (x

_) will probably dif-

fer from the true mean. By placing confi-dence limits it can be stated that with a95% probability the true mean for this lo-cation will be x

_± 2y, i.e. between x

_– 2y

and x_

+ 2y, where y is the standard devia-tion of the data set.

confining pressure (in geophysics) Thepressure exerted on a point resulting fromthe weight of the overlying rock or watercolumn.

confluence 1. In air flow, the rate ofconvergence or approach of one streamlineto the adjacent streamline relative to the di-rection of flow. It differs from CONVER-GENCE in that it does not imply an increaseof mass but simply the approach of stream-lines of air flow. Compare diffluence.2. The junction of two or more streams toform one.

conformable 1. Comprising an unbro-ken sequence of rock strata with no angu-lar discordance between them, marking acontinuous period of uninterrupted deposi-tion. See also discordant (def. 1).2. See accordant.

conformal projection See orthomor-phic projection.

congelifraction /kŏn-jel-ă-frak-shŏn/ Theweathering of rocks through the freezing ofwater within pore spaces. The accompany-ing expansion creates stresses, which resultin fracture of the rock and its consequentdisintegration. The effectiveness of theprocess varies considerably with rock type,being dependent on the size of the pores. Ifthese are too large, or open to the surface,the water may be forced out as tempera-tures fall, whereas if they are too small, the

water remains in a liquid state despite su-percooling. The process is most effective inschists and certain limestones, notablychalk, within which very rapid shattering ispossible. See also frost-shattering.

congeliturbation The movement of soilmaterials in the PERIGLACIAL environment,as a result of freezing and thawing of watercontained within the soil. Freezing is ac-companied by an expansion in the soil, butthis is irregular because in a heterogeneousmaterial water will not be evenly distrib-uted. Similarly, thawing takes place at un-even rates, and the result is that particlesare transported considerable distances,even on flat ground. On slopes, materialmigrates downslope under the influence ofgravity. The ice causing the disruptiontends to be concentrated in the silt andsand fraction, coarser material beingmoved to the surface.

conglomerate A RUDACEOUS sedimen-tary rock in which the constituent clasts orfragments are more or less well rounded.Compare breccia.

conical projection A MAP PROJECTION

produced by projecting the meridians andparallels onto a cone. (See diagram above.)The center of curvature of the cone is notone of the poles, although the meridiansare drawn radiating from it. The parallelsare concentric circles and are their true dis-tance apart. The scale is only correct along

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conical projection

North Pole

50° N

50°

Equator

standard parallel

Simple conical projections

the standard parallels; all others are toolong. This projection is neither equal-areanor orthomorphic and is usually modified,e.g. BONNE’S PROJECTION and the conicalwith two standard parallels.

coniferous forest A forest of cone-bear-ing GYMNOSPERM trees. The trees aremainly evergreens (larches are an excep-tion) with needle-shaped leaves, such asfirs, pines, and spruces. Coniferous forestsoccur farther north than DECIDUOUS

FORESTS because the trees are better able towithstand cold temperatures. Trees fromthe forests, which are often managed, pro-vide timber for building and wood pulp forpaper making.

conjugate joint Any of a system ofjoints in rocks, in which the sets of jointsare of related origin. It generally consists oftwo sets, which intersect at 90° but share acommon dip or strike.

conodont /kon-ŏ-dont/ A small tooth-like structure up to 2 mm long and com-posed of calcium phosphate. Conodontsare found in rocks of marine facies, espe-cially shales, from Ordovician to Permianage and also in later strata, in which theyare suspected as being DERIVED FOSSILS. Be-cause they are small, change in morphol-ogy through time, and are widelydistributed, conodonts are particularly use-ful in the correlation and identification ofthe rocks in which they occur. They havebeen variously attributed to most phyla buttheir zoological affinities remain unknown(see species); their widespread distributionsuggests that they may have belonged to agroup of pelagic animals.

Conrad discontinuity A seismicallydetectable boundary within the Earth’scontinental crust, separating it into a lower(basic) layer and an upper (granitic) layer.Beneath the ocean floors the upper layer ismissing. It is named for the Austrian seis-mologist Victor Conrad (1876–1962).

consanguinuity /kon-sang-gwă-new-ă-tee/ The common features of different ig-neous rocks that probably arose from the

same original magma. They usually occurclose together, are of about the same age,and have similar chemical compositions.See also comagmatic.

consequent stream A stream whosecourse is dictated by the slope of the land.See also obsequent stream; subsequentstream.

conservation (in ecology) Any mea-sures taken to preserve resources, such asthe soil, minerals, and living organismswithin the environment. Methods includepreserving habitats and eliminating wasteof raw materials, and recycling those thatare used.

conservative plate boundary See plateboundary.

conservative property A property ofair that remains unaltered when affectedby specified processes. For example, whenair is cooling at the DRY ADIABATIC LAPSE

RATE, its potential temperature remainsconstant and is therefore a conservativeproperty under these conditions.

constancy of wind An index of the de-gree of constancy of wind direction. It ismeasured by determining the ratio of thevector mean wind to the scalar wind, andconverting to a percentage value. It rangesfrom 0 if all winds are equally strong andfrequent in all directions to 100 if thewinds are constant or unvarying in direc-tion. For the British Isles, values of between15 and 50 are typical, but in the trade windzones it can approach 90.

constant slope The scree or talus slope,i.e. the foot slope produced by the accumu-lation of weathered rock debris fromabove. The evolution of the constant slopebegins with a vertical rock face at the startof the cycle and at its peak it covers the freeface and buries it. At this stage the constantslope, whose angle is defined by the ANGLE

OF REST of the debris of which it is com-posed, dominates and largely determinesthe hillslope form. Later on, the washing offines from the constant slope leads to its

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burial by a wash slope, or alluvial toe-slope, which accumulates at the foot of theconstant slope. See also colluvium.

constructive plate boundary See plateboundary.

constructive wave A wave that assistsin the aggradation of a beach. For aggra-dation to occur the material moved land-ward must exceed that moved seaward, sothe SWASH must be more efficient than theBACKWASH. For a powerful swash the wavesmust be flat, although their frequency can-not be too high, because the swash of anew wave would be interfered with by thepreceding wave’s backwash. The usual fre-quency of constructive waves is betweensix and eight per minute. Compare de-structive wave.

consumer (in ecology) An organismthat is above the first level in a FOOD CHAIN,which is occupied by producers. Most PRI-MARY CONSUMERS are herbivores, feedingon plants. Secondary consumers are carni-

vores, feeding on the herbivores or othercarnivores.

contact The surface forming the bound-ary between adjacent and distinct bodies ofrock. It may be conformable within a sedi-mentary succession, unconformable, orproduced by intrusion or faulting. Therecognition of contacts is fundamental toLITHOSTRATIGRAPHY and much geologic in-terpretation.

contact metamorphism (thermal meta-morphism) The changes resulting fromthe intrusion of hot magma into preexist-ing cold country rock. The thermal effectof the magma causes a recrystallization ofminerals in the country rock and thegrowth of new minerals. Contact meta-morphic rocks have equigranular textures,have suffered little or no deformation, andin general do not possess any foliation.Thermal effects die out rapidly with in-creasing distance from the contact with theigneous rock. Sedimentary rocks are con-verted to hornfels, quartzite, and marble.Hornfels often contain porphyroblasts orspots of contact metamorphic mineralssuch as biotite, andalusite, cordierite, andgarnet. With the exception of the expulsionof water from the country rocks, the meta-morphism is largely an isochemicalprocess.

Four facies of contact metamorphismare recognized:

albite-epidote hornfels facieshornblende hornfels faciespyroxene hornfels faciessanidinite facies.

In an ideal metamorphic aureole, pyroxenehornfels rocks occur adjacent to the intru-sion and pass into hornblende hornfels fa-cies rocks farther from the contact.Inclusions of country rock caught up in theigneous magma reach the high tempera-tures associated with the sanidinite facies.Such rocks may become partly fused, re-sulting in the formation of glass, and areknown as buchites. See also metamor-phism; skarn.

continent A large landmass, which risesmore or less abruptly from the deep ocean

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continent

Fig. 1:Initiation

Fig. 2:Growth ofconstant slope

Fig. 3:Domination ofhill slope byconstant slope

free face

cons

tant

slopefree

face

free

face

rock

wal

l

consta

nt

slope

face

free

constant slope

wash slope

Fig. 4:Wash of fines from constant slope evolveswash slope, which buries constant slopeand ultimately dominates hill slope form

Stages of slope evolution

floor. Associated with the exposed land-mass are marginal areas (CONTINENTAL

SHELVES) that are shallowly submerged.The continents occupy about 30% of theEarth’s surface.

continental borderland A marine re-gion adjacent to, or lying in place of, a con-tinental shelf, and displaying markedtopographic irregularity. A frequentlyquoted example lies off the coast of South-ern California, a region of very diverse sub-marine morphology, which includessubmarine ranges and basins. Some of thebasins are quite deep. Often the adjacentlandmass displays similar topography. Thedepths of continental borderlands are usu-ally far in excess of the depths normally en-countered over shelf areas.

continental climate The generalizedcharacteristic climate of continental interi-ors, implying a low precipitation total, alarge annual range of temperature withwarm summers and cold winters, and lowrelative humidities.

continental crust The part of theEarth’s crust constituting the continents,often referred to as the SIAL. It is composedof granite rocks and reaches a thickness inexcess of 50 km under mountain regions,although it is generally only 33 km thick.The lower continental crust was formerlybelieved to be of basaltic composition, sim-ilar to that of the oceanic crust. It is nowthought that it may simply be a more densephase of the granitic rocks. See also oceaniccrust.

continental drift The movement of thecontinental blocks relative to one anotheracross the surface of the Earth, as a resultof SEA-FLOOR SPREADING. The hypothesis ofcontinental drift was proposed in the early1900s, but it was only with the advent ofthe PLATE TECTONICS theory that a viablemechanism was available to explain themovements of the continents. It is thoughtthat the present continents were groupedtogether in pre-Mesozoic times into twolarge supercontinents, Pangaea and Gond-wanaland. These continents subsequently

broke up to form the present continents.The presence of these supercontinents wasbased upon the work of the German mete-orologist and geophysicist Alfred Wegener(1880–1930) in 1910. His evidence wasonly qualitative but included distributionsof rock types, flora, fauna, geologic struc-tures, and similarity of the shape of thecoastlines on either side of the Atlantic.Geophysicists of the time dismissed the hy-potheses as impossible. However, in thelate 1950s and 1960s geophysical evidencesupported the theory.

continental heat flow The flow of heatreaching the surface of the Earth in a con-tinental as opposed to oceanic region.

continental margin (continental terrace)The combined continental shelf and slopezones, whose modes of origin are generallyclosely related. The depth range of the con-tinental margin may, therefore, be taken asextending from the coast out to a depth ofsome 2000 m. Some continental marginsare of a constructional kind, others dias-trophic. Warping, faulting, and long-con-tinued sedimentation have been shown tohave played dominant roles in their evolu-tion. Deep seismic soundings have revealedthat many have thick layers of sedimentaryrocks overlying older basement rocks of acrystalline nature.

continental rise The submarine surfacebetween the abyssal floor and the base ofthe continental slope. Together with theslope, the rise forms a separation zone be-tween the continental shelves and the deepocean basins. The surface of the rise is gen-erally smooth, with gradients between1:100 and 1:800. The minor irregularitiesrarely amount to more than 20 m or so inheight or depth. The rise may be up to sev-eral hundred kilometers wide, for exampleoff part of the North African coast where itis over 600 km wide. On the other hand, itmay be virtually absent, as off the Bay ofBiscay. In some areas, quite largeseamounts are found on the rise.

continental shelf That part of a conti-nent that is shallowly submerged by the

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sea. It is a gently dipping area extendingfrom the shoreline down to the continentalslope at the SHELF EDGE, or to a depth of200 m if the slope is absent. The averageshelf width is about 70 km but shelves tendto be wide off low-lying regions and nar-row off mountainous regions, varyingfrom less than 1 to more than 1000 kmwide. Average gradients tend to be steepernear the coast than over the outer shelf.Globally, shelves occupy some 7.6% of theocean floor. Most shelves exhibit evidenceof marine erosion, especially in shallowcoastal waters. Some have carved withinthem valleys, troughs, and basins (see sub-marine canyon), often formed during lowsea-level stands. Many are veneered withloose sediments that are being moved andsorted by wave and tidal action, and thenmolded into tidal banks and channels.

continental slope A portion of the con-tinental margin, dipping more steeply thanthe continental shelf, with a gradient ofmore than one in forty, although this variesconsiderably. It is bounded by the conti-nental shelf on the landward margin, andthe continental rise on the seaward side.Globally, the continental slope zone makesup about 8.5% of the total ocean floor, butapart from some SUBMARINE CANYONS thathave been carefully explored and surveyedits precise form is not known. Where thereis abundant sediment, especially mud,slumping and sliding is common and re-sults in very irregular topography.

contour A line drawn on maps andcharts joining points of the same elevation,above or below sea level (see diagram).This line enables the user of the map to de-termine the elevation and slope of the landor sea bed (contours below sea level arecalled submarine contours). Measurementsmay be in meters, feet, or fathoms (subma-rine). See also contour interval; hypsomet-ric tinting; supplementary contour.

contour interval The difference inheight between two consecutive index con-tours. The interval will depend on severalfactors: the scale of the map, the terrain themap portrays, and the purpose of the map.

The scale of the map determines the maxi-mum possible number of contours per areato retain clarity of detail; the terrain influ-ences the number of contours required todepict accurately the true nature of theland (in level areas a few contours willshow the land characteristics whereas inhilly areas more contours are needed toshow the elevation variations within thearea); the purpose of the map also affectsthe interval: plans for builders and landsurveyors require the maximum number ofcontours per area to enable accurate mea-surements to be made.

control point A point on a map whoseposition and usually elevation are known.Control points form a network aroundwhich other map detail is plotted in its cor-rect position, azimuth, and elevation.

Conulariidae /kon-yŭ-lair-ee-I-dee/ Afamily of fossil animals of uncertain zoo-logical affinities. It has been suggested thatthey belong to the MOLLUSCA, the ANNEL-IDA, or the classes Hydrozoa or Scyphozoa

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Conulariidae

contours

submarinecontours

sea level

Fig. 1: Cross section of coast and hill to show contour lines

150150150

100100100

505050

Fig. 2: Hill viewed as from above (as on maps)

Contours

of the CNIDARIA. Some authorities placethem, with a few other similar groups ofanimals, in a separate and extinct phylum,the Conulariida. They occur in sedimentsof marine facies and typically have a skele-ton constructed from four flat triangularplates joined at the edges to form an in-verted four-sided pyramid. The plates areformed of calcium phosphate and have anornamentation of transverse ridges. Projec-tions from the free basal edges of theseplates fold over to protect the terminalopening. Conulariids are found in rocksdating from the Cambrian to the Triassic;in size they range from about 5 cm to over20 cm.

convection (in meteorology) The mostimportant form of heat transfer within theatmosphere. It takes place by the physicaltransfer of air resulting from density varia-tions, warmer air being less dense than coolair. Two types of atmospheric convectionare distinguished: (1) free or natural con-vection, which is essentially the transfer ofwarm air from lower levels of the atmos-phere through the mechanism of thermalsor bubbles; (2) forced convection, which isthe vertical movement of air produced bymechanical rather than thermal forces,such as occurs through movement over up-lands or irregular vegetation surfaces.

convection cell A continuous convec-tion system, air that rises being replaced byair from elsewhere. This can occur on asmall scale with a single thermal or con-vection cell rising above a plowed fieldheated by the Sun. At the other extreme,the meridional circulation within the Trop-ics has been likened to a vast convectioncell with zones of rising air and the tradewinds linking the vertical parts of the cell.This system is called the HADLEY CELL.

convection current A current within afluid created by differences in temperaturebetween layers causing differences in den-sity. Convection currents are thought tooperate in and beneath the oceans. Theyoperate in the ocean when a portion of awater mass that is denser than the waterbeneath it tends to sink, thereby being re-

placed by less dense water. In this context,the currents caused in such regions as theRed Sea and the Mediterranean Sea by ex-cessive surface evaporation, increase insurface salinity, and consequent sinkingare claimed by some to constitute a con-vection current. Convection currents arealso held, by some authorities, to occurwithin the Earth’s mantle beneath theoceans. One suggestion is that the highertemperatures under the oceans at certaindepths could be a consequence of convec-tion flow that must be operating at somedepth within the mantle. This could resultin an increase in the temperature of certainparts of the ocean floor, such temperatureshaving been recorded. See also density cur-rent.

convective precipitation Precipitationproduced by the strong but localized verti-cal motion in convectional clouds found inan unstable atmosphere. This is usuallyproduced from cumulonimbus clouds, butcumulus clouds may also give slight rain.Because of the intensity of the upwardmovement within the clouds, convectiveprecipitation is normally heavy but re-stricted in distribution at any instant.

convergence 1. A measure of the rate ofinflow of a fluid into a certain volume. Thereverse state is DIVERGENCE. In the atmos-phere, these terms are used to indicate thehorizontal components of wind velocity:convergence indicates retardation and di-vergence indicates acceleration. Surfaceconvergence is usually accompanied by up-ward air currents. In wave refraction phe-nomena, convergence refers to thebunching together of the various wave or-thogonals in the direction of wave ad-vance. Wave energy tends to beconcentrated in a zone of convergence. Inthe case of opposing currents, a conver-gence is the boundary or zone at whichthese currents meet. Usually convergencezones experience the sinking of some oceanwater to greater depths.2. See convergent evolution.

convergence zone An area of the at-mosphere in which convergence prevails

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82

and which is characterized by a belt of con-vectional clouds, often with rain. Suchzones can be found within otherwise uni-form air streams giving a short period ofheavy rain as they pass. The best known isthe INTERTROPICAL CONVERGENCE ZONE,which is frequently seen on satellite pho-tographs and can extend for several thou-sand kilometers.

convergent evolution (convergence)The evolution of similar characters andstructural modifications in organisms thatare not closely related. For example, dol-phins and ichthyosaurs, although onlydistantly related, have a very similar exter-nal form that is presumed to have beenacquired through adaptation to similar en-vironmental conditions. Convergent evolu-tion also occurred in marsupial andplacental mammalian groups, which wereisolated on different landmasses but occu-pied similar habitats. Such organisms areknown as homeomorphs.

convergent plate boundary (convergentboundary) A region where lithospheric(or tectonic) plates are moving togetherand the crust occupies a smaller area. Itmay involve subduction, in which oneplate dips below the other and is consumedinto the mantle. Alternatively, one platemay force the crust on the other plate topile up and form a new range of moun-tains. See also plate boundary; plate tec-tonics; subduction zone.

convex slope A hill slope that is steeperat the bottom than at the top. On a map ofthe feature, contour lines are closer to-gether at the bottom of the slope. See alsoconcave slope.

convolute bedding Highly disturbed,folded, or crumpled laminae that are foundwithin a single well-defined sedimentarylayer, resulting from deformation of thesediment by slumping, gliding, loading, up-ward expulsion of water, or the effect ofpassing eddies in the overlying water mass,while the sediment is still in an unlithifiedstate.

coombe /koom/ A small hollow in asteep chalk hillside. It generally resultsfrom weathering, which opens and en-larges a joint in the rock. A mound ofweathered material, COOMBE ROCK, mayslide down the hill and accumulate at thebottom of the slope.

coombe rock (head) A structureless de-posit consisting of unweathered flintswithin a matrix of chalk mud and disinte-grated chalk, resulting from SOLIFLUCTION

movements during the PLEISTOCENE Epoch.The name is especially applied to the de-posits found in the chalk areas of S Eng-land.

coordinate A linear or angular quantityin a frame of reference such as a grid, bywhich positions of points on the map aredefined.

copper pyrites See chalcopyrite.

coprolite /kop-rŏ-lÿt/ The fossilized fecesof an animal. Coprolites may contain otherfossils, such as fragments of other animals,leaves, or pollen, that formed the diet ofthe creature concerned and they thus pro-vide valuable information for paleoecol-ogy.

coral One of a group of animals belong-ing to the cnidarian class ANTHOZOA. Theysecrete a hard calcareous skeleton and areimportant geologically in BIOSTRATIGRAPHY

and because of their reef-building activi-ties. See coral reef.

coral bleaching A condition that occursin reef corals when the symbiotic unicellu-lar algae (zooxanthallae), which give thecoral its color, are expelled from the coralpolyp hosts leaving the coral with ableached white appearance. Coral bleach-ing is caused by stresses induced bychanges in environmental conditions suchas elevated sea temperatures. The coralusually recovers under cooler conditionsbut if too many algae are lost it may die.Widespread bleaching events occurred onthe GREAT BARRIER REEF off Australia in1998 and 2002 (when temperatures over

83

coral bleaching

much of the reef were more than 2°C abovenormal for more than two months).

coral reef A ridge of CORAL that formsoffshore in mainly tropical regions. Forcoral animals to survive, the water shouldbe shallow (allowing the penetration ofsunlight) and temperatures should beabove 21°C. The reef may be partly ex-posed at low tide but is always covered athigh tide. Coral reefs are often very steep-sided. Close to the shore they are termedFRINGING REEFS; BARRIER REEFS form fartherout to sea. See also reef.

corange line A line drawn on certainhydrographic charts joining points of equaltidal range.

cordierite An orthorhombic mineral,bluish in color and of compositionAl3(Mg,Fe2+)2Si5AlO18. The basal sectionof cordierite has a hexagonal outline andoften shows concentric twinning. The min-eral is usually found in thermally meta-morphosed argillaceous rocks, where it isassociated with andalusite and biotite. Italso occurs in some high-grade gneissesand rarely in gabbro that has been contam-inated by argillaceous sediments.

cordillera A chain of mountains, suchas the parallel chains of the Andes in SouthAmerica or the Rockies, Sierra Nevada,and Coastal Ranges of North America.

core The central sphere within theEarth, separated into inner and outer units.The INNER CORE has the properties of asolid, whereas the OUTER CORE prevents thepassage of S waves, suggesting that it is aliquid. It is composed of a mixture of ironand nickel, together with some dissolvedsilicon and sulfur, and is separated fromthe mantle above at a depth of 2900 km be-neath the Earth’s surface by the GUTENBERG

DISCONTINUITY.

coreless winter (kernlose pattern) InAntarctica, the condition in which surfacetemperatures remain almost the same fromthe beginning of winter to the end. Tem-peratures decrease toward winter, but by

the end of the long winter night, tempera-tures are little lower than they were at itscommencement. It represents a balance be-tween surface radiational cooling and at-mospheric counterradiation from abovethe intense surface inversion. Some theo-ries suggest that advection of warmer mar-itime air may play a part in the corelesseffect, but this is disputed.

core sampling A method of obtainingsamples of rock, sediments, soil, snow, orice by drilling a hollow tube into the ma-terial and extracting a reasonably undis-turbed core sample. The core samples aresliced into thin sections for analysis. Coresampling can be used underwater to obtainsamples from the bed of a stream, lake, orsea. A wide range of coring devices havebeen developed. Gravity cores drop undertheir own weight and drive themselves intothe bed. Piston cores are placed on the bedand forced into it to retrieve a sample. Inthe case of hard-packed sand a vibro-corermay be used, the vibrator operating afterthe coring apparatus has reached the bed.

corestone /kor-stohn/ A rounded hardblock of rock within a matrix of soft de-composed material, occurring in well-jointed rocks, notably granites. When thejointed rocks are subjected to extensiveweathering, it proceeds more rapidly in thejoint areas, where water can penetrate, re-sulting in a series of corestones, but contin-ued weathering will bring about theireventual disappearance. Individual core-stones, however, may be especially resis-tant to weathering and these will remain asisolated unweathered blocks within a massof incoherent weathered rock.

Coriolis effect /kô-ree-oh-lis/ A hypo-thetical ‘force’ that is required to relatemovement in the atmosphere to the rotat-ing coordinate system of the Earth’s sur-face. For example, if a projectile waslaunched from the North Pole with a con-stant southward trajectory, from space itwould appear to maintain this track. How-ever, as the Earth is rotating from west toeast, to an observer on the ground surfaceit would appear to be constantly deviating

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84

to the right (see diagram). In Newtonianmechanics, a body will maintain its courseunless acted on by other forces, so to ex-plain this deflection the Coriolis ‘force’ isintroduced. Its magnitude is obtained fromthe equation 2ωv sin φ, where ω is the valueof the Earth’s angular velocity, v is air ve-locity, and φ the latitude. It therefore variesfrom zero at the Equator to a maximum atthe poles. Without the Coriolis effect, windflow would be at right angles to the pres-sure gradient, as found in equatorial areas,instead of being parallel to it. It is namedfor the French physicist Gustave-GaspardCoriolis (1792–1843).

cornelian See carnelian.

corona structure The growth of a con-centric mineral zone around a core min-eral. The second mineral may be aREACTION RIM or the result of EPITAXIAL

GROWTH upon the primary mineral.

corrasion /kŏ-ray-zhŏn/ A type of ero-sion involving abrasion of a rock surfaceby small fragments of rock carried along bya river or glacier. See also abrasion.

correlation 1. A statistical techniquethat determines the degree of associationbetween two sets of data. The most com-mon method in climatology is to use theproduct moment correlation coefficientwhose value ranges from +1 for a perfectdirect relationship through zero for no as-

sociation whatsoever to –1 for a perfect in-verse correlation. The data must have anormal distribution for this coefficient, butothers that do not have this constraint areavailable. The product moment coefficientis most frequently used for comparing rain-fall values at different locations, e.g. thewell-known work on Indian monsoonrainfall by the British physicist Sir GilbertThomas Walker (1868–1958), who wasDirector General of the India Meterologi-cal Department, 1903–24. He found signif-icant associations between rainfall in Indiaand sea-level pressure at various locationsaround the world.2. (in stratigraphy) The process of equatingparts of isolated geologic successions, usu-ally in relation to time (see chronotaxis;homotaxis). Strata may be correlated onthe basis of similarities in lithology or bio-stratigraphically on the basis of similarcontained fossils. Other methods employradiometric age determinations or paleo-magnetic information (see paleomagneticcorrelation).

corrie /kô-ree/ See cirque.

corrosion The process of erosion bychemical solution. All common rock-form-ing minerals are to some extent soluble inwater, even quartz, generally considered tobe highly resistant, and so removal in solu-tion is a significant process. In humidareas, corrosion renders limestone one ofthe most easily eroded of rocks, whereas inarid zones, where water is lacking, lime-stone is highly resistant, demonstrating theeffectiveness of corrosion. Rivers drainingareas of organic materials, especially bogs,swamps, and marshes, are rich in organicacids, and these accelerate corrosive ac-tion. See also abrasion; corrasion.

corundum A mineral consisting essen-tially of aluminum oxide, Al2O3, but minoramounts of other ions give rise to a varietyof colors. The blue of sapphire is due to thepresence of iron and titanium whereas thevariety ruby contains chromium. Corun-dum occurs in some silica-poor igneousrocks such as nepheline-syenites. It is alsofound in hornfels.

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corundum

path of projectileviewed from space

North Pole

path of projectileviewed from groundsurface

direction of Earth’s rotation

Coriolis effect

cotidal line /koh-tÿ-dăl/ A line drawnon certain hydrographic charts joiningpoints at which average high water (or lowwater) occur simultaneously. These linesare often given as differences from thetimes of high (or low) water at a ‘standardport’, or they may be expressed as time in-tervals after the time of the moon’s transit.A cotidal chart depicts both CORANGE LINES

and cotidal lines.

coulee /koo-lee/ A series of deep branch-ing channels or gorges that are formed asMELTWATER is suddenly released from aGLACIAL LAKE dammed by ice at the foot ofa glacier, or from glacial ice that has beenmelted by volcanic activity. Well-knowncoulees occur on the Columbia plateau inWashington, USA, e.g. Grand Coulee.

coulee flow A steep-fronted and com-paratively short flow of viscous lava. Itmay be up to 100 m in thickness.

country rock The preexisting envelopeof rocks into which an igneous magma isintruded. The country rock often showsthe marked thermal effects of CONTACT

METAMORPHISM. Xenoliths contained in thesolidified magma are usually derived fromthe country rock.

covellite /koh-vel-ÿt/ (covelline) An in-digo-blue iridescent mineral form of cop-per sulfide, CuS. It usually occurs ascompact or slaty masses in metasomaticzones or hydrothermal veins, associatedwith other copper minerals.

crag A rocky outcrop on a hillside, char-acterized by very steep edges. Most cragsoccur in hard-rock regions, resulting fromerosion and weathering.

crag and tail A glacial erosion featurefound in areas of relatively low relief,formed where a knob of particularly resis-tant rock exists on the floor of a valley be-neath a glacier. This knob will causedeflection of the ice, protecting any less re-sistant rock behind it. The knob, which isinvariably striated and fluted on the stossend (facing the oncoming ice) is known as

the crag, while the protected rock behind,which may also receive a covering of glac-ier-derived material, is known as the tail.Together they form a semistreamlinedform. These features become visible onlyon the disappearance of the ice.

crater A large bowl-shaped depressionin the Earth’s surface, resulting from vol-canic activity, out-gassing, or the impact ofa meteorite.

crater lake A lake that forms in a vol-canic crater or CALDERA. The water comesfrom direct PRECIPITATION and runoff fromthe crater sides. The water may be forciblyejected if the volcano becomes active.

craton /kray-tŏn/ Any of those parts ofthe Earth’s crust that have been relativelystable (i.e. unaffected by orogenic activity)during the last 1000 million years, e.g. theZambia nucleus of Africa. Frequently theyrepresent the roots of deeply eroded an-cient mountain chains.

creep The slow movement of soil androck debris down gentle slopes under theinfluence of gravity: it is the slowest of allthe types of MASS MOVEMENT, and it is notcertain whether it is a flow, which is move-ment by internal deformation and over-turning, or a slide whereby the wholemantle slips over the bedrock beneath.Current evidence rather supports the latterexplanation. Creep occurs in all environ-ments but is most important in the mar-itime periglacial areas, where it is of a typeknown as SOLIFLUCTION. It is said to occuron all nonvegetated slopes of more than 5°,but in the areas subject to solifluction it canoccur on much gentler slopes.

cotidal line

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protectedrock

boulderclay

direction ofice movement

cragcragcragtailtailtailst

riatio

ns

Crag and tail

Evidence of creep is commonly seenwhen structures such as tombstones andtelephone poles lean upslope, as the ma-terial in which they stand has moveddownslope. Many processes are involved,including freeze-thaw, wetting and drying,needle ice, and the preferential filling ofcavities from upslope. It is suggested thatcreep is the dominant process in shapingthe convex parts of slopes. See also slopeconvexity.

crenulation cleavage See strain-slipcleavage.

crepuscular ray An optical phenome-non of the lower atmosphere produced bythe alternations of sunlight and shadow ra-diating from the Sun, made visible by haze.The shadowing may be produced by gapsin low-level layer cloud, or by the irregularprofile of convective cloud. A similar effectcan sometimes be seen at sunset when theSun is just below the horizon and distinctrays are seen radiating from it, the darkbands indicating parts cut off from theSun’s rays by clouds or hills.

crest The highest point of an ANTICLINE.A line joining the highest points of a givenbed is the crest line; a plane including allthe crest lines of successive folds is the crestsurface.

Cretaceous /krĕ-tay-shŭs/ The final pe-riod of the MESOZOIC Era. Beginning about135 million years ago and lasting about 70million years, it followed the JURASSIC andpreceded the PALEOGENE (which markedthe start of the Cenozoic Era). The namederives from creta, the Latin word forchalk, the characteristic rock of the period;extensive deposits form the white cliffsalong the English Channel. The lower partof the Cretaceous System consists of sixstages: the Berriasian, Valanginian, Hau-terivian, Barremian, Aptian, and Albian.The six stages forming the Upper Creta-ceous are the Cenomanian, Turonian, Co-niacian, Santonian, Campanian, andMaastrichtian.

During the period South America beganto move westward away from Africa; India

was moving north, and Australia andAntarctica were also breaking away. NorthAmerica was moving away from Eurasia,and by the end of the Cretaceous the At-lantic Ocean extended as far as the ArcticOcean. Sea levels were higher than at anyother time in the Earth’s history. Creta-ceous rocks are widespread, the Early Cre-taceous characteristically deltaic andlacustrine, and the Late Cretaceous includ-ing marine deposits, such as sandstone andthe characteristic chalk, a pure fine-grainedwhite limestone formed largely of plank-tonic COCCOLITHS. In the seas in which thecoccoliths were deposited, bivalves, am-monites, belemnites, echinoids, and bonyfish also flourished. On land angiospermplants appeared, and dinosaurs and otherreptiles reached their peak of development.At the end of the period the ammonites andmany other invertebrate groups and mostof the reptiles became extinct. See K/Tboundary event.

crevasse A crack of variable width in thesurface ice of a glacier, caused by shearstresses set up by differential movementswithin the ice. These movements occur in aVALLEY GLACIER, the edges of which movemore slowly than the center, owing to thefrictional effect of the valley sides. In thiscase crevasses form pointing up the glacier.Crevasses also occur where the ice movesover a steeper section of ground, in whichcase they are transverse to the valley. De-spite the fact that their depths rarely exceed30 m, crevasses are very important becausethey assist in the downward penetration ofboth rock debris and meltwater.

crinanite /krin-ă-nÿt/ A type of ALKALI

GABBRO.

Crinoidea /kri-noi-drr-ă/ The class ofthe phylum ECHINODERMATA that containsthe sea lilies. They are typically sessile ben-thonic marine animals, consisting of astem, formed of ossicles of calcite, whichsupports a cup (calyx) of calcite plates. Onthe upper side of this is a centrally placedmouth surrounded by five feathery armsthat bear pinnae. These are used in foodcollection. Fossils are known from the

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Crinoidea

Cambrian Period onward and the class isstill in existence today. Crinoids formed animportant group in the Lower Carbonifer-ous and contributed to the formation ofbioherms and rocks. Most became extinctat the end of the Paleozoic. Crinoids havebeen used biostratigraphically as ZONE FOS-SILS.

cristobalite /kris-toh-bă-lÿt/ A white mi-crocrystalline type of silica, SiO2, whichoccurs in cavities in volcanic rocks and insome thermally metamorphosed rocks. Seesilica minerals.

critical angle The angle of incidence ofa light ray which, if exceeded, will lead tototal internal reflection.

critical distance (in seismic refractionsurveying) The distance between the sourceof seismic waves and that point in an upperrock horizon at which the arrival time of adirect wave is matched by the arrival timeof a higher-velocity wave that has been re-fracted from a lower horizon.

Cromwell Current See equatorial cur-rent.

cross-bedding (cross-stratification) Thedevelopment of internal laminationswithin a stratum inclined at an obliqueangle to the main bedding planes, resultingfrom changes in the direction of water orwind currents during deposition. It is mostcommonly developed in sandstones.

Crossopterygii /kro-sop-tĕ-rij-ee-ÿ/ Agroup of bony fish (see Osteichthyes)whose sole present-day representative isthe coelacanth. The group is important forhaving given rise to the AMPHIBIA and henceto all terrestrial vertebrates. Unlike mostmodern fish, the Crossopterygii had fleshypaired fins. They were common from theDevonian to the end of the Paleozoic.Compare Actinopterygii.

cross profile A transverse section of ariver’s channel or valley. A stream can ad-just these parameters as well as its longprofile in response to environmental

changes. Valley cross profile can showsuch features as valley-in-valley forms,river terraces, extent of floodplains, sym-metry or asymmetry of the valley-sideslopes, angle of valley-side slopes, etc., allof which contain clues to the geomorphichistory of the river and the region throughwhich it flows. See also long profile.

crude oil Petroleum as it occurs in theground. It consists of a mixture of impurehydrocarbons which, after purification,can be separated to yield various fuels andcompounds used by the petrochemical in-dustry.

crust The outermost shell of the Earth,varying in thickness from 5 km under theoceans to 60 km under mountain ranges.The lower boundary is marked by the Mo-horovičić discontinuity. The crust is com-posed of two units, the CONTINENTAL CRUST

(also known by the acronym SIAL) and theOCEANIC CRUST (SIMA).

Crustacea /krus-tay-shă/ A class of in-vertebrate animals of the phylum ARTHRO-PODA, characterized by the presence of twopairs of antennae and one pair ofmandibles. Crustaceans are almost exclu-sively aquatic and range in size from thelarge lobsters to almost microscopic forms.Included in the six subclasses are the OS-TRACODA, important geologically in strati-graphic correlation, the Malacostraca(crabs and lobsters), and the Cirrepedia(barnacles). Fossil crustaceans, mostly os-tracods, are known from the CAMBRIAN Pe-riod onward.

cryolite /krÿ-ŏ-lÿt/ (Greenland spar) Amineral form of sodium aluminum fluo-ride, Na3AlF6. Rarely crystalline, it occursas colorless or white masses. It is used as aflux in the electrolytic extraction of alu-minum and in making various ceramics.

cryoturbation /krÿ-oh-ter-bay-shŏn/ Themodification of the soil through theprocesses of CONGELITURBATION and the ef-fects of NEEDLE ICE in a periglacial environ-ment.

cristobalite

88

cryptic layering See layered igneousrock.

cryptocrystalline /krip-toh-kris-tă-lin, -lÿn/ Describing material that is crys-talline yet so fine-grained that individualcrystals cannot be resolved even under themicroscope.

crystal A chemical substance with a def-inite geometrical form, having plane facesat regular angles to each other, which is theexpression of the regular arrangement ofatoms or ions composing the substance.See crystal symmetry; crystal system.

crystal axes Imaginary lines that passthrough the center of a CRYSTAL. There aregenerally three axes, but four in the hexag-onal system. The angles between the axesand their relative lengths characterize thestructure of any crystal. See crystallo-graphic index.

crystal form A complete set of regularplanes forming a crystal. The diagram il-lustrates different forms found in the cubicCRYSTAL SYSTEM: A and B are simple crystalforms and C is a combination of the twoforms cube and octahedron. The form ex-hibited by a crystalline mineral may be agreat aid to identification.

crystal habit The shape of a crystal aris-ing from the shape and size of faces and thedevelopment of different forms. In the dia-gram the two crystals possess the samecombination of crystal forms but A has aprismatic habit and B a pyramidal habit

owing to the relative development of prismand pyramid faces.

crystalline rock /kris-tă-lin, -lÿn/ Rockformed by the process of crystallization,i.e. metamorphic and igneous rock.

crystallite /kris-tă-lÿt/ A microscopicembryonic crystal found in glassy rocks asa result of DEVITRIFICATION. Crystalliteshave rodlike, stellate, or feathery forms.

crystallographic index /kris-tă-lŏ-graf-ik/ A crystal face may be indexed accord-ing to a notation that depends upon theintercepts made by the face with the crys-tallographic axes. The distances along theaxes from the origin to the points wherethe plane cuts are termed the intercepts.The ratios of the intercepts (referred to astandard parametral plane or axial ratio)are termed the parameters of that crystalface. According to the system of Miller, thereciprocals of the parameters of a crystalface are called the indices.

crystal symmetry The crystal faces ofminerals are symmetrically arranged andreflect the internal regularity of the atomicstructure. Crystal symmetry is described byreference to the following elements: 1.plane of symmetry – a plane along which acrystal may be cut into two equal halves,one being the mirror image of the other(fig. 1). 2. axis of symmetry – a line aboutwhich a crystal may be rotated through360° so that the crystal assumes a congru-ent position every 180°, 120°, 90° or 60°.

89

crystal symmetry

A B C

100 100

III

III

I

Crystal form

A

B

Crystal habit

Such axes are termed diad (2-fold), triad(3-fold), tetrad (4-fold), and hexad (6-fold)axes respectively (fig. 2). 3. center of sym-metry – a central point about which everyface and edge of a crystal is matched by oneparallel to it on the opposite side of thecrystal (fig. 3).

On the basis of these symmetry ele-ments, 32 different crystal classes are rec-ognized together with 7 CRYSTAL SYSTEMS:

cubic – 4 triad axestetragonal – 1 tetrad axishexagonal – 1 hexad axistrigonal – 1 triad axisorthorhombic – 3 diad axesmonoclinic – 1 diad axistriclinic – no axes.

Other symmetry elements may be pos-sessed by crystals in addition to thoselisted. For example, a cube has three

crystal system

90

•

planes ofsymmetry

center ofsymmetry

Fig. 3

Fig. 2Fig. 1

hexad axis ofrotation symmetry

Crystal symmetry

c

β

monoclinic

b

a

αβ

triclinic

γ

c

ab

tetragonal

c

a2

a1

a3

a2

cubic

c

ba

orthorhombic

c

a2

a1

a3

120°

hexagonal andtrigonal

a1

Crystal systems

tetrads, four triads, six diads, nine symme-try planes, and one center of symmetry.

crystal system A category of crystalwith reference to the position of the crystalfaces in relation to the intercepts that theplanes containing the faces make withthree (or four) axes, which intersect at anorigin. All crystals of the 32 different sym-metry classes can be referred to seven crys-tal systems as follows (see diagram): 1. cubic (isometric) – 3 orthogonal tetrad

axes of equal length, a1, a2, a3. 2. tetragonal – 3 orthogonal axes, 2 hori-

zontal diads of equal length, and onevertical tetrad, a1, a2, c.

3. orthorhombic – 3 orthogonal diad axesof unequal lengths, a, b, c.

4. hexagonal – 4 axes, 3 horizontal diadsof equal length at 120° apart and onevertical hexad at right angles, a1, a2, a3,c.

5. trigonal – 4 axes, 3 horizontal diads ofequal length at 120° apart and one ver-tical triad at right angles, a1, a2, a3, c.

6. monoclinic – 3 unequal axes, one verti-cal, one horizontal diad, and a thirdmaking an oblique angle with the planecontaining the other two, a, b, c.

7. triclinic – 3 unequal axes, none at rightangles, a, b, c.

Different CRYSTAL FORMS can be re-ferred to the same set of crystal axes andhence belong to the same crystal system.For example the cube, rhombdodecahe-dron, and octahedron are different crystalforms of the same cubic system.

crystal tuff A type of tuff that containsmany broken crystals. See pyroclastic rock.

crystal zoning Many crystals, especiallyin igneous rocks, are zoned such that theircomposition changes systematically andgradually from the core of the crystal to therim. The succession of zones reflectschanges in the composition of the melt inwhich the crystal is growing or changes inthe physical conditions as crystallizationproceeds.

cubic (isometric) See crystal system.

cuesta /kwess-tă/ An asymmetrical ridgeproduced by differential erosion of gentlydipping strata. A cuesta is characterized byits steep scarp face, well-defined crest, andgentle backslope, which largely conformsto the dip of the strata. It is constructed ofa more resistant geology (e.g. sandstone orlimestone) than the etched out foot (e.g.clay).

culmination The highest point on astructural feature. As the axis of a foldoften undulates up and down, a series ofculminations separated by depressions candevelop. (See diagram at FOLD.)

cummingtonite /kum-ing-tŏ-nÿt/ Amonoclinic type of AMPHIBOLE.

cumulate rock A type of igneous rock

that is formed largely by the accumulationof early-formed PRIMARY MINERALS thatcrystallize from a magma and sink underthe influence of gravity to settle in layers onthe floor of the magma chamber. Often thecumulus crystals trap small quantities ofliquid in the interstices and this liquid sub-sequently crystallizes to produce intercum-ulus minerals. Cumulate rocks may befound in large basic layered intrusions,often of lopolithic form. Monomineralicand bimineralic rocks are characteristicand cumulate dunites, peridotites, pyrox-enites, anorthosites, and gabbros com-monly occur.

cumulonimbus /kjoo-myŭ-loh-nim-bŭs/The main rain cloud of convective origin. It

is heavy and dense with considerable verti-cal extent to give the appearance of a vastcauliflower, each protuberance being atower of rapidly rising saturated air withvelocities of up to 30 meters per second. Atthe top of the cloud there is often a more fi-brous flattened cloud composed of ice crys-tals: this is the ANVIL. Most thunderstormsare associated with this type of cloud.

cumulophyric /kjoo-myŭ-loh-fi-rik/ Seeglomerophyric.

cumulostratus /kjoo-myŭ-loh-strat-ŭs/See stratocumulus.

cumulus /kjoo-myŭ-lŭs/ A type of cloudindicating convectional activity extendingabove the CONDENSATION LEVEL. Cumulusclouds range in vertical extent from shal-low fair-weather cumulus clouds indicat-ing weak convection topped by aninversion, to the much more extensive cu-mulus congestus, which almost approachesa cumulonimbus cloud in size.

cup anemometer See anemometer.

cupola /kjoo-pŏ-lă/ A slender upwardprojection from a large igneous body intothe overlying country rock. See also roofpendant.

cuprite /kjoo-prÿt/ (ruby copper) A redto black mineral form of copper oxide,Cu2O. It crystallizes in the cubic systemand occurs in oxidized copper deposits,sometimes associated with limonite. It isused as a source of copper.

Curie point /kyoo-ree, kyoo-ree/ Thetemperature above which permanent mag-netism disappears. Each element has itsown specific Curie point; in the case of ironit is 760°C. It is named for the Frenchphysicist Pierre Curie (1859–1906).

current (in oceanography) A horizontalflow of water, the movement affecting thewhole water column or only a part or partsof it. The speed at which currents flow, andtheir direction, may vary markedly, or thecurrent velocity may be remarkably con-

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cupola

nonresistant bed,nonresistant bed,nonresistant bed,typically claytypically claytypically clay

resistant bedresistant bedresistant bed(e.g. sandstone(e.g. sandstone(e.g. sandstoneor limestone)or limestone)or limestone)

backslope(mainly dipslopeof resistant bed)

crest

scarp

Cuesta

stant in a temporal sense. Often variationsare seasonal, or they may be due to chang-ing meteorological factors. Permanent cur-rents operate independently of the tidesand weather, although they are indirectlyaffected by these. These include the generalcirculatory current systems in the oceans.Other currents flow on account of fresh-water discharge from rivers, the action ofwaves and tidal motion, differences of seawater density, and wind-drag. All orcertain of these currents may be superim-posed, one upon the other. See also densitycurrent; ocean current.

current bedding A sedimentary struc-ture resulting from the action of eitherwind or water currents. It includes CROSS-BEDDING and RIPPLE BEDDING.

cusp A crescent-shaped mass of beachmaterial, which may range from sand toquite large shingle or cobbles. The coarsermaterial accumulates on the promontoriesor horns between the bays and the finermaterial in the bays. Cusps are regularlyspaced and generally display coarser ma-terial than is found over the remainder ofthe beach surface. Those formed purely insandy material are less common than thosedeveloped in shingle or in sand and shinglemixtures. They point down the beach to-ward the sea and the margin betweencoarser and finer material has a scallopedshape. They vary greatly in size, rangingbetween several centimeters in height togiant cusps or megacusps, such as thosefound on parts of the coast of West Africa.It seems that there exists a relationship be-tween cusp spacing and swash length, andthat a type of cellular water flow togetherwith swash periodicity are important addi-tional factors in their development, buttheir origin has yet to be explained satis-factorily. Waves breaking perpendicular tothe beach are thought to be the most con-ducive to cusp formation.

cuspate delta /kuss-payt/ A DELTA

within which material is evenly depositedon either side of the river mouth. It is usu-ally found on straight coasts where waveaction is fairly strong.

cuspate foreland An approximately tri-angular accumulation of beach materials,usually shingle, the apex of which extendsout to sea and produces an irregularity inthe coastal outline. Such forelands are cre-ated as a result of the combination of SPITS

or BEACH RIDGES approaching each otherfrom opposite directions, owing to the ac-tion of two major wave sets in the area,each being more active on one side of theforeland. The growth of these features canoften be traced by the existence of manyparallel shingle ridges. The maintenance ofa sharp projection out to sea largely de-pends upon the presence of a nearby is-land or coastline, which provides shelterfrom direct frontal attack by destructivewaves.

cutoff See oxbow lake.

cut-off high A warm anticyclone thathas moved poleward from the main sub-tropical high-pressure belt, often produc-ing BLOCKING in the westerlies. This systemusually forms as a ridge in the upper west-erly circulation and this intensifies into acellular form becoming detached from themain flow like an oxbow lake (or cutoff) ina river. Its influence then extends down-ward toward the surface. (See diagram.)

cut-off low A low-pressure system ofsimilar origin to the CUT-OFF HIGH but ex-tending toward the Equator after develop-ing from a trough in the upper westerlycirculation. Cut-off lows frequently driftinto the Mediterranean Sea area after cut-off has taken place farther north.

cwm /kûm/ See cirque.

cyanite See kyanite.

cycle of erosion A concept explainingthe evolution of dissected land surfacesfrom uplifted areas of little relief to dis-sected landscapes and then to level surfaces(peneplains) via a series of stages (youth,maturity, and old age). The process is saidto be cyclic because it will begin again onfurther uplift (see rejuvenation). The con-cept of strict erosional cycles is not now ad-

cuprite

92

hered to, because climatic changes meanthat processes in any one area are not con-stant, and most landscapes have evidenceof evolution under a series of cycles orparts of cycles of different climatic types asfollows:AridThe key to the cycle of arid erosion is theprocess of PEDIMENT formation. The Ger-man geologist Walther Penck and SouthAfrican geologist Lester King developedthis idea, and as a result arid lands are saidto evolve by a process of slope retreat (seeparallel retreat) not decline, such that thetrend is not the increase of relief to a max-imum in maturity and thereafter increas-ingly subdued relief but decrease of thearea of plateau and increase of the area ofpediment. In youth the rivers incise them-selves, and firstly retreat produces valley-side pediments; by maturity the divideshave shrunk and the initial topography isnearly lost; by old age retreat of slopes hasreached the point where divides are lost al-together, and pediments of individualbasins coalesce to give the ultimatePEDIPLAIN. See also river.CoastalThis cycle, evolved by Douglas Johnson in1919, differs on emergent (recently up-lifted) and submergent (recently drowned)coasts. On both types, the general patternis a simplification of coastal outline. In theemergent case, the coastline will be initiallystraight, with a shallow offshore slope.

Waves break offshore and build up a bar-rier island, with a lagoon behind. As theoffshore profile is deepened, the island isattacked by larger waves which push it on-shore, migrating over the lagoon, until atmaturity the barrier is coterminous withthe original coast. It then continues to mi-grate, eroding the shore, until at theoreticalold age a plain of marine planation is cre-ated. On a submergent coast, the initialoutline is very complex, as existingdrainage patterns are drowned. The head-lands are subject to concentrated attack byrefraction of waves, and as they are cutback spits begin to build out across thebays, eventually cutting them off and lead-ing to their infill with waste. At submatu-rity the headlands are cut back and thebays closed; continued erosion pushes thecoast to maturity, at which point the shore-line lies behind the heads of the initialdrowned valleys. Thereafter the course isas in the emergent case.GlacialThe major process involved is the creationof cirques and the retreat of headwalls, de-stroying the preglacial landscape. Initiationinvolves the collection of snow fields andthe creation of the cirques by nivation.Once created, headwall retreat eats intothe original surface, eventually leavingonly horned peaks and arêtes. Such fea-tures typify the passage from youth to ma-turity. By full maturity the main regionalvalleys are filled with ice, only the highestpeaks projecting above the ice surface asNUNATAKS. The retreat of the headwall ofthe glacial valleys extending from thecirques eventually exceeds the rate of re-treat of cirque headwalls, and the mergingof the two marks the end of maturity. Theold age phase has never been observed: aswith the coastal cycle, only the phases ofyouth and maturity are well defined.Davisian cycle (Normal cycle)Developed by the American geographerWilliam Morris Davis (1850–1934) in thelatter part of the nineteenth century, thiswas an attempt to devise an orderly se-quence for the evolution of landscapesunder a humid temperate climate. The se-quence involved is as follows: 1. Uplift ini-tiates the stage of youth, when rivers begin

93

cycle of erosion

Fig. 1 Fig. 2

Fig. 3 Fig. 4

low

lowlowlow

highhighhighhighhighhigh

Cut-off high and low

to incise themselves into the landscape.Steep V-shaped valleys are separated bybroad flat interfluves. 2. Maturity of thelandscape, when widening of the river val-leys has completely destroyed the originallandscape and valleys meet at sharp inter-fluves. In the rivers, this stage occurs whenthe valleys are graded, and down-cutting isreplaced mostly by side-cutting and flood-plain creation. 3. By old age the landscapeis subdued owing to SLOPE DECLINE. Thestreams meander widely on broad flood-plains, losing their adaptation to structure,a feature of maturity, due to its maskingbeneath debris. Only MONADNOCKS onrocks of great resistance in high divides riseabove a peneplain.PeriglacialThis scheme revolves around the role offrost shattering and solifluction. Originat-ing on a dissected landscape, frost shat-tering attacks the valley-side slopes,producing a frost-shattered cliff, which mi-grates into the divide. The debris from thefrost shattering is transported downslopeby solifluction and meltwater to the val-leys. As this process continues, the frost-shattered cliffs retreat until no solid rockremains. The scree slopes then dominatethe landscape, being progressively flat-tened until by old age a landscape of faintrelief is formed.

cyclogenesis /sÿ-kloh-jen-ĕ-sis/ The ini-tiation or development of a depression orcyclone.

cycloidal wave /sÿ-kloi-dăl/ (trochoidalwave) A wave with a flatter trough and asharper crest than the typically smooth si-nusoidal wave. It is a steep symmetricalwave with a crest angle of some 120°. Thewave form is that of a cycloid or trochoid,i.e. a curve that would be described by apoint rotating within a circle that itself wasbeing rolled along a straight path. With anincrease in wave steepness, the cycloidalwave sharpens its crest and increases itsasymmetry. Because the flow is of a rota-tional type, no mass transport is possible(see mass transport current).

cyclolysis /sÿ-klol-ă-sis/ The process ofdecay or weakening of the cyclonic circula-tion around a low-pressure center.

cyclone An area of low pressure with aseries of closed isobars, usually of circularor oval form, around its center. In the Nhemisphere, it is surrounded by a counter-clockwise wind circulation and in the Shemisphere by a clockwise rotation, butboth are known as cyclonic circulation foreach respective hemisphere. Compare anti-cyclone.

In mid- and high-latitudes, the cycloneis usually referred to as a DEPRESSION. Intropical areas, it is a storm system of greatintensity with wind stronger than 64 knots(120 km per hour) and is synonymous witha hurricane or typhoon, although thenomenclature is not fully standardized. Seetropical cyclone.

cyclonic precipitation See frontal pre-cipitation.

cyclostrophic winds /sÿ-kloh-stroff-ik/A class of winds in which there is extremelystrong curvature of the airflow, such as atropical cyclone or tornado. Under theseconditions, the centripetal acceleration be-comes the major control of the gradientwind.

cyclothem /sÿ-klŏ-th’em/ A series ofsedimentary beds deposited as part of a sin-gle cycle. The term is generally applied toCarboniferous strata, where the sequencebegins with a sandstone layer, followed byshale and freshwater limestone. Above thelimestone is a clay layer, above which restsa coal seam. Following the coal there is areturn to marine conditions, with the de-position of a marine shale, followed by amarine limestone. Each cyclothem is sepa-rated from the next by a disconformity.The cycle represents an episode of emer-gence and erosion.

cylindrical equal-area projection SeeLambert’s cylindrical equal-area projec-tion.

cyclogenesis

94

cylindrical projection A MAP PROJEC-TION constructed as though a cylinder wereplaced around or cutting through theEarth, onto which the relevant details areprojected (see diagram), meridians andparallels being drawn as straight parallellines. There are three basic types: the sim-ple cylindrical, LAMBERT’S CYLINDRICAL

EQUAL-AREA PROJECTION, and the MERCA-TOR PROJECTION. In the simple cylindrical,the cylinder touches the Earth at the Equa-tor, which has the effect of making thepoles the same length as the Equator andtherefore causes considerable distortion inthe higher latitudes. The scale along themeridians is correct, but directions are nottrue. It is a projection that is seldom usedowing to the existence of more practicalmodified versions.

95

cylindrical projection

EquatorEquatorEquator

Cylindrical projection

96

dacite /day-sÿt/ A dark igneous rockconsisting mainly of quartz and plagio-clase. See rhyolite.

darcy /dar-see/ A unit used to measurethe porosity of a rock (the permeability co-efficient). It represents the resistance toflow through the rock of a fluid (gas or liq-uid). The usual practical unit is the milli-darcy (md).

Darcy’s law The rate of flow of a fluidthrough a porous material (suck as a rock)is proportional to the product of the per-meability of the material and the pressurecausing the flow. The law is named for theengineer and French scientist Henry Philib-ert Gaspard Darcy (1803–58).

datolite /day-tŏ-lÿt/ A mineral form of hydrated calcium borosilicate, CaB-SiO4(OH). It crystallizes in the monoclinicsystem as colorless or white prismatic crys-tals. It occurs in veins and in AMYGDALES.

datum /day-tŭm/ A fixed referencepoint. The vertical datum is a horizontalline or a point used as the origin (zero)from which heights and depths are mea-sured. On most maps and charts it is thatof mean sea level, e.g. in Britain the Ord-nance Datum (O.D.) of Ordnance Surveymaps is mean sea level at Newlyn, Corn-wall, calculated from tidal observations be-tween 1915 and 1921. Horizontal datum isgenerally used in connection with geodeticinformation where it forms the basis forhorizontal control. Horizontal datum isdetermined, using the reference spheroid,from bearings (azimuths) and longitudeand latitude readings.

daughter element An element thatforms as a result of the radioactive decay ofanother element (e.g. radon is a daughterelement of radium, which is in turn adaughter element of thorium).

Davisian cycle See cycle or erosion.

day Astronomically, a solar day is theperiod of time between successive occa-sions on which the Sun is in the meridian ofany fixed place; the sidereal day is the timebetween successive transits of a fixed dis-tant star. As the Earth varies in its rate ofmovement during its orbit round the Sun,the solar day is not constant. In Britain theclimatological day extends for a 24 hourperiod from 9 a.m. GMT. This is the timeat which most climatological observationsare made and so rainfall for April 21, forexample, would in reality be the amountthat fell between 9 a.m. April 21 to 9 a.m.April 22.

death assemblage See thanatocoenosis.

debris An accumulation on the surfaceof soil or rock fragments. It may resultfrom glacial action (and is found when theice melts) or from processes (such as frostaction) that cause rock to break into frag-ments.

debris load That part of a river’s totalload carried as solid material. It includesmaterial moving by suspension whollyabove the bed, by SALTATION, and byrolling and sliding along the bed. The totaldebris load is the sediment discharge of ariver. See also load.

decalcification /dee-kal-să-fă-kay-shŏn/

D

(in soil science) The removal of calciumions from a soil. The process involved iscarbonation, whereby rainwater percolat-ing through humus becomes enriched withcarbon dioxide, forming an acid, whichcombines with calcium carbonate in thesoil and carries it away in solution as cal-cium bicarbonate. It operates best at lowtemperatures, because the solubility of cal-cium carbonate decreases as temperaturerises. If the content of carbon dioxide in therainwater falls, then its ability to dissolvecalcium also falls. Decalcification is said bysome pedologists to precede LESSIVAGE andPODZOLIZATION: in a given soil it influencesthe upper profile first, and so there is apoint at increasing depth beyond which de-calcification has not proceeded, called theacid point.

deciduous forest Areas occupied bytrees and shrubs that periodically shedtheir leaves, usually in winter or during thedry season. Deciduous trees are also calledbroadleaved trees.

decile /dess-ăl/ (in statistics) A tenth partof a series of values that are ranked inorder of magnitude, normally quoted asthe first or second decile, etc. If a series ofvalues are ranked in order of magnitudethey can be subdivided into any number ofgroups depending upon the amount ofdata.

declination See angle of declination.

décollement /daykolmawn/ The planemarking the boundary between two differ-ent types of deformation. The rocks aboveare generally deformed, whereas thosebelow may be unaffected. It is caused bythe upper rock series sliding over the lowerduring folding.

decomposer (in ecology) Any organismthat breaks down dead plants and dead an-imals or their excreta. They are importantin the FOOD CHAIN because they set freesuch substances as nitrogen compounds,which pass into the soil or atmosphere.Bacteria and fungi are the main decom-posers, but some invertebrate animals

(such as earthworms) also fulfill this func-tion.

decomposition The weakening andbreak-up of a rock mass through CHEMICAL

WEATHERING. A decomposed rock will lackits former cohesion owing to the washingaway of cementing materials or the alter-ation of the constituent minerals.

deep (abyss) A depression in the floor ofthe ocean, generally at depths in excess of5000 m.

deep-focus earthquake An earthquakewhose focus is at a depth of more than 300km. Most are found along the BENIOFF

ZONES.

deep ocean One of the fundamental di-visions or zones of the Earth’s surface. Thetotal volume of deep oceans far exceeds thetotal volume of land lying above sea level.Also, the oceans are, in general, markedlydeeper than the continents are high. ThePacific Ocean is the largest ocean, occupy-ing, together with its adjacent seas, some50% of the total ocean area, which is 361million sq km. The Atlantic Ocean occu-pies roughly 106 million sq km and the In-dian Ocean 75 million sq km. The averagedepth of the deep oceans is 3800 m, with alarge percentage of the oceans varying indepth between 3000 and 6000 m below sealevel.

deep-sea trench See trench.

deep weathering Weathering at greatdepths, occurring when suitably porous,permeable, or chemically reactive rocks aresubjected to humid tropical or subtropicalweathering in areas where erosion is veryslight. REGOLITHS over 100 m deep havefrequently been reported and others up toand over 300 m are known. There is a tran-sition in all profiles from solid rock (atdepth), through partly weathered rock(containing CORESTONES), to totally weath-ered material that has lost the former struc-tures of the parent material. Evidence ofdeep weathering occurs in areas that arenow totally unsuitable for its development,

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deep weathering

in which cases weathering is assumed tohave occurred during former periods ofwarmer climate.

deflation Wind removal of fine material(silt and clay) from areas of increasedweathering. Typically, a chance depressionin the desert surface increases moistness atthat point, producing increased chemicalweathering. This leads to clay formation.On drying, the clay is susceptible to windaction, and when picked up can be carriedlong distances in suspension, often in theform of sandstorms. This fine material isdropped elsewhere, leaving behind a con-centration of coarse material as a deflationsurface. Deflation surfaces usually take theform of gravel-strewn areas, although thisprocess can also produce large depressions,and sometimes OASES may result. See alsoaeolian erosion.

deforestation /dee-fô-rĕ-stay-shŏn/ Thepermanent removal of trees and shrubs,usually as a result of human activity for ob-taining timber or clearing land for agricul-ture or mining. The exposed soil is easilywashed away by rain or blown away by thewind, resulting in an eroded infertile land-scape and loss of natural habitats. See alsobadlands; desertification.

deformation /dee-for-may-shŏn, deff-er-/The alteration of rock formations thatgenerally results from tectonic plate move-ments. The consequences include compres-sion, extension, faulting, and folding.

deglaciation /dee-glay-see-ay-shŏn/ Theshrinkage of an ice sheet or glacier throughmelting exposing formerly ice-coveredland. The term is applied especially to theprocesses operating during an INTER-GLACIAL period once climatic conditionsbegin to improve and the ice sheets thatmay have developed in the previousGLACIAL PHASE start to shrink. Althoughconsiderable melting takes place at themargins, accumulation can still be main-tained in the source zones, thereby contin-uing the transportation of debris bymoving ice. Most melting takes place dur-ing the summer when the ice margins may

recede toward the source or alternativelythe whole ice mass may become shallower,resulting in the landforms characteristi-cally created by stagnant ice. During thewinter an ice sheet may remain static oreven advance again slightly, causing depo-sition of TERMINAL MORAINES or PUSH

MORAINES. Successive moraines may be dis-tinguished in order to trace the recessionalpattern of the ice sheet. The complete dis-appearance of an ice sheet may take thou-sands of years, and crustal adjustments andsea-level changes may continue long after-ward.

degradation 1. In general, the wearingdown of the land surface by processes ofEROSION.2. The lowering of stream beds over a timeperiod measurable in years. It is not ap-plied to the short-term cut and fill ofstream beds extending over a period of ayear or less, which is an integral part of theequilibrium regime of streams. Degrada-tion can arise through a fall of sea level,causing REJUVENATION, or a change of flowconditions giving the river increased ero-sive power, such as decreased load or in-creased runoff. Evidence of degradationcan be found in river terraces and knick-points. See also aggradation.

degree 1. A unit of measurement alongthe lines of latitude and longitude. One de-gree is equal to l/360 of the Earth’s circum-ference on the lines of longitude. However,a degree of latitude decreases in length to-ward the poles. Each degree is subdividedinto 60 minutes, and each minute into 60seconds.2. A unit of temperature. See also absolutetemperature; Celsius scale; kelvin.

degree-day The basic unit used for AC-CUMULATED TEMPERATURE. It can refer to ei-ther degrees Celsius or Fahrenheit and isthe number of degrees above (or below) aspecified datum during the day.

delayed runoff Rainwater that flowsinto streams (usually from springs) afterhaving first been absorbed into the soil.

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Delay may also be caused if rainwater istemporarily locked up in snow or ice.

delta A large accumulation of sedimentdeposited at the mouth of a river where itdischarges into the sea (or a lake). Deltasare formed as a result of the decrease inload-carrying capacity following the decel-eration of river water on entering the com-paratively static sea (or lake). Fine claymaterial, normally carried in suspension, isalso deposited because the very small par-ticles coagulate and sink in the presence ofsaline water. For the sediment to accumu-late the amount of material deposited bythe river must exceed that removed bycoastal erosional processes. Deltas grow insize because the river tends to bifurcateonce a certain amount of deposition hasoccurred; the smaller streams (distribu-taries) then deposit material over a widerarea. They may become abandoned later,and that part of the delta no longer receiv-ing sediment will become eroded. See alsoarcuate delta; bird’s-foot delta; cuspatedelta; delta deposit.

delta deposit Prior to the formation of aDELTA proper, fine material is deposited onthe seabed at the river mouth. These mater-ials (bottomset beds) are laid down hori-zontally and extend a considerable way outto sea. Above these, and making up themajor volume of the delta, are materials(foreset beds) deposited with an inclinationfrom top to bottom of up to 35°; progres-sive deposits extend farther out to sea andthereby cause enlargement of the delta.These beds are overlain by the topset beds,which are again horizontal and are a sea-ward extension of the river’s alluvial flood-plain.

demersal /di-mer-săl/ Describing a fishthat lives on or near the sea floor. Flatfish,including the flounder, halibut, and sole,are specially adapted for life on the bot-tom, with special forms of camouflage.The other main types of demersal fish arethe round fish, which include haddock,cod, and hake.

demography /di-mog-ră-fee/ The use of

statistics and mathematics to study humanpopulations geographically to determinesuch factors as composition, distribution,and size.

dendrite /den-drÿt/ A fernlike branchingpattern of material deposited within a rockor mineral, often resembling a plant fossil.Most dendrites are black, composed ofsuch minerals as pyrite (iron sulfide) andpyrolusite (manganese dioxide). See mossagate.

dendritic drainage /den-drit-ik/ Thedrainage pattern that develops wherestructural controls of slope, variable lithol-ogy, or fault and joint patterns are absent;as a result the drainage net is entirely ran-dom, with equal probability of stream flowin all directions. It characterizes areas offlat rocks with uniform lithology, notablyplains, plateaus, and massive crystallinerocks. One example is the US Great Plains,notably the Badlands of South Dakota.

dendrochronology /den-droh-krŏ-nol-ŏ-jee/ (tree-ring analysis) The method ofusing the annual growth rings of certaintrees, in order to calculate the year inwhich they formed, for dating purposes. Itis used for dating certain fossils of woodand wood specimens, and for the interpre-tation of past environments through analy-sis of the widths of the tree rings. See alsodendroclimatology.

dendroclimatology The branch of

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Dendritic drainage

DENDROCHRONOLOGY that focuses on theinterpretation of former climates fromchanges in width of annual growth rings ofcertain tree species. By using the bristle-cone pine, a long-lived conifer from thesouthwestern USA, and preserved speci-mens, it is possible to go back as far as ap-proximately 5000 BC. However, the widthof the tree ring does not bear a simple lin-ear relationship with one climatic factor,being related to an amalgam of many, ofwhich temperature and amount of precipi-tation during its growing season are themost important. This limits the reliabilityof the method.

denitrification /dee-nÿ-tră-fă-kay-shŏn/The process by which bacteria in the soilbreak down nitrates to produce nitrogengas, which passes into the atmosphere.Denitrification is an important part of theNITROGEN CYCLE.

density The mass of unit volume of asubstance at a specified temperature andpressure. In the case of the atmosphere,density is not normally measured directlybut is calculated from its relationships withtemperature, pressure, and humiditythrough the GAS LAWS. For dry air at 1000mb and 290 K, density is about 1.2 kg/m3

(see diagram at ATMOSPHERE).

density bottle See pycnometer.

density current A CURRENT caused bydifferences in density. The density of amass of water may become different fromthat of the surrounding water; if it isgreater, the density-driven flow is usually aBOTTOM CURRENT or underflow. On theother hand, the density of a mass of watermay be intermediate between that of thewater above and beneath it, in which casethe density-driven flow may develop as aninterflow. A common cause of density-dri-ven flow arises when relatively light freshwater discharging from a river rides outacross and above denser saline water enter-ing from the sea; in this case, a salinewedge tends to drive upstream along thebed of the river or estuary, often transport-ing appreciable amounts of fine sediment.

density log A subsurface logging tech-nique, which records the variations withdepth in the density of strata in an uncasedborehole.

denudation /dee-new-day-shŏn, den-yû-/The weathering of rocks, the entrain-ment of debris, and its subsequent trans-port and deposition. Denudation is highestin areas of high relief, heat, humidity, steep slopes, and rocks with abundantsediment yield. The highest rates of de-nudation recorded, reaching 3000 mm/1000 years, occur in glaciated areas; lowest rates, about 1.2 mm/1000 years,occur in hot dry lowlands. Compare ero-sion.

denudation chronology The study ofhow a landscape has evolved through timeto its current state, by arranging in se-quence the pieces of evidence discovered,to obtain a series of pictures of the evolu-tion of the relief through a series of stages.For each major stage in the evolution of aregional landscape the following broadgroups of features are researched. Climate,geology, and base level (the independentvariables) dictate the geomorphologicalsystem, consisting of erosion, transport,and deposition. This leads to the creationof landforms, the remains of which consti-tute the basic research material. Partly re-flecting the above factors and in turninfluencing them are the flora and fauna.As flora and fauna reflect so many envi-ronmental variables, especially climate,their study in the form of pollen, insect,and mammalian remains is significant inenvironmental reconstruction. Once thetype of environment is known, inferencecan often be made of the geomorphologicalprocesses operating.

Dating is done by various methods: or-ganic matter of 60 000 years old or less canbe dated by radiocarbon methods, and forgreater time range other methods are avail-able. Paleobotany has been used to estab-lish pollen zones through time, with similarchanges occurring over wide areas allow-ing organic remains to be correlated be-tween regions and assigned to a relativedating system. Pedological methods can be

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used to study superficial deposits of pastclimates still preserved.

deposition The laying down of materialsubsequent to EROSION and TRANSPORT. De-position could be described as the creativepart of the geomorphological system.

In the fluid transport media of sea,rivers, and wind, deposition occurs whenthe forward movement and turbulence inthe transporting medium falls below thesettling velocity of the load, i.e. when thetransporting medium lacks the COMPE-TENCE or CAPACITY to carry the load anyfarther. Low-energy environments aretherefore zones of deposition; in rivers,these occur on the inside of meander bends;on coasts, in bays and estuaries; in an airstream, in the lee of obstructions. All theseenvironments are characterized by lack ofstrong upward eddies and divergentstreamlines of flow.

In the solid transport medium of icemovement, most deposition occurs wherechanges in the climatic regime stop the ad-vance of ice and lead to its stagnation andmelting, rendering it incapable of trans-porting any farther.

depression 1. (in meteorology) A mid-latitude CYCLONE (low) or a weak TROPICAL

CYCLONE (with wind speeds of less thanForce 6 (BEAUFORT SCALE)). The term mayalso be used for smaller extratropical de-pressions not associated with surfacefronts such as lee lows, thermal lows, orpolar lows. However, its most commonusage is for the main synoptic disturbancesof mid-latitudes. These are areas of lowpressure surrounded by several closed iso-bars, frequently accompanied by surfaceFRONTS and moving toward the northeastin the N hemisphere and the southeast inthe S hemisphere. They appear to undergoevolution during their movement east-ward, commencing as shallow lows withwidely separated fronts (see diagram) andonly moderate winds. With time the pres-sure at the center of the depression fallswith a strengthening of the winds and anarrowing of the warm sector. Eventuallythe warm sector disappears from the sur-face (the process being known as OCCLU-

SION), the central pressure starts rising, andthe system begins to decay. This is the gen-eral pattern for depressions, but they varyappreciably in their intensity, size, direc-tion of movement, and time of existence.

Depressions are the main source of pre-cipitation in most lowland parts of themid-latitudes, the amount falling being de-termined by distance to the depression cen-ter, intensity of the depression, and thetime taken for the depression to pass. As aresult, the area of highest precipitation to-tals is about 50–60° latitude where depres-sions are most frequent and at their mostintense stage. In the early stages of the de-pression, the troposphere is warm and thecyclonic circulation is shallow, but as it de-velops the troposphere cools and the cy-clonic rotation extends to much greaterheights. The origins of the depression havebeen disputed, but they appear to be due to

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depression

Fig. 1

low

warm front

cold front

low

warm front

warm sector

cold front

Fig. 2

low

warm sectorwarm sectorwarm sectorcold frontFig. 3

occlusionocclusionocclusion

occlusionocclusionocclusion

warm frontwarm frontwarm front

back-bentback-bentback-bentocclusionocclusionocclusion

Depression evolution

DIVERGENCE in the upper atmosphere abovesuitable thermal gradients near the ground.2. (in geology) A structurally low area inthe Earth’s crust, lying between culmina-tions along a fold axis. See fold.

deranged drainage A type of drainagepattern characteristic of recently glaciatedareas, such as the Canadian Shield andSiberian tundra, where the drainage pat-tern has not yet adjusted to the structuresin the relatively recent glacially depositedsurface, which masks the previously ad-justed drainage pattern developed on thesolid geology beneath. As a result, drainageis not coordinated, and is characterized bymany small local drainage basins andlakes.

derived fossil A fossil that was origi-nally preserved in a stratum older than thatin which it is ultimately found, having be-come incorporated in the more recent stra-tum by processes of erosion anddeposition. If its true nature is not sus-pected, misleading conclusions regardingage and stratigraphical relations of the de-posit may be made.

desert An area of the Earth’s surfacewhere precipitation (usually taken asbelow 250 mm mean annual rainfall) is toolow to compensate for evaporationthroughout the year. This reduces the typesof vegetation that can survive and the ab-sence of surface runoff produces distinctivetypes of landform. The atmospheric stateproducing deserts is either a persistenthigh-pressure area, such as the subtropicalanticyclones in the Sahara, or areas inwhich natural atmospheric stability is em-phasized by surface cooling due to coldwater ocean currents, such as the Hum-boldt and Benguela currents. Deserts arealso found in continental interiors wheremountain barriers prevent the passage ofmoisture-bearing winds; examples are theGobi Desert and the deserts of the south-western USA. On the margins of a desertthere are areas of climatic variability thatexperience true desert conditions in someyears but not in others.

Soils are usually very poorly developed,stony, or saline (see sierozem). Weatheringdebris accumulates close to its source be-cause surface drainage cannot be main-tained. When rain does fall it is usually inthe form of sudden downpours, which cancause brief surface runoff, moving consid-erable amounts of debris. Mechanicalweathering is more important than chemi-cal owing to the lack of water, and wind isa very active agent of erosion and trans-portation of fine material. Vast areas of un-consolidated sand and dunes are lesstypical of most deserts than stony scrub-lands with occasional resistant rock up-lands.

desertification /dez-er-tă-fă-kay-shŏn/The process by which a desert is formed byeither changes in climate or human inter-vention. Natural causes also include firesthat destroy vegetation. Most often, how-ever, the cause is overgrazing by farm ani-mals or DEFORESTATION, resulting inerosion and infertility (see badlands). Theprocess can sometimes be reversed, e.g. byplanting special grasses or incorporatinginto the soil water-absorbent grains ofplastic. See also salinization.

desert pavement An area of gravellydesert plain or REG, over which the abra-sive effect of wind-blown sand has createda closely packed level surface. The individ-ual stones may be cemented together byprecipitated salts drawn to the surface insolution by capillarity.

desert rose A flowerlike arrangement ofplatelike mineral crystals, often foundamong the sand of deserts. The most com-mon types consist of BARYTES or GYPSUM.

desert soil A characteristic type of soilthat has poorly developed horizons (seeprofile). There is little vegetation and onlya thin organic layer, because it is too dryfor the formation of humus.

desert varnish A thin coating, varyingin color from pale yellow to very dark red,found on the surfaces of pebbles andblocks in stony deserts. It is thought to be

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caused by the deposition of iron and man-ganese oxides from solutions drawn to thesurface by capillarity and then evaporated.Highly polished surfaces of this type canoccur through the extremely abrasive effectof fine sand carried by strong winds.

desiccation crack /dess-ă-kay-shŏn/ (mudcrack; sun crack) A type of crack that de-velops in fine-grained deposits as a result ofshrinkage owing to the evaporation of thewater they contain, producing polygonalpatterns. These patterns are usually irregu-lar and polygons can vary from a few mil-limeters up to a meter in diameter.Subsequent wetting of a dried-up sedimentwill cause swelling and the disappearanceof the cracks. PATTERNED GROUND may bemaintained, however, if long-lasting cracksbecome infilled by some extraneous ma-terial, such as wind-blown sand. Whenpreserved they act as useful indicators ofpaleoenvironments and orientation.

desilication /dee-sil-ă-kay-shŏn/ (in soilscience) The removal of silica from a soilprofile by intense weathering and leaching.The process is typical of tropical areas andleads to the development of latosol profiles(see ferrallitic soil).

destructive plate boundary See plateboundary.

destructive wave A wave that movesmore beach material seaward than land-ward, resulting in a diminution in the sizeof the beach. Characteristic destructivewaves are steep, so that on breaking theirBACKWASH is more active than their SWASH.They occur at high frequency, usually be-tween 13 and 15 per minute, which meansthat the backwash of a preceding wave caninterfere with the swash of the next, reduc-ing the potential landward movement ofmaterial. Destructive waves frequentlyoccur in association with local onshorewinds, which cause the setting up of a sea-ward current on the sea floor, assisting intransporting the material stirred up by thebreaking waves away from the beach.Compare constructive wave.

detritus 1. Rock or mineral waste pro-duced by the breaking up and wearingaway of rock surfaces; debris.2. Organic debris from dead or decayingorganisms, particularly the remains ofaquatic creatures that fall to the bottom ofa lake or the sea.

detrivore /det-ră-vor, -vohr/ An animalthat feeds on organic detritus, the smallparticles of matter formed when deadplants and animals decompose. Detrivoresare most common at the bottom of the sea,where they feed on matter that drifts downfrom above.

deuteric changes /dew-te-rik/ Small-scale textural and mineralogical changesbrought about by a residual hot volatilephase during the final stages of crystalliza-tion of a magma. These include the alter-ation of feldspars to albite, analcime, andzeolites and the alteration of mafic miner-als to chlorite. Deuteric changes are ex-ceedingly difficult to distinguish fromMETASOMATISM when material is intro-duced from outside. Compare hydrother-mal process; pneumatolysis.

development equation In the atmos-phere there exists a high degree of compen-sation, so that the change in pressureobserved at the surface is often only a smallnet effect between strong DIVERGENCE orCONVERGENCE in the surface layers and thereverse flow in the upper atmosphere. Thiswill result in vertical motion to compensatefor the changes. If there is divergence in theupper levels and convergence at the sur-face, the vertical motion will be upwardwith the likelihood of precipitation.

The development equation, deduced byR. C. Sutcliffe, relates the difference in di-vergence between high and low levels tothe thermal wind, vorticity, and the Corio-lis effect, the first two of which can be mea-sured from charts and the last easilycalculated. If the result is positive it impliescyclonic development and if negative anti-cyclonic development.

devitrification /dee-vit-ră-fă-kay-shŏn/The slow crystallization of natural volcanic

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glasses such as obsidian and pitchstone,which are metastable, often accompaniedby secondary hydration. Devitrification isindicated by the presence of incipient CRYS-TALLITES and SPHERULITIC growth of quartzand feldspar. The process often goes tocompletion and many rocks show little orno evidence of a former vitreous state.

Devonian /di-voh-nee-ăn/ The first pe-riod of the Upper PALEOZOIC. Beginningabout 408 million years ago and lasting forsome 45 million years, it followed the SIL-URIAN and was succeeded by the CARBONIF-EROUS. The period is named for the countyof Devon, England, where these rocks werefirst recognized as a major group. The De-vonian System is divided into seven stages:the Lochkovian, Pragian, and Emsian formthe Lower Devonian, the Eifelian andGivetian the Middle, and the Frasnian andFamennian the Upper Devonian. These aredivided on the basis of fossils from rocks ofthe shallow-water marine facies, where in-vertebrates including corals, brachiopods,ammonoids, and crinoids flourished.Graptolites became completely extinct andthe trilobites declined.

Outcrops of Devonian rocks occur inall continents with extensive deposits un-derlying areas of North America, SouthAmerica, Europe, and Russia. A giantlandmass, Gondwana, was located in the Shemisphere with smaller landmasses inequatorial areas. At the close of the Sil-urian and during the early Devonian thecollision of the continents of what is nowNorth America and Europe was accompa-nied by extensive volcanic activity andmountain uplift, especially in a belt that in-cluded New England, Nova Scotia, New-foundland, Scotland, Scandinavia, and EGreenland. Extensive continental depositsaccumulated, consisting of conglomerates,red silts, and sandstones. This facies isknown in Europe as the Old Red Sand-stone and contains remains of a large vari-ety of ostracoderm fish.

Other Devonian sedimentary rocks in-clude the carbonate reef deposits of West-ern Australia, Europe, and Canada. Blackshale deposits formed locally and therewere widespread evaporite deposits.

Pelagic limestones rich in fossilcephalopods occur in Europe and theUrals. Gnathostome (jawed) fish, includingplacoderms, were also common. By the endof the period primitive amphibians hadevolved from certain crossopterygianfishes. There is also evidence of land plants,such as ferns and horsetails, and of associ-ated insects and spiders.

dew Water droplets deposited on theground after radiational cooling has re-duced the temperature of the ground sur-face below the dew-point temperature ofthe air in contact with the surface. Thesource of the moisture may be either dew-fall from the atmosphere during conditionsof light wind and a downward transfer ofwater vapor to the ground, or diffusion ofwater vapor from the soil and condensa-tion onto vegetation, which is also beingcooled by long-wave radiation losses. Thelatter process takes place only when the airnear the surface is calm, but is one of themost frequent sources of dew.

dew-point The temperature of air atwhich saturation will take place if the air iscooled while remaining at a constant pres-sure and moisture content. Although it canbe measured directly, dew-point is nor-mally determined indirectly from tablesbased on dry- and wet-bulb temperatures.

dew-pond A shallow artificial depres-sion lined with clay that collects water andis used in fields without running or pipedwater. The ponds are particularly commonon chalk or limestone rock. It was origi-nally believed that their main source ofwater was dew because they appeared to bewell maintained with water even duringdry periods. However, later investigationshowed conclusively that natural precipita-tion was most important and that dew wasof minor significance.

dextral fault A transcurrent (wrench ortear) fault in which the rocks on the oppo-site side of the fault plane are offset to theright. See diagram at FAULT. Compare sinis-tral fault.

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diabase /dÿ-ă-bayss/ (dolerite; microgab-bro) A rock that differs little mineralogi-cally from gabbro, of which it is themedium-grained equivalent. Calcic plagio-clase and augite are essential; in addition,diabase may contain olivine, hypersthene,quartz, or feldspathoids. The characteristictexture is ophitic but many examples areintergranular and porphyritic. The termepidiorite is sometimes applied to altereddiabase. Diabase occurs mainly as dikes,plugs, and sills. See also alkali basalt;basalt. In the UK the rock is known as do-lerite and the term diabase has been used torefer to altered dolerite.

diabatic /dÿ-ă-bat-ik/ Describing a ther-modynamic process in which heat enters orleaves a system. There are many examplesof this in the atmosphere, such as evapora-tion, turbulent mixing, and radiation ab-sorption. Compare adiabatic.

diachronism /dÿ-ă-kroh-miz-ăm/ Thecondition of a lithological unit whose baseis not a time plane, i.e. whose age is differ-ent in different successions. Diachronismoccurs when the boundaries of facies movein time. It can often be detected only if suit-able zone fossils are available, and failureto recognize diachronism can lead to falseimpressions of past events and geography.

diagenesis /dÿ-ă-jen-ĕ-sis/ A collectionof processes by which loose accumulatedsedimentary material becomes sedimentaryrock. Diagenetic processes are postdeposi-tional. In time, the pressure on a sedimentincreases owing to the increasing load ofsuperposed material, and compaction re-sults, involving a reduction of pore space.Chemical reactions take place between thesediment and entrapped and circulatingfluids, leading to the cementation of grainsby materials precipitated from the fluids.Calcite, silica, and hydrated iron oxides arecommon cementing materials. The diage-netic replacement of calcite by dolomitemay take place in calcareous marine sedi-ments. Such processes taking place near theEarth’s surface at low temperatures andpressures ultimately lead to the indurationof loose aggregates and to lithification. Di-

agenetic changes grade into those takingplace at higher temperatures within the do-main of METAMORPHISM.

diallage /dÿ-ă-lij/ A mineral name fordiopside and augite when displayingSCHILLER.

diamagnetic /dÿ-ă-mag-net-ik/ Describ-ing a substance that has a magnetic suscep-tibility of slightly less that 1. When adiamagnetic substance is placed in a mag-netic field, its induced magnetization is di-rected opposite to that of the applied field.The most diamagnetic substance is bis-muth.

diamond A crystalline form of the ele-ment carbon, which occurs mainly in pipesof KIMBERLITE and in alluvial deposits. Itcrystallizes in the cubic system, formingcolorless or colored crystals (tinted by im-purities), and is the hardest known min-eral. It has long been valued as a preciousgemstone; nongem varieties are used as in-dustrial abrasives.

diapir /dÿ-ă-peer/ A vertical body of IG-NEOUS ROCK that rises into the Earth’s crustbecause it is less dense that the surroundingrocks. SALT DOMES also rise for the samereason. See diapirism.

diapirism /dÿ-ă-pi-riz-ăm/ The upwardintrusion of a less dense rock mass throughoverlying more dense rock. It was origi-nally applied to SALT DOMES, but is also animportant mechanism for the inplacementof granitic rock types.

diaspore /dÿ-ă-spor, -spohr/ A hydratedform of alumina, AlO(OH), which occursin ALUMINA and some BAUXITES. It crystal-lizes in the orthorhombic system as gray,green, or pinkish aggregates.

diastem /dÿ-ă-stem/ A depositional breakof a very short time period, with or withouterosion. The beds above and below the bedhave the same dip and strike. The absenceof beds can be determined only by paleon-tological evidence.

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diastrophism /dÿass-trŏ-fiz-ăm/ Move-ment within the lithosphere, includingfolding, faulting, orogenesis, and the for-mation of new ocean floor, causing large-scale deformation of the Earth’s crust.

diatom /dÿ-ă-tom/ A microscopic ma-rine or freshwater alga (see algae). Diatomsform an important constituent of plank-ton, providing food for a great variety ofaquatic animals. Their geologic impor-tance derives from the fact that many di-atoms possess a case of silica, which maybecome fossilized, and fossilized diatomsoften form extensive deposits (see di-atomite). The earliest diatoms are found inrocks of Cretaceous age.

diatomite /dÿ-at-ŏ-mÿt/ A very fine-grained siliceous rock consisting of theskeletal remains of DIATOMS. They areformed under both freshwater and marineconditions.

diatom ooze A deep-sea siliceous ooze(see pelagic ooze), containing over 30% or-ganisms. It is a cold-water deposit, espe-cially prominent in an elongated belt in theN Pacific Ocean and flanking Antarctica.On a global scale, it occupies some 9% ofthe total ocean floor. Living diatoms con-sist of siliceous algae belonging to the phy-toplankton. They thrive in zones ofupwelling water, where nutrients are abun-dant.

diatreme /dÿ-ă-treem/ A volcanic vent,often filled by brecciated material, that hasbeen cut from the sides of the conduit byhigh-pressure gas charged with particles.The best-known examples are the dia-mond-bearing kimberlite pipes of SouthAfrica.

differential compaction The reductionin volume of sediments during compactionto different degrees, depending on theirporosity, grain size, and the rigidity of theparticles that compose the rock. For exam-ple, shales are more compressed than sand-stones.

differential erosion The erosion of

rocks subject to differential weathering.Well-weathered rocks are obviously moresusceptible to subsequent erosion than re-sistant types. After a period of active ero-sion, easily weathered rocks will formlowland areas, whereas resistant strata re-main as upstanding blocks. Even within asingle rock type certain bands may be moreor less resistant than the main mass, result-ing in ridges and depressions respectively.TORS and INSELBERGS represent good exam-ples of differential weathering and erosionwithin homogeneous rocks, different jointspacings accounting for the varied suscep-tibility.

differential shear A type of rock defor-mation in which movement takes placethroughout the whole rock, just as in aflow, but it occurs in distinct laminae orplanes.

differential weathering The more in-tensive weathering of certain parts of arock mass even when the same weatheringprocesses have been acting on the wholemass for the same length of time. Withinmany theoretically homogeneous rocksthere may be variations in mineral compo-sition or grain size, which can explain suchdifferences. Joints play an important part,especially in the case of rocks with lowporosity, as the percolation of waterthrough them causes preferential weather-ing around the joints. There will be differ-ential weathering between different rocktypes subjected to the same weatheringprocesses.

differentiation See magmatic differenti-ation; metamorphic differentiation.

diffluence /diff-loo-ĕns/ The rate of sep-aration of adjacent streamlines in the di-rection of airflow. It is the reverse ofCONFLUENCE.

diffusion The slow process by whichdifferent fluids or fluids having differentdensities mix together as a result of molec-ular movements. It obeys similar laws tothermal conduction but is too slow to be ofimportance in the atmosphere. Far more

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important is mixing achieved by eddytransfer (EDDY DIFFUSION) in turbulent air.

digital cartography (digital mapping)See cartography.

dike (dyke) A tabular body of igneousrock that is intruded vertically and discor-dantly to the structure of the rocks throughwhich it passes. See also dilation dike; nep-tunian dike; radial dike; sill.

dike swarm A large number of dikes,often arranged in either a radial or parallelpattern.

dilatancy /dÿlay-tĕn-see/ An increase inthe volume of a rock deformed by pressure,caused by the expansion and extension ofsmall cracks within it. The effect can be de-tected in strained rocks just before anearthquake, and is the basis of one type ofearthquake prediction.

dilation dike /dÿlay-shŏn/ A discordantigneous intrusion that causes the walls oneither side of a fracture to move apart. Theterm is also applied to swarms of dikes thathave filled fractures in the Earth’s crustwhen the area has been subjected to ten-sional forces.

dimorphism /dÿ-mor-fik/ (in mineral-ogy) The existence of an element or com-pound in two different crystal forms, suchas marcasite and pyrite (both forms of ironsulfide, FeS2) or diamond and graphite(both forms of carbon).

dinosaur /dÿ-nøbrevel;-sor/ A memberof a group of extinct archosaur reptiles thatwere the dominant form of terrestrial lifefrom the end of the Triassic period to theCretaceous. They evolved into a great vari-ety of both carnivorous and herbivorousspecies, some of very large size. They areclassified in two orders: the ORNITHISCHIA

and the SAURISCHIA.

diopside /dÿop-sÿd, -sid/ A clinopyrox-ene with composition CaMgSi2O6. See py-roxene.

diorite /dÿ-ŏ-rÿt/ A coarse-grained inter-mediate igneous rock containing plagio-clase of oligoclase-andesine compositionand mafic minerals. The An50 (anorthite)composition of plagioclase feldspar marksthe division between diorite and GABBRO. Indiorites, the most common mafic mineralsare hornblende and biotite although somerocks contain pyroxene. A little alkalifeldspar may be present together with ac-cessory magnetite, apatite, and sphene.More acid varieties of diorite containquartz up to 10% and may be calledtonalites. With a further increase in quartz,tonalites pass into granodiorites. With anincrease in the amount of alkali feldspar,diorites pass into syenodiorites. In theUSA, the term tonalite is applied to allrocks containing quartz, sodic plagioclase,and mafic minerals and is equivalent to theBritish tonalite (quartz-diorite) and gran-odiorite (in part).

Both leucocratic and melanocratic dior-ites occur. Meladiorites containing euhe-dral hornblende crystals are termedappinites and often appear to be a peg-matitic facies of more normal diorite. Dior-ites usually have equigranular textures.The medium-grained equivalents, microdi-orites, are often porphyritic. Markfielditeis an oversaturated microdiorite consistingof a groundmass of graphically intergrownalkali feldspar and quartz with phenocrystsof plagioclase and hornblende. The vol-canic equivalents of diorites are andesites.Dioritic rocks tend to occur in small intru-sive masses associated with granodioriteand gabbro bodies and many diorites arethought to be hybrid rocks. Compare syen-ite; syenodiorite.

dip 1. The angle made between the hori-zontal plane and that of the bedding plane,measured perpendicularly to the strike, in astratified rock or any planar structure. Seealso apparent dip. Compare hade.2. (angle of magnetic inclination; magneticdip; magnetic inclination) The angle be-tween the Earth’s magnetic field at anypoint on the Earth’s surface and the hori-zontal. It is 90° at the MAGNETIC POLES and0° at the Equator. It is measured using adip circle, an instrument in which a mag-

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netic needle is free to rotate, in the verticalplane, around a circular scale.

dipole field /dÿ-pohl/ That portion ofthe Earth’s magnetic field that can best bedescribed as originating from a dipolemagnet in the Earth’s interior, inclined at11° to the Earth’s axis of rotation. See alsonondipole field.

dip slope The slope of the surface of theland that more or less mirrors the slope ofthe rocks beneath.

direct circulation (in meteorology) Cir-culation in which potential energy is con-verted into kinetic energy through therising and sinking of juxtaposed lighterand denser air, respectively. This occurs atscales ranging from land and sea breezes tothe meridional cells of the GENERAL CIRCU-LATION OF THE ATMOSPHERE.

direct wave A wave that travels fromone point to another along the path ofshortest distance.

discharge The volume of water flowingthrough a cross section of a stream. It ismeasured by gauging mean velocity in thecross section, which is multiplied by thecross-sectional area to give an expressionof flow in m3 per second. Within a river,discharge increases downstream, except inarid areas where volume is lost by evapo-ration. By far the greatest discharge of allthe world’s rivers is that of the Amazon,with a flood-stage discharge of 180 000 m3

per second, which is at least as great as thecombined discharges of its nearest rivals,the Congo, Yangtze, Mississippi-Missouri,Yenisei, and Lena.

disconformity /diss-kŏn-for-mă-tee/ (pa-raunconformity) An unconformity thatmarks a considerable time gap but wherethe beds of rock above and below the planeof the unconformity have a similar dip andstrike (compare angular unconformity),the absence of beds being detectable onlyby paleontological means. It is of greatermagnitude than a DIASTEM. Compare un-conformity.

discontinuity /diss-kon-tă-new-ă-tee/ 1.(in meteorology) A sharp change in thevalue of a meteorological variable, for ex-ample at a cold front, where there is usu-ally a discontinuity in the temperature,humidity, and wind velocity fields acrossthe frontal surface. Most meteorologicalproperties are continuous functions ofspace and time and their values can there-fore be mapped in the forms of pressurecharts or isotherm maps.2. (in geophysics) A marked change withdepth in one or more of the physical prop-erties of the materials constituting theEarth’s interior. For example, a boundaryat which the velocity of earthquake waveschanges is a seismic discontinuity.

discordant 1. Decribing a rock unit thatcuts across the bedding or foliation of ad-jacent rocks. Intrusive igneous rocks, suchas dikes, show discordant relationships.2. (unconformable) Denoting a drainagepattern characterized by streams cuttingindiscriminately across structures. Discor-dance at a local level can be a product ofvarious detailed factors, but at a regionallevel it can be due to glacial blocking (seederanged drainage), tectonic activity (seeantecedence), or to superimposition of thedrainage from a preexisting cover of rocks.Compare accordant.

discordant coastline A coastline run-ning at right angles to structural featuresshaping the landscape immediately inlandof it. It is most obvious where lines of hillsrun inland, giving an undulating coastlinewith alternating cliffs and level beaches. Ifthe land sinks relative to the sea, the valleysbetween the hills become deep inlets (seefiord; ria). Such coastlines are common onthe western side of the Atlantic Ocean.

disharmonic folding /diss-har-mon-il/Folding in which differences in the COMPE-TENCE of the various beds of rock result invariation in types of folding within thefold, the less competent beds forming nu-merous folds that are smaller than thoseformed in the more competent beds enclos-ing them.

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dislocation metamorphism See meta-morphism.

dispersion The separation of SEISMIC

WAVES into groups with different fre-quency, resulting from variations in the ve-locity of the waves. Such differences, morenoticeable in surface waves than bodywaves, are caused by variations with depthin the density and elasticity of the rocksthrough which they travel.

disphotic zone /dis-foh-tik/ Oceandepths at which photosynthesis is not ef-fective because of the small amount of lightthat penetrates to this layer, which extendsfrom about 80 m to 200 m (the edge of theCONTINENTAL SHELF) below sea level. Seealso aphotic zone; euphotic zone.

dissected plateau See plateau.

disseminated deposits Deposits ofminerals that are formed when hydrother-mal fluids fill small fissures and pores in arock (see hydrothermal process). They areusually found with igneous intrusions (seeintrusive rock), and often comprise usefulmetallic minerals.

dissolved load That part of a river’sload carried in chemical solution. Rainwa-ter, being mildly acidic, can dissolve rocks,especially limestone, and then feed that dis-solved content into a river via groundwaterflow. Rivers may also be mildly acidic, es-pecially if they pass through areas of bogsor marshes, where organic acids are pro-duced, and they may dissolve minerals oftheir beds. In the Mississippi 29% of theload by weight is carried in solution, butthis proportion varies in other streams ac-cording to climate and according to whatproportion of runoff is contributed viagroundwater flow. The major ions carriedare bicarbonate, sulfate, chloride, calcium,and sodium, although in small basins thereis variation according to the nature of thelocal rocks, and in populated areas pollu-tion can significantly alter chemical con-tent. See also debris load; load.

distributary /diss-trib-yŭ-tair-ee/ A branch

of a river that flows from the main courseand does not rejoin it; the distributarymakes its own way to the sea. Most areshallow and narrow. See also tributary.

disturbance (in meteorology) Anysmall-scale synoptic feature causing a dis-turbance from the normal gradient windflow, especially a shallow DEPRESSION (lowor cyclone).

diurnal variation /dÿ-er-năl/ The changesin magnitude of any climatological prop-erty recorded during the solar or climato-logical DAY. Most elements exhibit somevariation. Temperature and relative hu-midity have an inverse cyclic fluctuationreaching peak and trough respectivelyabout 14.00 hrs. Atmospheric pressurealso shows a daily variation, but it is basedon a 12-hourly oscillation that proceedsaccording to local solar time. Maximaoccur about 10.00 and 22.00 and minimaat 16.00 and 04.00 hours. Because the syn-optic variations in mid-latitudes are large,it is only within the tropical areas that thediurnal variation of pressure is immedi-ately apparent. Some proposals have beenmade for a nocturnal maximum of precip-itation but this has never been proved.

divergence A measure of the rate of out-flow of a fluid from a certain volume. It isthe opposite state from CONVERGENCE andthe mathematical term used to describeboth, convergence being negative diver-gence. It has important implications in theatmosphere as the cause of pressurechanges and vertical motion, but is verydifficult to measure directly or accurately.Values of divergence in the free atmos-phere range up to 10–5 per second, al-though locally, as in fronts, it can reachhigher values.

In wave refraction phenomena, diver-gence refers to the spreading out of thewave orthogonals in the direction of waveadvance. Wave energy and wave heighttend to decrease in areas of divergence.

In the case of ocean currents, a diver-gence is the zone in which currents flowaway from each other, for example atroughly latitude 10°N where the North

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Equatorial Current and the countercurrentassociated with it separate.

divergent plate boundary A region inwhich two of the Earth’s lithospheric (ortectonic) plates are diverging, always oc-curring on the ocean floor. As the platesmove apart, material wells up from themantle beneath to form new oceanic crust.Such boundaries are often associated withMID-OCEAN RIDGES (the best known beingthe MID-ATLANTIC RIDGE) and active vol-canic action.

divide (drainage divide) The area ofhigher ground that lies between two sepa-rate drainage systems. The term watershedis also used for a divide but as watershed isalso used synonymously with drainagebasin it can cause ambiguity. See also in-terfluve.

division 1. An informal word for anyunit in any scheme of stratigraphical classi-fication, including LITHOSTRATIGRAPHY,BIOSTRATIGRAPHY, and CHRONOSTRATIGRA-PHY. See also stratigraphy.2. In traditional plant classifications, a tax-onomic grouping corresponding to a PHY-LUM.

doldrums Equatorial oceanic areas inwhich winds are light and variable andwhere navigation by sailing ships was diffi-cult. It coincides with the intertropical con-vergence zone or thermal equator withfrequent thunderstorms, heavy rains, andsqualls. It is now only used as a graphicterm for this belt of variable winds any-where near the Equator between the tradewind zones.

dolerite /dol-ĕ-rÿt/ See diabase.

doline /dol-een/ (dolina) A large SINK-HOLE formed by solution in KARST country.

dolomite /dol-ŏ-mÿt/ A magnesium-containing CARBONATE MINERAL of compo-sition CaMg(CO3)2. The term dolomite isalso used to denote a rock with a high ratioof magnesium to calcium carbonate.

dolomitization /dol-ŏ-mÿ-tă-zay-shŏn/The process by which a calcium carbonaterock is transformed into a calcium-magne-sium carbonate rock through the partial orcomplete replacement of calcite or arago-nite by dolomite. See carbonate minerals.

dome An anticlinal fold in which thebeds dip in all directions away from thecentral point of folding. A dome may bemerely structural or it may constitute anexposed landform. See also salt dome.

Doppler effect A change in the mea-sured frequency of a wave due to the rela-tive motion of the source and the recorder.As the source moves toward the recordingstation the wavelength decreases, whereaswhen the source moves away from therecorder the wavelength increases. It isnamed for the Austrian physicist ChristianJohann Doppler (1803–53).

Doppler radar A type of radar that re-lies on the Doppler effect. If the radar tar-get is approaching or moving away fromthe transmitted signal, it will affect the fre-quency of the returning signal, being lessthan that transmitted if moving away orgreater if approaching. In meteorology,precipitation drops are the usual target andindicate the horizontal air motion, the fallspeed of the particles, and, with certain as-sumptions, the vertical air motion. Thistechnique is being increasingly used for in-vestigations of subsynoptic levels, such asat frontal zones. Networks of radar sys-tems, e.g. in the USA, are used to providewarning of severe weather and its location.

dormant volcano A volcano that haserupted within recorded history but is ap-parently not active at present. Because no-body can predict if it will one day becomeactive again, it is best described as an inac-tive volcano.

dot map (in ecology and demographics)A map on which the distribution of a par-ticular measurable quantity is denoted bysame-sized dots (each representing a par-ticular number). For example, populationdensities of plants, animals, or even people

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can be represented graphically in this way(with each dot standing for, say, 50 trees or10 elephants).

double refraction See anisotropic; bire-fringence.

downland An area of pasture on hillyterrain. The term is used mainly in Aus-tralia and New Zealand.

downs Open rolling uplands, usually onchalky soil. The soil is thin and there arefew trees; the chief vegetation is grassland,often used as pasture. Plowed areas of thedowns are used to grow cereal crops. Theterm is used mainly in S England.

downthrow side The side of a fault thathas moved downward relative to the other,i.e. the side that has younger beds broughtdown against the older beds of the otherside. (See diagram at FAULT.)

drag fold A small fold associated with alarger folding structure, often found adja-cent to faults where the rock strata havebeen bent as a result of the movementalong the fault plane.

drainage Any process that removesrainwater from the land, whether by meansof streams or by artificial means (such asdrainpipes and channels. See also drainagebasin; drainage pattern.

drainage basin The area that supplieswater to a particular network of drainage

channels. At the edge of the basin a DIVIDE,usually in the form of hills, mountains, ora plateau, separates it from the adjacentbasin. The drainage basin constitutes theunit of study for drainage pattern, rates ofdenudation, relation between precipitationand runoff, and various other geomorpho-logical factors. Little is known of the con-trols on basin size and shape, but regionalgeologic structure and tectonic history areclearly important.

The term watershed is often used for adrainage basin but as watershed is alsoused synonymously with divide it cancause ambiguity.

drainage density The ratio between thetotal length of stream channel in a basin,and the area that the channel occupies.Closely related is stream frequency, whichis the number of streams divided by thetotal channel area. Drainage density is highon rocks yielding impermeable soils, e.g.shales and clays; it is also high where sedi-ment yield is high, on easily erodible rocks,e.g. clays.

Other things being equal, drainage den-sity is governed by climate, through its con-trol on runoff via rainfall and vegetation.Relief is also significant: high-relief areastend to have high drainage densities.Drainage density is an integral part ofdrainage pattern description: in particularit shows the degree of dissection of thedrainage basin.

drainage divide See divide.

drainage pattern The spatial arrange-ment of streams within a basin. It is a re-flection mainly of the area’s geologicstructure, i.e. the nature and arrangementof faults, folds, rocks (especially litholo-gies), and relative relief. Past and currentclimate and the tectonic and geomorphichistory of the basin are also important.These variables control the rate and natureof stream incision and headward erosion,the basic processes by which the patterndevelops. Field studies show the patternevolves very rapidly at first, then reaches asteady state of little further change, con-trary to the American geographer William

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divide divide

divid

e

Drainage basins

Morris Davis’s original idea that the rate ofdevelopment was constant through time.See also accordant; angulate drainage;barbed drainage; dendritic drainage; de-ranged drainage; discordant (def. 2);drainage density; parallel drainage; radialdrainage; rectangular drainage; streamorder; trellis drainage.

drainage wind See katabatic wind.

dravite /drav-ÿt/ A brown variety ofTOURMALINE that is rich in magnesium. Itgenerally occurs in metamorphic rocks,and is sometimes used as a semipreciousgemstone.

dredging The deepening of a river, port-approach channel, or other such area byexcavating loose sediment or in-situ rockfrom the river or seabed. The dredger ordredge is a vessel specially designed to un-dertake this. Some dredgers actually dig upthe bed using a combination of buckets orgrabs; others suck up loose sediments me-chanically using suction devices, as in atrailing suction-dredger. Dredging may beperformed to improve navigable depths orto remove obstructions such as shoals orbanks that are dangerous to navigation.Other dredgers operate farther offshoreand recover useful minerals, such as sand,gravel, tin, ore, and diamonds from theseabed. Dredging can have far-reaching en-vironmental consequences, as when it in-creases turbidity, to the detriment of fishand shellfish production.

dreikanter /drÿ-kahn-ter/ A pebble withthree facets, formed by the erosive actionof windblown sand in desert regions. Thewind moves the pebbles back and forth, be-cause they are too heavy to be lifted en-tirely. See also ventifact.

drift Glacial and fluvioglacial deposits.Great thicknesses of this drift accumulatedduring the PLEISTOCENE Epoch, althoughmuch has subsequently been removed byerosion. An area of glacial deposition maybe referred to as one of drift topography. Adrift map is a geologic map on whichglacial drift and other superficial deposits

are shown, as contrasted with a solid edi-tion showing only the underlying rocks.

drizzle Water droplets ranging in diam-eter from 0.2 to 0.5 µm. It forms by coa-lescence in stratiform cloud with onlyweak vertical motion, otherwise thedroplets would be unable to fall. It also re-quires a high relative humidity betweencloud base and ground surface or only ashort distance between cloud and surface.In both cases, evaporation of the dropletswould occur without these constraints.

drought (drouth) A period of drynessdue to the absence of significant precipita-tion. It is an unsatisfactory term unless verycarefully specified because it implies someeffect on agriculture and vegetation, butthis is dependent upon factors other thanrainfall alone. In purely climatic terms, itcan be used to indicate lack of rain. In theUSA a drought is defined as a period of 21days or more when rainfall is 30% belowaverage for the time and place.

drowned coastline A strip of landalong the coast that has been submergedunder the sea, either because the sea levelhas risen or because the land has sunk rel-ative to the sea. Valleys are flooded, form-ing FIORDS or RIAS, and hills may becomeislands.

drowned valley A river valley cut at atime when sea level was at a much lowerlevel than it now is, usually during thePLEISTOCENE Epoch, and subsequentlydrowned as sea level rose again on meltingof the large ice sheets. These former inlandvalleys now exist as indentations in thecoastline, their internal shapes beinglargely hidden underwater. Differentlyshaped coasts are produced by the floodingof geologically different areas. If the for-mer drainage system was perpendicular tothe general outline of the present coast a se-ries of inlets separated by headlands willexist; if the drowned valleys lay parallelwith the present coastline numerous elon-gate islands, the former interfluves, will bepresent, with narrow flooded areas be-tween them. See also fiord; ria.

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drumlin /drum-lin/ A streamlined glacialform tending to occur in groups, all withtheir long axes parallel with the directionof ice movement. They may be composedof TILL, which can vary considerably incomposition, of preexisting DRIFT, or ofrock. Some contain stratified deposits.Since these forms are so variable a numberof explanations of their origins have beensuggested: there are two principal explana-tions. Those composed of rock or preexist-ing drift materials must have involvederosion, although if erosion were the onlyprocess, one would expect shapes similarto those of ROCHES MOUTONNÉES, which isnot the case. Drumlins composed of newlydeposited till must involve deposition, andit has been suggested that they formaround a nucleus of frozen till, or rock. De-spite their variability, all drumlins arelinked by their common shape, and the factthat their formation is associated withmoving ice.

druse /drooz/ (drusy) A cavity in a rockor mineral vein into which large euhedralcrystals project. A rounded cavity contain-ing crystals that project toward the centeris called a geode. See also miarolitic cavity;vugh.

dry adiabatic lapse rate The constantrate at which air will cool upon up-lift, prior to saturation being reached. Itsvalue of 0.98°C/100 m depends upon theproperties of the gases in the atmosphereand the gravitational force. See also envi-

ronmental lapse rate; saturated adiabaticlapse rate.

dry- and wet-bulb thermometer Seethermometer.

dry site A small hill in a wetland ormarshy region, often occupied by humansettlement.

dry valley A typical subaerial river val-ley in all respects except that it contains nostream or river. Such valleys are mostlyfound in areas of soluble limestone wherethe waters responsible for the cutting of thevalley, at an early stage in the erosional his-tory of the area, have subsequently disap-peared underground through enlargedjoints and subsurface cavern systems, de-veloped as a result of LIMESTONE SOLUTION.At times of exceptional discharge the un-derground systems may be incapable ofcarrying all the water, and surface drainagemay return temporarily.

Some dry valleys may have been formedin former periglacial conditions, when per-mafrost rendered the ground impermeable,or in times of wetter climate, when thewater table was higher.

dumortierite /dew-mor-tee-ĕ-rÿt/ A blueor green fibrous mineral form of hydratedaluminum borosilicate, Al8BSi3O19(OH).

dune A mound or hill of unconsolidatedmaterial, usually sand. Dunes are charac-teristic landforms of deserts, in an unvege-tated form, and of certain coastal andriverside areas, where they are usually veg-etated. Desert dunes are simpler in formowing to the lack of moisture and plantgrowth, the two major forms being theBARCHAN (crescentic dune) and the SEIF

DUNE (longitudinal dune). Coastal dunesshow a development inland from theyoung FOREDUNE through the main MOBILE

DUNE to the STABILIZED DUNE.The major requirement for dune-build-

ing is a sand supply; in the deserts this istransported by wind from areas of erosion,notably scarps and areas of DEFLATION,whereas in coastal areas it is transported bythe longshore drift from a source that may

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direction ofice movement

profileprofileprofile

planplanplan

Drumlin

be a preexisting supply or an area of ero-sion creating a supply, and then left on theforeshore ready for wind action to take itinland and build the dune.

Coastal dunes are hence due to a three-way relationship between littoral drift ofsand, blowing of sand onshore, and vege-tation growth trapping the sand and build-ing a dune, whereas desert dunes may bewave patterns resulting from the interac-tion of air flow and land surface, the dunesaccumulating in areas between turbulenteddies. See also parabolic dune; star dune.

dune-bedding A type of CROSS-BEDDING

that occurs in DUNES, resulting from vari-able deposition of sand by the wind.

dunite /dew-nÿt/ A monomineralic ultra-mafic rock consisting wholly of OLIVINE.

dust bowl An area in the western GreatPlains region of the USA that suffered ex-tensive wind erosion (deflation) during themid 1930s, which removed the fertile top-soil. It lies mainly within W Kansas, Okla-homa, and Texas and extends intoColorado and New Mexico. A smaller dustbowl occured in central Nebraska, NorthDakota, and South Dakota. See also bad-lands.

dust devil A WHIRLWIND in which sandand dust rise into the atmosphere reachingan average height of around 200 m andwith a diameter ranging from about 3 m toover 30 m. It forms by strong convectionabove an intensely heated sandy surface

and is distinguished as a dust devil only ifthe surface is sufficiently sandy for the sur-face material to be drawn into the rotatingcolumn. Less developed whirlwinds arealso known as dust whirls.

dust storm A cloud of fast-movingwindblown dust, which occurs when thewind is strong enough to lift the dust parti-cles from the ground. The dampness, den-sity, shape, and size of the particlesdetermines how strong the wind has to be.Local dust storms are often associated withrain and thunder; extensive dust stormsusually occur in regions of low atmos-pheric pressure. In desert areas, very smalloccurrences are known as DUST DEVILS.

dyke See dike.

dynamical meteorology The branch ofmeteorology concerned with the causesand nature of motion within the atmos-phere.

dynamic equilibrium (in geomorphol-ogy) The state of balance between erosionand deposition to which rivers seek to ad-just. The emphasis on dynamic is becauserivers never actually achieve a balance ofno erosion or deposition, but are alwaysactively adjusting to the constant changesof load and discharge, etc., by some minorerosion and deposition. See also equilib-rium profile; river.

dynamothermal metamorphism Seemetamorphism.

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earthflow A mass movement of soil on asteep slope, well mixed with water. Suchflows are commonest in areas with littlevegetation, where rainfall comes in sharpbursts, saturating the soil, and where thesoil has a high content of fines. The differ-ence between earthflows and mudflows ispurely of degree: earthflows occur on gen-tler slopes and move at a lesser velocitythan mudflows. The soil eventually comesto rest as a tongue at the slope foot, thethickness of the tongue becoming less asthe velocity of flow increases. Like theother mass movements, flows act on unsta-ble slopes, reducing them to a more stableangle.

earth hummock A rounded mound offrozen soil, up to 20 cm high, that has finematerial at its core and is covered with veg-etation. Common in alpine and arctic re-gions, earth hummocks create a type ofPATTERNED GROUND.

earth movement Any movement of theEarth’s crust caused by processes occurringbeneath it. Sudden earth movements ac-company earthquakes and volcanic erup-tions; slow movements cause folding anduplifting of rock strata.

earthquake A series of shocks, subdi-vided into FORESHOCKS, PRINCIPAL SHOCKS,and AFTERSHOCKS, which generate SEISMIC

WAVES within the Earth, as a result of thefracturing of brittle rocks within the lithos-phere. They result from the accumulationof forces within the rocks until they arestrained to a point beyond which they frac-ture. The magnitude of an earthquake isthe amount of energy involved and is de-termined from recordings of seismic waveson seismographs. Depending on the quan-

tity of energy liberated when the over-strained rocks fracture, the earthquakemay vary from mild quiverings to violentoscillations of the land surface. A numberof earthquake magnitude scales have beendeveloped, the RICHTER SCALE being thebest known.

earthquake focus See focus.

earthquake intensity The degree of vi-olence of an earthquake at a particularpoint on the Earth’s surface, expressed ona descriptive scale. Many intensity scaleshave been developed; in the USA the Mod-ified Mercalli scale (see Mercalli scale) iscurrently used. It is composed of 12 levelsof increasing intensity based on observa-tions and descriptions of damage.

earthquake zone See seismic zone.

earth science Any one of the sciencesthat study the Earth. The earth sciences in-clude geology, geomorphology, meteorol-ogy, and oceanography.

earthslide The movement of the soilmantle over a shear plane. For the processto operate there must be an unstable slopeand a dry soil. If the soil becomes wet, itmay turn into an EARTHFLOW. An impor-tant part may be played by water, however,in lubricating the shear plane. See alsomass movement.

earth temperature The temperature ofthe ground surface as determined by thecharacteristic wavelengths of long-waveradiation emission.

earthworm A terrestrial annelid wormthat is the most important of the macro-

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fauna in the soil. Under favorable condi-tions, as when the soil is moist and rich inlime and organic matter, there may be upto a million earthworms per acre. Theymay pass as much as 10 tonnes per acre ofsoil through their bodies each year. Thismaterial is humified and excreted in theform of worm casts, which contain morehumus, nitrates, and exchangeable basesthan the surrounding soil. Besides theirfunction in the chemical and physicalbreakdown of plants they are also impor-tant in improving the texture, aeration,and drainage of the soil.

easterlies Any winds in which the zonalcomponent of air flow is from the east.They are subdivided into the tropical east-erlies or trade winds and the polar easter-lies. The presence of easterly winds at theground surface is an essential requirementto counteract the effect of the mid-latitudewesterlies on the Earth’s rate of rotation.

easterly wave A shallow trough or dis-turbance in the trade-wind flow associatedwith an increase of cloudiness and precipi-tation to its rear. After initial debate aboutthe status of the easterly wave, satellitephotographs have confirmed characteristiccloud patterns associated with the troughalthough they do exhibit greater varietythan was first thought. In some instancesthey probably act as initial disturbances forthe development of hurricanes.

easting Any of the north-south grid lineson a map, quoted before the NORTHING

when coordinates are being given, showingdistance east from the origin of the grid.

ebb tide The outgoing of the tidalstream; the retreating tide, i.e. that part ofa tide cycle following the high-water stageand preceding the low-water stage. Com-pare flood tide.

Echinodermata /i-kÿ-noh-der-mă-tă/ Thephylum of marine invertebrate animalsthat includes the starfish, sea urchins, andcrinoids. The body of an echinoderm usu-ally has five radii, along which are groupedhydrostatic tube feet, functioning in loco-

motion and feeding and operated by thewater vascular system, an internal systemof fluid-filled canals. These radii areknown as ambulacra and the areas be-tween as interambulacra. An externalskeletal system (test) composed of plates ofsingle calcite crystals is usually present andsometimes bears spines. There are five liv-ing classes: the Asteroidea (starfish), Ophi-uroidea (brittle stars), ECHINOIDEA (seaurchins), Holothuroidea (sea cucumbers),and the CRINOIDEA (sea lilies). In addition,there are a number of important extinctclasses, such as the Cystoidea. Echino-derms extend from the early Cambrian Pe-riod to the present day.

Echinoidea /ek-ă-noi-dee-ă/ The class ofthe phylum ECHINODERMATA to which thesea urchins belong. They are protected by ahemispherical and usually spiny test of in-terlocking calcite plates, which in somelater forms is flattened. Typically the anusis situated in the center of the upper surfaceand the mouth is diametrically opposite. Inlater forms the mouth and anus may havebecome displaced, the anus sometimesopening on the oral surface. Echinoids aremarine, benthonic, and free living and areknown as fossils from the Ordovician Pe-riod onward. The group was affected bythe widespread extinctions at the end of thePaleozoic Era but revived in the Mesozoicand extends to the present day. Echinoidsare used as biostratigraphic ZONE FOSSILS inthe Cretaceous System.

eclogite /ek-lŏ-jÿt/ A coarse-grainedgranular rock consisting essentially ofbright green omphacite and deep red al-mandinepyrope garnet. Diopside, quartz,and kyanite may also be present in smallamounts. Eclogite has the chemical compo-sition of basalt and may be considered tobe the high-pressure high-temperaturemetamorphic equivalent. Eclogite is foundas lenses and bands in regionally metamor-phosed rocks of the highest grades and asinclusions in basalt and kimberlite.

ecology The scientific study of how liv-ing organisms affect, or are affected by,their natural environments and by other

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organisms. Both living and nonliving com-ponents of the environment are considered(including plants, animals, soil, climate,temperature, etc.). Ecology may be con-cerned with individual organisms, popula-tions, or even whole communities. See alsoautecology; ecosystem.

economic basement Strata below whichthere is little chance of finding economicmineral resources, particularly oil.

ecosystem An ecological unit that con-sists of the physical environment (both liv-ing and nonliving) and the organisms thatoccupy it. It may be as small as a pond ortidal pool, or as large as a tropical rainfor-est or even the whole global system. Nutri-ents and energy pass through an ecosystemin a certain way (see food chain). The or-ganisms living in it may occupy variousTROPHIC LEVELS, from plants (producers) atthe bottom to animals (consumers) at thetop. Left alone, an ecosystem achieves abalance, with all the organisms living suc-cessfully together. It will even usually re-cover from a disaster such as a drought orflood. Human intervention, however, maycause changes from which the ecosystemcannot recover, for example the large-scalecutting down of forests or extensive pollu-tion of the air, land, or oceans. See also car-bon cycle; nitrogen cycle.

ecoulement /i-Kool-mĕnt/ The down-ward sliding of large masses of rock as a re-sult of gravity.

ectoparasite See parasite.

Ectoprocta See Bryozoa.

edaphic /i-daf-ik/ Describing those fac-tors of the soil (chemical, biological, andphysical) that affect the growth of plants.Examples of such factors include moisture,mineral content, and texture.

eddy A rotational feature of a fluid,which retains its identity for a limited timewhile moving within the main body of thefluid and eventually amalgamating with it.Such eddies can frequently be seen in river

flow after the current has been disturbedby a bridge or shearing. See also whirlpool.

In the atmosphere, eddies are found in awide range of scales. Near the ground sur-face they are important in transferring mo-mentum, heat, and moisture. On the largescale, eddies in the form of depressions area necessary mechanism for much of themeridional transfer in the GENERAL CIRCU-LATION OF THE ATMOSPHERE.

eddy diffusion The exchange of atmos-pheric matter and properties broughtabout by eddies. See also diffusion.

edenite /ee-dĕ-nÿt/ A monoclinic AMPHI-BOLE.

edge dislocation A defect in a crystallattice resulting from the insertion of anextra plane of atoms.

effluent 1. (in geology) A stream thatflows out of a lake or other stream.2. In general usage, waste material dis-charged as a liquid, e.g. from an industrialplant, and a potential source of pollution.

ejecta /i-jek-tă/ 1. Material that is thrownout of an erupting volcano. See pyroclasticrock.2. Material that is thrown out when a ME-TEORITE impacts with the ground. It con-sists mainly of rock fragments, but mayalso include glassy particles that resultfrom the melting of the meteorite or therock it hits.

ejecta blanket A deposit of EJECTA thatsurrounds an impact crater after a METE-ORITE hits the ground.

ejectamenta /i-jek-tă-men-tă/ See tephra.

Ekman flow /ek-măn/ (in oceanogra-phy) The movement of surface sea water inthe direction of a wind, which, as it blows,exerts a force on the sea surface in the di-rection of the wind and, because of fric-tional forces operating within the watercolumn, causes a certain thickness of waterto flow. The ocean’s response is highlycomplicated, especially because of Earth

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Ekman flow

rotation and the fact that water is fluid, butalso because of such factors as land-oceanconfiguration. In the N hemisphere, aver-age Ekman flow is some 45° to the right ofthe wind, and the speed decreases and thedirection swings increasingly to the right ofthe wind direction with increasing depth.Ekman flow has several far-reaching ef-fects; for instance, off the coasts of Califor-nia and Peru, Ekman flow tends to be in anoffshore direction, thereby causing zonesof upwelling of fertile deep water near thecoast, which provides nutrients for fishpopulations. It is named for the Swedishoceanographer Vagn Walfrid Ekman(1874–1954).

Ekman layer (in oceanography) Theshallow surface layer of the upper watermasses of ocean circulation in whichEKMAN FLOW occurs. The thickness of theEkman layer has been calculated theoreti-cally. The uppermost layer of the circula-tion of ocean water is relatively thin, about100 m deep. Beneath it, and extendingdown to the ocean floor, is the deep-watercirculation involving deep water masses.

Ekman spiral (in meteorology) TheEKMAN FLOW (or effect) as it applies in theatmosphere. The ground surface has a fric-tional interaction with the atmosphere andits effect decreases with height until, be-tween 500 and 1000 m, the GEOSTROPHIC

WIND is observed. The spiral illustrates invector form how the wind velocity changes

with height (see diagram), blowing acrossthe isobars at low speed near the surfaceand parallel at higher speeds in the free at-mosphere.

elastic wave A seismic wave that ispropagated through a medium by elasticdeformation.

elbow of capture A right-angled bendin a river, downstream of which is the cap-turing river and upstream of which is thecaptured river. See river capture.

electrical logging A subsurface loggingtechnique in which an electrode or series ofelectrodes are lowered into an uncasedborehole. As the sonde is raised a continu-ous record of the electrical properties of therocks through which it passes is recorded.By an examination of the variations in sev-eral properties an assessment of the rocktypes present in the borehole can be made.

electromagnetic radiation The formof energy that travels in waves from itssource without the necessity of an inter-vening medium. The waves move with thespeed of light (3 × 108 m per second) inspace and only slightly slower through theatmosphere. The total spectrum of thisform of radiation includes wavelengths asshort as gamma rays (10–13 m) to longradio waves (up to 105 m), as shown on thediagram overleaf. Within this range is vis-ible light between 0.4 and 0.7 µm, to whichour eyes are sensitive. The behavior of elec-tromagnetic radiation is explained by thefollowing physical laws: 1. All matter witha temperature above absolute zero(–273°C) emits radiation. 2. Some sub-stances emit radiation of certain wave-lengths only. 3. A substance that emits themaximum amount of radiation for a giventemperature in all wavelengths is known asa black body (see black-body radiation).The amount of radiation emitted is thenproportional to the fourth power of thesubstance’s absolute temperature. 4. Sub-stances will only absorb radiation of wave-lengths that they can also emit. 5. Thehotter a substance, the shorter will be thewavelengths at which most of the radiant

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Ekman spiral

height

geostrophic wind

at approx. 1 km

Wind increases in speedand veers in directionwith increasing height

Ekman spiral

energy is emitted. 6. The amount of radia-tion received at a point is inversely propor-tional to the square of the distance of thatpoint from the radiation source.

element A substance that cannot be bro-ken into simpler substances by chemi-cal means. All the atoms in an element havethe same atomic number although, inisotopes, some of an element’s atoms havea different mass (because they containdifferent numbers of neutrons). There are 92 elements that occur naturally onEarth.

elevation The vertical distance of anypoint on the Earth’s surface above theDATUM level, measured in meters or feet.

El Niño–Southern Oscillation /el neen-yoh/ (ENSO) A periodic disturbance ofthe ocean–atmosphere system that occursin the tropical Pacific at irregular intervalsof some 3 to 8 years, and which may lastfor over a year. The term El Niño (babyboy) was originally applied by Peruvianfishermen to a warm southward-flowingocean current that displaces the normalnorth-flowing cold current in an allusionto the Christ child because it appearsaround Christmas time. It was subse-quently used for the periodic warmingsthat disrupted the fish and bird popula-tions off Peru and Ecuador and has sincebecome synonymous with the larger-scalewarm events. It now refers to the large-scale weather effects that are associatedwith the weakening of the trade winds andunusually warm surface ocean layers in theeastern and central Pacific Ocean. El Niñoevents are connected to fluctuations in theSouthern Oscillation (SO), which is a see-sawing in atmospheric pressure betweenthe eastern South Pacific (east of Tahiti)and the Indian Ocean (west of Darwin,Australia). The coupled air–water flow iscalled the El Niño–Southern Oscillation(ENSO) and has repercussions for climateand weather around the world.

Under normal, non-El Niño, conditionsthe prevailing trade winds blow westwardand push large volumes of water west to-ward the coast of Indonesia. The coldHumboldt current flows northward off theSouth American coast and then, at theequator, turns westward as the SouthEquatorial Current. The upwelling of coolwater off the South American coast is nu-

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10–15 –

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1 –

10 –

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– 1020

– 1019

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– 109

– 108

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– 1

gamma rays

X-rays

ultravioletradiation

visible light

infrared(heat)

radiation

EHF

SHF

UHF

VHF

HF

MF

LF

VLF

radiofrequencies

freq

uen

cy/k

Hz

wav

elen

gth

/m

Electromagnetic radiation

trient rich and supports diverse marine lifewith important fisheries. When an El Niñoevent occurs there is a change in the windpattern with a weakening of the tradewinds in the central and W Pacific. The up-welling of cool water ceases, to be replacedby warmer water that flows back from thewest, and the nutrient supply is cut off witha consequent fall in marine life and adverseeffect on the commerical fisheries, such asthe anchovy catch off Peru. The normallow-pressure system over the W tropicalPacific and Indian Ocean is replaced in an

El Niño event by weak high pressure andpressures drop in the SE tropical Pacific.Drought conditions occur in Australia andsoutheastern Asia while in contrast thewarm waters bring heavy rains and flood-ing to parts of the coasts of North andSouth America, such as the normally desertareas of Peru. Recent El Niños have oc-curred in 1982–83, 1986–87, 1991–92,1994, and 1997–98. As part of this cyclethere is, in some years, formation of a coldregion in the eastern Pacific. This is calledLa Niña (the little girl).

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P A C I F I CP A C I F I CP A C I F I C

O C E A NO C E A NO C E A N

P A C I F I CP A C I F I CP A C I F I C

O C E A NO C E A NO C E A N

Under normal conditions the trade winds push water from east to west and warm wateraccumulates around Indonesia

INDONESIAINDONESIAINDONESIA

AUSTRALIAAUSTRALIAAUSTRALIA

warmwarmwarm

seaseasea

normal ocean currentsnormal ocean currentsnormal ocean currents

UNITED UNITED UNITED STATESSTATESSTATES

PERUPERUPERU

normal trade windsnormal trade windsnormal trade winds

El Niño currentEl Niño currentEl Niño current

increase inincrease inincrease insurface surface surface temperaturestemperaturestemperatures

weakerweakerweakertrade windstrade windstrade winds

INDONESIAINDONESIAINDONESIA

AUSTRALIAAUSTRALIAAUSTRALIA

UNITED UNITED UNITED STATESSTATESSTATES

PERUPERUPERU

In El Niño years, the trade winds weaken and the warm water flows back toward the coast ofIn El Niño years, the trade winds weaken and the warm water flows back toward the coast ofIn El Niño years, the trade winds weaken and the warm water flows back toward the coast ofLatin AmericaLatin AmericaLatin America

elutriation /i-loo-tree-ay-shŏn/ The nat-ural sorting of rock fragments into finerand coarser particles. It most commonlyoccurs when the fragments are transportedby water, but may also happen during py-roclastic flow, when volcanic materialflows down the side of an erupting vol-cano.

eluviation /i-loo-vee-ay-shŏn/ The wash-ing out of fine material from a soil, espe-cially from the upper part. The eluvialhorizon is the A horizon, which has lessclay than the rest of the profile as a result.

emerald A bright green transparent va-riety of BERYL (the color is caused bychromium impurities), valued as a preciousgemstone. It crystallizes in the hexagonalsystem and occurs mainly in mica schistsand veins of calcite.

emery A black or dark gray impure formof the mineral CORUNDUM. It occurs as finegranules, often with hematite or magnetiteimpurities. It is used as an abrasive.

emission A substance (particulate, gas,or aerosol) that is discharged into the at-mosphere as a waste product or pollution,either from an anthropogenic source (e.g.,exhaust emissions from automobiles andemissions from industrial processes) orfrom a natural source (e.g., volcanic activ-ity). See also air pollution.

emissions trading A mechanism thataims to reduce pollutants and greenhousegas emissions. In emssions trading schemescredits or permits are allocated to thoseparties responsible for emissions (e.g. acompany or country) by a regulatory orga-nization. The scheme allows the party toemit in excess of its allocation by purchas-ing credits or allowances from the market;a party that emits less than its allocationcan sell or trade the surplus. The conceptwas proposed by the KYOTO PROTOCOL,which sets limits on total emissions ofgreenhouse gases by the world’s majoreconomies. The European Union Emis-sions Trading Scheme (EU ETS) came intoeffect in January 2005.

emissivity /em-ă-siv-ă-tee/ The ratio ofemission of radiation from a substance tothe emission from a black body at the sametemperature and wavelength (see black-body radiation). Values range from slightlybelow 1.0 to 0.85 for most substances.

enclave An inclusion or XENOLITH.

enderbite /en-der-bÿt/ A rock of theCHARNOCKITE group, rich in plagioclasefeldspar.

endomorphism /en-doh-mor-fiz-ăm/ Thechange in composition of igneous rock de-rived from magma as it assimilates materialfrom the COUNTRY ROCK surrounding it.See also metamorphism.

endoparasite See parasite.

en echelon /awn esh-ĕ-lawn/ Denotingoverlapping or offset geologic structures.

energy balance A concept applied tothe static energy of the Earth–atmospheresystem, rather than its dynamic energy. Itrelates the net radiation flux at a surface tothe utilization of this available energy. Thisis consumed by the conduction of heatbelow the ground surface, the transfer ofsensible heat to the atmosphere by turbu-lence and convection, and the transfer oflatent heat by similar methods. All theseprocesses are reversible.

The energy balance equation is oftencalled the heat balance equation and is ex-pressed in the form R = H + LE + G whereR is net radiation, H is sensible heat, LEthe latent heat transfer, and G the compo-nent transferred into the ground.

englacial /en-glay-shăl/ Describing ma-terials or meltwater contained within anice mass. Such debris or water can reachthe interior ice either by movement upwardfrom the bed or downward from the sur-face.

enhanced greenhouse effect See green-house effect.

enrichment The natural increase in the

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proportion of one of the elements or min-erals that constitute a given type of rock. Itmay result when additional amounts of aconstituent are introduced from outsidesources, or when one constituent is selec-tively removed from the rock. It can becaused by chemical processes, as whenmineral-laden water filters through aporous rock, or by mechanical processes,as when relatively light quartz is trans-ported away from heavier metallic miner-als.

ENSO See El Niño–Southern Oscilla-tion.

enstatite /en-stă-tÿt/ An orthopyroxene.See pyroxene.

entisol /en-tă-sôl/ One of the twelve soilorders of the US SOIL TAXONOMY, approxi-mately equivalent to the azonal category ofthe old classification. These are recent soilswithout natural genetic horizons and theyinclude lithosols (shallow stony soils),regosols (thin soils on unconsolidateddrift), and alluvial soils or fluvisols (soilsthat are constantly being added to becauseof their site on active floodplains). Thesesoils are not extensively used for agricul-ture but may become productive whenmanaged properly.

entrainment (in meteorology) Theprocess of mixing between the environ-ment and a rising thermal in the updraft ofa cumulus or cumulonimbus cloud. It hasthe effect of diluting the warmer rising airand so slightly reduces its buoyancy. Thisprevents the rate of cooling in the thermalfrom being adiabatic, but in practice thedifference is slight.

entrenched meander The type of IN-CISED MEANDER that results from the REJU-VENATION of floodplain meanders andsubsequent incision into the bedrockbelow. They are therefore the product of atwo-stage process, unlike the one-stageorigin of INGROWN MEANDERS. Entrenchedmeanders are often distinguished fromingrown meanders on the basis of theirsymmetrical valley cross profile, contrasted

to the asymmetrical cross profile of theingrown type. However, some flood-plain meanders do not incise themselvesvertically into their floodplains, but at anangle, producing an asymmetric cross pro-file.

environmental lapse rate The actualrate of change in temperature with heightof the atmosphere, at a given time, usedparticularly when investigating the stabil-ity of the lower atmosphere. The differ-ences between the DRY ADIABATIC LAPSE

RATE, SATURATED ADIABATIC LAPSE RATE, andenvironmental lapse rate are especially im-portant in this respect when drawn on aTHERMODYNAMIC DIAGRAM. The lapse rateis steep in the lower layers during daytimeheating, decreasing with height, so that bythe upper atmosphere it approximates tothe saturated adiabatic lapse rate. Inver-sions are frequently present indicating awarming of the air with increasing height,but within the troposphere they are rarelyvery deep.

Eocene /ee-ŏ-seen/ The epoch of the PA-LEOGENE Period that extended from the endof the PALEOCENE, 57.8 million years ago,to the beginning of the OLIGOCENE Epoch,about 36.6 million years ago. Some classi-fications dispense with the Paleocene, thusadding another 8.6 million years to theEocene, which then becomes the firstepoch of the Paleogene. During this epochmammals were abundant, perissodactyls,including large and small horses, and artio-dactyls appeared and the modern carnivorefamilies became established. The first evi-

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stream

level of floodplain

bedrockbedrockbedrock

Section through an entrenched meander

dence of bats and whales also comes fromthis epoch.

eolian erosion /ee-oh-lee-ăn/ (aeolian ero-sion; wind erosion) The direct erosive ac-tion of wind. This is the least importantaspect of wind action, generally of littleconsequence in landscape formation whencompared with the role of wind transportand deposition. Sand blast, from the im-pact of saltating grains, is limited to belowone meter or so from the ground surface; itcan undercut rocks leaving pedestals, al-though in some instances increased weath-ering at the foot of the rock weakens itbeforehand. More significant than eolianerosion by abrasion is the production ofDEFLATION hollows. These can reach 100km across, and although partly due insome cases to faulting or rock solution,they are mainly due to wind removal ofpreweathered material down to the watertable, which halts further removal and pro-duces OASES.

eolian form (aeolian form) A landformproduced by material transported by wind.Large-scale features include DUNES, sandshadows, and sand sheets, while small-scale features include sand ripples andridges. Sand shadows, unlike true dunes,are deposited in the shelter of an obstacle,while sand sheets or seas are amorphoussheets with gentle swellings reaching 3–6m. Ripples are the products of irregulari-ties in the surface over which the sand ispassing: these initiate local concentrationsof sand grain impacts on the slopes facingthe wind, which become built up as ripples.Further, each ripple acts as a take-off pointfor grains in SALTATION, and since the aver-age length of travel per jump is related towind speed, an even repetition of areas ofconcentrated grain impacts occurs down-wind, leading to regular ripple patterns.

eolian transport (aeolian transport) Themovement of sediment by wind. Below athreshold of 16 km per hour wind is inca-pable of moving sand, but thereafter sandmovement rises as a cubic function of windspeed. Short periods of high-velocity winds

are therefore very much more effectivethan longer periods of gentle winds.

Eolian transport takes place concur-rently in three forms: suspension, SALTA-TION, and surface creep. A smallproportion of grains of less than 0.2 mm indiameter can be carried wholly in suspen-sion; the particle is totally buoyed up bythe rising eddies in the air and carriedalong parallel to the air stream. Surfacecreep accounts for about 25% of actualsand movement, and involves the move-ment of grains of coarse sand along the sur-face by the impact of the saltating grains.Saltation accounts for about 75% of sandflow and involves the bouncing of grainsalong the surface at heights of less than 1m, and mostly within 200 mm of theground surface.

eon /ee-ŏn, ee-on/ The largest interval ofgeologic time in the Chronomeric Standardscheme of chronostratigraphic nomencla-ture (see chronostratigraphy), formed ofseveral ERAS grouped together.

Eötvös unit /ay-ot-vos; Hungarian ay-œt-vœsh/ The unit employed to expressgravitational curvature or gradient. 1Eötvös unit equals 10–6 mgal cm–1. It isnamed for the Hungarian physicist BaronRoland von Eötvös (1848–1919).

epeiric sea /e-pÿ-rik/ See inland sea.

epeirogenesis /i-pÿ-roh-jen-ĕ-sis/ Upliftor subsidence of large areas of continentsor ocean basins. Compare orogenesis.

ephemeral stream /i-fem-ĕ-răl/ A streamthat contains water only immediately afterrainfall, found mainly in arid and semiaridareas. For most of the year, its channel isdry (see gully).

epicenter /ep-ă-sen-ter/ The point on theEarth’s surface situated directly above thefocus of an earthquake.

epicentral angle /ep-ă-sen-trăl/ The an-gular distance between the epicenter of anearthquake and a recording station, ex-pressed in terms of the angle subtended at

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epicentral angle

the center of the Earth between verticalsfrom these two points.

epidiorite A rock with a dioritic min-eralogy derived from the low- to medium-grade metamorphism of labradorite-pyroxene assemblages of basic igneousrocks (gabbro and dolerite), which resultsin the formation of hornblende and a pla-gioclase of more sodic composition.

epidote A group of minerals having thegeneral formula X2Y3Si3O12(OH), whereX is mainly Ca but also Mn2+, Ce3+, andother rare earths, and Y = Al, Fe3+, Mn3+,and Fe2+. The following compositionsoccur:

zoisite Ca2Al3Si3O12(OH)clinozoisite Ca2Al3Si3O12(OH)epidote Ca2Fe3+Al2Si3O12(OH)piemontite Ca2(Mn3+Fe3+,Al)3-

Si3O12(OH)allanite (orthite) (Ca,Mn2+,Ce)2(Fe2+,

Fe3+Al)3Si3O12(OH)The epidote minerals are monoclinic

except for zoisite, which is orthorhombic.Zoisite may be gray, green, or brown incolor; clinozoisite is colorless to green; epi-dote is green to yellow-green; piemontite istypically red-brown; allanite is brown toblack. A pink manganiferous variety ofzoisite is called thulite. Minerals of the epi-dote group are found in the medium-graderegionally metamorphosed rocks of thegreenschist and amphibolite facies. Epidoteis produced during retrograde metamor-phism and may be found on joint surfacesand along fractures. Plagioclase feldspar,clinopyroxenes, and hornblendes can all bereplaced by epidote minerals and basic ig-neous rocks in particular may suffer exten-sive epidotization. Allanite is found as anaccessory mineral in some acid igneousrocks.

epilimnion The warmer upper layer ofwater in a lake or shallow sea. Photosyn-thesis may occur and green plants growwithin the epilimnion because light canpenetrate it. See also hypolimnion.

epipedon /ep-ă-pee-dŏn/ A diagnosticsurface horizon, constituting that part of

the soil with organic matter or the uppereluvial horizon or both. Introduced by theUS SOIL TAXONOMY, there are six commonepipedons: mollic, histic, plaggen, an-thropic, umbric, and ochric, each being in-dicative of a certain class of soil. Forexample, MOLLISOLS are identified by themollic epipedon; soils influenced by humanuse are identified by the anthropic epi-pedon (>250 ppm acid soluble salt phos-phate due to farming) and the plaggenepipedon (a layer >50 cm deep producedby manuring).

epitaxial growth /ep-ă-taks-ee-ăl/ Theparallel or orientated overgrowth of onemineral on a crystal of another such thatthere is some form of continuity main-tained between the crystal structures ofboth minerals. Examples include the over-growth of augite on orthopyroxene andidocrase on garnet.

epithermal deposit /ep-ă-th’er-măl/ Seehydrothermal process.

epoch An interval of geologic time in theChronomeric Standard scale of chrono-stratigraphic classification (see chrono-stratigraphy). The equivalent StratomericStandard term, indicating the body of rockformed during this time, is the SERIES. Sev-eral epochs together form a PERIOD and arethemselves formed of a number of AGES.

equal-area projection See homolo-graphic projection.

Equator An imaginary line that girdlesthe Earth at latitude 0°. It is a GREAT CIRCLE

40 076 km long. North of it is the N hemi-sphere; to the south is the S hemisphere.

equatorial current Among the promi-nent currents that flow in the oceans arethose in the equatorial regions. The cur-rents flowing at or close to the Equator inthe Pacific Ocean are, in general, similar inpattern to those flowing in equatorial re-gions of the Atlantic Ocean, there being aSouth Equatorial Current flowing astridethe Equator, which is separated from thewestward-flowing North Equatorial Cur-

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rent by an eastward-flowing EquatorialCountercurrent. However, this countercur-rent is much more strongly developed inthe Pacific than is its counterpart in the At-lantic. There are also currents that flow atsome depth; for instance, the CromwellCurrent (also known as the Pacific Equa-torial Undercurrent) flows as a narrowswift current in an easterly direction and ata fairly shallow depth beneath the SouthEquatorial Current. The flow rate in theEquatorial Countercurrent probably at-tains some 25 million cubic meters per sec-ond, with speeds of up to 2 knots.

equatorial rainforest See tropical rain-forest.

equatorial trough The belt of low pres-sure that occurs in equatorial areas and os-cillates in its mean location dependingupon the position of the overhead Sun. Itmarks the convergence zone of the tradewinds blowing from each hemisphere andis most clearly observed over the oceanareas where continental modifications ofairflow are least.

equatorial westerlies A zone of rathervariable winds, found when the INTERTROP-ICAL CONVERGENCE ZONE extends morethan 5° from the Equator. They probablyrepresent trade-wind air that has acquireda westerly component on crossing theEquator but their origins may vary de-pending upon the precise location of for-mation.

equatorial zenithal gnomonic projec-tion A MAP PROJECTION in which the tan-gent touches the surface of the Earth at theEquator. All the great circles are straightlines but the angle of intersection of themeridians and the parallels (shown bycurves) becomes increasingly more acutepoleward, and therefore the distortion isgreater. Places best represented on this pro-jection are those near the Equator, e.g.Africa; beyond about 30° north or souththe distortion is too large for the projectionto be of any use.

equigranular /eek-wă-gran-yŭ-ler/ De-

scribing a rock that has crystals of a uni-form grain size.

equilibrium profile (graded profile) Along profile of a river in which there is adelicate balance between erosion and de-position. Slope of the equilibrium profile isadjusted so that the sediment load pro-vided to the river can just be transported bythe available discharge and channel char-acteristics. Traditionally this is thought ofas being a concave profile, but in practice itneed not be perfectly smooth. Streams withuneven long profiles have been found to bein equilibrium in the sense that in no partof their profile is active erosion or deposi-tion taking place. See also dynamic equilib-rium; equilibrium regime.

equilibrium regime The regime of anatural system, such as a river channel,coastline, slope profile, etc., when it is in astate of balance considered over time, nei-ther eroding nor depositing. The term ismost often used in connection with rivers,where it is largely synonymous with GRADE.

equinox /ek-wă-noks/ The period of theyear when the Sun is overhead at localsolar time at the Equator. This happenstwice a year, about March 21 and Sept. 22,when day and night are equal time periodsof twelve hours. Because of refractionthrough the atmosphere, actual day lengthis slightly longer than the true astronomi-

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equinox

projected viewon tangent plane

0°

45°

30°

0°

pole

Equator

Equatorial zenithal

cal day. In NW Europe, the first and sec-ond equinoxes are taken to mark the be-ginning of spring and of fall respectively.

era An interval of geologic time in theChronomeric Standard scheme of chrono-stratigraphic nomenclature (see chrono-stratigraphy). It is formed of severalPERIODS grouped together and a number oferas may be compounded to form an EON.For example, the Paleozoic Era is com-posed of six periods (the Cambrian, Or-dovician, Silurian, Devonian, Carbonifer-ous, and Permian) and lasted for some 325million years, from about 570 to 245 mil-lion years ago.

erg 1. A type of arid desert landscapeconsisting of a very extensive sand cover,especially in the Sahara. See also ham-mada; reg.2. A unit of work or energy equal to thework done by a force of 1 dyne actingthrough a distance of 1 cm. 1 erg is equalto 10–7 joules.

erosion The lowering of the land surfaceby agents that involve the transport of rockdebris. Unlike degradation, erosion tendsto be an episodic process, varying greatlyover time and space (see cycle of erosion).For erosion to occur, the eroding medium(gravity, river flow, waves, currents, wind,ice, etc.) must exert a force on the land sur-face greater than its shear strength. Theelastic limit of the material then being ex-ceeded, fractures form and the materialmoves in the direction of the force appliedto it. The movement may be sharp and sud-den, as in LANDSLIDES and ROCKFALLS, orslow and imperceptible, as with CREEP.Compare denudation. See also differentialerosion.

Soil erosion is a great problem becausethe surface part is the most fertile and plantnutrients have to be replaced artificially.Wind erosion of soil can be severe in semi-arid regions (see dust bowl). Water erosion(sheet wash, rill action, gullying, etc.) isusually most prominent in humid lands,e.g. in the Tennessee Valley. See also ab-normal erosion.

erratic A rock that has been transportedby ice and deposited in an area of dissimi-lar rock type. Volcanic and metamorphicrocks frequently make the most impressiveerratics because they are often distinctivelycolored and their high resistance to erosionpermits their continued existence despiteconsiderable transportation. By linkingsource outcrop with final resting place,some insight into directions of ice move-ment can be gained, although one erraticmay, in turn, be moved by several icemasses in different directions. The exis-tence of some erratics hundreds of metersin extent indicates the phenomenal poweravailable in glacial transportation.

eruption plume A cloud of gas, rockfragments, and molten magma that risesinto the air above an erupting volcano. Itsheight depends on its density and the pres-sure that forced it upwards.

escape velocity The initial velocity thata particle, space probe, etc., on the surfaceof a body such as the Earth would requireto overcome the gravitational influence ofthat body and move away from it.

escarpment See scarp.

esker /esk-er/ An elongate ridge ofrounded stratified FLUVIOGLACIAL deposits,consisting primarily of sands and gravelswith some finer and coarser materials; theycan be only tens of meters long but someextend for hundreds of kilometers. Eskersform in contact with stagnant or very slowmoving ice from materials deposited bymeltwater streams, which may be under-neath, within, or above the ice mass. Thecross-sectional shape of an esker will de-pend largely upon the position of the melt-water stream. If it is above the ice, the eskerwill have an ice core and melting will causecollapse; if it is in tunnels within the ice,collapse will also occur when the wallsmelt. If the tunnels are high and narrow,sharp-crested ridges are produced, whereaslow wide tunnels result in flat-topped andcomparatively undisturbed eskers. See alsobeaded esker.

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essential mineral Any of certain diag-nostic minerals on the presence or absenceof which is based the classification andsubdivisions of igneous rocks. For exam-ple, the presence of calcic plagioclase andpyroxene is implied in the term gabbro.

essexite /ess-iks-ÿt/ A type of ALKALI

GABBRO.

estuarine (in ecology) Describing an en-vironment at a river ESTUARY, where thereare tidal effects and fresh and salt watermix.

estuary The part of a river mouth withinwhich tides have an effect and thereforewhere fresh and saline water are mixed.Most present-day estuaries are DROWNED

VALLEYS, owing their existence to the post-glacial rise in sea level, and for this reasonthey usually contain much deposited sedi-ment. Deposition will continue if the riverintroduces more sediment than can be re-moved by tidal streams and by what littlewave action is possible within the estuary.Many estuaries exhibit intricate patterns ofchannels, largely the result of erosion byboth incoming and outgoing tidal streams,which give flood and ebb channels respec-tively. See also braiding; delta.

etesian wind /i-tee-zhăn/ A wind blow-ing from a northerly direction in theAegean Sea or E Mediterranean during thesummer period. Such winds are a responseto the shallow thermal low pressure overthe Sahara and the northward movementof the Azores anticyclone to be centeredover the W Mediterranean.

eucrite /yoo-krÿt/ A variety of coarse-grained basic igneous rock containingclinopyroxenes and orthopyroxenes,olivine, and plagioclase of bytownite-anor-thite composition.

euhedral /yoo-hee-drăl/ Describing crys-tals that are well developed and have goodcrystal faces. Compare anhedral; subhe-dral.

Euler’s theorem /oi-lerz/ A theorem

that provides a mathematical explanationfor the distribution of conservative PLATE

BOUNDARIES. Plate tectonics requires thatall conservative plate boundaries (trans-form faults) lie on small circles, the axes ofwhich form the axis of rotation for the rel-ative motion of the plates on each side. Italso indicates that the velocity of relativemotion across a destructive or constructiveplate boundary is proportional to the an-gular distance of the particular point fromthe axis of rotation and to the angular ve-locity about the axis of rotation for the mo-tion of the plates. Thus velocities will varyalong plate boundaries. The theorem isnamed for the Swiss mathematician Leon-hard Euler (1707–83).

eulittoral zone /yoo-lit-ŏ-răl/ See lit-toral zone.

euphotic zone /yoo-foh-tik/ Ocean depthsto which abundant sunlight for photosyn-thesis penetrates, extending from the sur-face to about 80 m. See also aphotic zone;disphotic zone.

eustasy /yoo-stă-see/ Worldwide move-ments of sea level. The origin of thesemovements is most commonly attributedto the withdrawal and release of water dueto the growth and decay of ice masses inthe Quaternary (GLACIO-EUSTATISM). Buteustatic movements can also be caused bytectonic movements of the sea floors orlandmasses. It may be difficult to distin-guish between the two, but most effects canbe attributed to glacio-eustasy. Since thelast ice age there has been a gradual rise insea level as conditions have become rela-tively warmer. This is a positive eustaticchange. See also base level.

eutaxitic /yoo-taks-it-il/ Denoting thestreaky or banded appearance common inignimbrites. Individual streaks or fiammeare discontinuous flattened bodies. Theterm parataxitic is applicable when the fi-amme are extremely elongated. See pyro-clastic rock.

Eutheria /yoo-theer-ee-ă/ The placentalmammals. Their young are born at a late

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Eutheria

stage in development, having been nour-ished inside the body of the mother bymeans of a connection called the placenta.Fossils suggest that the Eutheria divergedfrom other mammalian groups in the Cre-taceous, but their main evolutionary radia-tion took place during the Cenozoic, afterthe extinction of the dinosaurs. Theyrapidly became the dominant land animalsand now include such diverse forms as theflying bats and the marine whales (whichare the largest living animals known).Compare Marsupialia.

eutrophic /yoo-troff-ik/ (in ecology) De-scribing a freshwater HABITAT in which thewater is rich in plant nutrients. See eu-trophication.

eutrophication /yoo-trŏ-fă-kay-shŏn/The process by which a freshwater HABITAT

becomes excessively enriched with nutri-ents. These nutrients include nitrates andphosphates (generally from artificial fertil-izers washed off farmland by rainwater)and possibly sewage. As a result, there israpid growth of algae (an algal bloom),some of which die because insufficient lightcan penetrate the overgrown water. Thedead algae decompose, using up dissolvedoxygen and leading in turn to the death offish and other aquatic animals.

euxinic environment A marine envi-ronment in which the bottom water ispoorly ventilated, deficient in oxygen, and(in extreme cases) characterized by thepresence of hydrogen sulfide, i.e. chemi-cally reducing conditions. Such conditionsapply, for example, to parts of the BlackSea and to certain of the Norwegian fiords,where sea-floor basins occur at differentdepths. One effect is that the floor of theBlack Sea is sparsely populated with livingorganisms. The seabed deposits in euxinicenvironments are characteristically fine-grained ones containing a high percentageof decomposable organic material.

evaporation The process by which aliquid is changed into a gas or vapor. Therate of evaporation in the atmosphere rep-resents a net effect between the removal of

water molecules from the surface layersand replacement by other water moleculesin the lower layer of the atmosphere, whichthrough continual motion strike the sur-face and become absorbed. As the mol-ecules are being transferred from a lower toa higher energy state through evaporation,a supply of energy must be provided for theprocess to be maintained. This is a very im-portant aspect of the Earth’s energy bal-ance; by this process surplus energy intropical areas is transferred by the flux ofwater vapor to the radiation-deficit areasnearer the poles.

If evaporation continued into still air,the overlying layer would very soon reachsaturation and prevent further evapora-tion. However, air movement usuallymixes lower layers by turbulence, reducingthe water vapor content and enabling fur-ther evaporation to take place. Clearly, thestronger the wind, the greater will be therate of evaporation as long as the air is notsaturated. This introduces the second fac-tor influencing evaporation, that is the re-quirement for the air to be capable ofabsorbing moisture. An index of this is thesaturation deficit, which is the differencebetween the saturation vapor pressure andactual vapor pressure. If the saturationdeficit is large, as in warm dry air, the gra-dient between the moist surface and the at-mosphere will be high, and so the rate oftransfer will be large. With moist air, thehumidity gradient will be less and the rateof evaporation correspondingly smaller.

The measurement of evaporation is dif-ficult because of the problem of adequatelysimulating the evaporation surface. Thestandard method is to measure the loss ofwater from an evaporation pan surface,but it is accepted that this does not equatevery well with true atmospheric evapora-tion. Measurements of the changes inweight of soil samples are a closer ap-proach to reality but still differ from thenatural situation.

evaporimeter See lysimeter.

evaporite /i-vap-ŏ-rÿt/ A rock or depositformed as a precipitate from a saturatedsolution. Evaporation from virtually closed

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128

bodies of saline water is the most commonprocess involved. Evaporites are classifiedaccording to their chemical composition.Great thicknesses of evaporite depositsoccur, with minerals layered in a sequencedepending upon their relative solubilities.

evapotranspiration /i-vap-oh-tran-spă-ray-shŏn/ The combined system of vaportransfer by evaporation and transpirationfrom the ground surface and its vegetationlayer. Except in areas of sparse vegetation,such as deserts, transpiration by plants isthe dominant factor in the total loss ofwater from the land surface. See hydro-logic cycle.

evergreen trees Trees that do not shedtheir leaves in winter or the dry season.Typical evergreens include conifers andtrees of the tropical rainforest. Comparedeciduous forest.

evolution The gradual change in organ-isms through time, resulting in the origin ofnew SPECIES. In 1859 the British naturalistCharles Darwin (1809–82) suggested theprocess of NATURAL SELECTION as the meansby which this could come about. The di-versity of extinct and modern organismshas resulted from their becoming adaptedto particular habitats. Evolution is gener-ally a progressive change, with organismsbecoming more able to deal with ever moreextreme environments. This theory ofmodification and change is opposed to thedoctrine of Special Creation, which holdsthat species are immutable and are re-placed at intervals by newly created forms.Much fundamental evidence for the vari-ous pathways of evolutionary changecomes from paleontology. See also conver-gent evolution; parallel evolution.

exfoliation The splitting of rocks into aseries of concentric shells by a number ofdifferent weathering processes: UNLOAD-ING, SPHEROIDAL WEATHERING, and FLAK-ING.

exhumation The exposing by denuda-tion of a surface or feature that had previ-ously been buried by deposition. Theories

of exhumation have been used to explainthe creation of a number of landforms,such as TORS and INSELBERGS. A first stageconsists of progressive DEEP WEATHERING

on a rock type embodying considerablevariation in its susceptibility to decomposi-tion, which largely reflects variation in itsjointing pattern. A highly irregular WEATH-ERING FRONT develops and may remain fora considerable length of time. Subsequenterosion of the REGOLITH, initiated by earthmovements or sea-level changes, will even-tually exhume the least weathered rockmasses.

exosphere /eks-ŏ -sfeer/ The upper layerof the Earth’s atmosphere, extending up-wards from a height of about 600 km. Itconsists mainly of hydrogen and helium invarying proportions. See also atmosphere.

exotic A boulder or large rock body thathas been transported by one of a variety ofprocesses into an area of unrelated rocktypes. Where the transporting mechanismis tectonic in origin the rock body is said tobe ALLOCHTHONOUS, whereas if ice is thetransporting mechanism such blocks arecalled ERRATICS.

exposure (in meteorology) The degreeof openness of a site and the amount ofinterference to airflow by natural and arti-ficial obstacles. For meteorological instru-ments to give representative values, theymust be sited correctly with a uniformity of exposure. The correct exposure variesfrom element to element, but is most im-portant for rain gauges and anemometers,where airflow can affect the recorded val-ues. It is never measured quantitatively al-though attempts to do so have been made.

exsolution /eks-sŏ-loo-shŏn/ (unmixing)Several minerals, notably feldspars and py-roxenes, are homogeneous solid solutionsof two or more chemical end-members athigh temperatures. During cooling, the ho-mogeneous mix becomes unstable andsuch a mineral unmixes, producing twodistinct mineral phases. For example, analkali feldspar rich in potassium may un-dergo exsolution to produce a perthite in-

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exsolution

tergrowth consisting of separate structuralunits of sodium and potassium feldspar, in-dicated by the occurrence of blebs of albitewithin an orthoclase host.

external magnetic field That part ofthe Earth’s magnetic field that results fromeffects produced above the Earth’s surface.

extinction The disappearance of aSPECIES or other group of organisms repre-senting a particular evolutionary line. Itmay occur when an organism that has be-come highly specialized for living in a par-ticular environment is unable to adapt toextreme changes in its HABITAT. Some ex-tinctions affect a wide variety of habitats,as occurred at the end of the PALEOZOIC andat the end of the MESOZOIC Eras. The latterextinction affected the dinosaurs on land,pterodactyls in the air, and ichthyosaurs inthe sea; many invertebrate groups, such asthe ammonites, also disappeared (see K/Tboundary event). The causes of these wide-spread extinctions are not clearly under-stood. Major climatic changes combinedwith unsuccessful competition with othergroups of organisms have been suggestedas likely causes: the extinction of one group

is often broadly contemporaneous with theevolutionary radiation of another unspe-cialized and often insignificant group.

extrusion The process by which magmaemerges onto the Earth’s surface throughvents and fissures to form lava flows. Theterm is also applied to the structure or formthat results from this process. See also ex-trusive rock. Compare intrusion.

extrusive rock A rock that forms afterigneous materials cool and solidify on thesurface of the Earth. The rapid cooling gen-erally results in a fine-grained type of rock.Compare intrusive rock.

eye The central area of a tropical cyclone(hurricane or typhoon), where wind speedsare light, breaks in the main cloud sheetsappear, and the driving rain stops. The di-ameter of the eye varies, reaching up to 80km in the larger storms, although its dura-tion depends upon the speed of movementof the storm. The weather in the eye feelsparticularly oppressive after the strongwinds preceding it, but it is soon followedby even stronger winds in the opposite di-rection.

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face 1. One of the surfaces of a CRYSTAL.2. The chief or most obvious surface of alandform (e.g. the face of a CLIFF).

facies /fay-shee-eez/ 1. A collection ofmetamorphic rocks that have formed overthe same range of physical conditions. Dif-ferences in mineralogy are attributed to thevariations in chemical composition of theoriginal rock types. For example, at theamphibolite facies, a shale may be repre-sented by the assemblage staurolite + gar-net + biotite + plagioclase + quartz, a basaltby hornblende + plagioclase + quartz, andan impure limestone by diopside + calcite +quartz. A number of metamorphic faciesand subfacies are recognized in bothregional and contact metamorphism.Metamorphic facies and their relative tem-peratures (T) and pressures (P) are as fol-lows:Contact metamorphic facies – low P, lowto high T.Zeolite facies – low T, low to intermediateP, gradational from DIAGENESIS.Greenschist facies – low to intermediate Pand T.Amphibolite facies – intermediate to high Pand T.Glaucophane schist facies – low T, veryhigh P.Granulite facies – high P and T.Eclogite facies – high T and very high P.See also grade (def. 1); zone (def. 1).2. A rock unit or group of associated rockunits having particular features, lithologi-cal, sedimentological, and faunal, that re-flect some specific environmentalconditions. Thus, rocks deposited at thesame time in different regions may be dis-similar because they are of different facies,as is the case, for example, with the PERMO-TRIASSIC rocks of marine facies and conti-

nental facies in Britain. Correlation of onefacies with another may be difficult, espe-cially if the fossils they contain are re-stricted to each particular facies. If this isthe case they are known as facies fossils. Aunit of a particular facies may be diachro-nous, that is, it is present in different areasat different times.

facing direction The direction in whichyounger beds of rock lie in relation to therocks in a FOLD. If the younger rocks lieabove the fold, the facing direction is up-ward; if they lie below, it is downward.

Fahrenheit scale /fa-rĕn-hÿt/ A scale oftemperature in which the freezing point ofwater is at 32° and the boiling point at212°. The scale is named for the Germanphysicist (Gabriel) Daniel Fahrenheit(1686–1736).

fair-weather cumulus A CUMULUS cloudwith limited vertical extent. The clouds areprevented from rising farther by an inver-sion, usually formed by subsidence in anarea of high pressure. Because such areasrarely give rain, clouds formed in this wayare known as fair-weather cumulus.

false bedding An obsolete term forCROSS-BEDDING.

family A group in the taxonomic classi-fication of organisms. A number of relatedfamilies together form an ORDER, and afamily itself is composed of one or moregenera (see genus). For example, the Ho-minidae, to which MAN belongs, is a familyof the order Primates. See taxonomy.

fan cleavage CLEAVAGE in which theplanes are not parallel but form a fanlike

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F

arrangement with either an upward ordownward convergence. This arrangementoften results from rotation of beds in thelimbs of a developing fold.

fathom A unit of depth equal to 1.83 m(6 feet).

fault A fracture in the Earth’s crustalong the plane of which there has beendisplacement of rock on one side relative tothe other, either in a horizontal, vertical, oroblique sense. (See illustration opposite.)

fault basin A depression separated fromthe enclosing higher ground by faults.

fault block 1. An area of the Earth’scrust that behaves as a single unit duringfaulting.2. A rock body bounded by at least twofaults, which can be either elevated or de-pressed relative to the surrounding region.

fault breccia The mass of broken rockfragments along a fault plane producedduring dislocation.

fault line The line along which a FAULT

intersects the surface of the ground.

fault-line scarp A type of cliff, formedoriginally by a FAULT, whose face has re-treated because of erosion. The originalfault line lies in front of the scarp under ac-cumulated rock fragments. See also faultscarp.

fault plane The surface along which aFAULT forms.

fault scarp A type of cliff formed whena block of rocks is forced upward by aFAULT. It is located on the FAULT LINE. Seealso fault-line scarp.

faunal province See province.

fayalite /fay-ă-lÿt, fÿ-ah-/ A mineral ofthe OLIVINE group.

feldspar /feld-spar, fel-/ The most im-portant type of rock-forming minerals and

the most abundant minerals in igneousrocks. Feldspars have a framework struc-ture in which (Si,Al)O4 tetrahedra arelinked together with calcium, sodium,potassium, and barium ions occupying thelarge spaces in the framework. Feldsparsmay be considered to be mixtures of thefour components:

CaAl2Si2O8 – anorthite (An)NaAlSi3O8 – albite (Ab)KAlSi3O8 – orthoclase (Or)BaAl2Si2O8 – celsian (Ce).

Barium feldspars are rare and mostfeldspars are members of the ternary sys-tem An-Ab-Or. Members of the series be-tween NaAlSi3O8 and CaAl2Si2O8 areplagioclase feldspars and those betweenNaAlSi3O8 and KAlSi3O8 are alkalifeldspars.Plagioclase feldspars. The replacementNaSi↔CaAl results in a complete grada-tion between the two end members of theseries, calcic (anorthite 100–90% An,0–10% Ab; bytownite 90–70% An,10–30% Ab; labradorite 70–50% An,30–50% Ab) and sodic (andesine 50–30%An, 50–70% Ab; oligoclase 30–10% An,70–90% Ab; albite 0–10% An, 100–90%Ab).

Plagioclases are triclinic and exhibitmultiple lamellar twinning. They are milkywhite or colorless. Plagioclase occurs bothas phenocrysts and in the groundmass ofmost basic and intermediate igneous rocks.Calcic plagioclases are characteristic ofbasic rocks and anorthosites. Sodic plagio-clases are found in intermediate rocks. Al-bite occurs in some granites and also inspilites. Plagioclase is also found in a widevariety of metamorphic rocks.Alkali feldspars. At high temperatures thereplacement Na↔K results in a continuouschemical series between NaAlSi3O8 andKAlSi3O8. In most volcanic rocks, thefeldspar has crystallized at a high tempera-ture and has been quenched, thus retainingits high-temperature crystal structure. Slowcooling under plutonic conditions or crys-tallization at relatively lower temperaturesresults in the formation of two separatefeldspars, one rich in NaAlSi3O8, the otherin KAlSi3O8. The homogeneous high-tem-perature structural state of an alkali

fathom

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133

Fig. 1: Thrust fault

P = principal stress

P max

P min

P inter

Fig. 2: Dextral wrench fault

Fig. 3: Transform fault

Fig. 5: Horst

Fig. 6: Normal fault

Fig. 7: Reverse fault

P inter

P maxP min

spreading direction

sense of fault movement

ridg

e

oce

an

mid

-

graben

Fig. 4: Graben

horst

P maxP interP min

footwall

hangingwall

downthrow

Types of fault

feldspar may become unstable at a lowertemperature and unmixing occurs by therearrangement of the alkali ions in the solidstate. The resulting intergrowth of the twocomponents is called perthite.

Sanidine is a high-temperature mono-clinic potassium feldspar found in volcanicrocks, such as trachytes and rhyolites.Anorthoclase is a high-temperaturesodium-potassium feldspar. Orthoclase(also monoclinic) and microcline (triclinic)are intermediate- to low-temperaturepotassium feldspars found in plutonic acid-and alkali-rich rocks. Adularia is a verylow-temperature monoclinic potassiumfeldspar found in pegmatites. Alkalifeldspars also occur in a wide variety ofmetamorphic rocks. Sanidine and ortho-clase exhibit simple twinning but micro-cline and anorthoclase have multiple twinsin two directions at right angles, producinga tartan effect. Alkali feldspars are color-less, white, pink, or red.

feldspathic /feld-spath-ik, fel-/ Describ-ing a rock or mineral aggregate that con-tains FELDSPAR, usually in a specifiedamount. See also feldspathoid.

feldspathoid /feld-spă-thoid, fel-/ Amember of a group of undersaturated min-erals in which the alkali cations sodiumand potassium are located in cavitieswithin the framework structure of(Si,Al)O4 tetrahedra. Feldspathoids appearin place of feldspar in rocks deficient in sil-ica. They do not occur with free quartz be-cause they react with silica to formfeldspar.

Nepheline has an ideal compositionNaAlSiO4 but always contains somekalsilite (KAlSiO4), the amount of solid so-lution increasing with increasing tempera-ture. Both nepheline and kalsilite are whiteor colorless and hexagonal. Carnegieite is ahigh-temperature cubic form of nephelineand kaliophilite is a rare form of kalsilite.Nepheline is a primary mineral in many al-kaline igneous rocks and is produced dur-ing the metasomatic alteration of rocksthrough the process of FENITIZATION.Nepheline also occurs as a result of the re-action between basic magma and lime-

stone. Kalsilite is found in potassium-richvolcanic rocks.

Leucite is a white pseudocubic mineralof composition KAlSi2O6 and often showscomplex twinning. It occurs as euhedralicositetrahedral crystals in potassium-richbasic lavas. Analcime (analcite) is a whiteor pink cubic mineral of compositionNaAlSi2O6.H2O with some replacementK↔Na. It occurs as a late-stage mineral inbasic and intermediate igneous rocks.

The minerals of the sodalite subgroupare as follows:

sodalite 3(NaAlSiO4).NaClnosean 3(NaAlSiO4).Na2SO4haüyne 3(NaAlSiO4).CaSO4

These minerals are cubic, commonly bluein color, and are found in alkaline igneousrocks such as nepheline syenites andphonolites. Lazurite is the major con-stituent of lapis-lazuli, formed by the meta-morphism of limestone. Cancrinite is ahexagonal white or colorless mineral of ap-proximate composition Na8(AlSiO4)6-(HCO3)2. It is formed as a late-stage orsecondary mineral as a result of the reac-tion between nepheline and carbon diox-ide.

fell An upland in northern England, usu-ally covered by heathland or rough pasturefor the summer grazing of sheep.

felsic /fel-sik/ Denoting light-coloredminerals such as feldspar, feldspathoids,and quartz. Compare mafic.

felsite /fel-sÿt/ Any crystalline acid vol-canic rock.

femic /fem-ik/ Denoting the iron- andmagnesium-rich minerals of the CIPW nor-mative classification (see norm). Comparesalic.

fen A flat marshy low-lying region, oftenlocated near the coast where rivers andlakes have silted up. Such terrain is namedafter the Fens in E England, where themarshes have been drained and turned intofertile farmland.

fenite /fen-ÿt/ A type of igneous rock

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formed by METASOMATISM through contactwith a CARBONATITE intrusion. It is an al-kaline SYENITE.

fenitization /fen-ă-tă-zay-shŏn/ The al-kaline metasomatism of country rock,typically gneissose, surrounding ijolite-carbonatite complexes. Quartzo-felds-pathic rocks may be completely made over to syenitic alkali feldspar-nepheline-aegirine assemblages, termed fenites. Feni-tization precedes or accompanies the em-placement of the carbonatite magma.

fen soil See organic soil.

fenster See window.

ferrallitic soil /fe-ră-lit-ik/ (latosol; lat-eritic soil; red earth) A soil that is typi-cally found on the old planation surfaces ofthe humid tropics. The chief soil-formingprocess is ferrallitization where weatheringand leaching are intense. Rapid formationresults in a deep profile rich in clay and hy-drated oxides of iron, aluminum, and man-ganese. Free drainage gives a redcoloration to the soils but they may be yel-lowish where drainage is poorer. Any plantremains are rapidly broken down and thereleased nutrients are immediately usedagain by the plants so that little organicmatter accumulates. This factor plus thekaolinitic clay results in a low cation ex-change capacity, which makes these soilsof little agricultural value. They fall intothe OXISOL order of the US SOIL TAXONOMY.

ferrimagnetism /fe-ri-mag-nĕ-tiz-ăm/ Atype of weak FERROMAGNETISM in whichthe magnetic domains of a substance arealigned in opposite directions, althoughthere are slightly more of one alignmentthan the other. The mineral MAGNETITE is aferrimagnetic material.

ferrisol /fe-ră-sôl/ A soil that has devel-oped similarly to FERRALLITIC and FERSIAL-LITIC SOILS, its profile being less intenselyweathered and leached than ferrallitic soilsbut more so than fersiallitic soils. Unlikeferrallitic soils these are found on slopingsites where surface erosion is an important

process. The profile is similar to a ferralliticsoil, being red in color and having a similarstructure, but often clay movement resultsin a textural B horizon. These soils have alow cation exchange capacity but are morefertile than ferrallitic soils. They fall intothe ULTISOL order of the US SOIL TAXONOMY.

ferroaugite /fe-roh-aw-jÿt/ A mono-clinic PYROXENE.

ferromagnesian mineral /fe-roh-mag-nee-shăn, -zhăn/ A mineral that is rich iniron and/or magnesium and usually dark incolor, for example olivines, pyroxenes, am-phiboles, and biotite.

ferromagnetism /fe-roh-mag-nĕ-tiz-ăm/A type of magnetism exhibited by sub-stances (usually metals) that have a mag-netic permeability very much larger than 1.Cobalt, iron, nickel, and many of their al-loys are ferromagnetic materials, used formaking permanent magnets. They showsuch behavior because, in a magnetic field,their magnetic domains become perma-nently aligned in one direction.

ferruginous /fĕ-roo-jă-nŭs/ Describing asubstance that contains iron, such as a sed-imentary rock (e.g. types of sandstone)whose constituent grains are cemented to-gether with iron oxide (see cementation).

fersiallitic soil /fer-shă-lit-ik/ (ferrisialliticsoil) A tropical soil formed in a lesshumid climate than FERRALLITIC SOILS. Theprofile differs in that weathering and leach-ing are not so dominant, producing a soilthat is not so deep. Where the parent ma-terial is particularly resistant only an A Cprofile occurs, although sometimes an A BtC profile develops. The whole profile isricher in weatherable minerals and has ahigher cation exchange capacity than fer-rallitic soils.

fertilizer Any organic substance or inor-ganic salt applied to the soil to improvecrop production. The nutrient elementsmost important for plant growth are nitro-gen, phosphorus, and potassium.

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fetch The distance of the stretch of openwater over which the frictional effect of awind that is blowing in a constant directionis actually generating waves. In small seaareas, fetch distance is usually limited bythe dimensions of that sea area; in the openocean, however, the limit is usually im-posed by the constantly changing meteoro-logical situation. There is also a relationbetween fetch distance, wind strength,wind duration, and the maximum size ofwaves produced. For a given wind speed,the largest possible waves require a mini-mum fetch distance; and for a given fetchdistance, there is an optimum size wavethat can be generated, however hard orlong the wind blows. Very large oceanwaves may require fetch distances of theorder of 1000 km or more.

fiamme /fyam-ay/ Flattened disk-shapedglassy bodies found in ignimbrites and im-parting a eutaxitic structure. See pyroclas-tic rock.

fibrolite /fÿ-brŏ-lÿt/ A fibrous or acicu-lar form of SILLIMANITE. See aluminum sili-cates.

field capacity (field moisture capacity,soil moisture storage capacity) The waterthat remains in the soil after it has been sat-urated and then left to drain by gravity. Itis usually expressed as a percentage of theoven-dried soil.

field completion A check of a mapunder compilation carried out in the fieldto ensure that all the details are shown andto make any corrections or amendmentsthat may be necessary. Details usually af-fected by the field check are names andcivil boundaries.

field intensity The strength of a mag-netic field at a particular point on theEarth’s surface.

filtering (in climatology) A method ofseparating components of a selected timeperiod from all other possible periodiccomponents. It is most frequently used forinvestigations into changes of climate.

filter pressing See magmatic differentia-tion.

finger lake A deep long narrow lake,usually occupying a valley carved byglacial erosion. The Finger Lakes in NewYork State are well-known examples.

fiord /fyord/ (fjord) A long narrow seainlet lying between steep mountain slopes,which often reach up to several hundredmeters above sea level. Fiords are glaciatedvalleys which, owing to a relative rise ofsea level after the melting of the Pleistoceneice sheets, have become flooded by the sea.Many fiords have very considerabledepths, some well in excess of 1000 m. Be-cause of glacial action, they are character-ized by the existence of gouged basinsseparated from the open sea by sills of solidrock, often capped by moraine, markingthe point where the glacier that cut the val-ley lost much of its erosive power, eitherthrough melting or slowing down. Thecross section of a fiord is often U-shaped.

fireclay A type of clay that can be bakedto high temperatures without crumbling orfusing. Fireclays are composed mainly ofhydrous aluminum silicate, as in illite,kaolin, or kaolinite (see clay minerals), andoften occur beneath beds of coal.

firn /fern/ (névé) A half-way stage in thetransformation of fresh snow into glacierice. Fresh snow is very loosely packed andhas a density between 0.1 and 0.2. Com-paction and recrystallization reduce thepore space between grains, increasing thedensity. Firn exists when the densityreaches 0.5. The word means ‘of last year’,and in this case refers to snow that has sur-vived one summer. Further compactioncontinues until glacier ice of density 0.89 to0.90 is formed, although this may takemany years.

firth (in Scotland) A lengthy estuary orarm of the sea. Firths bear some similarityto FIORDS and some have the closed anddeep depressions and rock sills typical offiords. They developed from river orglacial valleys that later experienced the

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postglacial transgression, becoming partlyflooded by the sea. Firths on Scotland’s Ecoast are not classed as the fiord type. Thelochs and firths of the W coast of Scotlandare fiords but have rather flatter hillsideslopes than, for example, the fiords of Nor-way and Alaska.

fish One of a variety of aquatic verte-brates belonging to any of the followingclasses: AGNATHA, PLACODERMI, CHON-DRICHTHYES, and OSTEICHTHYES.

fissure Any extensive break, cleft, orfracture in the Earth’s surface, generallycaused by earth movements (earthquakesand faulting) or volcanic action. See alsocrevasse.

fissure eruption A volcanic eruptionthrough a FISSURE rather than a centralvent. The largest occur along MID-OCEAN

RIDGES, where molten magma rises throughfissures in the oceanic crust.

fjord /fyord/ See fiord.

flaking The splitting of rocks aroundtheir margins into curved flakes, whichvary in thickness from a few millimeters toa few centimeters, depending on theprocess involved. The most commonprocess is the chemical decay of minerals,especially HYDRATION, causing expansion.The growth of salt crystals from solutionbeneath the surface of porous rocks cangive rise to flaking, the flakes in this casebeing frequently backed by a thin layer ofsalt. In both these cases there tends to be aseries of concentric layers of flakes aroundone boulder. Flaking results in a roundingof the block, because there is a preferentialattack on edges and corners. The processcan take place only above ground level,and the final development is a flat rock sur-face. Flaking on concave surfaces produceshollows.

flame structure A sedimentary struc-ture in which an underlying bed has beensqueezed up into an overlying horizon inflamelike tongues as a result of loading.

Generally there is some degree of horizon-tal displacement.

flap structure A structure resultingfrom gravity collapse, consisting of a bedoriginally on the limb of an anticline thathas bent as it slipped downward and cometo rest in an inverted position.

flash flood A flood of sudden occur-rence and short duration that results fromrapid runoff after heavy rainfall. Flashfloods are commonest in desert and semi-arid areas, where there is little or no vege-tation to prevent fast runoff, although theyalso occur in wetter regions, especiallywhere streams occupy small drainagebasins with steep slopes. The water mayflow along gullies and cause a flood manykilometers away from the site of the rain-fall. Flash floods often occur too rapidly tobe forecast and may cause considerabledamage and loss of life.

flexure fold /flek-sher/ A type of foldbest developed in rocks where there is goodlayering. As the fold develops the beds slipover one another.

flint A nodular variety of CHALCEDONY

formed in chalk. See also silica minerals.

flocculation /flok-yŭ-lay-shŏn/ (in soilscience) The process by which colloidal(i.e. clay-size) particles join together form-ing groups, called floccules. Colloidal ma-terial tends to adopt this habit in thepresence of neutral salts, notably salts ofcalcium; in the presence of alkaline salts,especially sodium, the particles adopt theopposite habit and separate as independentunits. Calcium therefore improves thestructure of the soil by helping aggrega-tion, and this in turn improves its agricul-tural value; flocculation of otherwiseintractable clays is often encouraged by theaddition of lime to make them more pro-ductive.

flood The state of a river when the vol-ume of water flowing in it exceeds BANK-FULL, and water commences to spreadaway from the channel over the FLOOD-

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PLAIN. Floods are produced by dischargeincreases due to exceptional rainfall andrunoff. In hydrology, much effort has beendevoted to the study of floods because oftheir great geomorphic significance andtheir cost to life and property on flood-plains. Flood frequency curves have beenproduced for many rivers, showing thelikely recurrence interval of floods of vary-ing magnitudes, based on the analysis ofpast DISCHARGE records from gauging sta-tions. Geomorphologically, floods can ac-complish great landscape modifications inshort periods, because the increased veloc-ity of flood rivers produces an increase inerosional capacity proportional to thesquare of that velocity: thus if velocity dou-bles, erosional capacity increases fourtimes. See also geomorphic process.

flood basalt Very fluid basaltic LAVA

(usually from a FISSURE ERUPTION) thatflows over large areas; there may be a seriesof such flows one after the other. Thelargest known area of flood basalt covers250 000 sq km of the Deccan Trapsplateau on the Indian subcontinent;smaller examples occur in southern Africaand the northwestern USA (ColumbiaRiver Plateau).

floodplain A relatively level area bor-dering a river, subject to periodic floodingand made up of sediments deposited by theriver, which bury the rock-cut valley to avariable depth. In the larger rivers it maycontain the following features: the riverchannel itself, OXBOW LAKES, POINT BARS,scars of former MEANDERS resulting fromtheir migration downvalley, areas of stag-nant sedimenting water (sloughs), LEVÉES,backswamp deposits of fine sediments inslack water away from the river, and sandyflood debris. In smaller streams all thesefeatures will not be present, but the Missis-sippi and Mekong, for example, displaythem all.

Floodplains originate from depositswithin the river channel on the point barsand from overbank deposits in time offlooding. It is now thought that depositswithin the river channel are the majorsource, constituting possibly 60–80% of

floodplain material, but actively aggradingrivers probably owe more to overbank de-posits.

The existence of a floodplain does notmean a river is actively aggrading: mean-ders migrate downstream, depositing ma-terial on the inside of bends and erodingmaterial on the outside. Once deposited, aparticle will remain static in the floodplainfor a considerable period (about 1000years in one stream studied) until the nextmeander upstream had eroded its waydown to the position of that grain and re-entrains it. Floodplains in streams in equi-librium therefore represent a temporarystorage for sediment on its passage throughthe valley. See also lateral erosion; river.

flood tide The incoming of the tidalstream; the rising tide, i.e. that part of atide cycle following the low-water stageand preceding the high-water stage. Com-pare ebb tide.

Florida Current A major surface cur-rent on the W periphery of the AtlanticOcean. It forms one of two branches of theNorth Equatorial Current (see equatorialcurrent). At about longitude 60° W, theNorth Equatorial Current splits up intotwo important currents: one flows into theCaribbean Sea but then re-enters the At-lantic Ocean via the Gulf of Mexico, andthereafter flows as the Florida Current.The Florida Current extends from theStraits of Florida to roughly the latitude ofCape Hatteras. As such, it constitutes the Sportion of the GULF STREAM system. Thedischarge rate of the Florida Current isprobably of the order of some 26 millioncubic meters per second, but this figurevaries significantly from one period to an-other, and almost certainly seasonally. It isa fast-flowing current that is particularlyconfined to the surface waters (reachingdown to depths of between 600 and 800m), probably attaining speeds in excess of150 cm per second close to the coasts ofFlorida. An important factor in the genera-tion of this current is the significant differ-ence in water level that exists between theGulf of Mexico coast and the Atlanticcoast of Florida.

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flowage /floh-ij/ Rock deformation re-sulting from the stressing of the rock be-yond its limit of elasticity, but withoutfracture.

flow cleavage A type of cleavage result-ing from solid-state flowage in a rock, ac-companied by the regrowth of minerals.Traces of bedding are almost destroyed.See also slaty cleavage.

fluid inclusion See inclusion.

flume /floom/ 1. A deep narrow gorge orravine containing a fast-flowing stream orseries of cascades.2. A channel constructed in a stream tocontrol streamflow; some specializedflumes are used to measure streamflow.

fluorescence /floo-ŏ-ress-ĕns/ The emis-sion of light as a result of previously ab-sorbing radiation. Many minerals fluoresceon exposure to ultraviolet light, and thisproperty may be used as an aid to identifi-cation.

fluorite /floo-ŏ-rÿt/ (fluorspar) A cubicmineral form of calcium fluoride, CaF2.The purple color of the familiar varietyblue john is due to the presence of colloidalcalcium; other varieties are white, green,and yellow. Fluorite is found in hydrother-mal vein deposits and as a late-stage min-eral in some alkaline and acid igneousrocks.

fluorspar /floo-er-spar/ See fluorite.

flute A sedimentary structure in theform of a hollow on the surface of a poorlyconsolidated bed of sediment, resultingfrom the scouring action of passing water.When this is infilled by an overlying bed ofsilt or sand the form of this hollow is pre-served, and is exposed as a flute cast on theundersurface of the overlying bed.

fluvial /floo-vee-ăl/ (fluviatile) Of or re-lating to a river.

fluvioglacial /floo-vee-ŏ-glay-shă/ (glacio-fluvial) Describing any process or resul-

tant landform involving the presence ofwater derived from a glacier or ice sheet.Glacial meltwater can transport and erodematerial just as a normal river, but if it lieswithin or beneath the associated ice, it maybe capable of extremely efficient erosiondue to increased pressures or velocities.Fluvioglacial deposits can generally be dis-tinguished from those of purely glacial ori-gin because they exhibit rounding, sorting,and stratification.

flux (flux density) A rate of flow of somequantity, usually some form of energy,through a unit area.

flysch /flish/ Thinly bedded marinesandstones, marls, shales, clays, conglom-erates, and graywackes, which fill a troughadjacent to a rapidly rising mountain chainand represent rapid erosion and depositionprior to the main period of orogeny. Theterm was originally used by Alpine geolo-gists and applies strictly to the sedimentsassociated with the Alpine orogeny. Com-pare molasse.

focal depth The shortest distance be-tween the focus of an earthquake and theEarth’s surface.

focus The center of an EARTHQUAKE,where strain energy is converted into elas-tic waves. It is the source of SEISMIC WAVES

produced during an earthquake. For con-venience it is thought of as being a pointsource, although strictly speaking earth-quake waves originate from a nonfinitesource, such as a fault plane.

foehn See föhn.

fog A cloud near the ground surfacecomposed of minute water droplets in sus-pension. It is sufficiently dense for thedrops to produce a reduction in visibility toless than 1 km. Condensation of water toform the droplets in a fog can occur inmany ways, and fogs are often classified bytheir method of formation – RADIATION

FOG, ADVECTION FOG, etc. The most fre-quent method is by some form of cooling,such as radiational cooling at night; in ad-

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vection fog the lower layers of the atmos-phere are cooled through turbulent contactwith the ground. More rarely, condensa-tion can be achieved through evaporationfrom the surface.

In industrial areas, the greater supply ofcondensation nuclei and hygroscopic parti-cles enable fogs to form even when relativehumidity is less than 100%. Where smokeis abundant, this can produce a thick andparticularly unpleasant type of fog, knownas SMOG (an abbreviation of smoke fog).See also mist.

föhn (foehn) A warm dry wind that de-scends the leeward side of mountain rangesto give extremely rapid thaws of lyingsnow in winter. Such winds occur when-ever rapid descent favors adiabatic warm-ing, whether or not there has been coolingat the SATURATED ADIABATIC LAPSE RATE onthe upwind side of the mountains. How-ever, they are of climatological significanceonly where the descent is sufficient to pro-duce a marked warming, as in the Alps,Rockies, or Andes, where airflow is at rightangles to the mountain barrier and themaximum rate of descent achieved.

fold A buckling of bedded sedimentaryrocks due to deformation processes or theeffect of gravity. See also orogenesis.

fold axis An imaginary line parallel tothe HINGE LINE of a fold, from which therock strata dip away in different directions(downward from a crest; upward from atrough). See diagram.

fold belt See mobile belt.

folding The action of forming a FOLD inrock. See orogenesis.

fold limb Either of the two flanks on ei-ther side of the axis of a fold. The angle be-tween the limbs is called the interlimbangle.

fold mountain A mountain that isformed by large-scale folding. See orogene-sis.

foliation A banded or laminated struc-ture within a metamorphic rock, resultingfrom the metamorphic segregation of min-erals into compositionally different layers,parallel to the schistosity. The term is alsocommonly used to describe any parallelplanar element in a metamorphic rock,such as schistosity and cleavage.

food chain A series of linked feeding re-lationships between organisms. Each or-ganism depends for its food on the onenext lowest in the hierarchy (called aTROPHIC LEVEL. The lowest level is occupiedby PRIMARY PRODUCERS (autotrophs), suchas plants, which provide food for the her-bivores (PRIMARY CONSUMERS) in the levelabove them. These, in turn, become foodfor carnivores (secondary consumers).Consumers also include decomposers (DE-TRIVORES), which break down the wasteproducts and dead bodies of all the otherorganisms. A better picture of complexfeeding interactions between many organ-isms is provided by a FOOD WEB.

food web A sequence of complex inter-linked FOOD CHAINS that demonstrates thefeeding relationships between all the or-ganisms in a natural COMMUNITY.

fool’s gold See pyrite.

foot wall The surface of rock beneath afault plane or ore body. Compare hangingwall.

Foraminifera /fŏ-ran-ă-nif-ĕ-ră/ An orderof planktonic organisms (see protozoa).Most species display jellylike bodies sur-rounded by a shell or casing of calcium car-bonate. The casing is often flask- orsphere-shaped. Foraminiferans are mostabundant in the surface layers of the sea,and tend to reach their maximum develop-ment near the Equator. The tests or casingsof dead foraminiferans make up a largeproportion of the organic ooze deposits,for example, GLOBIGERINA OOZE, whichcontains some 30% foraminiferal tests.

forecast (weather forecast) (in meteo-rology) A statement about the probable

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weather events for specific areas for certaintime periods. Forecasts are normally givenat three levels of detail: (i) short-range,dealing in depth with the probable eventsfor the next 24–48 hours; (ii) mediumrange, covering about one week, in whichthe expected movements of the main highand low pressure systems are covered; (iii)the long-range forecast for a month or evena season, in which the only details refer toclimatological averages and whether the

weather is expected to be warmer orcooler, wetter or drier.

The methods required for working outthese forecasts differ for the time period in-volved. For the short-range forecast, satel-lite images show the major cloud belts,radar detects the areas of precipitation,and over land areas there are large num-bers of stations continuously recording theweather elements. Together they provide afair assessment of likely events supple-

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Types of fold

mented by computer calculations of pres-sure fields for 24–48 hours ahead (seenumerical weather prediction). Interpreta-tions can then be made on the basis of theseforecast maps.

Medium-range forecasting for the USAand Europe is largely based on experienceof previous weather sequences togetherwith a knowledge of the upper atmos-phere, particularly the Rossby waves (seelong wave) of the mid-latitude westerlies.As these to some degree control the direc-tion of movement of low-pressure systems,information about the state of the upper at-mosphere aids forecasting for thistimescale. However, as even quite smallinitial disturbances can dramatically andrapidly change the weather, success is onlymoderate.

For long-range forecasts, different tech-niques are required. In Britain, the ANALOG

method is often used. In this method for-mer weather charts are compared with pre-sent charts and it is assumed thatsimilarities in the charts can be used to pre-dict weather events. Sea-surface tempera-tures and their anomalies are also includedas these are important in affecting depres-sion formation and energy transfer to theatmosphere. In the USA, long-range fore-casts are based on computer simulations ofthe atmosphere. Details of the present stateof the atmosphere at many different levelsare fed into powerful computer systemsand then run for a time period equivalentto one month in real time to forecast pres-sure movements. Neither method is en-tirely satisfactory and there is considerabledebate as to whether it is theoretically pos-sible to forecast movement in the atmos-phere accurately for such long timeperiods.

foredeep See trench.

foredune /for-dewn, fohr-/ A coastaldune growing in the newest line of dunesnearest to the sea. Some authorities put amaximum size of about 3 m on foredunes.The usual distinguishing characteristics aredominance by the grass Agropyronjunceum, which is capable of tolerating theoccasional drenching with salt water to

which the foredune is subject, and a veryopen nature with low cover values andlarge areas of bare sand. Since the foreduneis produced by the coalescence of individ-ual grass and sand mounds, at a very earlystage it will appear broken with an unevencrest line.

foreland /for-lănd, fohr-/ A stable areaof older cratonic rocks marginal to an oro-genic belt, generally part of the continentalcrust. When two continental plates collidethe intervening wedge of sediments issqueezed between two forelands, ontowhich these sediments are thrust and over-folded.

foreset bed See delta deposit.

foreshock /for-shok, fohr-/ One of a se-ries of seismic waves recorded before theprincipal shock of an earthquake, resultingfrom small slips or fractures in brittle rocksas they reach their YIELD POINT, as a resultof a buildup in stress.

foreshore The part of a beach that be-comes covered and uncovered by waterduring the process of tidal rise and fall. Itextends from low-water spring tide level tonormal high-water springs level, and itswidth will naturally depend upon the tidalrange and gradient of the beach.

foresight See leveling.

formation The fundamental unit in thelithostratigraphical classification of bodiesof rock (see lithostratigraphy; stratigra-phy). A formation is formally defined by aTYPE SECTION at a type locality, on the basisof readily observable lithological featuresthat distinguish it from bodies of adjacentrock. It should preferably be marked offfrom these by a distinct lithologicalchange, such as might be found at an un-conformity, but it may have more arbitraryboundaries in a gradually changing rocksequence. Thickness is not a criterion in itsdefinition; a single formation will often dif-fer in thickness at different points. Strati-graphically adjacent and relatedformations may be associated to constitute

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a GROUP. A formation may contain MEM-BERS but need not be entirely composed ofthem.

form line A line drawn on a map to givean impression of the terrain where there isinsufficient information to construct accu-rate contours. Form lines are usually repre-sented by dashed lines rather thanunbroken ones. In areas mapped by aerialphotography there may be parts obscuredby cloud and these are often covered byform lines in the absence of alternativeheighting data.

forsterite /for-ster-ÿt/ A mineral of theOLIVINE group.

fosse /foss/ A long narrow depressionbetween the side of a glacier and the valleythat contains it. The depression forms assome ice melts because of heat absorbed byor reflected from the walls of the valley.

fossil The remains of any organism thatlived in the past. The term was originallyapplied loosely to anything ancient anddug up and as an adjective may still be usedin this sense (as in FOSSIL FUEL). A fossil canbe the whole or any part of an animal orplant, usually chemically altered (i.e. fos-silized); it may be an impression of theshape of an organism that has been pre-served in some way (see cast; mold) or itcan be simply the remains of the effects ofan organism in the past, such as fossiltracks, excrement, or burrows (see tracefossil). See also derived fossil; zone fossil.

fossil fuel Any naturally occurring hy-drocarbon fuel such as coal, natural gas, oil(petroleum), and peat. The fuels formedslowly underground by the action of pres-sure on the remains of dead plants and ma-rine animals. Fossil fuels represent anonrenewable resource.

Fourier analysis /foo-ree-ay, -er/ (har-monic analysis) A mathematical methodof analyzing a data series that contains pe-riodic variations, such as seasonal precipi-tation. These can be represented by a seriesof sine or cosine functions. The method is

named for the French mathematicianBaron (Jean Baptiste) Joseph Fourier(1768–1830).

foyaite /foy-ă-ÿt/ A type of alkali SYEN-ITE.

fractocumulus /frak-toh-kyoo-myŭ-lŭs/Broken or fragmented cloud rising onlyslightly above the CONDENSATION LEVEL.Such clouds are most frequently seen onearly mornings when there has been suffi-cient convection for the clouds to approachthe condensation level.

fractostratus /frak-toh-stray-tŭs/ (scud)Cloud that is broken and ragged, similar inappearance to fractocumulus but found be-neath altostratus or nimbostratus clouds,frequently preceding a warm front. It is theresult of condensation taking place in themoist layer beneath the main cloud base,either by turbulent mixing or through theaddition of moisture by precipitationfalling from higher levels.

fracture cleavage A series of closelyspaced fractures and parallel joints pro-duced in deformed rocks that have beensubjected to only minor metamorphism.

fracture zone A region that lies astride aline along which faulting has occurred. It isoften the site of earthquakes and volcanicactivity, and may run across a continent oralong the seabed.

franklinite /frank-lin-ÿt/ A rare blackcrystalline mineral of the SPINEL group con-sisting of oxides of iron, manganese, andzinc, (Fe2+,Zn,Mn2+)(Fe3+,Mn3+)2O4. Itcrystallizes in the cubic system and is usedas a source of zinc.

frazil ice /fră-zil/ Small crystals of icethat form at the edges of fast-flowingstreams or in moving seawater.

free air anomaly A gravity anomalythat takes account of altitude but does nottake account of attraction effects resultingfrom topogaphy and isostatic compensa-tion.

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free face A vertical or near vertical facetof slope cut in bare rock. It is rare to findfaces actually at 90°, and various authori-ties have adopted different critical valuesfor the minimum allowable slope; slopes ofabove 40–45° are usually termed free faces.They occur in consolidated materials,which can maintain their verticality whenthe removal of debris from the footslopeexceeds the rate of debris production fromthe face above. Wave erosion at the foot ofa sea cliff, or lateral planation by a streamat the side of its valley, are situations pro-ducing free faces. Owing to this removal ofdebris from its foot, the slope becomes con-stantly oversteepened, and slips and slideslead to stripping of material from thewhole face equally, thereby maintainingthe face by PARALLEL RETREAT. If the agencyremoving the debris at the foot ceases to doso, for example the accumulation of abeach at a cliff foot cutting off direct waveaction, retreat at the foot of the free faceslows down compared with its upper part,and the face will tend to be gradually oblit-erated by decline to a gentle angle. See alsobasal sapping.

freeze-thaw Describing weathering pro-cesses involving the freezing and thawingof water within preexisting rock fissures.When water expands on freezing, it tendsto enlarge cracks; on thawing, the enlargedcracks can contain more water than before,which will cause increased enlargement onrenewed freezing. Eventually, particles be-come broken away from the main mass ofrock. Freeze-thaw processes weaken rocksbefore they are subjected to other agents ofweathering and erosion. A notable exam-ple is GLACIAL PLUCKING.

freezing level The height above sea levelof the layer of the atmosphere that is at atemperature of 0°C. More frequently, itsheight is quoted in terms of pressure level,e.g. 850 mb. As water does not automati-cally freeze at 0°C (see freezing nucleus), itwould be more accurately called the melt-ing level, representing the point at whichsolid precipitation would melt. If theground temperature is below freezing, thefreezing level would be stated as being at

station level, although if there is an inver-sion aloft the situation may be complex.

freezing nucleus A tiny particle withinthe atmosphere that, in the presence of su-percooled water droplets, initiates thegrowth of ice crystals around the nucleus.Water droplets in clouds do not automati-cally freeze when the temperatures fallbelow freezing point. Being very small, thedroplets require some form of NUCLEUS onwhich the crystalline growth can developbut such nuclei are not as effective or nu-merous as cloud condensation nuclei (seecondensation) and do not produce totalfreezing at 0°C. Instead a few ice crystalswill appear at between –5°C and –10°Cand become more frequent with lower tem-perature. The nuclei are composed of vol-canic dust or clay particles with a similarcrystalline structure to that of ice, and insome cases fractures or splinters of ice crys-tals may themselves act as nuclei. At tem-peratures below –40°C, spontaneousfreezing takes place even in the absence ofnuclei.

freezing point The temperature atwhich the liquid and solid phases of a sub-stance are in equilibrium at standard pres-sure (1013.25 mb); for water this is 0°C.Because nuclei (see freezing nucleus) arerequired for cloud droplets to freeze, watercan exist at temperatures far below its truefreezing point. The term is therefore pri-marily used for ground-surface tempera-ture observations and forecasts, partic-ularly in connection with road surface con-ditions.

freezing rain Rain or drizzle that fallsonto a ground surface that is below 0°C,the water freezing to give a coating of ice.It can occur when a layer of rising warmair, such as at a warm front, is movingslowly over a cold surface where the tem-perature remains below freezing point. Be-cause the warm air eventually reaches theground surface this state is normally ofshort duration.

fresh water Water that contains less

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than 2% of dissolved minerals. It is notnecessarily safe to drink.

friable Describing a mineral or rock thatcrumbles easily (using the pressure of thefingers).

friction The mechanical force of resis-tance operating between two substances incontact. In the atmosphere, it is the resis-tance between the ground surface and thelayer of air above it, although the thicknessof this layer varies appreciably. The termcan also be used when referring to windshear.

The effects of friction on airflow are nu-merous. Wind speeds are reduced near theground and there is a movement of windacross the isobars from higher to lowerpressure, compared with the GEOSTROPHIC

WIND when friction is excluded. The rate ofincrease of speed with height away fromthe surface is affected by the roughness ofthe ground, extremes being represented byoceans and urban areas.

frigid zone A cold area of the Earth’ssurface, either in polar regions or at highaltitude.

fringing reef A REEF that grows directlyon bedrock at the seashore, especially intropical regions. It forms a shelf whoserough surface appears above the water atlow tide. See also barrier reef.

front A transition zone between air ofdifferent thermal characteristics and ori-gins. On surface pressure charts, fronts aredepicted in two-dimensional form alongthe line of maximum thermal gradient,where this can be determined. In cross sec-tion they represent a surface of separationbetween the two AIR MASSes, sloping gentlyaway from the surface warm air. Distinc-tive weather phenomena are associatedwith fronts and hence they are very impor-tant in short-range weather forecasting (seeforecast). Problems of precise definition ofa front occur because it represents a ther-mal gradient, the strength of which is notnecessarily related to weather activity, sothe same gradient may have quite different

degrees of vertical motion. In weather fore-casting, a front is often called a trough oflow pressure. See also depression; coldfront; occluded front; polar front; warmfront.

frontal precipitation (frontal rainfall; cy-clonic precipitation) Precipitation thatoriginates from the movement of frontswhere there is large-scale rapid uplift andcooling of warm moist air along the junc-tion of cold and warm fronts in a DEPRES-SION (low-pressure system).

frontal wave An undulation on a frontbetween two air masses that travels alongas a wave-like perturbation. The frontalwave initially has a warm front and coldfront but over time it starts to occlude.

frontal zone The zone of transition, inwhich mixing has occurred, between twoadjacent air masses. The zone may rangefrom tens to several hundred kilometers inwidth.

frontogenesis /frun-tŏ-jen-ĕ-sis/ Theprocess of intensification of the thermalgradient along a frontal zone: part of theformation of a DEPRESSION. It can takeplace whenever the isotherms are suitablyorientated during conditions of CONFLU-ENCE and CONVERGENCE in the lower at-mosphere.

frontolysis /frun-tol-ŏ-siss/ The weak-ening of the thermal gradient at a frontalzone. This is produced under the reverseconditions of frontogenesis, i.e. with sur-face DIVERGENCE of air and often subsi-dence. When frontolysis is taking place,precipitation along the front stops and thecloud sheet breaks up and eventually dis-appears.

frost The condition that occurs when thetemperature falls below 0°C. Subdivisionsare made into ground frost and air frostbased on thermometer readings at grasslevel and screen level (1.2 m) respectively.It is normally recognized by the icy depositthat usually forms under such temperatureconditions, but if the air is very dry, frost

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may occur without forming icy deposits.For weather-forecasting purposes, gradesof severity of frost are distinguished with–0.1 to –3.5°C slight, –3.6 to –6.4°C mod-erate, –6.5°C to –11.5°C severe, and below–11.5°C very severe. Allowances are madefor wind speed and wind-chill index, whichinfluence the sensation of cold.

frost action A type of MECHANICAL

WEATHERING that results in the breakup ofrock through repeated freezing and thaw-ing of water in its crevices.

frost heaving (frost heave) The swellingof the ground surface through a combina-tion of CONGELITURBATION and the effectsof NEEDLE ICE.

frost hollow An area that has a greaterliability to frost incidence than its sur-roundings. Because cold air has a slightlygreater density than warm air, it tends tocollect in hollows or low ground and soareas of high frost incidence are usually inhollows where free drainage of air is pre-vented. The most famous frost hollow isthe Gruenloch (or Gstettneralm) sinkholein the Austrian Alps where temperatures aslow as –52°C have been recorded.

frost-shattering The breaking apart ofmasses of rock by the continued enlarge-ment of cracks within them throughFREEZE-THAW action, in areas where thetemperature fluctuates for considerable pe-riods around freezing point. The resultantoutcrops and debris are all highly angularin appearance, and this is the major char-acteristic of frost-shattered forms. See alsocongelifraction.

fugitive See volatile.

fulgurite /ful-gyŭ-rÿt/ A tubular or den-dritic mass of fused SILICA originating whenlightning strikes sand.

full A shingle BEACH RIDGE or a BERM

composed of sand. When ridges are called

fulls, the separating parallel depressionsare known as swales.

fuller’s earth A clay rich in montmoril-lonite. See clay minerals.

fumarole /fyoo-mă-rohl/ A vent in theground in a volcanic region which emitssteam and other hot gases. Temperaturesmay be as high as 1000°C within fu-maroles, which occur mainly on lava flowsand in the calderas and craters of activevolcanoes. If sulfurous gases are also pre-sent, it is called a solfatara.

fumigation (in meteorology) The resultof the influx of heavily polluted air downto the ground surface. It most frequentlyoccurs after night-time cooling has given atemperature inversion a few hundred me-ters above the ground surface, beneathwhich industrial pollution has accumu-lated. When convection starts after solarheating next morning, rising air motionwill resume and compensating downwardflow will take the polluted air to theground, giving sudden high values of pol-lution.

Fungi A kingdom of heterotrophic or-ganisms, important in soil. They are lessabundant in numbers than bacteria (up to1 000 000 per gram of soil) but greater inbulk, because they have a filamentous na-ture. They require moisture and may befound in alkaline, neutral, or acid soils.Thus in the latter type they number morethan bacteria. Although, unlike bacteria,they cannot oxidize ammonium com-pounds or fix atmospheric nitrogen, theyare more important than bacteria in theformation of humus and in the promotionof soil stability. Fungi are capable of de-composing even the most resistant plantmaterials such as cellulose and lignin. Themycelia of certain mushroom fungi mayform associations with plants called my-corrhizas. This association is of mutualbenefit to plant and fungus and where pre-sent there is a marked increase in the up-take of nutrients.

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gabbro /gab-roh/ A coarse-grained basicigneous rock, the product of plutonic crys-tallization of basalt magma. The essentialconstituents are plagioclase that is morecalcic than anorthite (usually labradorite)and clinopyroxene (usually augite or ti-tanaugite). Many gabbros contain olivinein addition. Accessory minerals includemagnetite, ilmenite, apatite, biotite, andhornblende.

Rocks in which orthopyroxene, usuallyhypersthene, predominates over clinopy-roxene are called norites. With an increasein the proportion of plagioclase, gabbrosand norites pass via leucocratic varietiesinto anorthosites; with an increase inolivine and pyroxene they pass viamelagabbros into ultramafic rocks. Withan increase in the proportion of olivine atthe expense of pyroxene, gabbro passesinto troctolite and allivalite. With a changeto plagioclase of more sodic composition,gabbros pass into diorites. Eucrite is a vari-ety of gabbro that contains olivine, twopyroxenes, and plagioclase of bytownite–anorthite composition. Gabbroic rockscontaining pyroxenes and olivines rich iniron are termed ferrogabbros. Alkali gab-bros (syenogabbros) are marked by the in-troduction of feldspathoids or alkalifeldspar.

Most gabbros have equigranular tex-tures; others are ophitic or, rarely, orbicu-lar. Reaction rims are common featuresand rarely multiple rims involving olivine-pyroxene-hornblende-biotite may be pre-served. The medium-grained equivalent ofgabbro is termed microgabbro or DIABASE.Gabbroic rocks occur in stratiform layeredbodies, ophiolite complexes, and as nod-ules in basalts.

gaging station See gauging station.

Gaia hypothesis /gÿ-ă/ An idea champi-oned in the 1970s by the British scientistJames Lovelock (1919– ) which suggeststhat all the living and nonliving systems onEarth form a unity that is regulated, andkept suitable for life, by the organismsthemselves. The whole planet can thereforebe regarded as a huge single organism. Thehypothesis stresses the interdependence ofliving things and the environment.

gal A unit of acceleration equal to onecm per second per second (named afterGalileo). In practice this unit is too largeand measurements of the strength of theEarth’s gravitational field at a point aremade in milligals (one thousandth of a gal);1 milligal is equivalent to 9.8 × 10–3 N kg–1.See gravity unit.

gale 1. In general, any unusually strongwind.2. A wind speed averaged over a 10-minuteperiod of 34 to 47 knots. On the BEAUFORT

SCALE Force 8 is a gale; Force 10 a wholegale (or storm). Gales most frequentlyoccur during the passage of an intense de-pression.

galena /gă-lee-nă/ The chief lead ore(lead sulfide, PbS), found as gray metalliccubes of high density and often associatedwith sphalerite, barytes, fluorspar, and cal-cite in hydrothermal veins. It is sometimesmined for its silver content.

Galileo A global navigation satellite sys-tem under development by the EuropeanSpace Agency and European Union. Thefirst experimental satellite (Giove-A) waslaunched in 2005. When fully deployed,the system will consist of 30 satellites (27operational) with two control centers lo-

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G

cated on the ground surface in Europe andunder civilian control. It will be compatiblewith the GLOBAL POSITIONING SYSTEM of theUSA and the GLONASS system of Russia.

Gall’s projection /gawlz/ A cylindricalMAP PROJECTION with the cylinder cuttingthe Earth’s surface at 45° north and south(see cylindrical projection). The meridianshave true scale only at the points of inter-section, the scale increasing poleward anddecreasing toward the Equator. It is neithera homolographic nor orthomorphic pro-jection, and is not often used. The projec-tion was developed in 1855 by the Scottishcartographer Reverend James Gall(1808–94).

gamma-ray log A logging techniquerun in wells that records the natural ra-dioactivity of the formations throughwhich it passes. High values are given byshales, whereas sandstones give low read-ings. It can be run in cased boreholes, be-cause it does not rely upon the electricalproperties of the formation.

gangue /gang/ Minerals found in an oredeposit that have no commercial value andmust be removed during the refiningprocess.

gap An opening in a ridge, usuallyformed by the action of a glacier or river.Gaps provided routes through mountainranges and often became the sites of humansettlement.

garnet A member of a group of cubicminerals with a general formulaR3

2+R23+Si3O12 where the divalent metals

are magnesium, iron, manganese, or cal-cium and the trivalent metals are alu-minum, iron, or chromium. The followingsix end-members are recognized:

pyrope – Mg3Al2Si3O12almandine – Fe3

2+Al2Si3O12spessartite – Mn3Al2Si3O12grossular – Ca3Al2Si3O12andradite – Ca3(Fe3+,Ti)2Si3O12uvarovite – Ca3Cr2Si3O12

Compositions corresponding to pure end-members are rare and garnets are divided

into two series, pyralspite (pyrope, alman-dine, spessartite) and ugrandite (uvarovite,grossular, andradite). Little chemical vari-ation between pyralspite and ugranditegarnets occurs.

Garnets commonly develop dodecahe-dral and icositetrahedral forms and are avariety of colors. Pyralspite garnets aretypically pink, red, or brown. Melanite is adark brown to black variety of andradite,rich in titanium and occurring in ijolitesand nepheline syenites. In general, garnetsare characteristic of metamorphic rocks.Almandine is found in medium- to high-grade regionally metamorphosed argilla-ceous sediments. Pyrope is found inultrabasic rocks and eclogites, and grossu-lar occurs in metamorphosed limestones.Many types of garnet are used as semi-precious gemstones.

garnierite /gar-nee-er-ÿt/ A bright greenamorphous form of hydrated nickel mag-nesium silicate. It occurs as a mineral inSERPENTINITE and is used as a source ofnickel.

gas cap An underground pocket of nat-ural gas that occurs above a reservoir ofoil. It may be tapped for use as a fuel.

gas laws The basic physical equationsthat relate the pressure (p), volume (V),temperature (T), and density (ρ) of a per-fect gas. These include the equation of state(p = RρT/M, where R is the universal gasconstant and M is the molecular weight ofthe gas) and Boyle’s law, which states thatat constant temperature, the volume of agiven mass of gas is inversely proportionalto the pressure of the gas. This law controlsthe cooling of air as it rises and expandsinto regions of lower atmospheric pressure.

gas sand A type of sandstone or sanddeposit that contains useful quantities ofnatural gas, which may be extracted.

gas thermometer See thermometer.

gastrolith /gas-trŏ-lith/ A stone or peb-ble ingested by reptiles and birds to assistin the breakdown of food in the gizzard.

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Fossil structures associated with the re-mains of extinct reptiles are thought to begastroliths.

Gastropoda /gas-trop-ŏ-dă/ The class ofthe phylum MOLLUSCA that includes theslugs and snails. Typically they have a shellthat is often coiled in a helical spiral; unlikethat of the CEPHALOPODA it is not dividedinto chambers. Most gastropods are ma-rine but some live in fresh water and othersare terrestrial. Fossils are known from theCambrian Period onward and the class hascontinued to flourish and diversify to thepresent day.

gauge height (gage height) See stage(def. 3).

gauging station (gaging station) (in hy-drology) A point along a stream or by alake, reservoir, or well at which instru-ments are installed to monitor and collectinformation, such as the height or stage ofthe water above a fixed datum, stream flowor discharge, water chemistry, and watertemperature.

GCM See general circulation model.

geanticline /gee-an-ti-klÿn/ A very largeanticlinal structure that has developedwithin geosynclinal sediments, as a resultof lateral compression.

gedrite /ged-rÿt/ An orthorhombic min-eral of the AMPHIBOLE group.

gelisol One of the twelve soil orders ofthe US SOIL TAXONOMY (the order was notin the original 10 orders but was added in1998); it comprises those soils underlain bypermafrost. Freezing and thawing are im-portant processes, accompanied by cry-oturbation (frost mixing). Gelisol soils aredefined as having permafrost within 1 m ofthe soil surface, or gelic material within 1m and permafrost within 2 m of the soilsurface.

general circulation model (GCM, cli-mate model) A computer model or simu-lation that represents the general cir-

culation of the atmosphere or oceans, orboth, in three dimensions. Increasingly re-alistic models of the general circulation arebeing obtained now that more is knownabout the controlling factors and powerfulcomputers with large memory stores areavailable. Over the years the models orsimulations have become more sophisti-cated, starting by trying to account for theprimary processes responsible for main-taining the large-scale features of the circu-lation of the atmosphere, using theprimitive equations of motion. Sophisti-cated models now simulate physical, ther-modynamic, and dynamic activity. Themost complex models are coupled atmos-phere–ocean general circulation models,which couple an atmospheric general cir-culation model (AGCM) with an oceangeneral circulation model (OGCM), andsome also include factors such as the bio-sphere, carbon cycle, aerosol transport, andatmospheric chemistry. These models canbe used to provide predictions of future cli-mates and the rate of climate change. Gen-eral circulation models can also be used tosimulate past climatic conditions, e.g. anice age, for paleoclimatology studies.

general circulation of the atmos-phere The mechanism by which energyis transferred from surplus to deficit areasof the world. It is constrained by a numberof processes that interact with the maindriving force of the circulation to result inthe observed system of winds and distur-bances. This circulation is basic to an un-derstanding of the climates of the worldbecause it determines the direction andstrength of winds in any latitudinal posi-tion.

The atmospheric circulation is verycomplex, but it can be simplified into a se-ries of belts (see Fig. 1). Within the tropicsthere are the trade winds converging to-ward the intertropical convergence zone(ITCZ). Poleward of the subtropical anti-cyclones are the main westerly winds witha small area of easterlies near the poles.These mean winds are essentially zonal indirection and do not appear to be an effec-tive system in transferring surplus radia-tional energy toward the temperate and

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polar latitudes. However, long-term stud-ies have shown that meridional exchangetakes place as shown in Fig. 2. In the trop-ics, air rises at the ITCZ, releasing latentheat and gaining potential energy. This ismoved toward the subtropics where it de-scends and maintains the trade winds andwhere much moisture is evaporated fromthe sea surface to continue the convectioncell toward the ITCZ. This cellular circula-tion has been called the HADLEY CELL. Pole-ward, the mean charts show a dominanceof zonal winds, but exchange here is ef-fected by depressions, which advect warmand cold air and aid their mixing. Becausethis process takes place at different longi-

tudes for only short periods of time, it isaveraged out on the mean maps.

The driving force for the atmosphericcirculation is the differential pattern of netradiation received at the Earth’s surface. Ifthe Earth were uniform and stationary thiswould give rise to a simple thermal cell be-tween equatorial (surplus) and polar(deficit) areas. However, the effects of theEarth’s rotation, the presence of mountainbarriers, the distribution of land and sea,and the positions of the ocean currents allhelp to modify the simple thermal cell intothe complex general circulation of the at-mosphere. See also general circulationmodel.

polar easterlies

westerlies

NE tradesEquator

SE trades

westerlies

polar easterlies

polar anticyclonesNorth Pole

South Pole

mid-latitude depressions

subtropical highs

intertropical convergencezone

Arctic cell Ferrel cell Hadley cell

polar front tropopause

subtropical jet

polar front jet

risingwarm air

tradewinds

thermalequator

sinkingcold air

ArcticArcticArcticfrontfrontfront

90°N

Fig. 1

Fig. 2

General circulation of the atmosphere

genus /jen-ŭs/ (pl genera) A group in thetaxonomic classification of organisms thatconsists of one or more related SPECIES. Oneor several genera form a FAMILY. For exam-ple, the genus Homo, to which man be-longs, is placed in the family Hominidae. Ageneric name always begins with a capitalletter and is italicized. It is always includedwith the specific name in naming a species,when it is often abbreviated to the initialletter. Thus, the species to which modernman belongs is Homo sapiens or H. sapi-ens. A genus is defined on the basis of thespecies it contains, the most typical ofwhich is designated as the type species. Seetaxonomy.

geocentric /jee-oh-sen-trik/ At or relat-ing to the center of the Earth.

geochemical cycle /jee-oh-kem-ă-kăl/(rock cycle) The route followed by an el-ement or group of elements as they circu-late through the Earth’s crustal andsubcrustal rocks. Elements may combineor separate during the cycle. Weathering ofigneous rocks produces particles that settleas sediments, which may become sedimen-tary rock or undergo metamorphism intometamorphic rocks. These may melt andform new magma, which rises to the sur-face and crystallizes as new igneous rock.

geochemistry /jee-oh-kem-iss-tree/ Thebranch of geology that deals with thechemical elements and compounds in theatmosphere, water, soil, and rocks of theEarth, particularly their composition andhow they are formed and distributed.

geochronology /jee-oh-krŏ-nol-ŏ-jee/The science of age determination of partsof the Earth. It has two branches: absolute(or numeric) dating and relative dating.The former is more modern, and involvestechniques such as carbon–14, potassium–argon, and radioactive decay methods toput actual ages in years BP (before present)on organic remains, rocks, or sediments.Relative dating includes fossil correlations,pollen analysis and correlation, matchingof tills from different areas on the basis ofweathering or geologic content, archeolog-

ical indicators, and several lesser used tech-niques that help establish stratigraphy.Relative dating seeks to put events in order;absolute dating tries to give precise dates ofoccurrence. In geomorphology both areused, especially in studies of the Pleistoceneand post-Pleistocene periods.

geode /jee-ohd/ See druse.

geodesy /gww-od-ĕ-see/ The science con-cerned with determining the exact shapeand size of the Earth and the exact positionof points on the Earth’s surface. In additionit investigates the Earth’s gravitationalfield, variations in tides, and the Earth’s ro-tation. It provides the reference surfacefrom which astronomical observations aremade.

geodetic surveying /jee-ŏ-det-ik/ Thelarge-scale surveying of the Earth’s surface,taking into account its curvature. Precisemeasurements from geodetic surveys pro-vide the horizontal control points for moredetailed surveying.

geodimeter /jee-ŏ-dim-ĕ-ter/ A survey-ing instrument used for the accurate mea-surement of distance. The measurement isindirect, being obtained by timing the pe-riod required for a beam of light to travelfrom one end of a line to the other and re-turn. Since the velocity of light is known,the distance can then be calculated. The in-strument sends out a light beam, oftenfrom a laser, which is reflected by a mirrorat the other end of the line to be measured.Suggested maximum ranges are 5 to 10 kmin daylight and 15 to 25 km in darkness,depending on the light source. The geo-dimeter is ideal for rapid baseline measure-ment.

Geographical Information System(GIS) A computer system that includesthe hardware, software, and data for cap-turing, storing, updating, manipulating,analyzing, and displaying geographical andspatial data. Within the earth sciences GISsystems have been applied, for example, tocreate maps displaying specific informa-tion, such as the geology of an area, ocean

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Geographical Information System

circulation, topography, the watershed fora stream system, and hazard risks fromnatural disasters for an area.

geography The scientific study of thefeatures of the Earth’s surface. Physical ge-ography includes CLIMATOLOGY, GEOMOR-PHOLOGY, METEOROLOGY, and PEDOLOGY;human activity is covered by political orsocioeconomic geography.

geoid /jee-oid/ The Earth pictured as asmooth oblate spheroid, coinciding withsea level and taken to continue across con-tinents at the same level. At all points onEarth the geoid surface is at right-angles tothe downward direction of the pull of grav-ity. Geodetic leveling employs the geoid asits reference surface.

geologic timescale (geologic column) Achronological series of events in the geo-logic history of the Earth, beginning in thePRECAMBRIAN and extending to the present(the HOLOCENE epoch of the NEOGENE pe-riod). It is divided into eons, eras, periods,epochs, and ages, each ascribed dates interms of millions of years ago (see table).See also chronostratigraphy.

geology The study of the structure andcomposition of the Earth. See geomorphol-ogy; geophysics; petrology. See alsochronostratigraphy; geologic timescale.

geomagnetic equator /jee-oh-mag-net-ik/A great circle equidistant between the geo-magnetic poles. It connects all points ofzero geomagnetic latitude and its plane isat right angles to the Earth’s magnetic axis.

geomagnetic field The Earth’s mag-netic field, which causes a compass needleto align north–south whatever its locationon the Earth’s surface.

geomatics A new discipline that encom-passes geographical information systems(GIS), land surveying, and map making.The term was first used in Canada in themid-1980s.

geomorphic process /jee-ŏ-mor-fik/ The

chemical and physical interaction betweennatural forces (e.g. running water, waves,ice, gravity, biological phenomena, etc.)and the Earth’s surface, which act to mod-ify the landscape. An increasing trend ingeomorphology is toward the study ofprocesses, inferring from these the land-forms being created, as contrasted withmore traditional geomorphology, in whichprocesses were merely inferred from land-forms. Processes are determined by the en-vironmental variables of climate, geology,and base level; a change in one or some ofthese will change the nature or rate ofprocesses. Processes fall into the broad cat-egories of weathering, erosion. transport,or deposition, within each of which manydifferent types can be recognized. Rates ofa process, as shown by measurements orinference, are characterized by great spatialand temporal variation.

A central concept in geomorphology isthe relative significance, in terms of workdone in landscape formation, of infrequentbut highly destructive events such asstorms, floods, and gales and the very fre-quent but far less effective events of aver-age conditions. In storms, whole beachescan come or go overnight; rivers canchange their courses and expand theirfloodplains during floods; sand dunes canbe built and eroded in a single severe gale.This is because capacity for work increasesas a power function of the increase of theagency concerned; erosional capacity ofrivers is proportional not to velocity, butthe square of velocity; wave energy is pro-portional to the square of wave height; thetransporting power of wind increases asthe cube of its velocity. Major landscapefeatures are largely shaped by the infre-quent severe events, but Leopold, Wolman,and Miller (1964) concluded that moder-ate events of moderate frequency are re-sponsible for most work done.

A better understanding comes from ac-tually measuring the processes: this showsthe rates at which the same process oper-ates in different environments, and thecomparative effectiveness of different pro-cesses in the same environment. It alsoshows the spatial and temporal distribu-tion of changes, and once this is under-

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stood, prediction to the future is easier andmore reliable, and the formation of exist-ing landforms is more easily appreciated.Measurements of processes are taken asthey operate, by the use of rods insertedinto slopes and river banks for example,recording losses, or by the use of map evi-dence recording, for example, the succes-sive growth of a spit across a river mouth,measurements from which can show varia-tion in rates at different periods.

geomorphology /jee-oh-mor-fol-ŏ-jee/The study of the evolution of landforms,excluding the major forms of the Earth’ssurface, such as mountain chains andocean basins. It includes the study of the re-lationships between structures and land-forms at the basic level, but then has twomajor branches: the inductive study of ex-isting landforms, from which the processesof their evolution are inferred, and the de-ductive study and measurement of actualprocesses operating, inferring their ulti-mate influence on the landscapes on whichthey are acting. These two are complemen-tary, because they approach the cause andeffect relationships between landforms andprocesses from two contrasted viewpoints.The inductive branch is older than themore scientific deductive branch.

geopetal cavity /jee-op-ĕ-t’l/ (spirit levelstructure) A WAY-UP STRUCTURE producedwithin a closed cavity in a rock, generallywithin a fossil. On death some mud-sizedmaterial entered the fossil and settled outwith its upper surface parallel to the hori-zontal. Once the cavity was closed no fur-ther sediment could enter. The trappedsediment hardens and leaves a record of thepast horizontal level. The empty spaceabove the sediment is generally later in-filled, usually by coarse calcite crystals.

geophysics /jee-oh-fiz-iks/ The sciencethat combines the principles of mathemat-ics and physics with the use of sophisti-cated equipment to study the Earth andother planetary bodies.

geosphere /jee-oh-sfeer/ The nonlivingpart of the Earth, as opposed to the living

BIOSPHERE. It includes the Earth’s crust(lithosphere), all bodies of water (hydros-phere), and the air (atmosphere).

geostrophic wind /jee-oh-trop-ik/ Atheoretical wind representing a balance offorces between the pressure gradient andthe CORIOLIS EFFECT. This produces a windflow parallel to the isobars. Because the ef-fects of FRICTION are ignored, the real windonly approximates to the geostrophic windabove the BOUNDARY LAYER. At these levelsthe geostrophic wind can easily be calcu-lated from the pressure gradient.

geosyncline /jee-oh-sing-klÿn/ (geotecto-cline) A large linear trough developedalong the margin of a continent, withinwhich a considerable thickness of stratifiedsediments including turbidites accumu-lates, with occasional extrusive volcanicrocks. After a long period of accumulationthe trough is folded and uplifted to formmountain chains. In the light of the theoryof plate tectonics and sea-floor spreading,this term has fallen into disuse. See alsotrench.

geotectocline /jee-oh-tek-toh-klÿn/ Seegeosyncline.

geothermal /jee-oh-th’er-măl/ Describ-ing heat that originates in the Earth’s inte-rior. Evidence of geothermal activityincludes fumaroles, geysers, hot springs,and boiling mud volcanoes.

geothermal gradient The increase inthe Earth’s temperature with depth fromits surface, expressed either in depth unitsper degree, or more generally degrees perunit depth.

geyser /gÿ-zer, -ser/ A type of hot springthat throws up jets of boiling water andsteam, which are formed when groundwa-ter comes into contact with hot rock un-derground. High pressure increases theboiling temperature of water, but after it isforced out as steam the pressure falls andwater boils at lower temperatures. Thiscauses an intermittent cycle of events, with

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eruptions taking place at intervals after pe-riods of apparent inactivity.

GIS See Geographical Information Sys-tem.

glacial /glay-shăl/ 1. Relating to a glacieror ice sheet.2. (glacial period; glacial phase) The periodduring an ice age when ice sheets expand tolower latitudes. This would normally beassociated with a global drop of tempera-ture and changes in position of the maincirculation belts. See also deglaciation;glacial maximum; interglacial; interstadial.

glacial deposition The accumulation ofrocky material that has been transportedby a moving glacier or ice sheet. The de-posits, collectively known as DRIFT, areusually left stranded and include drumlins,erratics, eskers, and moraines. See also till.

glacial drainage system See meltwaterchannel.

glacial drift See drift.

glacial erosion (glacial scouring) Thewearing away of rock by the action of amoving glacier. The process is acceleratedby the presence of meltwater streams androck fragments beneath the ice. See alsoglacial plucking.

glacial lake An ice-dammed body ofwater. A glacial lake may accumulate be-tween the margin of a glacier and the val-ley wall, or where a glacier is enclosed byhigher ground such as moraine, or where aglacier blocks a stream emerging from atributary valley, or where a glacier blocks amajor river valley. The sudden flood thatresults when the water breaks through theice is termed a GLACIAL OUTBURST FLOOD orjökulhlaup. Such events can be cata-strophic for settlements and constructionsdown-valley in the path of the suddenly re-leased water. In the Himalayas, rising tem-peratures (an average of 1°C from the1970s to 2002) have led to an accelerationin the melting of glaciers, estimated to beretreating at the rate of 30–40 m a year in

Bhutan, and the consequent growth ofmany glacial lakes, nearly 50 of which areconsidered to be at risk of bursting throughthe ice that contains them.

glacial maximum The greatest extentof an ice age. During the Pleistocene, at itsgreatest extent, ice extended across theArctic Ocean and joined with the ice sheetsthat covered much of North America,Greenland, N Russia, and NW Europe (theBaltic, Norway, Sweden, Denmark, andFinland were completely covered). This iceis believed to have extended across theNorth Sea to engulf all but the very southof the British Isles. Iceland was covered byan isolated ice cap. Antarctica was con-cealed by ice, as today, and mountainousareas in both hemispheres underwentglaciation. At this maximum an area ofsome 46 million sq km was beneath ice,over three times that of the present day.

glacial outburst flood (jökulhlaup) Thesudden short-duration release of waterfrom a glacier or ice cap. The flood mayoriginate from the sudden drainage of anice-dammed GLACIAL LAKE; from the sud-den release of water from an englacial lakewithin the glacier; or from the release ofmeltwater resulting from volcanic activitybelow a glacier or ice sheet. The erosivepower of glacial outburst floods may formdeep branching channels or gorges (seecoulee). Glacial outburst floods occur withsome frequency in Iceland, where suchfloods are named jökulhlaup (derived fromjökull (glacier) and hlaup (flood)). In No-vember 1996 a catastrophic flood was re-leased following a subglacial eruption ofthe Grimsvötn volcano below the Vatna-jökul ice sheet; the flood caused severedamage with several bridges demolishedand roads washed away.

glacial phase See glacial.

glacial plucking The erosive process bywhich rock material can be included withinthe ice of a glacier, later becoming a tool ofglacial ABRASION. It entails the freezing ofthe glacier ice onto bedrock and the subse-quent plucking out of blocks on move-

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ment. For this to occur FREEZE-THAW activ-ity must be taking place at the ice-rock in-terface, thereby restricting the process toTEMPERATE GLACIERS.

A prerequisite for plucking is the pres-ence of already loosened blocks, becausethe tensile strength of consolidated rock isfar greater than that of ice. Well-jointedrocks are most susceptible (an optimummean joint separation of from one to sevenmeters has been suggested). Freeze-thawactivity both before and after the appear-ance of the glacier will assist, while UN-LOADING joints can also form as a result ofthe replacement of eroded rock by lessdense ice, providing more susceptible lay-ers.

glacial scouring See glacial erosion.

glacial trough (U-shaped valley) Awell-developed glacial valley with a char-acteristic U-shaped cross profile, as op-posed to the V-shape of fluvially developedvalleys. Such development can take placeby simple widening of preglacial valleys, orby a combination of widening and deepen-ing, the latter process depending upon theglacier’s ability to cut into the bedrock.The typical steep sides of glacial troughs re-flect the fact that whereas rivers occupyonly the bottoms of their valleys, a glaciertends to fill much of it, erosion taking placewherever there is an ice-rock contact. Theextent to which a valley can be deepeneddepends greatly upon the amount ofweathering active in the valley before theappearance of the glacier, since GLACIAL

PLUCKING is the major process of rock re-moval.

glacial wastage A reduction in thick-ness or areal extent of an ice mass resultingfrom more material being lost by ablationthan is added by accumulation.

glaciated sea floor /glay-shee-ay-tid/One of the most characteristic types ofCONTINENTAL SHELF is found off coasts thathave been heavily glaciated. Such shelvestend to have highly irregular surfaces andin their deepest parts may have 200 m ormore of water covering them. Glaciated

shelves are, in general, very wide, averag-ing 160 km; the average shelf-edge depth isaround 220 m. Shelf areas that have beenheavily glaciated, such as that off the Nor-wegian coast, often display enclosed basinsand trough features. Such submarine topo-graphic forms are analogous to lake basinsfound in many glaciated valleys on the con-tinents. Not all of the sea-floor features tes-tify to erosion; there may also beaccumulations of morainic material andother deposits, built into or retaining aconstructional form. Drumlins have beenfound off the coast of Maine, while con-centric stone deposits on part of the floorof the North Sea are probably of morainicorigin. Many glacially-eroded featureshave become buried by shifting sedimentsand can be found only by seismic measure-ments. Many of the banks and channelsformed in glacial debris have been shapedby a combination of wave and tidal action,for example, in the S North Sea.

glaciation 1. (in meteorology) Theformation of ice crystals from supercooledwater droplets in the atmosphere. It occursin the upper layers of cumulonimbuscloud.2. A period of time during an ice age whenice sheets grow and spread to cover exten-sive areas.3. The modification of land surfaces byglaciers through the processes of glacialerosion and deposition.

glacier /glay-sher/ An accumulation ofice of limited width moving downslope,under the influence of gravity, from asource area. Glaciers are formed by the ac-cumulation and compaction of snow (seefirn). They may be contained within basins(see cirque glacier) or within preexistingvalleys (see valley glacier). See also coldglacier; temperate glacier.

glacier regime The pattern of relativegaining or losing of ice from a glacier or icesheet. A glacier having a positive regime isgaining ice and will therefore advance andthicken, whereas one with a negativeregime loses ice (see ablation) and will bereduced in size.

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glacioeustatism /glay-shoh-yoo-stă-tiz-a˘m/ (glacioeustasy) Worldwide changein sea level due to the effect of ice. Duringthe PLEISTOCENE glacial periods sea levelwas much lower than at present, owing tothe presence of much water on the conti-nents in the form of ice. Estimates give val-ues for this lowering of around 100 m.During the INTERGLACIAL periods the icesheets melted and sea level rose again,sometimes to higher levels than those oftoday. Such changes would have a world-wide effect on erosional processes, becauseof the lowering of BASE LEVEL. However,these changes were not exclusively the re-sult of the reduction in the volume of seawater, because deformation of the Earth’scrust at the same time also had an appre-ciable effect.

glaciofluvial See fluvioglacial.

glass (in geology) An amorphous (non-crystalline) material that is formed by therapid cooling of lava or magma, such asOBSIDIAN.

Glauber’s salt See mirabilite.

glauconite /glaw-kŏ-nÿt/ A bright greenauthigenic mica-like mineral with an ap-proximate composition

(K,Na)1.2–2(Fe,Al,Mg)4-(Si7–7.6Al1–0.4O20)(OH)4.n(H2O),

which is found only in marine sediments,particularly greensands.

glaucophane /glawk-ŏ-fayn/ A mono-clinic AMPHIBOLE common in certain kindsof metamorphic rock.

glazed frost See black ice.

gleying /glay-ing/ The permanent or sea-sonal presence of either perched water orgroundwater within a soil profile. This cre-ates anaerobic conditions, leading to adominant process of REDUCTION of ferriciron to its ferrous form, giving the soil ablue-gray color. The water may either beperched on a heavier impermeable horizonbelow, as in the PEATY GLEY PODZOL, or theresult of a high groundwater table in areas

near drainage lines. If the gleying is sea-sonal, the profile may have ochreous mot-tlings due to periodic oxidizing conditions,changing the iron back to its ferric state.The lack of oxygen inhibits microbiologi-cal breakdown of humus, which accumu-lates as acid peat. Anaerobic bacteria tendto produce gases toxic to large plants, suchas hydrogen sulfide, ammonia, and marshgas (methane). Together, the effects of hy-dromorphism are sufficient to completelychange the direction of development of thesoil profile.

gley soil /glay/ A type of intrazonal hydro-morphic soil that may be found in tundra,temperate, or tropical regions where thereis excessive moisture in the soil profile. Thecharacteristic poor drainage may be a re-sult of the profile characteristics, i.e. a fine-textured parent material, or the site of thesoil, i.e. a low-lying area. Two main typesof gley can be recognized: the surface-water gley and the groundwater gley. Theformer is found where, because of a poorlydrained subsoil, water is held within theupper part of the soil, giving waterloggedconditions for part of the year. A typicalprofile would consist of a very dark AIhorizon rich in humus. Below this therewould be Eg and Bg horizons with typicalgleyed characteristics (see gleying). The Chorizon would not show any gleying andwould be similar to the parent material.The groundwater gley often occurs wherethere are permeable sands overlying imper-vious clay. The high groundwater tablethat is formed results in increased gleyingwith depth. The surface horizons are typi-cally aerobic, gray or brown in color, andrich in humus. With depth a mottled Bgand a gray/blue G horizon are common.Most gley soils are under grassland vegeta-tion and are used mainly for grazing.

glide twinning A complex form of crys-tal TWINNING resulting from deformation.It results from one layer of atoms changingits position so that the lattice is reversed.This results in broad lamellae within thecrystal. It is commonly shown in the calcitecrystals of marble.

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Global Positioning System (GPS) Asatellite-based coordinate positioning toolthat can rapidly and accurately determinethe latitude, longitude, and altitude of apoint on or above the Earth’s surface. It isbased on a constellation of 24 satellites or-biting the Earth at a very high altitude andwas originally developed by the US for mil-itary purposes but was made available for civilian use during the 1980s. It hasbecome an important tool in cartography,increasing accuracy and speeding produc-tion. Among its applications in earth sci-ences it has been used in studies to measuremovements of the Earth’s crustal plates,providing important evidence to supportthe theory of plate tectonics. See alsoGalileo.

global warming The supposed gradualincrease in the Earth’s average air temper-ature, generally attributed to an increasingconcentration of GREENHOUSE GASES

(chiefly carbon dioxide, plus some othertrace gases) in the atmosphere (and theconsequent enhanced GREENHOUSE EFFECT).It has been calculated (using computermodeling) that a 1.5°C rise in temperaturewould result in melting of ice in the polarice caps, causing a 20-cm rise in sea levelsworldwide, and widespread flooding. Inaddition changes induced in climate pat-terns may lead to shifts in vegetation zonesand extensions of desertification. Actualevidence of global warming is not yet con-clusive but appears to be mounting. Duringthe 20th century the average temperatureson Earth rose by 0.5°C and according tothe Intergovernmental Panel on ClimateChange (IPCC) most of the warming in thepast 50 years is attributable to human ac-tivity. Arctic ice fluctuates with the seasonsbut on average has been shrinking in recentyears; in 2005 the ice cover fell to 5.35 sqkm, the lowest recorded since 1978 whensatellite records became available, and theice has also become thinner. In Antarcticastudies have shown that glaciers have beenretreating and the ice on the peninsula andthe West Antarctic ice sheet thinning, pos-sibly caused by warming ocean tempera-tures or volcanic activity, although the EastAntarctic ice sheet is actually thickening.

Scientists have observed that many of theglaciers in the Himalayas, Peru, and else-where in the world are also retreating. Theloss of ice masses may in itself have a criti-cal effect on global warming as ice andsnow provides a reflective white surfacethat reflects solar radiation (see albedo). Inaddition the melting of permafrost in partsof Alaska and Siberia, for example, re-leases methane, a greenhouse gas, into theatmosphere.

global warming potential (GWP) Anindex that measures the influence that theprincipal GREENHOUSE GASES have on thenatural GREENHOUSE EFFECT in combinationwith the differing length of time that the in-dividual gases remain in the atmosphere.Carbon dioxide is used as the reference gasand given the value of 1; the global warm-ing potential of the other gases is given as aratio of the radiative forcing that would re-sult from the emission of that gas over acertain time period. It is calculated as theratio of the radiative forcing that would re-sult from the emission of 1 kilogram of thegreenhouse gas to that from the emission of1 kilogram of carbon dioxide over a fixedtime period (e.g. 100 years).

globigerina ooze /glŏ-bij-ĕ-ree-/ A cal-careous deep-sea ooze (see pelagic ooze)containing over 30% organisms. It is verywidespread on the deep-sea floor. The or-ganisms largely comprise the calcareousskeletons or tests of minute FORAMINIFERA.Globally, globigerina ooze represents some130 million sq km covering something likehalf of the Atlantic Ocean floor, beingwidespread in the Indian Ocean, particu-larly on the W side, and very conspicuousin the S Pacific Ocean.

glomerophyric /glom-ĕ-roh-fi-rik/ (glom-eroporphyritic cumulophyric) Describ-ing a PORPHYRITIC texture indicated by theoccurrence of phenocrysts in aggregates orclumps set in the groundmass of an igneousrock.

GMT See Greenwich Mean Time.

Gnathostomata /nath-oh-stoh-mă-tă/

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Vertebrates possessing jaws. The Gnatho-stomata include most fish and all highervertebrates. Compare Agnatha.

gneiss /nÿss/ A coarse-grained metamor-phic rock in which quartz and feldspar pre-dominate over micas. The schistosity ispoorly defined and segregation banding isirregular and discontinuous. Coarse granu-lar bands of quartz and feldspar alternatewith thin often undulating schistose bandsin which micas and amphiboles are con-centrated.

Paragneiss has an unambiguous sedi-mentary composition. Orthogneisses aremetamorphosed igneous rocks. Rocks ofgranitic composition containing a planaror linear orientation of minerals, impartedby stress, are termed granite–gneiss.Gneisses are formed during high-grade re-gional metamorphism.

gneiss dome See mantles gneiss dome.

gnomonic projection /noh-mon-ik/ Aform of azimuthal map projection (see az-imuthal projection) in which the great cir-cles of the Earth are shown as straightlines. Great circle routes can thus be plot-ted for navigation purposes. This projec-tion is not often used because of its limitedapplication.

goethite /goh-th’ÿt/ A yellow-brownmineral, FeO.OH, formed by the oxidationand hydration of iron minerals or as a di-rect precipitate. See limonite.

Gondwanaland /gon-dwah-nă-land/ TheS hemisphere supercontinent, which takesits name from the Gondwana system ofIndia, thought to have existed over 200million years ago. Since then it has beenfragmented as a result of SEA-FLOOR

SPREADING into the present continents ofAfrica, Australasia, India, South America,and Antartica. See also Pangaea.

goniatite /goh-nee-ă-tÿt/ One of the ear-lier mollusks of the subclass AMMONOIDEA,whose shells had simply folded angular su-ture lines. Goniatites are used as ZONE FOS-SILS in the Devonian and Carboniferous

System but were replaced by the AM-MONITES in the PERMIAN.

goniometer /goh-nee-om-ĕ-ter/ An in-strument that measures the angle betweencrystal FACES. Because such angles are char-acteristic of the crystal, they provide a use-ful means of identification.

gorge A narrow steep-sided valley, gen-erally formed in hard rocks by the erosiveaction of a flowing stream or river. A largegorge is called a CANYON.

gossan /goss-ăn/ A mass of quartz andhydrated iron oxides that often marks theoutcrop of a sulfide-bearing vein, the sul-fides having been oxidized to soluble sul-fates. Metalliferous material removed insolution may be redeposited at depth in azone of secondary enrichment. See also hy-drothermal process.

GPS See Global Positioning System.

graben /gray-bĕn/ A generally elongatedblock of rock that has been downthrownbetween two parallel faults relative to thesurrounding area. It differs from a RIFT

VALLEY in that it is a structural feature andnot necessarily also a topographical one.Compare horst. (See diagram at FAULT.)

grade 1. The degree of metamorphicchange undergone by a rock. When suchchanges can be traced from unmetamor-phosed rocks to highly metamorphosedrocks, the grade of metamorphism is saidto increase. High-grade rocks are producedunder conditions of high temperatures orpressures; likewise low-grade metamorphicrocks form at low temperatures and pres-sures. See zone.2. The condition reached in the later stagesof a normal CYCLE OF EROSION when a riveror slope has an EQUILIBRIUM REGIME, with asmooth concave long profile. Since the the-ory was proposed by Davis, other factorsbesides the modification of the long profilehave been taken into account, especiallyadaptation to changes in the controllingenvironmental factors of base level, rain-fall, and the quantity and type of the load

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supplied to the stream. A river that is atgrade is able to maintain its COMPETENCE sothat it is just able to transport its load at agiven discharge. It does this by adjusting itslong profile, cross section, channel pattern,and bed roughness: any environmentalchanges will be met by changes in some orall of these factors, so that the river uses upits available energy and no more.

graded bedding Sedimentary beddingin which particles show a size distribution.The coarsest material forms the base andthen the sequence becomes progressivelyfiner upward. It is often present in turbid-ity deposits. On a larger scale such upwardfining sequences are developed in braidedriver channels, as they are infilled. Seebraided stream.

graded profile See equilibrium profile.

gradient wind An extension to the con-cept of the GEOSTROPHIC WIND. Isobars inthe upper atmosphere are rarely straight,so to account for the curved flow, follow-ing Newton’s laws of motion, anotherforce must be included. This is the CEN-TRIPETAL ACCELERATION. The equilibriumwind for these three forces is known as thegradient wind. Curvature of the isobarscan be either in the cyclonic or anticyclonicsense.

granite A member of a family of acidcoarse-grained plutonic igneous rocks con-taining essential quartz, alkali, and plagio-clase feldspars and small quantities ofmafic minerals. The rocks of the granite-granodiorite suite and their volcanic equiv-alents, rhyolites-dacites, are divided ac-

cording to the proportions and composi-tions of the feldspars.

Granites contain on average about 25%quartz and contain either a single feldspar,a perthitic intergrowth (hypersolvus gran-ites), or two kinds, oligoclase and potas-sium feldspar (subsolvus granites). All areleucocratic rocks (see color index), the onlycommon dark mineral being biotite. Acces-sory minerals include muscovite, zircon,apatite, and tourmaline. The typicallygranular texture in which most of the crys-tals are subhedral is often termed granitic.Porphyritic varieties containing largezoned phenocrysts of white, gray, or pinkfeldspar are common. Many granites ex-hibit graphic, orbicular, and rapakivi tex-tures and contain miarolitic cavities. Somestrongly alkaline granites are chemicallydistinctive, having low aluminum and cal-cium contents and characterized by thepresence of the soda pyriboles, aegirineand riebeckite, and an abundance of other-wise rare accessory minerals. With a de-crease in the amount of quartz, granitespass into quartz-syenites and syenites.Adamellites, of which Shap granite is anexample, contain oligoclase and potassiumfeldspars, quartz, and possibly hornblendein addition to biotite. With a decrease inthe amount of quartz, adamellites gradeinto monzonites; the term quartz-mon-zonite is sometimes used as a synonym foradamellite. Granodiorites that are almostdevoid of alkali feldspar are called trond-hjemites. Granitic rocks are particularlysusceptible to the pneumatolytic (see pneu-matolysis) processes of alteration, greisen-ing, tourmalinization, and kaolinization.

Microgranites are the medium-grainedequivalents of granites. Porphyritic vari-

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GRANITE–GRANODIORITE ROCKS

Dominant feldspar Coarse-plutonic Medium Fine-volcanic

alkali feldspar granite microgranite rhyolitealkali feldspar= plagioclase feldspar adamellite microadamellite rhyodaciteplagioclase feldspar granodiorite microadamellite dacite

eties have been termed quartz-porphyriesor granite-porphyries. Granophyre is a mi-crogranitic rock exhibiting a micrographic(granophyric) texture. Granodiorites arevolumetrically the most abundant plutonicrocks and constitute the bulk of thebatholiths in orogenic regions. Granodi-oritic magma may be produced by anatexisat deep crustal levels. When such sites ofmagma generation become exposed at thesurface, the association of granites,migmatites, and high-grade regionallymetamorphosed rocks is apparent. Oncegenerated, granitic liquids rise up in thecrust and undergo differentiation to pro-duce the more chemically extreme mem-bers of the suite. At high crustal levels,intrusive granite plutons produce markedmetamorphic aureoles. Although mostgranites crystallize from a magma, somegranitic rocks may be produced by graniti-zation, the pervasive metasomatism of pre-existing rocks. See also aplites; pegmatite.

granitization The process of metaso-matic transformation of preexisting rocksinto granite due to the action of graniticfluids (ichors) arising from depth.

granodiorite /gran-ŏ-dÿ-ŏ-rÿt/ A plu-tonic igneous rock containing plagioclasefeldspar of oligoclase-andesine composi-tion, subordinate potassium feldspar, bi-otite, or hornblende. See granite.

granophyre /gran-ŏ-fÿr/ A micrograniteexhibiting a micrographic (granophyric)texture. See granite.

granular disintegration The separa-tion of individual grains from the mainmass of a rock. It most frequently occurs incoarse-grained rocks, notably granite, andboth mechanical and chemical weatheringprocesses can be responsible. Mechani-cally, the differential expansion and con-traction of different mineral grains due toINSOLATION may cause disintegration,while crystallization of salt from solution,within the surface pores of a rock, mayprise grains apart. Chemically, HYDRATION

usually involves a considerable volumechange, and granular disintegration may

occur where there is differential expansionof various mineral grains. The usual prod-uct of this type of weathering is a coarsesand.

granule A small fragment of rock up to5 mm across. It is thus larger than a coarsegrain of sand but smaller than a pebble.

granulite /gran-yŭ-lÿt/ A granular tex-tured metamorphic rock occurring in areasof regional metamorphism of the highestgrade. Granulites lack the hydrous min-erals, micas and amphiboles, but maycontain pyroxene, garnet, kyanite, or silli-manite. Characteristically the quartz andfeldspar crystals are flattened and theirparallel alignment gives rise to a foliation.See also charnockite; metamorphic facies.

graphic intergrowth An intergrowth ofalkali feldspar and quartz commonlyfound in pegmatites and granitic rocks.The quartz is orientated along preferred di-rections within the feldspar and on a flatsurface resembles hieroglyphic or runicwriting.

graphite (plumbago) A soft iron-graymassive or laminar form of pure carbon,which is characteristically greasy to thetouch. Graphite is found mainly in meta-morphic rocks; it has many commercialuses.

Graptolithina /grap-tŏ-lith-ă-nă/ Aprimitive group of invertebrates of the phy-lum Hemichordata. Graptolites are totallyextinct, fossils being confined to the Paleo-zoic. The body consists of a number ofminute cups (thecae) of chitinous material,which house individual polyps. The thecaeare arranged along a stem (stipe) and grap-tolites show great variation in the arrange-ment and orientation of the thecae and thedisposition of the stipes. Biseriate formshave thecae on both sides of the stipe, unis-eriate forms along only one, and there maybe a single stipe or many.

Graptolites were benthonic and plank-tonic marine organisms and the planktonicforms attained a wide distribution. This,coupled with their great morphological

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variation and rapid evolution, makes themof considerable importance in biostratigra-phy and they are used as ZONE FOSSILS

throughout much of the Ordovician andSilurian Systems. Graptolites are firstknown from rocks of the Cambrian Sys-tem; they suffered widespread extinctionsat the end of the Silurian but some groupscontinued to the early Carboniferous.

grassland Any region where grass is thenatural vegetation, generally where there isinsufficient rainfall to support woodlandor forest trees but too much for a desert toform. Tropical grasslands, also called SA-VANNAS, occur mainly in E Africa, N Aus-tralia, and parts of N South America.Temperate grasslands include the PAMPAS

of South America, PRAIRIES of centralNorth America, STEPPES of central Asia andRussia, and VELD of southern Africa andAustralia.

grass minimum thermometer A ther-mometer used to record the minimum airtemperature just above (20–30 mm above)ground level, where it is often lowest dur-ing the night.

graticule /grat-ă-kyool/ The network oflines on a MAP PROJECTION representing thelines of latitude and longitude. On somemaps the graticule may not be shown overthe entire face of the map; in these cases thelines shown have short graticule ticks rep-resenting lesser lines not shown.

gravel According to some particle sizeclassifications, unconsolidated material(usually rock fragments rather than indi-vidual mineral crystals) in the 2–60 mmsize range. It is the size of material lying im-mediately above sand, and below cobbles.In practice, however, the term tends to beloosely used in geomorphological literaturefor all material of an unconsolidated na-ture of greater than sand size and less thanboulder size (i.e. from 2–200 mm in diam-eter).

gravimeter /gră-vim-ĕ-ter/ (gravity meter)An instrument used to measure slight vari-ations in the Earth’s gravitational field,

commonly employed in prospecting for oil and other minerals (whose depositscause local anomalies in the gravitationalfield).

gravitational constant (G) The con-stant used in the law of universal gravita-tion. It has the value 6.670 × 10–11

N m2 kg–2.

gravitational gradient A regional shiftin the measured value of gravity across anarea.

gravity See acceleration of free fall.

gravity anomaly On the assumption ofuniform density, calculations can be madeto predict the value of gravity for a partic-ular point on the Earth’s surface. Wherethis value differs from that actually mea-sured an anomaly exists. A negative gravityanomaly is an area in which gravity is lessthan that predicted; it is an area out of iso-static balance. An area of greater gravitythan that predicted is a positive gravityanomaly and usually results from a largemass of basic rocks near the Earth’s sur-face.

gravity collapse structure A tectonicstructure produced mainly as a result ofdownward movement under the influenceof gravity.

gravity fault See normal fault.

gravity fold A fold structure resultingfrom sliding in response to gravity, as dis-tinct from vertical or horizontal compres-sional forces operating during an orogeny.

gravity gliding The movement of alarge rock mass downhill as a result ofgravity.

gravity meter See gravimeter.

gravity unit (gu) A unit of accelerationused with gravity measurements. Onegravity unit equals 10–6 m s–2 and is oftenused in place of the milligal (see gal). Onegravity unit is roughly equivalent to one

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ten millionth of the value of gravity at theEarth’s surface.

gravity wave A wind-generated wave atsea whose length exceeds 50 mm andwhose speed of propagation is influencedmainly by gravitational forces.

gravity wind See katabatic wind.

gray-brown podzolic soils Soils foundcovering large areas of the northeasternUSA to the south of the brown podzoliczone. They are more mildly podzolizedsoils, less acid conditions resulting in moreabundant soil fauna incorporating humusdeep into the AI horizon. An Ea horizon ispresent but it is darker (gray-brown) incolor than the typical eluvial horizon of apodzol. Clay is illuviated in the B horizon,which normally exhibits a blocky struc-ture. They fall into the ALFISOL order of theUS SOIL TAXONOMY.

gray desert soil See sierozem.

gray forest soil A type of soil found inthe transition zone between forest andsteppe in North America and Russia. Thetypical profile consists of a thin litter belowwhich is a dark gray A horizon with acrumb structure. Organic and mineral mat-ter have been well mixed. Below is the Eahorizon, with its nutty structure, which islight gray in color due to silica coating theped faces. Clay and humus have been re-moved from this horizon and deposited inthe illuvial Bt horizon, which has a pris-matic structure. The gradation to parentmaterial, which is often loess, is usuallycharacterized by calcium carbonate con-cretions. Thus the soil exhibits the leachingcharacteristics of a podzol and the calcifi-cation of a chernozem. From the profilecharacteristics it has been suggested thatthese soils originally developed understeppe vegetation, which was replaced byforest when there was a change from awarm dry to a cool humid climate. Thesesoils may be subdivided zonally. Westwardfrom E Russia to European Russia thesesoils change from dark gray forest soils tonormal gray forest soils and finally to light

grayish-brown forest soils. The humuscontent decreases westward from 10% to3% owing to more podzolic conditions.They fall into the ALFISOL order of the US

SOIL TAXONOMY.

graywacke /gray-waks/ An ARENACEOUS

sedimentary rock in which fairly angularparticles of SAND grade, mainly lithic frag-ments, are suspended in a matrix of muchfiner material. See also turbidite.

gray wooded soil A soil belonging tothe podzolic group, morphologically re-sembling a podzol but chemically resem-bling a gray-brown podzolic soil. It istypically found in the mid-continentalstates of Canada on a base-rich parent ma-terial under boreal forest. It has a well-de-veloped Ea horizon with a platy structureand a Bt horizon with a blocky structure.Leaching is less intense than in a podzol,resulting in less movement of sesquioxidesin the soil profile and a soil with a higherpH value. Gray wooded soils fall into theALFISOL order of the US SOIL TAXONOMY.

Great Barrier Reef A CORAL REEF lyingoff the NE coast of Australia, extending forsome 1900 km and varying from 30–160km in width. It is an example of a barrierreef found off a continental coast in con-trast to those reefs that encircle volcanic is-lands. It is separated from the mainland bya discontinuous channel 40–90 m deep andtherefore too deep for coral growth. On theseaward side of the reef, the shelf attainsdepths of at least 420 m. The coral com-posing it covers about half of the Queens-land shelf region. The reef is consideredone of the best coral features in the world,and fears have been expressed concerningserious damage to the reef caused by cer-tain species of starfish, especially thecrown-of-thorns starfish. Episodes ofCORAL BLEACHING of varying intensitieshave also occurred, notably in 1998 and2002. Borings have indicated that recentcoral extends down to depths of between120 and 140 m, and that sand and mud in-tercalations occur within the coral. Thecoral tends to be of two quite differenttypes, the dividing line between the two

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being the Trinity Opening. The reef may bebased on a step-fault platform.

great circle A circle round the Earth’ssurface whose plane passes through thecenter of the Earth. Great circle routes,broken down into a series of RHUMB LINES,are often used in navigation because theyrepresent the shortest distance betweentwo points on the Earth’s surface.

green flash A transient flash of greenlight sometimes observed at sunset or sun-rise. When the Sun sinks below the hori-zon, the last rays may be seen to changefrom pale yellow, to orange, then to green.The rapidity of the process has given thisphenomenon the name of the green flash. It requires a flat, low horizon and occursonly when the air is exceptionally clear; it is caused by differential refraction of the spectral colors by the atmosphere.Normally the colors complement eachother when the Sun is high in the sky, butas the Sun disappears at sunset, or justbefore it appears at sunrise, the individ-ual colors may very occasionally be identi-fied.

greenhouse effect The process bywhich the Earth’s surface is warmed byheat reradiated back to Earth by gases, es-pecially carbon dioxide but also includingmethane, chlorofluorocarbons, nitrousoxide, water vapor, and other gases, in theatmosphere (see greenhouse gas). The pres-ence of the atmosphere maintains thetemperature of the Earth’s surface at amuch higher level than would be expectedon the basis of equilibrium with solar ra-diation input. This is because the gases are transparent to short-wave radiationbut almost opaque to long-wave radiation.Glass has similar properties and the ef-fect of the Earth’s atmosphere has beenlikened to that of a greenhouse. Increasesin carbon dioxide levels resulting from thecombustion of fossil fuels may intensify thegreenhouse effect. See also global warm-ing.

greenhouse gas A gas in the atmos-phere that is mainly transparent to solar ra-

diation but absorbs strongly in the infraredpart of the electromagnetic spectrum andre-emits such longwave radiation in all di-rections, thus contributing to the GREEN-HOUSE EFFECT. The greenhouse gasesinclude carbon dioxide (the most impor-tant), methane, nitrous oxide, water vapor,chlorofluorocarbons, and other gases.Concerns at an international level aboutthe impact that increased levels of green-house gases are having on the global cli-mate led to the drawing up of a treaty in1997, the KYOTO PROTOCOL, which took ef-fect in 2005. The USA, the world’s largestemitter of greenhouse gases, withdrewfrom the Kyoto Protocol in 2001. The sig-natories of the Protocol are required toachieve individual emission reduction tar-gets for six gases or classes of gas: carbondioxide, methane, nitrous oxide, hydroflu-orocarbons, perfluorocarbons, and sulfurhexafluoride. See also global warming;global warming potential.

greenland spar See cryolite.

greensand A greenish sandstone or sanddeposit that consists mainly of GLAU-CONITE.

greenschist /green-shist/ A type of greenmetamorphic rock whose color is causedby the presence of ACTINOLITE, CHLORITE,or EPIDOTE. It should not be confused withGREENSTONE.

greenstone Basic volcanic rock that hassuffered low-grade METAMORPHISM or al-teration.

greenstone belt A group of very ancientbasic and ultrabasic volcanic and sedimen-tary rocks that occur on most continents.The composition of KOMATIITE lavas withinsuch sequences suggests that the tempera-ture of the mantle was much higher at thetime of their formation.

Greenwich Mean Time /grin-ij/ (GMT)Local time at Greenwich, London, which islocated on the 0° meridian and from whichthe standard times of different areas of theglobe are calculated. 15° longitude repre-

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sents one hour in time from the Greenwichmeridian.

greisening /grÿ-zĕ-ning/ See pneumatol-ysis.

grid A network of parallel lines inter-secting at right angles and forming refer-ence squares over the face of a map. Gridlines are numbered, therefore each squarehas unique easting and northing readings.Grids are used to enable identification ofpoints on the map with respect to otherpoints, or distances between points. (In thediagram the junction of stream Y and riverX has a grid reference of 545 315). Theyare also used to determine the accuracy ofa map: by knowing the true position of apoint in relation to the grid and the posi-tion as shown on a map the discrepancy be-tween the two can be determined.

gridiron drainage A type of drainagesystem characterized by many parallelstreams that each have tributaries runningat right angles to them, resulting in a rec-tangular network of drainage channels. Seealso trellis drainage.

grid zone The Earth’s surface can be ar-bitrarily divided into grid zones, which areareas of different grid systems, each withits own origin. Grid zones enable the iden-tification of points on the Earth’s surface

without the use of geographical coordi-nates, i.e. longitude and latitude.

grike /grÿk/ (gryke) See limestone pave-ment.

grit A type of coarse SANDSTONE with alarge grain size, typical of the MILLSTONE

GRIT of Britain.

groin (groyne) An artificial barrier con-structed on beaches and extending seawardperpendicular to the coastline in order totrap beach materials moving along theshoreline. Groins are usually built ingroups, their length, height, and separationvarying with type of beach material and de-sired results. Although very effective atbuilding up the beach in their immediatevicinity, by halting or at least slowingdown longshore movement of material,they may cause considerable erosion ofthat part of the beach down-drift of thegroins.

grossular /gross-yŭ-ler/ A white, yellow,or greenish member of the GARNET groupof minerals, Ca3Al2(SiO4)3. It occurs inlimestone, usually as a product of contactmetamorphism. It is used as a semipreciousgemstone.

Grosswetterlagen /grohs-vet-er-lah-gĕn/(large-scale weather patterns) A conceptdevised by F. Baur to identify the majortrends in atmospheric events over a partic-ular area. It has subsequently been modi-fied and became used in Germany as amethod of assisting long-range weatherforecasting. In the classification the mainanticyclone or depression center is locatedand this is used to predict the likely windand weather systems.

ground frost In Britain, the outcome ofa fall in temperature below 0°C, measuredby a thermometer at ground level on agrass surface. In climatological statistics,note is made of whether a ground frost oc-curred, not its intensity. Data prior to Jan-uary 1 1961 took 30°F as the base forground frost.

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ground ice Ice that is locked within thepores of the soil and rock fragments in theREGOLITH. It represents frozen GROUNDWA-TER.

groundmass The finer-grained bulk ofcrystalline or glassy material that com-poses an igneous rock and into which thecoarser components (PHENOCRYSTS, XENO-LITHS) are set.

ground moraine A sheet of TILL, usu-ally some tens of meters thick, with a sur-face characterized by very low relief.Ground moraine was deposited from thebase of an ice sheet or glacier on the melt-ing of the ice, and forms a cover that tendsto mask former bedrock features, becausewhere hollows formerly existed the great-est thicknesses of till were deposited.

ground swell 1. Swell waves as theyenter shallow water, i.e. water with a depthapproximately half that of the wavelength.This causes the swell waves to decrease inlength and increase in height. In this sense,the waves are beginning to be influenced bythe ground (seabed).2. Swell waves in deep water that possessconsiderable height and length.

groundwater Water precipitated fromthe atmosphere that has percolated into theground and become trapped within pores,cracks, and fissures. Its presence is essentialfor practically all weathering processes andfor this reason more weathering takesplace in well-jointed rocks than in massivetypes. In the upper layers of permeablerock, groundwater can flow through thestrata quite easily; however, at depth alevel is reached below which all pore spacesare water-filled (the water table). Weather-ing still occurs below the water table, in theform of reduction and hydrolysis.

group A division in the hierarchy of thelithostratigraphic classification of bodiesof rock (see lithostratigraphy; stratigra-phy). It is formed of two or more adjacentand related FORMATIONS; adjacent groupsmay be aggregated to form SUPERGROUPS.

growing season The period of the yearduring which plant growth can proceedwithout temperature restriction. By tem-perature latitude convention, a daily meantemperature at screen level of 6°C is thecritical lower limit; the growing season canthen be determined from the monthly vari-ation of mean temperature. It is not a veryprecise concept because a mean tempera-ture of 6°C could be produced by a varietyof diurnal ranges, and different plants re-spond to different critical temperatures.

groyne See groin.

grumusol /groo-mŭ-sôl/ See tropicalblack soil.

grunerite /groo-ner-ÿt/ A monoclinicAMPHIBOLE.

gryke /grÿk/ (grike) See limestone pave-ment.

guano /gwah-noh/ Seabird excrementfound in large amounts on certain of thePacific Islands and along part of the coastof South America. It is of great economicimportance since, being rich in nitrogenousmatter and phosphates, it serves as a valu-able land fertilizer.

guide fossil A fossil of known age that,because of its location, helps to establishthe age of the stratum in which it is found.

Gulf Stream The largest and most im-portant permanent ocean current in the Nhemisphere. It is most prominent in itssouthern portion, beyond the meeting-point of the FLORIDA CURRENT and the An-tilles Current. In this region, it attains itsgreatest depth, probably some 1000 m,and achieves a maximum width of some50–70 km. Farther north, it becomes lesseffective at depth, while to the south of theNewfoundland Banks it begins to split intoseveral branches. It is not a very warm cur-rent, differing only little in its thermal char-acteristics from the Sargasso Sea regionwhich lies close to its path. Indeed, as itspreads out northward, it tends to mixwith colder surrounding waters and is fed

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also by the rather cold LABRADOR CURRENT.Where strongly developed, its sharply de-fined left-hand flank contrasts markedlywith its more diffuse right-hand flank,where it is common for large eddies to bethrown off. The Gulf Stream bringswarmer conditions to NW Europe, includ-ing milder winters in Britain: average an-nual temperatures are some 5° to 10°Cabove the average for the latitude. Someclimate models predict that the flow of theGulf Stream will slow down if greenhousegas emissions continue to increase with re-sultant GLOBAL WARMING. Studies, includ-ing one that has shown a 30% reductionbetween 1957 and 2005 in the warm cur-rents that carry water north from the GulfStream, appear to support these predic-tions. Such a slow down may lead to coolertemperatures in NW Europe. See alsoNorth Atlantic Drift.

gully A narrow channel on a hillsidelacking vegetation, generally formed by therapid runoff of surface water followingheavy rainfall. A gully is dry most of thetime. See also badlands.

gully erosion A type of erosion in whichgullies develop from RILLS when sheet flowof water becomes concentrated into dis-tinct channels. They originate naturallynear the head of the drainage pattern, butthey are better known as the most wide-spread and obvious signs of human in-fluence on geomorphology due to theremoval of vegetation cover in farming.Plowing down the slopes aids the concen-tration of sheet flow into gullies, whichthen erode their headwalls and advanceupslope, as well as widening their valleysand developing tributaries. Gullies removegreat volumes of sediment, especially top-soil, and are known and feared in manycurrently settled areas, such as centralsouthwestern USA and southern Australia.There is evidence that they were also oncewidespread in Europe. It is now knownthat the simple conservation practice ofplowing parallel to contours will retardgully development. See also abnormal ero-sion; arroyo.

gust A sudden increase of wind speed ofshort duration. Gusts are due to mechani-cal interference by the ground surface, sothat where the surface is aerodynamicallyrough, as in cities, gustiness is increased.Conversely, winds in exposed coastal sitestend to experience fewer gusts, althoughtheir mean wind speed can be high.

Gutenberg discontinuity /goo-tĕn-berg,gû-/ The boundary between the Earth’score and mantle, at a depth of about 2900km, at which seismic primary (P) wavesslow down and secondary (S) waves disap-pear. The behavior of the waves is thoughtto be caused by the change from solid rockin the mantle to molten rock in the core.The discontinuity is named for the Ger-man–American geologist Beno Gutenberg(1889–1960).

guyot /gÿ-ŏt/ A relatively smooth flat-topped SEAMOUNT or tablemount. Guyotsare best known in the Pacific and AtlanticOceans. Their flat tops are interpreted asbeing planation surfaces eroded by a com-bination of marine and subaerial processes.The tablemount is more or less conical inshape, and the summit surface usually liesin fairly deep water. The summit depthsoften range between 1000 and 2000 m;others have been sounded between 2000and 3500 m. Few guyots with summitdepths less than some 1000 m have beenlocated. Guyots may be veneered with sed-iments such as volcanic sand, gravel, andglobigerina ooze.

GWP See global warming potential.

gymnosperms /jim-nŏ-spermz/ A groupof primitive seed-bearing plants that differfrom the ANGIOSPERMS in having seeds thatare not protected by an outer covering.They include the conifers, cycads, andginkgos. Gymnosperms are known as fos-sils from the Carboniferous Period andflourished during the early Mesozoic. Mostgymnosperms became extinct during theCretaceous, presumably because of un-successful competition with the angio-sperms.

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gypsum A common and widespread col-orless or white mineral form of hydratedcalcium sulfate, CaSO4.2H2O. It crystal-lizes in the monoclinic system and is anevapotite mineral that occurs in limestonesand shales, often associated with anhydriteand halite. The fine-grained massive whiteform is alabaster, tabular colorless crystalsare selenite, and fibrous crystals form satin

spar. Gypsum is used in making plaster ofParis and cement.

gyre /jÿr/ The gross circulation of waterthat occurs in each of the Earth’s majorocean basins. It is caused by the convectionof warm water from the surface, the effectsof prevailing winds, and the rotation of theEarth. See also ocean current.

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haar /har/ A local term for advection fogon the east coast of Britain. It is also calledsea-fret or sea-roke in NE Britain. In springand early summer the North Sea is rela-tively cold and quickly cools moist air tosaturation point giving low stratus cloud.Whenever winds are onshore, the lowcloud drifts inland where it is gradually dis-persed by heating. It is most frequent underanticyclonic conditions when vertical mo-tion and mechanical turbulence are weak.

habitat The place where an organismlives, as defined by those aspects of its totalenvironment that affect it and to which it isadapted. For example, sponges have a ben-thonic habitat. See also niche.

haboob /hă-boob/ An Arabic word ap-plied to any dust storm in the Sudan, oc-curring chiefly from May throughSeptember, raised by strong winds, with-out reference to its origin. Many of themresult from the downdrafts of cumulonim-bus clouds with wind speeds reaching 90knots in extreme cases, and they are oftenfollowed by heavy rain or thunderstorms.

hachures /hah-shoor, hash-oor/ Shortlines drawn on a map to show the relief ofan area without the use of contours. Theypoint downhill and are thicker and closertogether where the gradient is steepest. Be-fore cartographic techniques were suffi-ciently advanced to enable the constructionof contour lines this method was exten-sively used. It can be of particular effect inmountainous areas.

hade /hayd/ The angle measured be-tween the vertical plane and that of the in-cline of a structural surface, generally

applied to faults but applicable to anystructural surface. Compare dip.

Hadean /hay-dee-ăn/ See Precambrian.

Hadley cell A cellular wind circulationin tropical latitudes with surface windsblowing from the subtropics at about 30°to the Equator and winds in the upper at-mosphere blowing in the reverse direction.This corresponds with the surface temper-ature gradient from the thermal Equatortoward the poles. This simple circulationwas first suggested by the English meteo-rologist George Hadley (1685–1768) in1735. Although it is modified by the effectsof the Earth’s rotation, it is still believed tobe a reasonable approximation to reality.See also general circulation of the atmos-phere.

hail Approximately spherical particlesof ice, generally 5 mm or more in diameter,that fall from cumulonimbus clouds. Thepieces of ice can be irregular in shape andhave been known to weigh up to almost 1kg, although they are normally far less.They often have a concentric structure

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with alternating layers of clear and opaqueice, providing some evidence of their evo-lution within the cloud. It is believed thatthis structure is the result of the stone beingtransported into different parts of thecloud during its development. Beginningwith an ice nucleus, opaque ice will formaround it when it is in the colder parts ofthe cloud. Small cloud droplets freezerapidly to produce a spherical aggregationof ice with many air enclosures to give thecharacteristic white and opaque appear-ance. In the lower parts of the cloud withtemperatures only slightly cooler than 0°C,the relatively larger droplets there tend tospread over the hail surface before freezingso that little air is trapped and the ice istransparent. The distribution of hail in re-lation to the main path of the storm willdepend upon the nature of updrafts and thecloud structure.

Hailstorms occur most frequently in thecontinental interiors of temperate latitudesand decrease toward the poles and Equa-tor, and over the sea. This is because con-ditions most favorable to cumulonimbusdevelopment are found here (intense sur-face heating, sufficient concentrations ofcloud water, and strong thermal gradi-ents).

half-life The time taken for one half of asample of radioactive element to decay (toanother element). It has a constant valuefor any particular radioisotope, and itsmeasurement is the basis of RADIOMETRIC

DATING.

halide /hal-ÿd, hay-lÿd/ A compound (asalt) whose negatively charged element isone of the halogens: fluorine, chlorine,bromine, or iodine. The halides are thusfluorides, chlorides, bromides, or iodides.

halite (rock salt) The common mineralform of common salt, sodium chloride,NaCl. It may be colorless or white, and isoften colored pink by impurities. It crystal-lizes in the cubic system and occurs in un-derground evaporite deposits, derivedfrom ancient seawater. It is used as asource of sodium compounds and chlorine.See also salt dome.

halomorphic soil /hal-ŏ-mor-fik/ A soildominated by sodium salts. There are threemain types: the SOLONCHAKS or saline soils,which are dominated by chlorides and sul-fates of sodium; the SOLONETZ or alkalisoils, dominated by carbonates of sodium;and the leached or degraded SOLOD. Thereare also many arid zone soils subject tosome SALINIZATION. These halomorphicsoils are not true INTRAZONAL SOILS, buttend to occur in certain climates, e.g. thearid to semiarid transition zone; they areoften related to the beds of former lakes orseas, such as the area north of the CaspianSea, or to depressed areas where a watertable is near enough to the surface to feedsalts upward by CAPILLARITY. In highly irri-gated but poorly drained areas, e.g. the SanJoaquin Valley of California, the watertable may be artificially raised by irriga-tion, leading to salinization of formerly fer-tile soils through increased capillary rise ofwater bringing up dissolved salts.

hammada /hah-mah-dă/ (hamada) Atype of arid desert plain, consisting of anextensive almost bare rock surface, espe-cially in the Sahara. See also erg; reg.

hanging valley A tributary valley in aglaciated area, the gradient of which be-comes much steeper on entering the mainvalley. A large glacier will cause greaterdeepening of a preexisting valley than willa smaller glacier, and therefore where glac-iers invade a major valley and its tribu-taries the major valley will be deepened toa greater extent. On the disappearance ofthe ice, the floor of the major valley will beat a lower altitude than those of the tribu-taries, which will hang above the main val-ley. Subsequent rivers will reach the mainvalley from these hanging valleys as water-falls.

hanging wall The surface of rock abovea fault plane or ore body. Compare footwall.

harbor A sheltered stretch of waterwhere ships may anchor or tie up to buoysor jetties, in order to be safe from storms orother adverse conditions, or to load or un-

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load their cargoes. Some are formed bynatural features, including sheltered baysand estuaries; others are created by theconstruction of jetties, breakwaters, moles,and other artificial structures. Artificialharbors are usually built, sometimes aftercareful scale hydraulic model tests, so as tominimize wave action, SEICHE action, andundesirable currents. Many harbors, bothnatural and artificial, have to be dredged(see dredging) in order to maintain or toimprove limiting depths.

hardness See Mohs’ scale.

hardpan A layer of hard material justbelow the surface of the ground, usuallydeposits of carbonates, hydroxides, oxides,silica or, most often, clay. It may also con-tain some organic material. It is common insandstone regions and is impervious towater, thus preventing good drainage.Marshy patches and puddles may form onthe surface after rain.

hard water Natural water that containsdissolved ions of calcium and magnesium.These ions form a scum with soap. Waterwith temporary hardness contains dis-solved calcium hydrogencarbonate, whichis formed by the action of dissolved carbondioxide on chalk or limestone. When tem-porarily hard water is boiled, calcium car-bonate precipitates out, which formslimescale on boilers, pipework, and kettles.Water with permanent hardness generallycontains dissolved calcium sulfate or cal-cium fluoride. These are not removed byboiling.

harmatome /har-mă-tohm/ A white orgray hydrated barium aluminosilicate,Ba(Al2Si6O16).6H2O. It occurs in basic ig-neous rocks and is a member of the ZEOLITE

group of minerals.

harmattan /har-mă-tan/ A dry windblowing from between north and east overWest Africa. Because the air has had a longtrajectory across the Sahara, it is very dry,cool by night but warm by day, and isladen with dust. It represents the normaldry season state of the area, oscillating in

its extent with the seasons, occasionallyreaching the Gulf of Guinea in January butrarely south of 15°N in July.

harmonic analysis See Fourier analy-sis.

harzburgite /harts-berg-ÿt/ An ultra-mafic rock consisting largely of olivine andorthopyroxene.

hastingsite /hayss-tingz-ÿt/ A mono-clinic AMPHIBOLE.

haüyne /haw-ween/ A member of the so-dalite subgroup of FELDSPATHOIDS.

Hawaiian See volcano.

hawaiite /hă-wÿ-ee-ÿt/ A type of alkalibasalt. See trachybasalt.

hazard A natural or man-made event orcondition that has the potential to causeharm or loss to humans and/or damage tothe natural and built environment.

haze Atmospheric obscurity due tominute suspended solid matter, such asdust or smoke particles, in the sky. It is nota very precise term. See also fog; mist.

head See coombe rock.

headward erosion (headwater erosion)The increase in length of a gully or valley,or a stream within it, caused by erosion atthe upper end (head) of the valley.

heat The form of energy to which allother forms will eventually revert. Heatcan be transferred by conduction, convec-tion, or radiation and may lead to an in-crease of temperature, but it can cause achange of state from liquid to vapor with-out any corresponding alteration in tem-perature. Heat is normally measured injoules, although the former unit, the calo-rie, is still sometimes used.

heat balance The static ENERGY BAL-ANCE between the Earth and atmosphere. It

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indicates the utilization of net radiation byevaporation, sensible heat, and advection.

heat island (urban heat island) Anurban area that has a higher temperaturethan its rural surroundings: on calm clearnights excesses of up to 10°C have beenrecorded. They are the result of the differ-ent nature of the surfaces in a city, which inturn affects the heat balance and airflow.See also urban climate.

heat low See thermal low.

heave 1. The horizontal displacementalong a fault.2. See frost heaving.

hectare /hek-tair/ (ha) A metric unit ofarea equal to 10 000 m2 (100 ares), andequivalent to 2.47 acres.

hedenbergite /hed-ĕn-berg-ÿt/ A mono-clinic PYROXENE.

helical flow (helicoidal flow) The mostsignificant type of turbulence in streams.Superimposed on the primary downstreamflow, a secondary flow moves across thesurface of the stream toward the outside ofthe meander bends, compensated by a re-verse flow along the bed toward the insideof the meander bends. This gives thestreamflow a net corkscrew movement,concentrating erosion on the outside of themeander bends, with deposition on the in-side, causing downstream propagation ofthe meanders.

helicitic structure /hel-ă-sit-ik/ Curvedor S-shaped trails of inclusions occurring inporphyroblasts (see porphyroblastic), espe-cially garnets, and representing an earlierfoliation.

hematite /hem-ă-tÿt, hee-mă-/ The min-eral form of ferric iron oxide (Fe2O3), animportant iron ore. It occurs in two maintypes, as a massive red botryoidal or reni-form ore (kidney ore) and as metallic crys-tals known as specular iron ore. Hematiteoften occurs as a cement in sandstones pro-ducing a red coloration. Most ore deposits

result from the alteration of iron carbon-ates and silicates in sedimentary rocks.

hemimorphite /hem-ă-mor-fÿt/ (calamine)A mineral form of hydrated zinc silicate,Zn4Si2O7(OH)2.H2O. It crystallizes in theorthorhombic system and generally occursas fibrous crusts or masses. It is used as asource of zinc.

herbivore /her-bă-vor, -vohr/ An animalthat eats green plants or algae. In ecologi-cal terms, it is a primary CONSUMER and isgenerally the source of food for secondaryconsumers (carnivores). See also foodchain.

Hercynian orogeny /her-sin-ee-ăn/ Aphase in the VARISCAN orogeny affectingEurope and characterized by a northwestfold trend. The term is used without timesignificance and covers the Carboniferousand Permian.

hercynite /her-să-nÿt/ See spinel.

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Fig. 1: Distribution of maximum velocity in a meandering stream due to helical flow

depositionerosion

maximum current velocityas directed by helical flow

Fig. 2: Section through the outside of a meander bend

depositiondepositiondeposition(inside of(inside of(inside of

bend)bend)bend)

erosion(outside of bend)

helical current

Helical flow

heterogeneous strain /het-ĕ-rŏ-jee-nee-ŭs/ The changes in shape or dimensionsoccurring when a body of rock does not de-form equally in all directions. A linearstructure would become curved and paral-lel structures would diverge after deforma-tion.

heterolithic unconformity See non-conformity.

heteromorphism /het-ĕ-rŏ-mor-fik/ Theexistence of rocks of very similar chemicalcomposition but with different mineral ag-gregates, resulting from different rates ofcooling of the magmas from which theyformed.

heterotrophic /het-ĕ-rŏ-troff-ik/ Describ-ing an organism that is a CONSUMER, i.e.one that feeds on plants or other organismsbecause it cannot synthesize its own food(unlike AUTOTROPHIC organisms). Such or-ganisms include some algae and BACTERIA,most protozoa, and all fungi and animals.

heulandite /hyoo-lănd-ÿt/ A white, gray,or brown hydrated calcium sodium alu-minum silicate, (Ca,Na2)Al2Si7O18.6H2O.It crystallizes in the monoclinic system andoccurs in basic igneous rocks. It is a mem-ber of the ZEOLITE group of minerals.

hexagonal /heks-ag-ŏ-năl/ See crystalsystem.

hiatus A break in a stratigraphic se-quence, either as a result of nondepositionor erosion. It represents the period of timemissing between beds above and below anunconformity.

high See anticyclone.

high index circulation (in meteorol-ogy) The phase of the INDEX CYCLE whenthe westerly circulation reaches its maxi-mum intensity in specified areas. This rep-resents a strong surface westerly flow withRossby waves of long wavelength andsmall amplitude. Meridional exchange is ata minimum value.

high-resolution satellite imagery Seesatellite.

hill fog Persistent low cloud that covershigh ground, associated with low atmos-pheric pressure. It does not affect low-lyingregions. See also fog.

hill shading (plastic shading) A meansof showing relief on maps, shading theeast- and south-facing slopes (the steeperthe slope, the darker the shading) to givethe effect of an oblique light shining fromthe northwest over a relief model.

hinge fault (pivot fault) A normal faultthat dies out along its trend by a gradualdecrease in throw. (See diagram at FAULT.)

hinge line 1. (in folding) An imaginaryline joining points of maximum curvatureof the folded strata.2. The boundary between a stable region ofthe Earth’s crust and one undergoingchanges in elevation, generally as a resultof isostatic readjustment but also includingareas of tectonic deformation.

histosol /hiss-tŏ-sôl/ One of the twelvesoil orders of the US SOIL TAXONOMY, whichincludes bog soils such as peat. Histosolsare characterized by accumulations of or-ganic matter, which remains more or lessundecomposed because of the waterloggedconditions. The histic EPIPEDON (surfacehorizon) has at least 20% of its weight inorganic matter, or over 30% if half theweight of the horizon is clay. Histosols aredivided into four suborders on the basis ofthe degree of decomposition of the organicmatter.

Although found throughout the world,histosols occupy the smallest area of all thetwelve orders of the US SOIL TAXONOMY

(under 1% of all soils). They tend to de-velop particularly in cooler, poorlydrained, more humid areas.

hoar frost A silvery-white deposit of icecrystals formed on surfaces cooled belowfreezing point by radiation. It may resultfrom the freezing of dew or by the directsublimation of ice crystals from water

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vapor in the atmosphere when tempera-tures are less than 0°C.

hodograph /hod-ŏ-graf, -grahf/ A dia-gram used to analyze the wind field abovean observing station. Winds at differentpressure levels are drawn as vectors fromthe origin and so give an indication of thechanges in wind speed and direction withheight. It is then possible to make deduc-tions about the thermal patterns at theseupper levels and about the advection ofwarm or cold air.

hogback A steep-sided narrow ridgethat stands above the surrounding plain. Itgenerally results from the folding or fault-ing of strata, followed by differential ero-sion that removes softer rocks fromalongside it.

Holocene /hol-ŏ-seen/ (Recent) The pre-sent epoch, covering the last 10 000 yearsor so of geologic time from the end of thePLEISTOCENE. In Britain it follows all themajor Pleistocene glacial episodes and istherefore often known as the Post-Glacial.However, some authorities consider it tobe no more than an interglacial phase of the Pleistocene. At the beginning of the Holocene, Britain was isolated fromthe rest of the continent of Europe by ageneral rise in sea level. Descendants ofmost fossil species of this epoch are stillalive today.

holocrystalline /hol-ŏ-kris-tă-lin,-lÿn/Describing a rock composed wholly ofcrystals.

holozoic /hol-ŏ-zoh-ik/ Describing anorganism that feeds on complex organicmatter (as opposed to taking in simple sol-uble materials). The term thus includesmost animals.

homeomorphism /hoh-mee-ŏ-mor-fik/The existence of crystals that have the sameform and habit but different chemical com-position. See crystal form; crystal habit.

homeomorphy /hoh-mee-ŏ-mor-fee/ Sim-ilarity in structure between organisms thatare not closely related as a result of evolu-tionary adaptation to similar habitats (seeconvergent evolution). The term is oftenrestricted to similarities between moreclosely related species, in which homeo-morphy leads to confusions in taxonomy(see parallel evolution).

homocline /hoh-mŏ-klÿn/ A series ofbeds of rock all of which have a similar dipand strike, including monoclinal structuresand isoclinally folded beds. The term is ap-plied only to small areas of the Earth’scrust.

homogeneous strain /hom-ŏ-jee-nee-ŭs/The changes in shape and dimensions thatoccur when a body of rock deforms equallyin all directions. It results in straight-lineand planar structures.

homolographic projection /hom-ŏ-lŏ-graf-ik/ A MAP PROJECTION possessing theproperty of equal area, i.e. the areabounded by adjoining meridians and paral-lels on the map is equal in area, by ratio, tothat area bounded by the same meridiansand parallels on the ground, e.g. MOLLWEI-DE’S PROJECTION and BONNE’S PROJECTION.

homotaxis /hom-ŏ-taks-iss, hoh-mŏ-/The occurrence of divisions of rock occu-pying the same position in the stratigraphicsequence in separate successions. Such di-visions are said to be homotaxial. Al-though the divisions may be of the same

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Hodograph

age, the concept does not take account ofconsiderations of time and simply reflectsthe order of deposition. Compare chrono-taxis. See also correlation (def. 2).

honeycomb weathering Weatheringwithin jointed rocks in which joint infill-ings are more resistant to weathering thanthe main mass of the rock, so that after ero-sion they project and surround recesses inthe rock. Typical infilling materials areiron oxides, secondary silica, calcite, andmanganese oxides. Migration of rock ce-ments into the joints may produce a corre-sponding decrease in the resistance of therock itself, thereby increasing the effective-ness of honeycomb weathering.

Hooke’s law A law stating that astressed body deforms to an extent that isproportional to the force applied. Ma-terials that obey Hooke’s law are said to beelastic. The law is named for the Englishphysicist Robert Hooke (1635–1703).

horizon 1. (in soil science) One of anumber of layers of soil arranged in a ver-tical sequence in the PROFILE. Each layer isreasonably uniform. Most soil-formingprocesses result in this layered arrange-ment but some, e.g. the action of earth-worms, are destructive in this respect.2. (in stratigraphy) A plane within a rocksequence that is assumed to be a timeplane. In practice, a horizon is not a sharpplane but a thin bed characterized by somedistinctive feature of lithology or fossilcontent. A particular horizon is sometimesmerely theoretical, and it may be regardedas extending through a contemporaneoussuccession that carries no internal evidenceof its presence.

horizontal equivalent The distance be-tween two points on a slope projected ontoa horizontal plane, as they would be repre-sented on a map.

horn (horn peak) See pyramidal peak.

hornblende /horn-blend/ A monoclinicmineral of the AMPHIBOLE group, commonin both igneous and metamorphic rocks.

hornfels /horn-fels/ A fine- to medium-grained nonfoliated rock composed ofequidimensional mineral grains showingno preferred orientation. Porphyroblasts ofminerals such as biotite, andalusite, andcordierite may occur (see porphyroblastic)and their incipient development is often in-dicated by a spotted appearance to therock. Hornfels are the product of contactmetamorphism and are found in aureolesbordering intrusive plutonic igneous rocks.See also contact metamorphism.

hornito /hor-nee-toh/ A small volcanicstructure resulting from the accumulationof small blebs of lava thrown out around aparasitic vent; a spatter cone.

hornstone A very fine-grained volcanicash. See pyroclastic rock.

horse latitudes The zone of light andvariable winds between 30° and 40° N andS where the subtropical anticyclones aredominant. The name arose during the daysof sailing ships when unfavorable windscould prolong the journey; when food be-came short, horses would be thrown over-board or eaten.

horst An elongated block of rocksbounded by faults. The sense of movementof the block is upward relative to the sur-rounding rocks. Compare graben. (See dia-gram at FAULT.)

hot spot An area on the Earth’s surfacewhere there is high thermal activity, oftenin the form of volcanoes. Hot spots arethought to represent the surface manifesta-tion of MANTLE PLUMES, which may origi-nate deep withn the Earth. Most volcanicactivity occurs on or near PLATE BOUND-ARIES, e.g. the hot spot of Iceland is locatedon a constructive plate boundary, but thereare exceptions. The islands of Hawaii, forexample, are of volcanic origin but locatedover 3200 km from the nearest plateboundary. In such cases it has been hy-pothesized that the hot spot is virtually sta-tionary and successive volcanic eruptionsproduce a trail of volcanoes that becomeprogressively older away from the hot spot

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as the lithospheric plate passes above it(e.g. the Hawaiian Ridge–EmperorSeamounts chain). A hot spot below theNorth American plate is located at Yellow-stone National Park in the USA.

hot spring See thermal spring.

Hudson canyon One of numerous sub-marine canyons that cut into or across thewide continental shelf lying off the E coastof the USA. It is one of the best-known sub-marine canyons in the world and has beencarefully surveyed several times. It cutswell into the shelf opposite the HudsonRiver and has been traced seaward as far asa wide submarine fan. The cross section ofthe canyon tends to be V-shaped, andflanking it are what appear to be small trib-utary valleys. Core samples have yielded avariety of deposits (rock, gravel, and clay),certain of which suggest that some sort ofpowerful turbidity flow may have oper-ated, perhaps when sea level was signifi-cantly lower.

human influence on geomorphologyIn the 8000 years of the Holocene geologicperiod during which humans have becomeestablished, their activities have had con-siderable geomorphological impact, princi-pally in accelerating existing processes butto a lesser extent in creating new land-forms. The major effects are those conse-quent on destruction and removal of thevegetation cover. Clearing the land foragriculture can cause gullying and soil ero-sion, leading to destruction of valuablefarmland, and wind erosion of bare soilcan create DUST BOWLS (see also badlands).In coastal areas, human use of sand dunesleads to vegetation destruction and cre-ation of BLOWOUTS. River basins can havehigher sediment yields than those not somuch used, owing to the increased erosionfrom farming, etc.

Landforms created by human action in-clude lakes resulting from subsidence ofthe land surface in mining areas, the Nor-folk Broads, created by peat-cutting in me-dieval times, and reclaimed land from thesea, severely changing the coastline, aswith the Zuider Zee in the Netherlands.

Spoil heaps, railroad cuttings, and theblowing out of hillsides to build towns areother deliberate results. Sometimes humanaction can produce quite unexpected re-sults, especially in the coastal zone wherethe erection of groins, harbor arms, andother artificial structures severs the littoraldrift leading to the decay or growth ofbeaches in quite unnatural situations.Large-scale recharge schemes may be usedto overcome these problems, with convoysof trucks shuttling beach materials that canno longer be moved by natural agencies.See also arroyo.

Humboldt Current /hum-bohlt/ (PeruCurrent) A current of cold ocean waterthat flows from the Antarctic northwardalong the western coast of South America.It veers westward as it reaches the Equator.It cools winds blowing onto the coast fromthe Pacific Ocean, reducing rainfall on thewestern slopes of the Andes. The current isnamed for the German explorer and scien-tist Baron Alexander von Humboldt(1769–1859). See also El Niño.

humic acid /hyoo-mik/ An acid derivedfrom resynthesized organic materials thatdominate the soil humus along with thefulvic acids. Humic acids are polysaccha-rides and contain more carbon and nitro-gen and less oxygen than fulvic acids.Fulvic acids tend to be more important innewly formed humus but eventually theyare overtaken by the humic acids.

humidity The amount of water vapor inthe atmosphere. It can be measured or cal-culated in a variety of ways. The most fre-quently used measure is the RELATIVE

HUMIDITY, which is the mass of water vaporin a given volume of air expressed as a per-centage of the mass of water vapor in anequal volume of saturated air at the sametemperature and pressure. As its name im-plies, this is only a relative index and itvaries in opposite phase to temperature,even though the absolute amount of mois-ture in the air may not change. A better sys-tem from the meteorological point of viewis the use of VAPOR PRESSURE, which indi-cates the water vapor content of the at-

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mosphere irrespective of temperature. Yetanother measure is the SPECIFIC HUMIDITY –the mass of water vapour per unit mass ofair. The ABSOLUTE HUMIDITY is the mass ofwater vapor per unit volume of air, given ingrams per cubic meter.

humidity mixing ratio See mixingratio.

humification /hyoo-mă-fă-kay-shŏn/ Theformation of HUMUS from organic matter.

humite /hyoo-mÿt/ A member of a groupof orthorhombic and monoclinic mineralsconsisting of layers of Mg2SiO4, whichhave an olivine structure, alternating withlayers of composition Mg(OH,F)2. Humiteminerals are found in metamorphosedlimestones.

hummock A mound or knoll that risesabove a generally level surface, or a smallmound of soil or turf in alpine areas (seeearth hummock).

humus Amorphous colloidal material inthe soil, dark in color and composed of re-sistant plant tissues, such as lignin, andnew compounds, such as polysaccharides,synthesized by microorganisms. It is im-portant to the soil both physically andchemically (its cation exchange capacityfar exceeds that of the clays). See alsomoder humus; mor humus; mull humus.

hurricane 1. A TROPICAL CYCLONE withsurface wind speeds in excess of 64 knots(117 km per hour) that occurs in theCaribbean and W Atlantic, the Gulf ofMexico, and in the E and central North Pa-cific east of the dateline. They are identicalto similar tropical cyclones in the W Pa-cific, Indian Ocean, and near Australia,which have their own local names (see cy-clone; typhoon). Hurricanes are classifiedaccording to their strength on the SAFFIR–SIMPSON HURRICANE SCALE. At the center ofa hurricane is an area of light winds andhigher temperature known as the EYE,about which clouds and rain bands spiralwith the associated winds.

The 2005 Atlantic hurricane seasonwas the most active on record with 14 hur-ricanes, 7 of which became major hurri-canes of category 3 or higher on the Saffir–Simpson Hurricane Scale. These includedhurricane Katrina, one of the most devas-tating hurricanes in US history, whichcaused considerable flooding and damageover the New Orleans area and along theMississippi Gulf coast, and an estimated1200 deaths. The average number of hurri-canes in a season is around 6.2. In the BEAUFORT SCALE, wind speedsabove 64 knots, whether or not they arelinked to a tropical cyclone. Such speedsrarely occur over land.

Huygens’ principle /hÿ-gĕnz, how-/ Aprinciple stating that each point of a wavefront acts as a source of secondarywavelets. It was proposed in 1690 by theDutch physicist and astronomer ChristiaanHuygens (1629–95).

hyaline /hÿ-ă-lin, -lÿn/ Describing a rockthat is glassy.

hyaloclastic rock /hÿ-ă-lŏ-klass-tik/ Seepalagonite.

hyalocrystalline /hÿ-ă-lŏ-kris-tă-lin, -lÿn/ Describing a rock composed of bothcrystals and glass.

hybridization /hÿ-bri-dă-zay-shŏn/ Seeassimilation.

hydrate 1. A chemical compound thathas water in its composition (as water ofcrystallization).2. A chemical compound produced by HY-DRATION.

hydration The chemical addition ofwater to a substance. If the substance is amineral, it usually involves a fairly consid-erable expansion in the mineral grains.This may be important in producing subse-quent mechanical weathering, in the formof EXFOLIATION and GRANULAR DISINTEGRA-TION. Minerals affected by hydration areprepared for further chemical weatheringprocesses, such as OXIDATION and CARBON-

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ATION, while the process is also very im-portant in the formation of CLAY MINERALS.

hydraulic action (hydraulicking) (ingeomorphology) The removal of loose, in-coherent, or weathered material by flowingwater, assisting in stream-bed and bankerosion. Material carried can range fromsingle minerals or aggregates of crystals topieces of layered bed strata, e.g. slate orshale, if the materials dip downstream. Seeabrasion.

hydraulic jump A sudden increase inmean water level in the direction of flow,which is of a rapidly varying type and leadsto a form of stationary wave. In the sim-plest terms, a surface flow at high velocityconfines a deeper flow, thereby resulting inthe dissipation of energy through turbu-lence.

hydrograph A chart or graph thatshows changes in a particular variable (e.g.discharge, stage (water level), or watertemperature) over time for a body of watersuch as a stream, lake, reservoir, or bore-hole. For example, a stream discharge hy-drograph shows changes in the dischargeof a stream at a particular point over time;a storm hydrograph shows the total dis-charge at a point for a particular rainevent.

hydrographic chart /hÿ-drŏ-graf-ik/ Amap of the seabed, showing depths ofwater, heights of underwater features, and,sometimes, geologic information. See hy-drography (def. 2).

hydrography /hÿ-drog-ră-fee/ 1. Thestudy of the oceans, seas, rivers, and otherwater bodies, together with the strips ofland bordering these. It involves descrip-tion and measurement and the subsequentpresentation of this information on hydro-graphic charts.2. The shape of the sea floor and the de-posits of which it is composed, includingnavigational information.

hydrolith /hÿ-drŏ-lith/ A type of rock

that was formed by chemical precipitationfrom water, such as gypsum or halite.

hydrologic cycle /hÿ-drŏ-loj-ik/ (watercycle) The continuous circulation ofwater between the oceans, atmosphere,and land. Water evaporates from theoceans (and lakes and rivers) as watervapor in the atmosphere, where it may con-dense into clouds. Clouds release precipita-tion (rain, snow, or hail), which falls on theland. Some evaporates, some is taken up byplants and released into the atmospherewhen they transpire, and some runs off toform streams and rivers that flow to theoceans. Any that penetrates the groundforms GROUNDWATER.

hydrology The scientific study of thewaters of the Earth on and within theground. It focuses on those aspects of thehydrologic cycle that occur close to theland surface: precipitation, evaporation,evapotranspiration, runoff, soil moisture,snow and ice accumulations, and ground-water.

hydrolysis /hÿ-drol-ă-sis/ A chemical re-action involving water. In geology, it is aform of chemical weathering involving areaction between the H and OH ionsformed by the decomposition of water andthe ions of rock minerals; it occurs wher-ever rocks and water are in contact. Hy-drolysis is a particularly important processin the weathering of FELDSPARS; when watercomes into contact with orthoclasefeldspar, potassium hydroxide and alu-mino-silicic acid are produced. These reactwith atmospheric carbon dioxide givingpotassium carbonate and water, while theacid breaks down into clay minerals andeasily removable soluble colloidal silica.Kaolin (china clay) is produced by weath-ering in this way.

hydromorphic soil /hÿ-drŏ-mor-fik/ Atype of soil found in a wide range of envi-ronments where drainage is poor, e.g. allu-vial flats, and therefore such soils areclassified as intrazonal. Reduction is amore dominant process than oxidation, re-sulting in a blue/gray subsoil, with

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red/brown mottles. The most importantsoil of this group is gley but also includedare peaty gley podzols, peaty gleys,meadow soils, planosols, groundwaterpodzols, and groundwater latosols. Withincreasingly poor drainage these soilsgrade into the organic soils.

hydrosphere /hÿ-drŏ-sfeer/ All the wa-ters of the Earth, as opposed to gases (at-mosphere), rocks (lithosphere) and livingorganisms (biosphere). In all, water coversabout 74% of the Earth’s surface and has atotal mass of 1021 kg.

hydrostatic equation /hÿ-drŏ-stat-ik/An equation relating the variation of pres-sure with height to the density of air andthe force of gravity: dp/dz = –ρg, wheredp/dz is the rate of change of pressure withheight, ρ is the density of air, and g is theacceleration of free fall. It is a useful ap-proximation when considering horizontalmovement relative to the Earth and is usedas one of the basic equations in models ofthe GENERAL CIRCULATION OF THE ATMOS-PHERE.

hydrostatic pressure 1. Pressure that isexerted by a liquid, often water. In a bodyof water, the pressure increases with depth.2. (in geology) The pressure exerted onrock in the Earth’s crust by the rocks thatoverlie it. It is also termed confining pres-sure.

hydrothermal process /hÿ-drŏ-th’er-măl/Following the pegmatitic stage during thecrystallization of an igneous melt is the hy-drothermal stage, when the residual fluid isa relatively low-temperature aqueous solu-tion. Such a fluid may effect considerablealteration of the crystallized portion of themagma by such processes as saussuritiza-tion, kaolinization, and the alteration ofmafic minerals to serpentine and chlorite.

Many mineral deposits are formed byprecipitation from hydrothermal solutions.A characteristic hydrothermal deposit isthe sulfide-bearing vein filling rock fis-sures. Hydrothermal deposits may be di-vided into three groups: hypothermaldeposits formed at temperatures of

300–500°C at considerable depth and in-cluding cassiterite, wolfram, and molyb-denite veins; mesothermal depositsincluding sulfides of iron, lead, copper, andzinc and formed at temperatures of200–300°C; epithermal deposits formed atthe low temperatures of 50–200°C and in-cluding stibnite, cinnabar, sulfur, silver,and gold. See also pneumatolysis.

hydrothermal vent (smoker) An activevolcanic vent on the deep-ocean floor thatemits hydrothermal fluids at high pressureand with temperatures of around 300°C.The vents frequently contain dissolved sul-fides, e.g. of copper, iron, manganese, andzinc, which give the water a black colorand these are often known as black smok-ers. Vents may also be white, gray, or clearaccording to the minerals ejected; whitesmokers are those releasing minerals suchas barytes and silica. Sometimes a tall ver-tical chimney of deposited minerals formsaround the site of the vent. Hydrothermalvents were first discovered in 1977 atdepths of 2.5 km on the Galapagos Rift (a MID-OCEAN RIDGE) off the coast of Ecua-dor and more have since been discovered at other sites along mid-ocean ridges. The vents support unique ecosystems: thedissolved sulfides are oxidized by chemo-synthetic bacteria to form organic com-pounds. Some of the animals around thevents actually harbor the bacteria withintheir body tissue in a symbiotic relation-ship, while others feed on the organic com-pounds. The abundant sea life includesgiant tube worms, giant clams, mussels,fish, and crabs.

Hydrozoa /hÿ-drŏ-zoh-ă/ A class of thephylum CNIDARIA. As few hydrozoans pos-sess hard parts they are not common asfossils, although the Paleozoic STROMATO-POROIDS may belong to this class.

hyetograph /hÿ-ĕ-tŏ-graf, -grahf/ 1. Achart that shows the distribution of precip-itation with respect to time, either at apoint or over an area.2. A type of rain gauge that records theamount and duration of rain as it falls.Rain is collected in a cylinder with a float

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on the water surface. As the water levelrises during precipitation, the float rises inproportion to the rate of rainfall and this isrecorded by a pen on a rotating chart.

hygrometer /hÿ-grom-ĕ-ter/ An instru-ment that measures HUMIDITY (the watervapor content of the air).

hygroscopic nuclei /hÿ-grŏ-skop-ik/ At-mospheric particles that have an attractionfor water vapor and help in the initiation ofprecipitation processes.

hygrothermograph An instrument con-sisting of a combined hygrograph and ther-mograph that records both humidity andtemperature on a single chart.

hypabyssal rock /hÿ-pă-biss-ăl/ See plu-tonic rock.

hypersthene /hÿ-per-sth’een/ An ortho-rhombic PYROXENE.

hypervelocity impact /hÿ-per-vĕ-loss-ă-tee/ The impact of an object (such as aMETEORITE) that is moving so fast that therocks it hits are not strong enough to with-stand the shock waves created. The result isgenerally a large crater, surrounded by IM-PACT BRECCIA and other EJECTA.

hypidiomorphic /hÿ-pă-dÿ-ŏ-mor-fik/Describing a rock consisting of EUHEDRAL

and ANHEDRAL crystals or in which the ma-jority of crystals are SUBHEDRAL. Compareallotriomorphic; idiomorphic.

hypolimnion /hÿ-pŏ-lim-nee-ŏn/ Thecool lower layer of water in a lake or shal-low sea. Insufficient light penetrates forphotosynthesis to take place, so there areno green plants and there is little dissolvedoxygen in the water. See also epilimnion.

hypothermal deposit /hÿ-pŏ-th’er-măl/See hydrothermal process.

hypsithermal (altithermal) The NorthAmerican term for the period of time fromabout 8000 to 3000 BP during theHolocene when warmer climatic condi-tions prevailed. See also Atlantic period;climatic optimum.

hypsographic curve /hip-sŏ-graf-ik/ Acurve showing the proportion of a land-scape lying at, above, or below particularelevations. The percentage of the landscapebetween successive contours is calculatedfrom a relief map, and the results expressedeither as simple proportions between dif-ferent heights, or as a cumulative measureshowing the percentage of area lying aboveeach level, starting with 100% at sea leveland working up to nil at the absolutepeaks. It has been used to look for devel-opment of erosion surfaces, e.g. terracesand former peneplains, which appear onthe curve as extensive areas at their partic-ular elevations. It is also used to display theproportions of the Earth’s surface at par-ticular elevations above and depressionsbelow sea level.

hypsometer /hip-som-ĕ-ter/ An instru-ment for determining atmospheric pressureor altitude by measuring the boiling pointof water. The air pressure can be deter-mined directly from the temperature atwhich boiling occurs, but altitude mustthen be obtained from the altimeter equa-tion. Accuracy is not very high: the boilingpoint must be known to within 0.01 K toobtain the height to within 3 m.

hypsometric tinting /hip-sŏ-met-rik/The differential coloring of elevation bandson a map to enable the user to determinequickly the higher and lower areas of theland portrayed. Greens usually portraylowland areas; as the land rises so the col-ors change from yellows to browns topurples. This technique is common insmall-scale maps, e.g. atlases and wallmaps.

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Iapetus Ocean See proto-AtlanticOcean.

ice The solid state of water. It melts at0°C, requiring approximately 340 000joules per kilogram (80 calories per gram)of ice (latent heat of fusion) or 2.8 × 106

J/kg (677 cal/g) if converted directly fromice to vapor (latent heat of sublimation).Pure water does not necessarily freeze toice when the temperature falls below 0°C,especially if it is in the form of smalldroplets such as those found within clouds.This has very important implications in theBERGERON–FINDEISEN THEORY of precipita-tion formation. Even when FREEZING NU-CLEI are present, droplets may still remainin the liquid phase to temperatures of–30°C or less. By –40°C the water willfreeze spontaneously irrespective of thepresence or absence of nuclei.

At the ground surface, ice can takemany forms: rime, hail, black ice, and, ona vast scale, glaciers. Because ice reflectssolar radiation, and requires heat for melt-ing, it is a very important aspect of the heatbalance in high latitudes.

ice age A period in the Earth’s historywhen ice spread toward the Equator ac-companied by a general lowering of sur-face temperatures, especially in temperatelatitudes. The PLEISTOCENE epoch, endingfrom about 10 000 years ago, experiencedat least four major ice advances (see glacialmaximum), with the margin reachingabout 52°N over NW Europe and about45°N in NE America. With this change inlocation of ice surfaces the whole atmos-pheric circulation altered, the main cli-matic belts being compressed and pushedtoward the Equator. At present we appearto be in an interglacial circulation but it is

assumed that further ice ages will be expe-rienced in the future; as the true cause orcauses of ice ages are not known it is im-possible to predict when this will be. It ispossible that changes in landmass altitudeand variations in solar radiation are at leastpartly responsible.

From geologic records, it has been de-duced that ice ages also existed in Permo-Carboniferous times about 250 millionyears ago, one in late Precambrian timessome 500 million years ago, and possiblyseveral earlier. These former ice-age de-posits have even been found in presenttropical latitudes, because the continentshave since changed their relative positions.

Between 1550 and 1850 temperaturesin much of the N hemisphere fell to theirlowest since the last ice age, and this periodhas been called the Little Ice Age. It wascharacterized in northern regions by heavysnowfalls and prolonged winters. Alpineglaciers advanced and settlement in manynorthern areas, such as Greenland, Iceland,and N Norway, had to be abandoned. Theworld’s climate, however, did not undergoany overall change.

iceberg A mass of ice in the sea that hasmoved from a land area to the coast, andfrom there into the sea, and which usuallyexceeds 5 m in height above sea level.These ice masses, many of which resultfrom the calving off of fragments of glacierice, float partly above but largely beneaththe surface of the sea. They vary greatly inshape one from the other: some are dome-shaped, some are sloping or pinnacled,while others have a more tabular form.Glacier bergs display very irregular shapes.

ice cap A dome-shaped permanent mass

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of ice such as might cover a highland area;a small-scale ICE SHEET.

icefall A section of a glacier at which theice moves down a steep slope. Many trans-verse crevasses form across the glacier, andthere may be piled up SÉRACS at the foot ofthe icefall.

ice field 1. A general term for a largearea of ice, smaller than, and without thedome shape of, an ice cap.2. An extensive mass of sea ice.

ice floe An area of floating sea ice thatmeasures 20 m or more across and is es-sentially flat and tabular. They range insize from small floes, 20 to 100 m across,to giant floes, over 10 km across.

ice fog A type of fog in which visibility isless than 1 km as a result of ice crystals sus-pended in the air. Such fogs have increasedin frequency in recent years as human ac-tivities extend to colder latitudes. Whenthe air is very cold, only a little water is re-quired before saturation takes place. If thetemperature is below –40°C the ice crystalswill form directly on saturation. Normallylittle evaporation takes place, but the re-lease of water vapor from motor-vehicleexhausts or power station cooling systemscan quickly saturate calm air, as often hap-pens in the Fairbanks area of Alaska.

Icelandic Low The mean low-pressuresystem of the North Atlantic. Low-pres-sure systems (cyclones or depressions)form in the latitude of Newfoundland,move northeastward as they intensify, thengradually decay. Although their tracksvary quite considerably, on average theyare most frequent and at their most intensein the Iceland area. As the circulation of theatmosphere varies with time, the precise lo-cation of the Icelandic Low changes.

Iceland spar A transparent variety ofCALCITE of optical quality. See also carbon-ate minerals.

ice-rafting The transport of debris insea ice. Widespread at high latitudes are

sediment deposits that have been rafted byice, either icebergs or ice floes. Theyreached the ice from meltwater streams orby being frozen into the base of ice thatlater became waterborne. These materialsrange from very large boulders to clays andsilts. They may be transported over consid-erable distances to be deposited amongmaterial of an entirely different character.Icebergs and floes reach nearer to theEquator in the N hemisphere than they doin the S hemisphere. Ice-rafted pebbles andboulders seem to be capable of making pitsor dents in soft sea-floor deposits, presum-ably as they fall to the sea floor, as wasshown by underwater photography in theArctic.

ice sheet An accumulation of ice hun-dreds of meters thick and covering vastareas. Many developed during the PLEIS-TOCENE Epoch, and are represented by thepresent Greenland and Antarctic ice sheets.The ice develops at high altitudes, spread-ing outward and submerging all the under-lying topography. Being unrestricted byrock walls, unlike CIRQUE and VALLEY

GLACIERS, the directions of ice movementwithin these ice sheets reflect the slopes ofthe ice surface, which depend upon relativeaccumulation rates within the variousparts of the sheet.

ice shelf Part of an ICE SHEET or ICE CAP

that spreads out over the sea, as aroundmuch of Antarctica. It is attached to theland and built up by accumulated snowand outward moving glaciers.

ichor /ÿ-kor, -ker/ See granitization.

Ichthyosauria /ik-th’ee-ŏ-sor-ee-ă/ Anorder of extinct reptiles that became secon-darily adapted to a marine life. In shapethey were similar to dolphins (mammals):they had fishlike tails and the limbs weremodified as paddles. Ichthyosaurs were to-tally aquatic and the young were probablyborn alive. They were prominent through-out the Mesozoic, especially in the JurassicPeriod, becoming extinct at the end of theCretaceous. Compare Plesiosauria.

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iddingsite /id-ingz-ÿt/ The red-brown al-teration products of forsteritic olivine con-sisting largely of iron oxides and clayminerals.

idioblastic /id-ee-ŏ-blas-tik/ Describinga metamorphic rock consisting largely ofcrystals having euhedral form.

idiomorphic /id-ee-ŏ-mor-fik/ Describ-ing a rock in which the majority of crystalsare anhedral. Compare allotriomorphic;hypidiomorphic.

idocrase /ÿ-dŏ-krayss, id-ŏ-/ (vesuvianite)A yellow, green, or brown tetragonalmineral which has an ideal formulaCa10(Mg,Fe)2Al4Si9O34(OH,F)4, found inthermally metamorphosed limestones.

igneous rock /ig-nee-ŭs/ Any of themajor rocks that have crystallized from ahigh-temperature molten silicate (or rarelycarbonate) liquid or magma. Igneous rocksinclude PLUTONIC, hypabyssal, VOLCANIC,and PYROCLASTIC ROCKS. The composi-tional range of igneous rocks is describedby the terms acid, intermediate, basic, andultrabasic. Classification may also bebased upon the SILICA SATURATION princi-ple. See andesite; basalt; carbonatite; dior-ite; gabbro; granite; ijolite; lamprophyre;layered igneous rock; rhyolite; syenite;syenodiorite; trachyte.

ignimbrite /ig-nim-brÿt/ See pyroclasticrock.

ijolite /ij-ŏ-lÿt/ A suite of strongly alka-line undersaturated plutonic rocks oftenassociated with carbonatites. They arecharacterized by varying proportions ofnepheline and clinopyroxene, an abun-dance of accessory minerals, and the vir-tual absence of feldspar. Members of thesuite are classified in terms of COLOR INDEX

as follows:urtite %t;30 (% dark minerals)ijolite 30–70melteigite 70–90pyroxenite >90

Thus an ijolite may consist of about equalproportions of nepheline and aegirine–

augite. Accessory minerals include apatite,melanite, phlogopite, sphene, and calcite.Uncompahgrite is a melitite pyroxeniteand jacupirangite consists largely of tita-nium-rich augite, magnetite, and biotite.

illite /il-ÿt/ A micalike clay mineral, a hy-drated silicate of aluminum and potassium.It occurs in mudstone and shale. See alsoclay minerals.

illuviation /i-loo-vee-ay-shŏn/ The accu-mulation of fine material in a soil, or a partof a soil, washed down from above. The il-luvial horizon is the B horizon: the degreeof development of the B is therefore a guideto the time and effectiveness of operationof the illuvial processes. The materials in-volved are sesquioxides, bases, clays, andcolloidal organic matter.

ilmenite /il-mĕ-nÿt/ A brown or blackmineral form of iron titanium oxide,FeTiO3. It crystallizes in the trigonal sys-tem, and occurs as an accessory mineral asaggregates in igneous and metamorphicrocks, as well as in some mineral sands. Itis a major source of titanium.

imbricate structure /im-bră-kit, -kayt/ Aseries of thrust sheets all dipping in thesame direction and displaced by roughlythe same amount.

impact basin A large IMPACT CRATER

with a diameter in excess of 200 km. It isformed when a meteorite strikes theground. There are usually terraced walls, asunken floor, and perhaps a hill at the cen-ter. The best examples can be seen on theMoon and on Mercury.

impact breccia Fragments of rock pro-duced when a meteorite hits the ground.Small pieces, including droplets of impactmelt, may fall back into the crater. Largerfragments are mixed with hot gases andflow out across the surface of the ground,or are hurled into the air as EJECTA. Thepreexisting rocks around the crater mayexhibit SHATTER CONES.

impact crater A crater formed when a

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meteorite hits the ground. See impactbasin.

impermeable Describing a type of rockthat is nonporous and therefore does notabsorb water. Igneous rocks such as gran-ite are good examples (although joints inthe rock may allow water through, makingit pervious). Clay is impermeable after ithas become saturated with water. See alsoimpervious.

impervious Describing a type of rockthat, because it contains no cracks or fis-sures, does not allow water to pass throughit. Metamorphic rocks such as shale andslate are good examples. See also imperme-able.

inceptisol /in-sep-tă-sôl/ One of thetwelve soil orders of the US SOIL TAXON-OMY, covering soils that are better devel-oped than entisols but not so advanced information as the alfisols, i.e. there is nohorizon of iron and aluminum accumula-tion. They are young soils in which thehorizons present have developed quickly.They are equivalent to the humic gley soils,acid brown soils, and andosols of the oldclassification system.

incised meander A meander of a streamthat is cut deeply into bedrock. This inci-sion could arise from original develop-ment, because meanders can developbefore a stream has completed downcut-ting (see ingrown meander). Alternatively,incision may result from REJUVENATION oftypical floodplain meanders causing themto cut down and incise themselves intotheir floodplain (see entrenched meander).

inclination See dip (defs. 1 and 2).

inclusion The occurrence of one sub-stance enclosed within another, e.g. a pre-existing rock fragment caught up in a laterrock (xenolith) or crystals of one mineralcontained in another. Small volumes ofvolatile constituents trapped during thegrowth of crystals are termed fluid inclu-sions. Inclusions of gabbro and peridotite

within alkali basalt are often called nod-ules.

incompetent Describing beds of rockthat deform plastically and flow whenstressed during folding, or folds in whichthe thickness of the beds changes duringfolding. See also competence.

index contour A contour line that is ac-centuated by its width, in order to facilitatethe reading of elevations on a map. For ex-ample, where the contour interval is 10 mthe 50 m and 100 m contours may be ac-centuated.

index cycle (in meteorology) A cyclicvariation that the ZONAL INDEX is believedto undergo from a high-index circulation,with its strong zonal westerlies, to the low-index state, with much meridional transfer,then back to the high-index phase, with awavelength of about 28 days. However,further work has cast doubt on the validityof a true cycle.

index fossil See zone fossil.

index map Usually a small-scale mapforming a base upon which certain infor-mation concerning larger scale maps, orother data, may be portrayed for easy ref-erence. In series mapping the individualsheet lines are drawn on a small-scale out-

impermeable

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111

222 333 444

555 666

7 888

Index map

line map and such information as sheetnumbers, dates of publication, and editionnumbers are shown for each sheet. This en-ables the user to find out immediatelywhich sheets cover any part of the area.

index mineral See zone.

Indian clinometer A surveying instru-ment used in PLANE TABLING for determin-ing the heights of points fixed byintersection or radiation methods. Theheight of the plane table and of the pointover which it is set up must be known. Theclinometer is placed on the plane table andan adjustable sight is used to view thepoints for which heights are required. Thisadjustable sight indicates a tangent value,positive for an elevation, negative for a de-pression, which is multiplied by the dis-tance between the points, measured fromthe table, to find the height difference.

Indian summer Any period of warmsettled weather in October or early No-vember in the N hemisphere. The term wasfirst used in New England when under suchweather conditions the Native Americansmade preparations for the coming winterby storing food and repairing tents. Mostyears do have at least one spell that couldbe called an Indian Summer but it has noprecise meaning.

induration /in-dew-ray-shŏn/ The hard-ening of porous rocks or of soils throughthe deposition of minerals, which act as acement, on or within the surface layers.Chemical weathering usually causes theweakening of rocks through the removal ofelements in solution. Where strong evapo-ration occurs, as for instance in the semi-arid regions, these elements are brought tothe surface by capillarity, where they com-bine to cement the soil or weathered rock.There are three main types of indurationrock forms. The cement in calcrete consistsmainly of calcite (CaCO3), that in ferricreteis of hematite (Fe2O3), while silcrete is ce-mented by silica (SiO2).

infiltration The movement of waterdownward into the ground. It is affected by

many factors: the amount and type of veg-etation cover, the compactness of the sur-face layers, the porosity of the underlyingrock, and the amount of rainfall. The pres-ence of water already in the ground also af-fects the rate of infiltration.

infiltration capacity The maximumrate at which rainfall enters soil and rockunder a given set of conditions. The ratechanges over time; it tends to decreasethrough time during a storm event. Therate is also dependant on factors such asthe soil texture, pore spaces, soil structure,and the type of vegetation cover.

infrared radiation Electromagnetic ra-diation with wavelengths in the approxi-mate range of 0.7 µm to 1 mm. It liesbetween the visible and microwave parts ofthe spectrum and is divided into three spec-trum regions: near-infrared, middle-in-frared, and far-infrared radiation.

infrared satellite imagery See satellite.

infrastructure A structure produceddeep within the Earth’s crust under condi-tions of high temperature and pressure. It ischaracterized by plastic folding, migma-tites, and granites. Overlying this area arethe less highly deformed rocks of theSUPERSTRUCTURE.

ingrown meander The type of INCISED

MEANDER that develops from origin. If awinding course develops in a stream beforeit has completed downcutting, it will me-ander and cut down simultaneously, low-ering its course into bedrock. As withfloodplain meanders, lateral erosion on theoutside of meander bends, complementedby deposition on the inside of bends, pro-

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ingrown meander

surface of bedrock

streamslip-off slopeundercutslope

Cross-section through an ingrown meander

duces an asymmetric cross profile of thevalley and downstream migration of themeanders. Compare entrenched meander.

inland basin (bolson) A closed depres-sion within an upland desert region towardwhich material is moved whenever rainfalls in sufficient amounts. The material ismoved both down wadis and as sheet washon broad slopes. The finest grains and ma-terial in solution are moved farthest, sothat there is a gradation from vast accumu-lations of unsorted material at the basinmargin, through sand and loam, to clayand salts in the center. Many of the mineralgrains also exhibit salt or calcareous coat-ings. The basins have a very flat surfacewhen dry, although extensive crackingmay be produced by the heat. Salt lakes orsalt marshes may form during periods ofrain. See also playa.

inland sea An extensive water bodylying in the interior of a landmass, often re-mote from the ocean. Inland seas may ormay not have access to the open coast;where they do have access it is usually re-stricted to a single or a few narrow sea pas-sages. Inland seas display a large depthrange. Those with depths generally lessthan 250 m are called shallow inland (orepeiric) seas, e.g. the Baltic Sea. Far greaterdepths are encountered in the deep inlandseas, for example the Mediterranean,where depths range from 2000 to 5000 m.Very large lakes have some of the charac-teristics of inland seas. All inland seas andlakes experience wave action, SEICHE move-ments, and wind-induced flow. Waves on alarge lake such as Lake Kariba can be morethan 3 m high, and very much larger wavesaffect some inland sea areas. The shoresdisplay many of the morphological formstypical of sea coasts in general. Comparemarginal sea.

inlier /in-lÿ-er/ An area of exposed rockthat is surrounded by stratigraphicallyyounger strata, usually a result of folding,faulting, and erosion (such as the erosionof the crest of an anticline). Compare out-lier.

inner core The innermost part of theEarth, believed on geophysical evidence toconsist of solid iron and nickel and sepa-rated from the OUTER CORE at a depth of5000 km. It has a diameter of 2600 km.

Insecta /in-sek-tă/ The largest class ofthe phylum ARTHROPODA, comprising theonly invertebrate animals adapted forflight. They are air-breathing, have threepairs of legs in the adult, and all but themost primitive have one or two pairs ofwings. This highly successful and diversegroup occupies an enormous range of habi-tats. Many fossil species are known, de-spite the rarity of suitable environments forpreservation. Fossil insects have beenfound in the Devonian, and some Car-boniferous dragonflies attained a wingspan of nearly 80 cm.

Insectivora /in-sek-tiv-ŏ-ră/ The orderof the class MAMMALIA that includes theshrews, hedgehogs, and moles. Modern in-sectivores are small and usually very un-specialized in their anatomy and feedinghabits. In these primitive features they re-semble the first Mesozoic placental mam-mals, which are generally regarded as earlyinsectivores and the ancestors of mostmodern placental groups.

inselberg /in-sĕl-berg/ A large prominentresidual hill of hard rock, characterized byvery steep slopes rising abruptly from thesurrounding surface of low relief. Insel-bergs were formerly thought to be semiaridlandforms, but it is now widely believedthat they are more typical of humid tropi-cal climates. Some scientists believe themain reason for their occurrence is differ-ential DEEP WEATHERING followed by subse-quent EXHUMATION, which exposes themore massive, less weathered parts of theweathering profile as inselbergs. Somelarge forms extend to a height of more than300 m and are possibly formed by succes-sive stages of weathering, which proceed atthe same time as REGOLITH removal. As aninselberg is exposed at the surface, UN-LOADING joints usually develop, and move-ment of rock slabs along such partings can

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bring about the destruction of these domedforms. See also bornhardt.

insequent stream /in-see-kwĕnt, in-sĕ-/ Astream that has a random course, whichcannot be predicted by reference to a slope,rock type, or faults or depressions in thesurface. As a result, it adopts a DENDRITIC

DRAINAGE pattern.

insolation /in-sŏ-lay-shŏn/ (from incom-ing solar radiation) The intensity of eitherdirect or global (direct and diffuse) solarradiation on a unit area at a specified timeon a specified surface. Its value is depen-dent upon the SOLAR CONSTANT, the time ofyear, the latitude of the receiving surface,the slope and aspect, if any, of the surface,and the transparency of the atmosphere.

insolation weathering Weathering re-sulting from the expansion and contractionproduced by temperature changes. It wasformerly believed that in arid areas practi-cally all rock fragmentation was caused bythe extreme variations in temperature andconsequent differential expansion betweenthe outside and inside of blocks, which setup stresses producing cracks. However, ex-perimental work has suggested that theprocess was extremely overrated, althoughthere are many examples of fragmentedrocks for which this process seems to be thebest explanation. In addition to large-scalecracking, GRANULAR DISINTEGRATION maybe produced by the differential expansionand contraction of several mineral types inone rock.

instability The state of the atmospherewhen thermals are able to rise freely bytheir own buoyancy forces. This occurswhen the ENVIRONMENTAL LAPSE RATE issteeper than the DRY ADIABATIC LAPSE RATE

and is known as absolute instability. Thisnormally is found only near the groundsurface. Other states of instability dependon the relative rates of cooling of the envi-ronment and dry and saturated thermals.Once condensation has taken place and theatmosphere remains unstable, cloud devel-opment will be extensive and heavy rainfallprobable.

instrument shelter (instrument screen,thermometer screen) A structure designedto house meteorological instruments, suchas psychrometers, thermometers, and hy-grothermographs. It is usually of wood, inthe form of a box, painted white to reflectsunlight, and the base stands about a meterabove the ground surface. The shelter pro-tects the instruments from direct solarradiation, wind, and precipitation; ventila-tion to allow the free flow of air and pre-vent condensation is usually provided bylouvered sides and vents. The STEVENSON

SCREEN is the standard instrument shelterfor thermometers for manned weather sta-tions.

insular shelf The shelf zone surround-ing a large island. The shelf extends fromlow-tide level out to the shelf-edge zonewhere, as with all shelves, there occurs amarked and steep descent to the deep-seafloor. Insular shelves, like continentalshelves, may be broad or narrow, but theytend to contrast with the zone surroundingcoral reefs and oceanic islands, for in thesecases the sea floor plunges steeply to thedeep-sea floor from close to the shore. Theshelf edge of insular shelves usually lies ata depth of some 200 m.

intensity scale See earthquake intensity.

interception The prevention, by thevegetation cover, of a portion of precipita-tion or irrigation water from directlyreaching the ground surface. Part of thiswater is evaporated directly back to the at-mosphere, but some will reach the groundsurface by running down the plant stems(STEMFLOW) and dripping from the plantleaves. See also throughfall.

interfluve /in-ter-floov/ The higher groundbetween one river and the next in the samedrainage system: an upstanding area notyet subject to fluvial denudation. Inter-fluves become lowered and narrowed asdenudation proceeds, until at the stage ofpeneplanation they are almost insignifi-cant. See also divide.

interglacial /in-ter-glay-shăl/ A period

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of warmer climatic conditions separatingGLACIALS of the PLEISTOCENE. Temperatureswere probably similar to those of today, orperhaps a little warmer. They are believedto have lasted longer than the glacials andvery much longer than the 10 000 years orso since the last glacial phase.

Intergovernmental Panel on ClimateChange (IPCC) An international panelthat was established in 1988 by the UnitedNations Environment Program (UNEP)and the World Meteorological Organiza-tion (WMO) to assess and provide infor-mation relevant to the understanding ofclimate change. The IPCC publishes peri-odic reports produced by scientific authorsand editors, including a series of major as-sessment reports: its First Assessment Re-port was published in 1990, the SecondAssessment Report in 1995, and Third As-sessment Report in 2001.

intergranular /in-ter-gran-yŭ-ler/ De-scribing a texture common in basic lavas inwhich the spaces between plagioclase crys-tals are occupied by relatively small grainsof pyroxene and olivine. Compare ophitic.

intergrowth An interlocking growth oftwo minerals that crystallize simultane-ously from a melt. Intergrowths may alsoresult from the EXSOLUTION of a homoge-neous mineral to form two discrete phases,one included in the other. See graphic in-tergrowth; myrmekite; perthite; symplec-tite.

interlocking spur One of a series ofspurs on alternate sides of a river valley.They are strips of high ground that remainafter a meandering river has eroded awaythe ground between them, and generallyoccur mainly in the upper and middlereaches of the river.

intermediate contour A contour linethat falls between the INDEX CONTOURS.

intermediate rock Strictly, a type ofrock containing 55–66% silica (by weight);in current usage intermediate rocks containless than 10% free quartz, sodic plagio-clase, and/or alkali feldspar. Typical inter-mediate rocks are syenite, diorite, trachyte,and phonolite. See also acid rock; basicrock; ultrabasic rock.

intermittent stream A stream thatflows at only certain times of the year, asafter heavy rain or when it receives waterfrom springs. Such streams dry up in thedry season, usually because the water tablebecomes lower.

intermontane /in-ter-mon-tayn/ 1. Lo-cated between mountains or mountainranges.2. A basin of deposition enclosed by moun-tains.

intermontane plateau See plateau.

internal magnetic field That portion ofthe Earth’s magnetic field that originateswithin the Earth itself.

internal wave (boundary wave) A wavethat occurs within a water mass, whichmay be stratified or in which density mayvary with depth. Internal waves have quitedifferent characteristics from those ofwaves that develop at the sea’s surface. Thedensity variations sometimes occur ab-ruptly, at sharp surfaces of discontinuitycalled interfaces, and sometimes quitegradually. The amplitude of internal wavesoften far exceeds that of waves on the sea’ssurface; for example, one measurementrecorded in the Atlantic Ocean amountedto some 60 m.

Intergovernmental Panel on Climate Change

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young landscape

mature landscape

peneplain

interfluve

Interfluves

International Date Line A line corre-sponding to the 180° meridian, deviatingto some extent around island groups. Thedate immediately to the east of the line isone day earlier than to the west, because180°E is 12 hours ahead of GREENWICH

MEAN TIME and 180°W is 12 hours behind.

International Map of the World Amap series at 1:1 000 000 scale that wasfirst proposed in 1891 by the Fifth Interna-tional Geographic Congress and publishedby a number of countries to common inter-nationally agreed specifications. It wastaken over by the United Nations in 1953but the project was officially abandoned in1986, by which time only 800–1000 of theoriginally proposed 2500 maps had beencompleted. The International Map of theWorld projection is a modified polyconicmap projection having two standard paral-lels and a scale of exactly 1:1 000 000.Straight lines joining points on the truly di-vided top and bottom parallels of the pro-jection represent the geographic meridians.Arcs of the same circle, rather than con-centric circles, form the parallels.

intersection A type of PLANE TABLING inwhich points of detail are fixed by settingup the plane table at two or more stationsfor each detail point and drawing lines ofsight to it, the point of intersection of theselines representing the location of the point.

intersertal /in-ter-ser-tăl/ Describing atexture common in basic rocks in which in-terstices are filled by a glassy MESOSTASIS ora fine-grained aggregate of quartz, alkalifeldspar, and deuteric minerals.

interstadial /in-ter-stay-dee-ăl/ A warmerstage within a GLACIAL, in which the ice re-ceded, to be followed by a readvance onthe return of colder conditions. Compareinterglacial.

interstice /in-ter-stiss/ The angular spaceinside a meshwork of lathlike crystals. Inbasalts, interstices between feldspar crys-tals may be occupied by grains of pyroxeneor glass. See also intergranular; intersertal.

intertidal /in-ter-tÿdăl/ Describing thepart of a seashore between the high- andlow-water marks. Organisms that livethere have to be able to withstand periodsof drying out and periods of being com-pletely covered by the sea.

intertropical convergence zone (ITCZ)The zone of convergence between theairstreams from the N and S hemispheres.It oscillates in position with the thermalequator, reaching its maximum northwardextent in July and maximum southwardextent in January. Over the ocean areas itmarks the meeting point of the tradewinds, but over the continents it becomesaffected by the monsoonal circulation andhas somewhat different properties. TheITCZ can be clearly seen over ocean areasfrom satellite photographs as a narrowband of enhanced convectional activity in-dicating upward motion and cloudy show-ery weather at the surface. Sometimes twobands are found separated by about 3–5°latitude, each exhibiting the usual ITCZcharacteristics. Many theories have beenproposed to account for this feature, but asyet the real origin remains in doubt. As thedual convergence zone is found only overthe oceans, it is presumed that it must beconnected with sea-surface temperaturesand upwelling.

Reference to this zone as the intertropi-cal front still remain, but as it has dissimi-lar features to temperate latitude frontsand is not really frontal, the use of thisterm is discouraged.

intraclast /in-tră-klast/ An ALLOCHEM

that is a penecontemporaneously erodedfragment of calcareous material. Intra-clasts are not derived from preexistinglimestones.

intraformational /in-tră-for-may-shŏ-năl/ Existing or formed with a forma-tion. For example, an intraformationalconglomerate is one in which the clasts areeroded fragments of newly consolidatedsediment. These clasts are then incorpo-rated immediately into the accumulatingsediment.

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intraformational

intrazonal soil /in-tră-zoh-năl/ A typeof soil differing from surrounding ZONAL

SOILS because of the dominance of localdrainage or parent material over the zonalinfluence of climate and vegetation. Thesesoils include hydromorphic soils (poordrainge), halomorphic soils (salinization),calcimorphic soils (calcareous parent ma-terial), soils dominated by human influ-ences, soils on organic matter, and varioushorizons in zonal soils regarded as intra-zonal. They differ from AZONAL SOILS inthat they have more or less well developedhorizons, whereas azonal soils still have lit-tle horizonation. They are often linked tozonal soils in catenary sequences, withpoorly drained soils occurring downslopefrom the well-drained zonal types. In the US

SOIL TAXONOMY, these soils are linked withthe nearest zonal soils as aquic or halicvariants, for example there is no separategroup of gleys.

intrusion 1. The process by which IN-TRUSIVE ROCKS are formed.2. See intrusive rock.

intrusive rock (intrusion) An igneousrock that forms by injection of magma orother plastic material into the Earth’s crust(the process of INTRUSION) and has solidi-fied beneath the Earth’s surface. Compareextrusive rock.

inversion An increase in temperaturewith height in the atmosphere. This can actas an upper limit to convection, hence in-versions are associated with atmosphericstability and usually dry weather. They canbe formed by cooling beneath the inversionor warming at the inversion level andabove. The former method occurs most fre-quently as a result of radiational cooling atnight from the ground surface, an extremecase being over the Antarctic ice cap, andthe latter by subsidence of air under anti-cyclonic conditions. In these circumstancesthe air may be extremely dry, as occursabove the trade wind inversion, andquickly evaporates any saturated thermalsreaching this level. It acts therefore as avery effective ceiling to convectional cloud.

inversion layer A layer of air in whichthere is a temperature INVERSION, i.e. thelayer is cooler than the air above it. A low-level inversion layer generally occurs inbasins or valleys; when it happens overurban areas pollutants are trapped and itmay cause SMOG.

invertebrates Animals that do not pos-sess a jointed backbone formed of verte-brae. The members of all animal phylaexcept the CHORDATA are invertebrates.Compare Vertebrata.

inverted relief Topographic relief thatis the opposite of the geologic structure, asfor example where anticlines form valleysand synclines form high ground. It is the re-sult of prolonged denudation.

involution /in-vŏ-loo-shŏn/ 1. A lobe ofone type of material projecting upward ordownward into material of another type,produced by the effects of CRYOTURBATION.These involutions are seen in the upper sec-tions of bedded deposits and are a clear in-dication of former PERIGLACIAL conditions. 2. The refolding of a NAPPE structure re-sulting in very complex fold patterns.

ionosphere /ÿ-on-ŏ-sfeer/ The layer ofthe atmosphere above the stratopause inwhich free ions and electrons occur as a re-sult of ionization of gas molecules by solarultraviolet and X-radiation. It consists ofseveral layers, indicated by the letters D toG, which reflect electromagnetic wavesback to Earth and thus it is very importantin radio communications.

IPCC See Intergovernmental Panel onClimate Change.

iridescence /i-ră-dess-ĕns/ A series ofrainbow colors produced by a mineralcaused by the interference of light withinits crystals or at its surface. The colorschange as the angle of incident lightchanges.

iron bacteria Bacteria that feed on solu-ble iron compounds and as a result precip-

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itate insoluble iron(III) hydroxide,Fe(OH)3.

iron meteorite See meteorite.

iron pan A type of HARDPAN that is richin iron oxide.

iron pyrites See pyrite.

ironstone A type of sedimentary rockthat is rich in iron minerals, found in beds,layers, or nodules. The iron may take vari-ous forms, including CHAMOSITE, HEMA-TITE, PYRITE, and SIDERITE.

irradiance /i-ray-dee-ăns/ The quantityof radiant energy received in unit time onunit area of surface. It is expressed in wattsper square meter.

irrigation The artificial application ofwater to land to provide the necessaryamount of water to grow or maximizeyields of crops or other plants.

isallobar /ÿ-sal-ŏ-bar/ A line on a chartconnecting places of equal pressure ten-dency. Such lines are used in forecastingthe movements of pressure systems, partic-ularly depressions.

isarithm /ÿ-să-rith-<m/ A line on a mapjoining points of equal value. There aremany forms of isarithm, depending on thevalues concerned (see isobar; isogon). Acontour is also a form of isarithm joiningpoints of equal elevation.

island arc A long chain of islands, usu-ally arcuate or bow-shaped. Many islandarcs are bordered on their ocean flank bydeep slits or TRENCHES in the deep-seafloor. The E seaboard of Asia, for example,has a whole series of island arcs – the Aleu-tians, the Kurils, the Japanese, and theMarianas – flanked on their ocean side bya series of trenches: the Aleutian Trench,Kuril Trench, Japanese Trench, RyukyusTrench, and Philippine Trench. On aglobal scale, the island arcs can be seen tocontinue the great folded mountain chainson land such as the Himalayas and West-

ern Cordilleras. The arcs usually includelines of active volcanoes; moreover, theyare associated with many of the world’smost severe earthquakes and with verypronounced gravity anomalies. The theoryof PLATE TECTONICS provides an explana-tion for the distribution of these features.

isobar /ÿ-sŏ-bar/ A line on a chart join-ing points of constant barometric pressure.At the ground surface, observing stationsare often at different altitudes and correc-tions have to be made to pressure readingsto provide a common level, normally sealevel. In some countries, e.g. South Africa,much of the land surface is well above sealevel and surface pressure readings are cor-rected to differences from the 850 mb pres-sure level. The patterns produced byjoining points of equal surface pressurerepresent synoptic systems, such as anticy-clones, depressions, troughs, ridges, andcols, from which interpretations of theweather can be made.

isochemical /ÿ-sŏ-kem-ă-kăl/ Describ-ing metamorphic processes that involve lit-tle or no change in chemical compositionof a whole rock.

isoclinal folding /ÿ-sŏ-klÿ-năl/ Gener-ally upright tight folds whose axial planesand limbs are parallel. (See diagram atFOLD.)

isogon /ÿ-sŏ-gon/ (isogonic line) 1. Aline joining points of constant wind direc-tion.2. A line joining points of equal magneticvariation.

isograd /ÿ-sŏ-grad/ See zone (def. 1).

isohaline /ÿ-sŏ-hal-ÿn, -hay-lÿn/ A linedrawn on certain oceanographic chartsand diagrams joining points of equal salin-ity.

isohyet /ÿ-sŏ-hÿ-ĕt/ A line joining pointsexperiencing the same amount of rainfall.This can be used for any time interval froma single period of rain to mean annual to-tals.

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isohyet

isoline /ÿ-sŏ-lÿn/ A line on a map orchart that joints points having the samevalue of some specified quantity, such as anISOBAR, ISOGON, ISOHALINE, ISOHYET,ISOPACH, and ISOPYCNIC.

isomeric line /ÿ-sŏ-me-rik/ A line thatjoins points having the same averagemonthly rainfall expressed as a percentageof the mean annual rainfall. It is helpful indisplaying similarities in rainfall regimesby removing the effects of large totals overmountainous areas.

isometric /ÿ-sŏ-met-rik/ See crystal sys-tem.

isomorphism //ÿ-sŏ-mor-fiz-ăm// Theexistence of crystals that have the sameform but different chemical compositions.Isomorphic minerals form a series of solidsolutions. See crystal form.

isopach /ÿ-sŏ-pak/ (isopachyte) A lineon a geologic map joining points of equalthickness for a particular rock bed.

isopleth map /ÿ-sŏ-pleth/ A map show-ing quantitative spatial distributions indi-cated by lines of equal value, such asisotherms.

isopycnic /ÿ-sŏ-pik-nik/ (isopycnal) Aline drawn on certain oceanographic andmeteorological charts joining points ofequal density.

isoseismal line /ÿ-sŏ-sÿz-măl/ A line ona map joining points on the Earth’s surfaceof equal earthquake intensity.

isostasy /ÿ-sos-tă-see/ A condition oftheoretical balance for all large portions ofthe Earth’s crust, which assumes that theyare floating on an underlying more densemedium. As a result of erosion or deposi-tion, this balance is put out of equilibriumand has to be compensated for by move-ments of the Earth’s crust. Areas of deposi-tion sink, whereas areas of erosion rise. Inthis way the roots of old mountain chainsformed many kilometers beneath the

Earth’s surface are brought up to the sur-face. See also Airy’s hypothesis of isostasy.

isotach /ÿ-sŏ-tak/ 1. A line of constantwind speed.2. A line joining points of equal distancetraveled in a period of time.

isotherm /ÿ-sŏ-th’erm/ A line joiningpoints of equal temperature. Isothermmaps are the most common methods ofshowing temperature changes on a climaticscale. As with pressure, the effects of alti-tude are important and corrections to sealevel are normally made.

isothermal layer /ÿ-sŏ-th’er-măl/ Anyvertical section of the atmosphere in whichthe temperature remains constant withheight, i.e. there is a zero lapse rate, as re-vealed by an upper-air sounding. Isother-mal layers are frequent but are usuallytransitory. The term formerly referred tothe stratosphere but it has now been super-seded as more information about the ther-mal structure of that layer becameavailable.

isotope /ÿ-sŏ-tohp/ One of two or moreforms of the same element whose atomsdiffer in their numbers of neutrons (al-though the numbers of protons and elec-trons is the same). Isotopes therefore havethe same atomic number; they differ inmass but not in chemical properties.

isotopic ratio /ÿ-sŏ-top-ik/ The ratio ofthe amount of one isotope of an element tothe amount of another isotope of the sameelement, in the same sample of rock.

isotropic /ÿ-sŏ-trop-ik/ Having the samephysical properties if measured in differentdirections. Compare anisotropic.

isthmus /iss-mŭs/ A narrow strip of landthat connects two large land masses. Forexample, North and South America areconnected by the isthmus of Panama.

ITCZ See intertropical convergencezone.

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jacupirangite /ja-kyoo-pi-răn-jÿt/ Anepheline-bearing basic plutonic rock. Seeijolite.

jade A green or white semipreciousstone. Jade is a tough compact variety of ei-ther jadeite or nephrite.

jadeite /jayd-ÿt/ A PYROXENE of compo-sition NaAlSi2O6, found in high-pressuremetamorphic rocks such as glaucophaneschists. See also jade.

jasper A red opaque variety of chal-cedonic quartz. See silica minerals.

jet A type of hard COAL that can becarved and polished for decoration.

jet stream A narrow band of winds athigh altitudes, with speeds above 60 knotsand having strong vertical and lateralshear. Two main regions of jet streams areknown in the troposphere, one in the sub-tropical westerly circulation (the subtropi-cal jet stream), which is fairly constant inlocation for any given season and so ap-pears clearly on mean charts, and a secondassociated with the polar front (the polar-front jet stream). This is highly variable inits location, being dependent on the transi-tory thermal gradients of mid-latitude de-pressions. Because surface depressionsform where divergence in the upper atmos-phere is strong, they are frequently initi-ated downstream of a trough in the upperwesterly circulation, with anticyclonesforming in areas of upper air convergenceupstream of the trough. This developmenttakes place in zones of large surface tem-perature gradient, which in turn causesmarked temperature contrasts in the upperatmosphere as depressions intensify. The

jet streams are the response to this upperatmospheric thermal gradient, and fre-quently encourage further development ofsurface pressure features by the markedconvergences and divergences that are ex-perienced in different parts of the jet.

Easterly jets have been reported be-tween India and Africa, and low-level jetsin East Africa, but these are weaker andless persistent than those of the westerlyzone. Above the troposphere, a POLAR

NIGHT JET STREAM occurs at high latitudesin winter at about 50 mb level (40 km).

joint A surface crack in a rock, with nodisplacement of the pieces each side of it.See jointing.

jointing The system of cracks in a massof rock constituting, in many rock types,the only means by which water can infil-trate the rock mass. Consequently, withinan area of a single rock type in which thereare variations in the extent of jointing,well-jointed areas will become weatheredmore quickly and to a deeper level than themore massive parts of the outcrop, whichmay even remain as solid rock.

jökulhlaup See glacial outburst flood.

joule /jool/ (J) The unit of work or en-ergy in the Système International (SI),equal to the amount of work done by aforce of one newton in moving an objectone meter in the direction of the force. It isequivalent to 1 watt second or 0.239 calo-ries. It is named for the British physicistJames Prescott Joule (1818–89).

junction The plane of contact betweentwo adjacent and distinct bodies of rock.Such a junction may be conformable, un-

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JK

jungle

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conformable, or produced by intrusion orfaulting.

jungle An imprecise term for a tropicalforest or mangrove region that has denseundergrowth. See tropical rainforest.

Jurassic /jucirc;-rass-ik/ The period ofthe MESOZOIC Era that followed the TRIAS-SIC and preceded the CRETACEOUS. It lastedfor about 65 million years, from 208 to144 million years ago. It was named for theJura Mountains in Europe, where rocks ofthis age were first studied. The Jurassic Sys-tem is subdivided into eleven stages. TheLower Jurassic consists of the Hettangian,Sinemurian, Pliensbachian, and ToarcianStages; the Middle Jurassic consists of theAalenian, Bajocian, Bathonian, andCallovian Stages; the Oxfordian, Kim-meridgian, and Tithonian Stages form theUpper Jurassic. Continental rifting of thesupercontinent Pangaea began toward theend of the Triassic; by the Middle JurassicNorth America was moving away fromEurasia and Africa and the Atlantic Oceanwas opening. Following a marine trans-gression, the characteristic rocks of theJurassic System are clays and limestones,some of them oolitic. There were coralreefs, and other important invertebrates in-cluded brachiopods, bivalves such as oys-ters, ammonites (on which much of thesystem is zoned), and irregular echinoids.On land, dinosaurs continued to flourishand diversify and other reptiles includedaerial pterosaurs and marine forms. Fossilsof both Archaeopteryx, the earliest knownbird, and the first mammals come fromJurassic rocks. A marine regression towardthe end of the period resulted in nonmarinedeposits.

juvenile water Water that exists as de-posits within the Earth’s magma and firstappears at the surface during volcanic ac-tivity.

kaersutite /kair-soo-tÿt/ A monoclinicAMPHIBOLE.

kainite /kay-nÿt, kÿ-/ A mineral consist-ing of a hydrated double salt of magnesium

sulfate and potassium chloride,MgSO4.KCl.3H2O. It crystallizes in themonoclinic system and occurs as under-ground granular masses, often associatedwith rock salt deposits. It is used for mak-ing other compounds of magnesium andpotassium.

Kainozoic /kÿ-nŏ-zoh-ik/ See Cenozoic.

kaliophilite /kal-ee-off-ă-lÿt/ A rare typeof KALSILITE. See feldspathoid.

kalsilite /kal-să-lÿt/ A potassium alumi-nosilicate that occurs in nepheline. Seefeldspathoid.

kame /kaym/ An isolated hummock ofstratified FLUVIOGLACIAL sediments, mainlysands and gravels, associated with slow-moving or stagnant ice. Kames may beformed in a variety of ways. Debris-ladenmeltwater streams may emerge from thesnout of a decaying glacier, depositingtheir load into an ice-dammed lake. Onmelting, that part of the accumulation for-merly adjacent to the snout may form asteep ice-contact slope, although a certainamount of slumping will occur. Alterna-tively, material may be deposited in melt-water pools on the surface of a stagnantglacier. The accumulated sediments will belowered to ground level on melting, form-ing a kame.

kame terrace An accumulation of FLU-VIOGLACIAL sediments deposited by melt-water flowing between a decaying glacierand its valley sides. As with KAMES, the icecontact slope suffers collapse on melting ofthe ice. Kame terraces can be continuous ordiscontinuous, deposition occurring onlywhere meltwater channels could widen,thereby reducing velocities of flow. Thesefeatures are flat-topped and steep-sided,sloping gently down-valley. Long contin-ued existence of these terraces afterdeglaciation depends upon the extent ofsubsequent fluvial and slope processes.

kaolin /kay-ŏ-lin/ (china clay) A softwhite clay used in medicine, as a filler forpaper, and in the manufacture of ceramics.

It consists of deposits of minerals of theKAOLINITE group that are produced by theweathering and hydrothermal alteration offeldspars in granitic rocks. See also clayminerals.

kaolinite /kay-ŏ-lin-ÿt/ A white or grayCLAY MINERAL consisting of hydrated alu-minum silicate, Al2Si2O5(OH)4. It crystal-lizes in the triclinic system and is derived byalteration of alkali feldspar, feldspathoids,and other silicates. It is the main compo-nent of KAOLIN (china clay).

kaolinization See pneumatolysis.

karren /ka-rĕn/ Relatively small irregu-lar hollows and elongated grooves cut onlimestone surfaces as a result of solution.At the most basic level they may be dividedinto rundkarren, having rounded separa-tions between grooves, and rillerkarren,with sharp separations. It is believed thatwhereas the latter are the result of runningwater, the former may be at least partlycreated beneath a cover of snow, soil, orpeat. The term is also used for the wholedissected surface and is the German equiv-alent of LIMESTONE PAVEMENT, the Frenchequivalent being lapiés.

karst Denoting any area within whichlimestone solution landforms such as SINK-HOLES, UVALAS, and LIMESTONE CAVERNS

have become well developed. The termstems from the regional name for the mas-sive limestone area on the Dalmatian coastof the Adriatic Sea, where practically allthe landforms have been produced by solu-tion and almost all the drainage is under-ground. Prerequisites for a fully developedkarst landscape include a considerablethickness of strong soluble well-beddedand well-jointed limestone, moderatelyheavy rainfall, and sufficient altitude toallow an extensive flow of undergroundwater.

katabatic wind /kat-ă-bat-ik/ (drainagewind; gravity wind) A downslope flow ofcool air that develops on calm clear nightswhen the ground surface cools by radia-tional losses and the air in contact with the

ground also cools. The density of air in-creases when the temperature falls, and ifthe ground surface is sloping, these heavierlayers will give rise to a downslope gravita-tional flow. Where concentrated in a val-ley, night-time MOUNTAIN WINDS can bequite strong, but the most extreme form ofkatabatic wind is off the Greenland andAntarctic ice caps. A mean hourly speed of156 km per hour was reported at CapeDenison (67.9°S, 142.7°E) under such con-ditions. See also anabatic wind.

katafront /kat-ă-frunt/ Any frontal sur-face at which the warm air is descendingrelative to the cold. As a result of this sub-sidence, weather activity at a katafront isnormally weak with only a belt of strati-form cloud indicating its presence. Somefronts vary in character along their lengthand may take the form of an ANAFRONT

near the depression center and a katafrontfarther away.

katophorite /kat-off-ŏ-rÿt/ A mono-clinic AMPHIBOLE.

kelp Certain species of seaweed of con-siderable length and thickness. These sea-weeds, which are usually brownish incolor, thrive below low-tide level, and maybe luxuriant enough to form kelp forests.Of particular importance are the large lam-inarians, which anchor themselves to rockyor stony bottoms and, depending on theirrigidity, either stand fairly erect or movewith the tidal currents. They influence themovement of bottom materials, and may, ifuprooted by strong tidal flow or wave ac-tion, carry their stony attachments shore-ward or alongshore. They are also effectiveat times in damping wave action. Kelpforests are important as fish feedinggrounds.

kelvin (K) The unit of thermodynamictemperature in the Système International(SI), equal to 1/273.16 of the temperatureof the triple point of water. 1 kelvin isequal to 1°C and a temperature expressedin kelvins is the same as the ABSOLUTE TEM-PERATURE. It is named for the British theo-

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kelvin

retical and experimental physicist BaronWilliam Thompson Kelvin (1824–1907).

Kelvin–Helmholtz instability The de-velopment of waves at the surface separat-ing two layers of the atmosphere havingdifferent temperatures and wind speeds.These waves sometimes increase in ampli-tude and become unstable, rolling into vor-tices, which break and give CLEAR AIR

TURBULENCE. Very occasionally moistureconditions may be such that clouds formon the waves and make the vortices visible.It is named for Baron William ThompsonKelvin (see kelvin) and the German scien-tist and philosopher Hermann von Helm-holtz (1821–94).

Kelvin wave A tidal phenomenon inwhich a progressive wave traveling along atidal channel is affected by the Earth’s ro-tation to the extent that the tidal range (inthe N hemisphere) is increased along theright-hand side of the channel and de-creased on the left-hand side (with the ob-server facing the direction in which theprogressive wave is traveling). Usually, theenergy of the progressive wave is much di-minished, so preventing a true am-phidromic tidal system from operating (seeamphidromic system). The Kelvin wavephenomenon can be observed in the Eng-lish Channel, where the tidal range alongthe English coast is small compared withthat along the French coast. During anytidal state, the sea surface slopes from oneside of the Channel to the other (seekelvin).

kelyphitic rim /kel-ă-fit-ik/ See reactionrim.

kentallenite /ken-tal-ĕ-nÿt/ A syenogab-bro containing olivine and augite togetherwith plagioclase and orthoclase feldsparsand large poikilitic crystals of biotite. Seealkali gabbro.

kenyte /ken-yÿt, keen-/ A TRACHYTE con-taining large rhombic orthoclase phe-nocrysts. Medium-grained varieties may betermed rhomb-porphyry.

keratophyre /ke-ră-tŏ-fÿr/ A fine-grainedigneous rock associated with basaltic lava.See spilite.

Kerguelen-Gaussberg Ridge /ker-gĕ-lĕngowss-berg/ A large ocean feature ex-tending southward from the East Africancoast, reaching almost to Antarctica. Theridge branches out, one branch reachingwestward to the MID-ATLANTIC RIDGE, andanother trending eastward, and then cir-cumscribing the Australian continent andextending toward the East Pacific Rise.Surveys conducted off the continent ofAntarctica show that the continental slopesouth of the Kerguelen-Gaussberg Ridge isin places made up of several tilted plainlikefeatures with gentle undulations. Rocksdredged from the Kerguelen-GaussbergRidge yield samples of basalt and conti-nental-type rock.

kernite /ker-nÿt/ A soft white or color-less mineral form of hydrated sodium bo-rate, Na2B4O7.4H2O. It crystallizes in themonoclinic system and is an importantsource of boron compounds, includingBORAX.

kernlose winter /kern-lohs/ See corelesswinter.

kettle hole A depression in the groundsurface in a glaciated area. As a mass of icestagnates, material is deposited from abovethe ice surface, and FLUVIOGLACIAL depositsalso accumulate. In this way masses of icecan become buried beneath DRIFT. Onceburied the ice takes a considerable time tomelt, and when this finally happens, col-lapse occurs and a depression is formed,which may subsequently become filled by alake or by sediments. Not all kettle holesare associated with stagnant ice, becausesmall lakes and streams may freeze whilesediments are accumulating, but those thatare will be found in association with KAMES

and ESKERS.

key See cay.

khamsin /kam-sin, kam-seen/ A hot drysoutherly wind blowing across Egypt from

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the Sahara. It precedes depressions passingeastward along the North African coastand hence is most common between Apriland June. According to Arab tradition itblows for 50 days.

kidney ore A form of the iron-contain-ing mineral HEMATITE, which occurs inmasses whose shape resembles solidifiedbubbles or kidneys.

kimberlite /kim-ber-lÿt/ (blue ground) Abrecciated carbonate-rich phlogopite-bear-ing PERIDOTITE found in pipes and dia-tremes piercing very old metamorphicrocks. Many kimberlites contain DIA-MONDS.

kinetic energy The energy of a body de-rived from its movement. Its magnitude isexpressed as ½ mv2 where m is the mass ofthe body, and v is its velocity. The atmos-phere possesses large amounts of kineticenergy by virtue of its motion, but this isconstantly being lost through surface fric-tion and turbulence. It is replenished byconversion of potential energy derivedoriginally from solar radiation and upwardmotion of air.

kingdom The largest group in the taxo-nomic classification of living organisms.Five kingdoms are usually recognized: An-imalia includes all animals; Plantae in-cludes all plants; Fungi includes all fungi;Protista (or Protoctista) includes algae,protozoa, and slime molds; and Monera(Bacteria or Prokaryotae) comprises thebacteria. Kingdoms are divided into phyla.See also taxonomy.

kink plane A deformation structure inwhich the orientation of a foliation ischanged as a result of slippage or gliding.

klippe /klip, klip-ĕ/ Originally, any rockbody isolated by erosion. The term is nowrestricted to the outlier or erosional rem-nant of a NAPPE.

knee fold A GRAVITY FOLD, generallyzigzag in shape. (See diagram at FOLD.)

knickpoint (rejuvenation head) A breakof slope in the long profile of a stream, re-sulting from a fall of base level rejuvenat-ing the seaward portion of the stream. ThisREJUVENATION leads to renewed downcut-ting, and the creation of a new lower longprofile; the point where this profile meetsthe established profile is the knickpoint.

Knickpoints are often marked by wa-terfalls if the break of slope is steep, orrapids if less steep. Once created, theknickpoint tends to migrate upstream andbe lowered in height; the rate at which itadvances and the degree to which it losesits height depends on the lithology in whichit is cut. In unconsolidated material it willrapidly disappear. The increased turbu-lence associated with break of slope accel-erates erosion and rapidly lowers itsheight.

knoll A small rounded hill. See alsohummock.

knot A unit of speed, being one NAUTI-CAL MILE per hour. 1 knot is equal to 0.515meters per second.

komatiite /kŏ-mat-ee-ÿt/ A series of an-cient extrusive igneous rocks that containsome of the oldest PERIDOTITE lavas. Theyare characterized by large branching crys-tals, indicative of rapid cooling of themagma from which they derived.

Köppen climate classification See cli-matic classification.

K/T boundary event (Cretaceous–Ter-

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K/T boundary event

former base level

current base levelknickpoint

Knickpoint

tiary boundary event) The time at theend of the Cretaceous period, when (ac-cording to fossil evidence) large-scale ani-mal extinctions took place. For example,dinosaurs, ammonites, and belemnitesceased to exist. The cause is thought to bea general and rapid rise in world tempera-tures. Because of high concentrations ofthe element iridium in rocks of the time,some geologists think that the climaticchange resulted from a major meteorite im-pact which filled the Earth’s atmospherewith debris, cutting off most of the sunlight(iridium is relatively abundant in mete-oroids).

Kuroshio Current /koo-rosh-ee-oh/ Anocean current constituting one of the im-portant western BOUNDARY CURRENTS in theworld’s ocean circulation system. It devel-ops following a gradual deflection of partof the North Equatorial Current (see equa-torial current) in the Pacific Ocean, beingat first rather diffuse, but in more northerntracts becoming concentrated between theshelf region off the Ryukyu Islands and asubmarine ridge. At about 35°N, the cur-rent swings away from the Japanese main-land and flows for some 8000 km beforeagain splitting up into branches. In con-trast to the water beneath the GULF STREAM,water that is relatively warm and fairlysaline because of its continual replen-ishment of very saline water from theEuropean Mediterranean Sea, the water

beneath the Kuroshio Current is onlypartly saline and cold, being of sub-Antarc-tic origin. The Kuroshio Current and theFLORIDA CURRENT have roughly the sameflow rates.

kyanite (cyanite) A light blue, white, orgray aluminum silicate mineral, Al2SiO5. Itcrystallizes in the triclinic system and oc-curs in pressure-metamorphosed rocks,such as amphibolite and some mica schists.See also aluminum silicates.

Kyoto Protocol An international agree-ment that aims to curb the air pollutionthat contributes to GLOBAL WARMING. Itsmeasures include targets to reduce world-wide emissions of GREENHOUSE GASES by in-dustrialized countries. It was originallyformulated in 1997 at the Third Confer-ence of Parties to the United NationsFramework Convention on ClimateChange (UNFCCC) in Kyoto, Japan. Theagreement commits the signatory industri-alized nations to specified reductions inemissions of six greenhouse gases or classesof gas: carbon dioxide, methane, nitrousoxide, hydrofluorocarbons, perfluorocar-bons, and sulfur hexafluoride. The Proto-col also introduced the concept ofEMISSIONS TRADING. Following ratificationthe Protocol came into force on February16, 2005. The USA indicated its decisionnot to ratify the Protocol in 2001. See alsoMontreal Protocol.

Kuroshio Current

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Laborde projection /la-bord/ A MAP

PROJECTION resembling Mercator’s trans-verse, the only difference being thatLaborde uses a spheroid and not a sphere.It is generally restricted to areas ofeast–west extent, for which it is orthomor-phic. The projection is named for theFrench artillery officer and general JeanLaborde in 1926.

Labrador Current One of severalmajor surface currents in the N AtlanticOcean, bringing cold water southwardfrom the Arctic Ocean region. TheLabrador Current flows in on the W flankof Greenland, and is matched on the Eflank by the East Greenland Current.These influxes of cold water meet, in thesurface layers, with the relatively warmand saline GULF STREAM and NORTH AT-LANTIC DRIFT water. Both cold currentscarry icebergs southward, and these tendto become concentrated in the ‘Gateway ofIcebergs’ east of the Grand Banks. This as-sembly of bergs is most common betweenmid-March and mid-July. Another clima-tological feature of the region is the highfrequency of fogs, especially off the coastof Newfoundland. Winter cooling of thecombined waters from the North AtlanticDrift, the Labrador Current, and the EastGreenland Current may increase theirdensity sufficiently to cause the surface wa-ters to sink, perhaps to depths in excess of1000 m.

labradorite /lab-ră-dor-ÿt/ A variety ofplagioclase FELDSPAR.

laccolith /lak-ŏ-lith/ A concordant in-trusive igneous body with a dikelike feeder,usually forming small lenslike features lessthan 5 km in diameter. As the magma is

injected it has sufficient pressure to arch up the overlying strata. Therefore althoughit has a flat base, its upper surface is con-vex.

lacuna /lă-kyoo-nă/ The time intervalmissing between beds above and below anunconformity, including both the nonde-positional and erosional components.

lacustrine /lă-kuss-trin/ Describing some-thing that lives in, is related to, or is pro-duced by a lake.

lagoon A stretch of water that is moreor less enclosed and often shallow, pro-tected from the open sea by a spit,tombolo, bay-mouth bar, or other kind ofbarrier. These barriers are not complete,for lagoons invariably possess a free con-nection with the open sea, however narrowthis may be. The barrier provides protec-tion from waves, although wave overwashis common to many lagoonal areas. Tidalcurrent action is usually minimal withinthe lagoon itself. The water may be fresh,saline, or brackish. Moreover, the salinitylevel in a lagoon may vary with time. Thedeposits of lagoons are characteristicallyfine-grained, with a significant proportionof organic matter.

lahar /lah-har/ A mudflow composedmainly of volcanic debris. If the unstabledebris accumulating on the sides of a vol-cano is lubricated by heavy rain, it willflow under gravity. The flow that coveredHerculanium in AD 79 may have been ofthis type.

lake A body of water completely sur-rounded by land. It lies in a depression inthe Earth’s surface or in an artificially cre-

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L

Lambert’s cylindrical equal-area projection

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ated location behind a dam. Most lakescontain fresh water, although some verylarge ones (such as the Caspian Sea andDead Sea) have saline water. A very smalllake is generally referred to as a pond orpool. Some lakes have streams or riversflowing into them, and some dry up at cer-tain times of the year. In ecological terms,a lake can be considered as a completeECOSYSTEM.

Lambert’s cylindrical equal-area pro-jection A MAP PROJECTION drawn so thatthe scale along the Equator is correct, butall the other parallels are exaggerated inlength. The scale on the meridians de-creases away from the Equator, causing theparallels to become closer poleward. Theprojection is equal-area because the ratiobetween the meridians and parallels re-mains constant. It is seldom used outsidethe equatorial zones because of the degreeof distortion of shape north and south ofabout 40°. The projection is named for theGerman mathematician, physicist, as-tronomer, and philosopher Johann Hein-rich Lambert (1728–77).

Lambert’s zenithal projection Anequal-area MAP PROJECTION used mainly inpolar areas, where the pole is the center ofthe projection and the meridians arestraight lines radiating from it, making thebearings true. The parallels are concentriccircles. Lambert also constructed anoblique (again equal-area, showing coun-tries within 40° with good shape) and anequatorial projection (used for countriesastride the Equator, equal-area).

Lamellibranchia /lă-mel-ă-brank-ee-ă/See Bivalvia.

lamina /lam-ă-nă/ A thin layer of sedi-mentary rock, up to 1 mm thick, thatdiffers in some respect from the layers oneach side of it. It may, for example, have aparticular color, particle size, or composi-tion.

laminar flow The nature of movementof a fluid over a smooth surface at very lowvelocities, whereby parallel ‘slabs’ of air or

water move by sliding over each other,with no mixing. Energy is transmitted fromone layer to the next by the viscosity be-tween the layers, while at the boundary(ground surface, stream bed, stream wall,etc.) movement is nil. Laminar flow is rarein natural streams or air flows, except atboundary contacts.

lamprophyre /lam-prŏ-fÿr/ One of agroup of medium-grained basic alkali-richigneous rocks occurring as dikes and sills.These rocks are characterized by the pres-ence of mafic phenocrysts, biotite, am-phibole, or augite, and a groundmasscontaining orthoclase or sodic plagioclasefeldspar.

Minette and vogesite are varieties con-taining orthoclase feldspar and in whichbiotite and hornblende respectively is thepredominant mafic mineral. Analogousrocks containing plagioclase are called ker-santite and spessartite. Camptonite con-tains plagioclase and a brown amphibolecalled barkevikite. Monchiquite is afeldspar-free lamprophyre containing phe-nocrysts of biotite, augite, amphibole, andolivine in an analcime-rich matrix.

Lamprophyres are melanocratic hy-pabyssal equivalents of diorites and syen-ites and are associated with such plutonicintrusions.

land breeze The weaker nocturnalequivalent of the SEA BREEZE. It results fromthe unequal cooling of the different sur-faces at night, but because the temperaturedifference is small, wind speeds are low. Itis most evident when the pressure gradientis weak and skies are clear, so allowing ra-diational cooling over the land to be at amaximum.

The surface wind is part of a larger con-vection cell, which maintains air continu-ity. Above the land breeze is a weak airflowfrom sea to land.

land bridge A strip of land that joinstwo larger landmasses and may provide aroute for the migration or spread of plantsand animals. See also isthmus.

landform The various surface features

of the Earth that contribute to the land-scape, such as plains, mountains, and val-leys.

Landsat Any of the unmanned orbitingearth resources satellites, launched byNASA from July 1972. They use remote-sensing devices, such as infrared camerasto provide information and images of theEarth’s surface, including land use, vegeta-tion, geology, and pollution.

landscape The geomorphological andartificial features that make up the Earth’sscenery. It thus includes the natural LAND-FORM as well as buildings, civil engineeringworks, and agricultural modifications.

landslide The sudden downslope move-ment of rock and debris due to failurealong a shear plane. The movement isstarted when the stress in the FREE FACE ex-ceeds the resisting power of the potentialshear plane: this loss of resistance may bedue to increased weight of the face, e.g. if itbecomes wetted; it may be due to under-cutting of support at the foot of the slopeby some process of BASAL SAPPING; or it maybe due to a sudden triggering by an earth-quake.

As a result, a convex scar is left upslope,with a mass of jumbled debris at the base ina tonguelike form. Areas of rapid down-cutting by rivers, or with variable litholo-gies, or swelling clays that expand onwetting are susceptible; so too are undercutsea cliffs and river banks, and areas of iceformation. Like all MASS MOVEMENTS, theeffect is to render an unstable slope morestable.

landslide surge See tsunami.

langley A unit of energy equal to 1 calo-rie per square centimeter. It is mainly usedfor radiation, but as the calorie has beenlargely superseded by the SI unit the JOULE,the langley is decreasing in use. It is namedfor the American aeronautics pioneerSamuel Pierpoint Langley (1834–1906).

lapiés /lap-ee-ay/ See karren.

lapilli /lă-pil-ÿ/ Round fragments of solidlava thrown out by an erupting volcano;small volcanic bombs. They are a form ofEJECTA up to 65 mm across. See also pyro-clastic rock.

lapis-lazuli /lap-is-laz-yŭ-lÿ, -lee/ Thesemiprecious variety of LAZURITE. Seefeldspathoid.

lapse rate The change of temperature inthe atmosphere with height. Distinctionsare made between the decrease of tempera-ture in the free atmosphere, the ENVIRON-MENTAL LAPSE RATE, and the decrease oftemperature under adiabatic conditions inthermals, the DRY ADIABATIC LAPSE RATE andSATURATED ADIABATIC LAPSE RATE.

larvikite /lar-vik-ÿt/ (laurvikite) A darkblue iridescent type of SYENITE, used as anornamental stone.

latent heat The energy required or re-leased on a change of phase. In the atmos-phere, water can exist as a solid, a liquid,or a vapor, representing increasing energystates respectively. Thus freezing and con-densation release latent heat, melting andevaporation require heat. Latent heat is ex-tremely important in atmospheric pro-cesses, enabling energy to be transferredwithin the general circulation, from groundto atmosphere, and as a reservoir of heat inthe atmosphere.

Specific latent heat is the latent heat re-quired or released per unit mass.

lateral accretion Material that accumu-lates at the side of a channel, such as thatdeposited on the inner sides of bends on ameandering river.

lateral erosion River erosion in a hori-zontal direction. Within the meanderingreach of a river, erosion on the outside ofbends leads to downstream migration andconstant paring away of the bluffs at theside of the valley; this effectively erodes thevalley width laterally. In floods, the ele-vated water level fills the channel andraises the water table, saturating the banks.When the floods subside, the support on

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the weakened (wetted) banks is removedand mass slumping occurs, hence wideningthe channel laterally. By these processes,some FLOODPLAINS are eroded rather thandeposited; they consist of a rock-cut benchwith a thin veneer of alluvium, unless theyform because of a base-level rise, when adeep purely aggradational floodplain willresult.

lateral fault See strike-slip fault.

lateral moraine A ridge of materialalong the side of a glacier (or along the sideof a valley formerly occupied by a glacier),primarily composed of angular and un-sorted material that has fallen from over-hanging rock outcrops as a result ofFREEZE-THAW weathering, although it is be-lieved that a certain proportion of the ma-terial is derived from within the glacieritself. The ridge may consist extensively ofice, because once a layer of waste about 0.5m thick has developed, the ice beneath isprotected from melting and hence it standsabove the surface level of the rest of theglacier. See also moraine.

laterite /lat-ĕ-rÿt/ A layer of depositsformed from the weathering of rocks inhumid tropical conditions and consistingmostly of iron and aluminum oxides. Ithardens on exposure to the atmosphereand is therefore used for building. It is nota soil, because it is produced by processesoperating on a far larger scale than typicalpedologic processes. It can occur in twoforms, either buried and soft or as a super-ficial capping (cuirass) of hardened red ma-terial on uplifted areas that have had theiroverburden stripped off. Most laterites de-veloped in the Cenozoic Era, and are hencerelic. In profile they display a strong hard-ened reddish surface horizon, dominatedby ferric oxide, which breaks up withdepth into a mottled horizon, which in turnpasses down to a depleted pallid zone,which may be 50 m thick and reach to thebedrock. In a pure laterite, the red horizonwill be composed of 90–100% iron, alu-minum, and titanium oxides. See also lat-erization; limonite.

lateritic soil /lat-ĕ-rit-ik/ 1. A soil devel-oped on part of a relic LATERITE, or with anindurated layer within its profile.2. A red tropical soil developing into lat-erite. It undergoes leaching of silica andbases, the concentration of aluminum andiron oxides increasing, as happened in theformation of the relic laterites. As leachingof silica proceeds, the silica:sesquioxideratio falls; in 1927 Martin and Doyne cal-culated that while laterites had a ratio of%t;1.33, lateritic soils had a ratio of1.33–2.0, showing that they were follow-ing the same development trend but hadnot yet been desilicified as much as true la-terite. However, relic laterite probably de-veloped by much larger-scale processes andthe term lateritic soil has now been re-placed by FERRALLITIC SOIL (latosol) or, inAustralia, red loam.

laterization /lat-ĕ-ră-zay-shŏn/ The con-centration of iron and aluminum oxides inthe upper layers of the soil and the removalof silica and bases, either in the formationof relic LATERITES or in current soil-formingprocesses in the tropics. In the case of reliclaterites, laterization is thought to be a re-sult of a fluctuating water table. Opinionsvary: the rising and falling groundwater(resulting from seasonal dryness and wet-ness) either brings up iron and aluminumfrom the pallid zone and concentrates it ata point of maximum upward extension, orit causes iron and aluminum being washeddown in percolating rainwater to be pre-cipitated on meeting the alkaline ground-water, or it causes a combination of thetwo. It is a large-scale process, operatingover geologic timescales. In the case of cur-rent tropical soil formation laterization de-notes the process of individualization ofiron and aluminum oxides, and to a lesserextent oxides of titanium, chromium, andnickel, by intense weathering and leaching,removing bases and silica. This processnow tends to be called ferrallitization,leading to the development of FERRALLITIC

SOIL (latosol).

laterolog /lat-ĕ-ră-log/ A subsurface log-ging technique, which measures variationsin the conductivity of strata.

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latite /lat-ÿt/ An extrusive igneous rockconsisting of alkali feldspar and plagio-clase. See syenodiorite.

latitude The angular distance of a pointon the Earth’s surface north or south of theEquator, measured as the angle subtendedat the center of the Earth by an arc along ameridian between the point and the Equa-tor. The Equator has no angle whereas thepoles are at 90°. Latitude is measured indegrees, minutes, and seconds. Parallels oflatitude form approximate concentric cir-cles, with the poles as the center (see dia-gram). See also longitude.

latosol /lat-ŏ-sôl/ See ferrallitic soil.

laumontite /law-mŏ-nÿt/ A hydrated sil-icate of calcium and aluminum,Ca(Al2Si4O12).4H2O. It crystallizes in themonoclinic system, and occurs in veins inschists and cavities in igneous rocks. It is amember of the ZEOLITE group of minerals.

Laurasia /law-ray-zhă, -ahă/ The Nhemisphere supercontinent believed tohave been present before 200 million yearsago. This supercontinent subsequently frag-mented to form the present Greenland, Eu-rope, Asia (excluding India), and NorthAmerica. See also Pangaea.

laurvikite See larvikite.

lava A molten or partly molten mass ofMAGMA extruded from a volcano or vol-canic fissure and the rock that forms as aresult of cooling and solidification. The ac-

cumulation of single extrusions, termedlava flows, produces a volcano or lava pile.

There are numerous textural terms usedto describe lavas, such as aphyric, por-phyritic, vesicular, glassy, holocrystalline,etc. See also aa; block lava; columnar joint;pahoehoe; pillow lava. See lava flow.

lava dome (volcanic dome) A mass ofsolidified lava, built up from one or morelava flows, that forms a rounded steep-sided mound. It forms from lava that is tooviscous to flow far and instead piles uparound the volcanic vent from which it isslowly extruded. Most lava domes areformed from dacite or rhyolite lavas. Asuccession of lava domes have formed inthe crater of Mount St Helens, Washing-ton, USA, following the major eruption of1980.

lava flow A stream of molten LAVA

flowing from a fissure or vent of an activevolcano, and the solidified rock it produceswhen it cools. An acidic siliceous lava, witha high viscosity, tends to move slowly inshort thick flows whereas basic basalticlava, with a low viscosity, flows quickly inlong thin streams.

lava plateau A large area of elevatedland gradually built up by a series of LAVA

FLOWS from a volcanic FISSURE. See alsoflood basalt.

lava tube (lava tunnel) A long hollowbelow the surface of a solidified LAVA

FLOW, created when lava that was stillmolten continued to flow after the surfacehad become solid. Such hollows occurmost frequently in basaltic lava, which hasa low viscosity when molten.

lawsonite /law-sŏn-ÿt/ A white mineralof composition CaAl2(OH)2(Si2O7)H2O,found in low-temperature regionally meta-morphosed rocks, particularly glauco-phane schists.

layer cloud A cloud having little verticaldevelopment. Such clouds are formedunder conditions of zero or weak uplift;precipitation may be extensive from nim-

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layer cloud

NorthPole

60°N

EquatorEquatorEquator

N

S

Latitude

bostratus clouds but because uplift isweak, intensity is low. Clouds of this typeare cirrostratus, altostratus, stratus, stra-tocumulus, and nimbostratus.

layered igneous rock Many large in-trusions of gabbroic igneous rocks arestratified, containing layers in which differ-ent minerals are concentrated, which sug-gests that the dominant differentiationprocess (see magmatic differentiation;metamorphic differentiation) was the grav-itational settling of early-formed crystals.Often mineralogically similar layers are re-peated in a rhythmic fashion. Systematicchanges in mineral composition from thebottom to the top of a layered sequence aretermed cryptic layering. Typically, olivinesand pyroxenes become progressively richerin iron (relative to magnesium) and plagio-clase trends to more sodic compositions,changes which are compatible with crys-tallization from residual liquids that aredifferentiating toward more acid composi-tions.

lazurite /laz-yŭ-rÿt/ A deep blue to violetcomplex sodalite mineral, (Na,Ca)8-(Al,Si)12O24(S,SO4), the chief componentof lapis-lazuli. It crystallizes in the mono-clinic system, and occurs in pegmatites andmetamorphic rocks. See also feldspathoid.

leaching The action of water movingdown through a soil profile carrying soilmaterials with it in suspension or solution.It includes the mechanical action of water,e.g. LESSIVAGE; its chemical action, as in theweathering processes of HYDRATION, HY-DROLYSIS, and CARBONATION; and its partlybiochemical action, as when it containsleaf leachates and moves material byCHELUVIATION. Leaching removes bases,clays, organic matter, and the varioussesquioxides; in its varying forms it is re-sponsible for the processes of lessivage,PODZOLIZATION and LATERIZATION. Soilssubject to leaching become increasinglyacid with strongly developed eluvial and, ifthe leached material is redeposited, illuvialhorizons.

lechatelierite /lesh-ă-tel-yĕ-rÿt/ A raresilica glass. See silica minerals.

lee The side of a hill or other prominencethat is sheltered from the prevailing wind.

lee waves Stationary waves of air gener-ated, under certain conditions of atmos-pheric stability, on the leeward side of hillsor mountain ranges. Their wavelength isusually between 3 and 30 km. Lee wavesare frequently seen when the crests of thewave reach saturation point to give a lens-shaped cloud outlining the upward mo-tion. This cloud is constantly forming onthe upwind side and being dissipated in thearea of descent while appearing to remainstationary; it is classified as altocumuluslenticularis. Lee waves occur when a stablelayer of air is sandwiched between layers oflower stability above and below, with astrong and steady wind flow.

lepidolite /lep-ă-dŏ-lÿt/ A pink, purple,or gray lithium-bearing mineral, a hy-drated silicate of aluminum, lithium, andpotassium, sometimes also containing fluo-rine and rubidium. It crystallizes in themonoclinic system as platy aggregates oc-curring in pegmatites, and is a member ofthe MICA group of minerals. It is a principalsource of lithium.

lessivage /less-ee-vahzh/ The mechanicalmovement of clay down a soil profile underthe influence of water and its redepositionlower down. It occurs in soils subject toLEACHING but where acidity is neutral orslight, so that the clay is not broken downinto its constituents but moves as a whole.It does nevertheless operate better in lessalkaline conditions, and DECALCIFICATION

generally precedes lessivage.Diagnostic signs include an increase in

the heaviness of texture with depth, andmicromorphological examination canshow clay skins on ped surfaces and alongchannels. Clay skins are evidence of rede-position of clay; increased heaviness aloneis only suggestive, as Bt horizons (textu-rally heavier B horizons) can be due to alighter horizon above or increased weath-ering in the B due to increased moisture.

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leucite /loo-sÿt/ A white or gray potas-sium aluminum silicate, KAlSi2O6, whichoccurs in basic lavas and other igneousrocks. It crystallizes in the tetragonal sys-tem and is used in making fertilizers. Seealso feldspathoid.

leucitite /loo-să-tÿt/ A volcanic rock thatis composed of LEUCITE and a pyroxene. Seenephelinite.

leucocratic /loo-kŏ-krat/ See color index.

levée /lev-ee/ A berm or bank elevatedabove the general level of the floodplain,bordering many streams in their alluvialfloodplain sections. Usually made of coarsermaterial than the rest of the floodplain,they originate in times of flooding whenthe overbank flow decreases in speed andvolume away from the stream, depositingthe coarse materials first as it rapidly losesCOMPETENCE. If the river lacks a coarsefraction, alluvium being mostly silt andclay size, levées may be absent.

leveche /lĕ-vesh/ A hot dry wind thatblows from the south in southern Spain. Itgenerally precedes a depression movingalong the Mediterranean Sea. See alsokhamsin; sirocco.

levéed channel A submarine channel inthe deep ocean flanked by marked levées,not unlike some of the levée features thatborder many river channels on land. Suchsubmarine channels usually lie at the lowerends of SUBMARINE CANYONS, such as the LaJolla Canyon off the coast of California.Here, at a depth of some 600 m, the truecanyon terminates in a channel flanked bynatural levées. At its seaward extremity,the channel merges into a submarine fan.Sand layers have been found in the fan andon the levées on each side of the channel.Channels with levées are also strongly de-veloped across many abyssal fans.

level A surveying instrument used inconjunction with a LEVELING STAFF to de-termine heights. A level is basically a tele-scope fitted to an adjustable base, whichclamps onto a tripod. Before use the in-

strument must be adjusted until the line ofsight is perfectly horizontal, by centering abubble that moves as the basal footscrewsare turned. Once leveled it can be used tosight onto the staff. The telescope incorpo-rates a diaphragm with engraved lines, thereading at the central line being that usedfor heighting calculations.

leveling A method of surveying used forobtaining heights of points in relation tothat of a point of known height. A LEVEL isset up between a point of known heightand one for which a height is required; it isthen adjusted to give a horizontal line ofsight. A sight, known as a backsight, ismade onto a LEVELING STAFF at the knownpoint and the staff reading noted. A read-ing is then obtained by making a foresightonto the staff at the other point. The dif-ference between the two staff readingsgives the difference in height between thetwo points. The process continues by mov-ing the level between the second point, nowof known height, and another unknownone, and so on. A leveling traverse (see tra-versing) returns to the first known point, asa check on accuracy.

leveling staff A staff used in surveyingin conjunction with a LEVEL for determin-ing heights. Most staffs consist of three sec-tions, which are either telescopic or simplyclip together, to reach a height of 4 m attheir full extent. They embody a small bub-ble to enable the user to hold them verti-cally. The staff has heights marked clearlyin black or red on white or yellow, whichcan be read to the nearest millimeter. How-ever, when viewed through the telescope ofa level, the figures appear upside-down,and care must be taken in reading them.

lherzolite /ler-zŏ-lÿt/ An ultramafic rockconsisting largely of olivine together withorthopyroxenes and clinopyroxenes.

life assemblage See biocoenosis.

lightning The visible discharge of athunderstorm and the natural mechanismfor neutralizing the high electrical fieldsthat build up. Air is a good insulator but as

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charge production increases the potentialgradient in a thunder cloud, and waterdroplets decrease the insulational proper-ties of air, eventually the insulation is over-come and the lightning flash results. Itconsists of a line of highly ionized air mol-ecules along which electricity can flow fora very short time.

High-speed film recordings of lightningdischarges have clarified the developmentof a flash. A cloud-to-ground discharge isstarted by a streamer, which developsdownward from the cloud in a series ofsteps. It is of low luminosity. When thisstreamer has approached to within about 5to 30 m of the ground, a highly luminousreturn stroke comes up to meet it and trav-els up the ionized channel to the cloud.This is the main visible flash, which travelsat speeds of about 0.1 times that of light,passing a current of 10 000 amperes butonly lasting for about 100 microseconds.Several more streamers and return strokesmay develop. See also thunder.

lignite /lig-nÿt/ (brown coal) Low-gradebrown coal with a high moisture content.

limb See fold limb.

limburgite /lim-berg-ÿt/ An undersatu-rated basic igneous rock occurring as flowsand dikes and consisting of phenocrysts ofaugite and olivine embedded in a matrix ofbrown basaltic glass.

lime An imprecise term with severalmeanings. To a chemist, lime (also calledquicklime) is calcium oxide, CaO, made byheating LIMESTONE. Slaked lime (made bytreating lime with water) is calcium hy-droxide, Ca(OH)2. To an oceanographer,the term lime is generally applied to CAL-CIUM CARBONATE, CaCO3, especially whenit is dissolved in seawater or laid down inthe shells of mollusks and other aquatic an-imals. See also carbonate minerals.

limestone A rock formed from carbon-ate minerals, principally CALCITE but in-cluding others such as DOLOMITE.Limestones can be classified broadly on agenetic basis as clastic, organic, precipi-

tated, or metasomatic. Organic limestonesare formed from the calcareous skeletonsof living organisms, and include in situ BIO-HERMS and accumulates such as CHALK.Among the various precipitated limestonesare those that are part of an EVAPORITE se-quence, and OOLITES. Clastic limestones arethose derived from preexisting calcareousrocks. Metasomatic limestones are otherlimestones that have undergone diageneticchange. The most common change in-volves the alteration of calcite to dolomite.Descriptive classifications employing therelative proportions of ALLOCHEMS, MI-CRITE, and SPARITE are increasingly used.

limestone cavern A subterranean cavedeveloped in limestone as a result of the en-largement of joints and fissures by solu-tion, larger caverns occurring in lessresistant well-jointed limestone. Most cavesystems comprise both large open cavesand small restricted passages, all of whichcontinue to grow in size as long as unsatu-rated water flows through them. Somecaves are considerably extended horizon-tally, which may reflect a position corre-sponding to the present or a former watertable. The largest caves are found in hardwell-jointed limestones of low porosity;soft limestones, such as chalk, are unsuit-able for major cave development.

limestone pavement A flat exposure ofbare limestone resembling a rough pave-ment and consisting of flat or ridged irreg-ularly sized and shaped blocks (clints),separated by clefts (grikes or grykes)formed by the widening of joints by solu-tion. Grikes can measure up to 0.5 macross and frequently 2–3 m deep.

limestone solution A weathering pro-cess that occurs in limestone. Limestone isthe only common rock type in which SOLU-TION is important in the development of itscharacteristic landforms. Because it is com-posed of calcium carbonate, solution iseasily achieved by very weak carbonic acid(formed from water and dissolved carbondioxide from the air and soil). Many lime-stones possess a well-developed system ofjoints and bedding planes; the vast major-

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ity of the water flowing through the rockpasses along these fissures, in which solu-tion takes place and they therefore becomeenlarged, forming better watercourses. Intheory more limestone is dissolved as thetemperature or acidity of the solution in-creases, but it is probably more effective tohave a rapid turbulent flow of ample sup-plies of water over a large surface area ofrock.

limnology /lim-nol-ŏ-jee/ The scientificstudy of lakes and other bodies of freshwater. It considers all the living and non-living factors that affect them.

limonite /lÿ-mŏ-nÿt/ The yellow tobrown amorphous and cryptocrystallineoxidation and hydration products of iron.Goethite and hematite are important con-stituents together with colloidal silica andclay minerals. Limonite is found as an al-teration product in all kinds of rocks, as aprecipitate in bog iron ore, and as a majorconstituent of gossan and laterite.

lineament A large-scale linear topo-graphic feature that is structurally con-trolled. Lineaments often appear as longlinear features on aerial photographs andfrequently result from faults, where ero-sion has worked selectively on the softermaterial of the fault zone.

line scale A divided scale given on a mapto enable distances to be directly measuredand read off in terms of distances on theground.

linkage analysis A statistical procedureused to group individuals into clusters. Themeasures of similarity used are coefficientsof association, coefficients of correlation,or distance measures, depending upon thetype of data and the nature of the problembeing analyzed.

litchfieldite /lich-feeld-ÿt/ A type of al-kaline SYENITE containing albite, nepheline,and potassium feldspar.

lithic /lith-ik/ Describing anything com-posed of or related to stone.

lithification /lith-ă-fă-kay-shŏn/ The pro-cess by which sedimentary deposits de-velop into sedimentary rocks.

lithographic limestone /lith-ŏ-graf-ik/ A dense fine-textured type of limestone, socalled because it was formerly used for lith-ography. It is a kind of MICRITE.

lithophile /lith-ŏ-fÿl/ An element thatoccurs mainly as silicates and is thereforemost concentrated in the Earth’s crust.Lithophile elements include lithium, oxy-gen, silicon, and sodium. See also at-mophile; chalcophile; siderophile.

lithosol /lith-ŏ-sôl/ AZONAL SOIL with anAC or (A)C profile (i.e. no illuvial B hori-zon) developed on fresh and imperfectlyweathered rock or rock debris. These soilsoften form on steep slopes where runofferodes developing soil and little water per-colates down through the profile to pro-mote LEACHING and soil development. Thesurface horizon is typically dark due to or-ganic matter accumulation, but may becoarse-textured and light colored if slopewash is intense. On eroded land, subject topoor agricultural practices, these soils canbe artificial. A special type is the ranker de-veloped on acidic rocks. These soils arepart of the ENTISOL order of the US SOIL

TAXONOMY.

lithosphere /lith-ŏ-sfeer/ That part ofthe Earth including the CRUST and theupper MANTLE above the ASTHENOSPHERE.

lithospheric plate /lith-ŏ-sfe-rik, -sfeer-ik/(tectonic plate) Any one of a series oflarge blocks of continental or oceanic ma-terial of which the Earth’s crust is com-posed. These plates, whose base is markedby the asthenosphere, move across the sur-face of the Earth as a result of SEA-FLOOR

SPREADING. The six major lithosphericplates are named Eurasian, Indian, Pacific,American, African, and Antarctic. See alsocontinental crust; oceanic crust; plate tec-tonics.

lithostratigraphy /lith-oh-stră-tig-ră-fee/The branch of STRATIGRAPHY concerned

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lithostratigraphy

solely with lithological features and withnaming and elucidating the spatial rela-tions of rock units. It is the most purely de-scriptive of the systems of stratigraphicclassification, incorporating the leastamount of inference, and does not take ac-count of the evolution of organisms asshown by contained fossils (see also bios-tratigraphy) or of time (see also chronos-tratigraphy). Hence lithostratigraphic unitsare often diachronous (see diachronism).The hierarchy of lithostratigraphic divi-sions consists of SUPERGROUP, GROUP, FOR-MATION, MEMBER, and BED (see alsocomplex). Formally designated lithostrati-graphic units (formations) are given geo-graphical names with capitalized initialletters.

lit-par-lit gneiss /lee-par-lee/ A coarselybanded or foliated gneiss consisting of al-ternating layers of quartz-feldspar andmafic minerals. The felsic material, whichmay have originated by partial melting, hasintruded along the schistosity planes of themetamorphic rock. See also migmatite.

litter The accumulation of leaves andtwigs on the surface of the soil. It is the rawmaterial for the formation of the soilHUMUS; beneath the fresh litter there is afermenting layer and below this the humus.

Little Ice Age See ice age.

littoral drift /lit-ŏ-răl/ See longshoredrift.

littoral zone The region immediatelyflanking the coast on its seaward side,

strictly the combination of the intertidaland SURF ZONES, i.e. the region lying be-tween highest high water and the outeredge of the surf zone. The width of the lit-toral zone varies according to the prevail-ing wave conditions and the slope of thesea floor. The eulittoral zone extends be-tween high water and the limit of attachedplants, which is usually at a depth between40 and 60 m. In global terms the littoralzone occupies an area of about 150 000 sqkm.

llanos /lah-nohz, lyah-nos/ A SAVANNA

type of grassland found in northern SouthAmerica, particularly in the Orinoco Riverbasin.

load (stream load) The material carriedalong by a river (stream). Material is incor-porated into a river by its erosive action onthe valley bed and walls. It can also be sup-plied by groundwater flow, or direct fromvalley sides by wash, CREEP, or LANDSLIDES.Once in the river, load can be split up as inthe table. The predominance of the differ-ent types of load varies between rivers andin the same river over time, e.g. DISSOLVED

LOAD is important in rivers receiving asteady groundwater flow and little over-land runoff; in floods, transporting abilityis increased owing to velocity increases,and debris that at low flow moves as BED

LOAD may start to saltate (see saltation),and saltating load may be carried in sus-pension (see suspended load (def. 1)). Gen-erally bed load is the least important part:in the Mississippi, only 8% of load movesas bed load, compared with about 65% insuspension and saltation.

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LOAD

Major division Subdivision Nature of load(1) dissolved load chemical content, dis-

solved in water(2) solid debris load (a) suspended load finest solid particles,

supported by water(b) saltation load sand/fine gravel, bounc-

ing along bed(c) bed load coarsest material, roll-

ing and sliding on bed

loam A type of soil with characteristicsbetween those of sand and clay. It is usu-ally rich in humus. It drains reasonablywell and yet retains sufficient moisture topromote good plant growth.

loch The Scottish name for a lake. Com-pare lough.

lode A deposit of minerals occupying aseam or vein. Lodes probably formed whenhot gases or liquids were forced throughexisting rocks before cooling and solidify-ing.

lodestone MAGNETITE that is stronglymagnetic.

lodgment till Unstratified glacial ma-terial deposited directly from beneath aglacier or ice sheet. Such deposits tend tobe somewhat compacted due to the weightof ice at one time above them. See till.Compare ablation till.

loess /loh-iss/ A fine-grained, unconsoli-dated yellowish sedimentary deposit origi-nating as windblown soil. It is very soft andeasily eroded to form gullies. It is porousbut can form extremely fertile soil. Thethickest deposits occur in northern China;it is also found widely in the USA, centralEurope, central Asia, and Argentina.

Lomonosov Ridge /lom-ŏ-noss-ŏf/ Avast submarine ridge that effectively di-vides the Arctic Ocean into two separatebasins. The ridge was first discovered byRussian scientists while they were conduct-ing their oceanographic surveys from fieldstations set up on drifting pack ice. Theridge runs directly underneath the NorthPole, stretching from Ellesmere Island,north of Canada, across to the NovoSibirsk Islands off the Siberian mainland.Such data as has been collected suggeststhat the ridge may be a faulted featurerather than volcanic. Soundings across theridge have revealed what appears to be atruncated upper surface lying at a depth ofsome 1400 m. The ridge is named for theRussian writer and chemist Mikhail Vasile-vich Lomonosov (1711–65).

longitude The angular distance of apoint on the Earth’s surface east or west ofa central meridian (see diagram), measuredby the angle between the plane of themeridian through the point and that of thecentral meridian. In 1884 it was interna-tionally agreed that the meridian fromwhich the readings would be taken wouldbe the one passing through Greenwich,London, now considered as 0 degrees east,west. In passing through 15 degrees of lon-gitude at the Equator there is a local timedifference of one hour. See also latitude.

longitudinal dune See seif dune.

longitudinal wave See primary wave.

long profile A section drawn fromsource to mouth along a stream or river,showing the changes in gradient through-out its length. The ideal form is an expo-nential concave curve, with far greatergradients at the head than at the mouth.This profile results from the fact that dis-charge increases downstream and hencevelocity increases, so less gradient isneeded to transport the load supplied. Si-multaneously the size of debris decreasesdownstream, most probably owing toabrasion of the debris, but possibly owingto sorting, and so there is a further reduc-tion in energy needed, allowing a furtherreduction in gradient.

Surveyed profiles in fact rarely appearas the ideal profile; rather they are a seriesof partial curves, due to past base-levelchanges, with further irregularities due tolakes, pools, bars, hard rock bands, etc. In-sufficient time to grade a smooth profile is

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long profile

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Longitude

a general cause of irregularity, as well asthe fact that if a river flows over differentlithologies, the size of debris supplied willvary: the coarser the debris the steeper theslope needed, so gradients over sandstoneare steeper than those on shale and lime-stone. See also cross profile; river.

longshore bar (ridge; full) A sand BAR

lying roughly parallel to the coastline, inthe intertidal zone or in the surf zone.

longshore current The current that isgenerated over the whole width of the SURF

ZONE at those times when waves approachand break at an angle to the shore. Long-shore currents tend to flow parallel to theshore, and probably attain their maximumrate when the wave crests near breakpointare angled approximately 30° to the shore-line. Field measurements have revealedthat such wave-induced currents areroughly of uniform speed across the wholewidth of the breaker zone, but that theyrapidly weaken seaward of the surf zone.Tidal and other currents may be superim-posed upon the longshore currents, tendingeither to strengthen or to nullify them.

longshore drift (littoral drift) Themovement of beach materials, sand orshingle, along the shore parallel with thecoastline. This process is basically due tothe effects of waves breaking on the beachobliquely. Despite WAVE REFRACTION,waves infrequently break perfectly perpen-dicularly to a beach and their SWASH tendsto move material landward diagonally.However, the returning water, in the formof the BACKWASH, moves directly down the

beach and the net effect of swash and back-wash is a small longshore movement.Longshore currents aid this process. Inmost areas one set of waves predominates,resulting in an undirectional longshoremovement, but some coasts may be influ-enced by two sets in opposition, causingthe material to move to and fro along thesame stretch of beach.

long wave (Rossby wave) The smoothbroad wave-shaped contour patterns on apressure chart of the upper atmosphere.They have wavelengths up to about 2000km and there are usually either four or fivesuch waves around the westerly wind beltof the N hemisphere. Both heat and mo-mentum are transferred poleward by thelong waves and they also have a significanteffect on CONVERGENCE and DIVERGENCE atthe surface. Long waves are also namedRossby waves for the Swedish–Americanmeteorologist Carl-Gustav Arvid Rossby(1898–1957).

lopolith /lop-ŏ-lith/ A large concordantigneous intrusion, which is saucer-shapedin cross section. The type example is theDuluth gabbro, Canada, which has a vol-ume of approximately 200 000 km3.

lough /lok/ The Irish name for a lake.Compare loch.

Love wave (L wave; Q-wave) A surfacewave produced by an earthquake in whichthe particle motion is in a horizontal direc-tion, perpendicular to the direction ofwave propagation. It causes multiple re-flections between the surface and the top ofthe layer of rock below. It is named for theBritish mathematician Augustus EdwardHough Love (1863–1940) who discoveredit. See also primary wave; Rayleigh wave;secondary wave; seismic wave.

low See depression (def. 1).

low index The state of the westerly cir-culation when flow is weak and there is ap-preciable meridional transfer, as in aBLOCKING situation.

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distance from sourcedistance from sourcedistance from source

mouth

source

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Ideal long profile

low-velocity zone See asthenosphere.

loxodrome /loks-ŏ-drohm/ See rhumbline.

luster The appearance of the light re-flected from the surface of a mineral, whichis a qualitative characteristic often provid-ing an aid to identification. The kinds ofluster are: 1. metallic – displayed by galena and

pyrites. 2. vitreous – displayed by glass and quartz. 3. resinous – displayed by amber. 4. silky – displayed by gypsum. 5. pearly – displayed by talc and muscovite. 6. adamantine – displayed by diamond.

lutite /loo-tÿt/ Any fine-grained sedimen-

tary rock formed of material that was oncemud. Examples include MUDSTONE, SHALE,and SILTSTONE.

luxullianite /luks-ul-ee-ă-nÿt/ A TOUR-MALINE-QUARTZ-FELDSPAR rock producedby the boron pneumatolysis of granite.

L wave See Love wave.

lysimeter (evaporimeter) An instrumentfor measuring the amount or rate of down-ward water percolation and water storagein a vegetated block of soil that is enclosed(in a tank, column, or plot) and open to thesky. From the measurements an estimationof the rate of EVAPOTRANSPIRATION can beobtained.

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maar /mar/ A shallow volcanic craternot associated with a single vent or cone.Maars are thought to be formed by manysmall explosive eruptions, sometimes at thetop of a DIATREME. Many fill with waterand form lakes.

macroclimate /mak-roh-klÿ-mit/ Thegeneral climate of a large area. Comparemicroclimate.

macrometeorology /mak-roh-mee-tee-ŏ-rol-ŏ-jee/ The study of the large-scale at-mospheric processes responsible for thedifferentiation of the macroclimates on theEarth’s surface, e.g. the general circulationof the atmosphere. Compare mesometeo-rology; micrometeorology.

maculose /mak-yŭ-lohs/ Having a knot-ted or spotted appearance, usually describ-ing a metamorphic rock. The irregularitiesare thought to result from the growth ofnew minerals during heating.

mafic /maf-ik/ Describing dark-coloredminerals. Mafic minerals are approxi-mately equivalent to FERROMAGNESIAN MIN-ERALS. Compare felsic.

magma /mag-mă/ A hot silicate liquidbeneath the Earth’s surface containing sus-pended crystals and dissolved gases. IG-NEOUS ROCKS are formed from thecrystallization and solidification of magmaand are said to be magmatic. A lava repre-sents magma extruded at the Earth’s sur-face whereas magma solidifying at depthgives rise to HYPABYSSAL and PLUTONIC

ROCKS. During the solidification of magma,the volatile constituents escape and thuscompositions of igneous rocks are only ap-

proximations of the compositions of themagmas from which they originated.

magma chamber A reservoir within theEarth’s lithosphere in which molten rockaccumulates before being emptied at theEarth’s surface through a volcano. Such achamber exists beneath most volcanoes,and is replenished from below after beingemptied during a volcanic eruption.

magmatic differentiation /mag-mat-ik/A number of processes causing the gradualevolution of the composition of an igneousmelt and the separation of the MAGMA intotwo or more fractions of contrasting com-positions. Several mechanisms are recog-nized: 1. Fractional crystallization. One ofthe main processes producing composi-tional variation in igneous rocks is thegravity settling of early-formed minerals.Such crystals having densities higher thanthe enclosing magma sink to the bottom ofthe magma chamber. In some cases, lightminerals such as leucite float upward. Thecomposition of the residual liquid differsfrom that of the original melt because ofthe removal of the solid crystalline ma-terial. 2. Filter pressing. During the laterstages of crystallization, residual liquidmay be squeezed out from a crystal mushand rise upward in the crust. 3. Flow dif-ferentiation. The movement of a magmaticliquid within a dike or lava flow often con-centrates phenocrysts toward the center ofthe body. 4. Liquid immiscibility. Duringcooling, two liquid fractions of a magmamay become immiscible and separate.Sulfides separate from a silicate magmaduring the early stages of crystallization.The role of liquid immiscibility in igneouspetrogenesis is limited, but this processmay account for the segregation of carbon-

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M

ate liquids from mafic alkaline magma. 5.Gaseous transfer. The upward streamingof gas bubbles and dissolved materials re-sults in the concentration of volatiles nearto the top of a magma body.

magnesite /mag-nĕ-sÿt/ A white, yellow,or gray major mineral form of magnesiumcarbonate, MgCO3. It crystallizes in thetrigonal system and occurs in masses insedimentary rocks. It is used as a refractorymaterial and as a source of magnesium. Seealso carbonate minerals.

magnetic anomaly A departure fromthe predicted value of the Earth’s magneticfield at a point on the Earth’s surface.

magnetic declination See angle of dec-lination.

magnetic dip See dip (def. 2).

magnetic division A stratigraphicalunit (see stratigraphy) consisting of therocks formed during one MAGNETIC INTER-VAL (see also paleomagnetic correlation).The establishment of magnetic divisionsconstitutes an important method in theCALIBRATION and CORRELATION of strati-graphic sequences, especially in the Neo-gene Period.

magnetic elements The seven parame-ters of the geomagnetic field at a particularlocality on the Earth’s surface. They in-clude a value for the field with respect togeographic north (X), east (Y), the vertical(Z), the horizontal (B), the total induction(F), its ANGLE OF DECLINATION (D), and in-clination (I) (see dip (def. 2)).

magnetic equator An imaginary lineround the Earth joining all points wherethe ANGLE OF DECLINATION is zero.

magnetic field A field of force that ex-ists as a result of a circuit carrying an elec-tric current, or the presence of a permanentmagnet.

magnetic inclination See dip (def. 2).

magnetic interval The variable periodof time between MAGNETIC REVERSALS in theEarth’s polarity. The paleomagnetic scalemust be calibrated by some independentmethod of assessing time, such as radio-metric dating. The rocks formed during amagnetic interval are known as a MAG-NETIC DIVISION, which can be used in thecorrelation of stratigraphic successions.See also paleomagnetic correlation.

magnetic iron ore See magnetic.

magnetic meridian An imaginary lineon the surface of the Earth joining the mag-netic poles. It indicates the direction of thehorizontal component of the Earth’s mag-netic field.

magnetic pole Either two of the pointson the Earth’s surface at which the lines ofmagnetic force are vertical. These pointsare slowly changing with time and do notcoincide with the geographical poles.

magnetic reversal (polarity reversal)Changes in the direction of the Earth’smagnetic field’s induction by 180° (i.e., theNorth Pole became the South Pole, andvice versa) that have been revealed by paleo-magnetic studies (see paleomagnetic cor-relation). Based upon these reversals,which have been recorded in rocks formingat that time, a polarity/time scale has beenestablished for the last 4.5 million years ofthe Earth’s history.

magnetic storm A more or less violentdisturbance of the Earth’s magnetic field,probably caused by the arrival of chargedparticles ejected from the Sun in solarflares.

magnetic stripes The roughly parallelbands of ocean floor with alternating di-rections of induction: a normally magne-tized stripe will be enclosed by tworeversely magnetized stripes. These pat-terns can be picked up by shipborne instru-ments and used to correlate areas of oceanfloor offset along TRANSFORM FAULTS.

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magnetite /mag-nĕ-tÿt/ (magnetic ironore) A shiny black magnetic mineral, anoxide of iron, Fe3O4, sometimes containingsome magnesium. It crystallizes in thecubic system and occurs as granular orcompact masses in various igneous andmetamorphic rocks. It is a member of theSPINEL group and an important source ofiron.

magnetometer /mag-nĕ-tom-ĕ-ter/ Aninstrument used for measuring the strengthof the Earth’s magnetic field.

magnetopause /mag-nee-tŏ-pawz/ Theregion where the Earth’s MAGNETOSPHERE

ends and interplanetary space begins.

magnetosphere /mag-nee-tŏ-sfeer/ Thearea around the Earth in which the Earth’smagnetic field is present. The field is con-fined to this area as a result of the interac-tion between the Earth’s magnetic field andthe solar wind. The magnetosphere ex-tends out away from the Earth’s surface fora much greater distance on the side awayfrom the Sun.

magnitude See earthquake.

malachite /mal-ă-kÿt/ A blue-green min-eral form of hydrated copper carbonate,Cu2CO3(OH)2. It crystallizes in the mono-clinic system and occurs in the oxidationzone of copper deposits. It is used for mak-ing ornaments and as a semiprecious gem-stone, and is a minor source of copper.

malignite /mal-ig-nÿt/ An alkalinenepheline-containing igneous rock, a typeof SYENITE.

mallee /mal-ee/ A type of scrubland thatoccurs in semiarid regions of Australia.The vegetation consist mostly of low-growing kinds of eucalyptus.

Mammalia /mă-may-lee-ă/ A class ofwarm-blooded vertebrates that nourishtheir young on milk secreted by mammaryglands in the female. Mammals have a rel-atively large brain, morphologically differ-entiated teeth, and (in most) a body

covering of hair or fur. They are skeletallydistinguished from reptiles by having onlyone bone in each lower jaw, which also dif-fers in its articulation with the cranium.

Mammals evolved from mammal-likereptiles in the late Triassic; the earliestforms were small flesh-eating active crea-tures similar to modern shrews. Fossils ofthese primitive Mesozoic mammals arerare and consist mainly of teeth. Mammalsexisted throughout the dominance of thedinosaurs but did not achieve supremacyon land until the Cenozoic, after the ex-tinction of most reptilian groups, whentheir main radiation took place. The twomain mammal groups, the Marsupialiaand Eutheria, separated from the ancestralstock by the late Cretaceous. They bothgive birth to living young (as opposed tolaying eggs). The most primitive group isthe Monotremata, which comprises egg-laying mammals whose modern represen-tatives are the duck-billed platypus andechidnas (spiny anteaters).

man Modern man, Homo sapiens, be-longs to the family Hominidae of the mam-malian order PRIMATES. Primates that aresometimes classified as hominids areknown from the Miocene and perhapseven earlier. In the Pliocene and early Pleis-tocene of Africa the remains of a variety ofmanlike creatures have been found; theyare usually placed in two genera, Australo-pithecus and Homo. Most authorities be-lieve that Homo evolved from one of thespecies of Australopithecus. Homo erectus,a hominid of robust construction, was pre-sent in Africa, Asia, and possibly Europeduring the mid-Pleistocene. In the laterPleistocene two species of Homo are re-cognized: modern man, H. sapiens, andNeanderthal man, H. neanderthalensis.Primitive stone tools have been found fromrocks dated as Pliocene in age.

manganese nodule A deep-sea mineralconcretion. Somewhat similar in form toPHOSPHORITE nodules, manganese nodules(one of the AUTHIGENIC minerals) are theonly potential deep-sea mineral depositthat could be economically recovered fromthe deep ocean floor utilizing present-day

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technology. They seem to be largely con-fined to deep-sea clays, but have also beenfound in association with oozes. The nod-ules are surprisingly heavy, averaging some24% manganese, 14% iron, 1% nickel,0.5% cobalt, and 0.5% copper. The vari-ous minerals frequently form roughly con-centric layerings on stones and other hardobjects, such as sharks’ teeth, present onthe sea floor. These layerings build upgradually on account of the slow precipita-tion from sea water of manganese oxidesand other mineral salts. Surveys have re-vealed that the nodules are widely distrib-uted on the deep ocean floor and aresometimes surprisingly abundant. For ex-ample, in certain parts of the PacificOcean, concentrations of 20 000 tonnesper sq km have been located.

manganite /mang-gă-nÿt/ A black orgray semiopaque lustrous mineral form ofhydrated manganese oxide, MnO(OH). Itcrystallizes in the monoclinic system andoccurs in hydrothermal veins, usually asso-ciated with BARYTES or CALCITE, and indeep-sea MANGANESE NODULES. It is a minorsource of manganese.

mantle That part of the Earth’s interiorbeneath the base of the CRUST and abovethe Earth’s OUTER CORE, roughly between30–2900 km beneath the Earth’s surface. Itis thought to consist of ferromagnesian sil-icate minerals such as olivine and pyrox-ene. Essentially it behaves as a solid,although within the asthenosphere it maybe partly molten.

mantled gneiss dome (gneiss dome) Astructure present in Precambrian shieldareas, consisting of a central core of gran-ite rock surrounded by regionally meta-morphosed rocks, mainly gneisses, that hasbeen exposed by erosion and covered by aseries of younger sediments. The area wasthen subjected to orogenesis, during whichthe granitic core was remobilized and roseup, causing the overlying strata to bearched upward.

mantle nodule A small fragment orblock of rock from the Earth’s MANTLE that

has been brought to the surface by a vol-canic explosion.

mantle plume An upwelling of buoyanthot mantle material that is hypothesized tooriginate deep within the Earth, possibly atthe core–mantle boundary. Mantle rockbegins to melt, thereby becoming lessdense, and as a result this material rises tothe base of the Earth’s lithosphere where itspreads out. In so doing it causes the over-lying crust to be domed up. The surface ex-pression of a mantle plume is known as aHOT SPOT.

map A graphic representation on a planesurface of the Earth’s surface or part of it,showing its geographical features. Theseare positioned according to pre-establishedgeodetic control, grids, projections, andscales.

map projection Any method of repre-senting the curved surface of the Earth ona flat plane surface such as a piece of paper.A grid printed on the map represents linesof latitude and longitude, which are drawnaccording to different mathematical for-mulas, depending on the area to bemapped. Some projections give the prop-erty of equal area (homolographic), otherstrue bearings (azimuthal), and othersachieve a compromise.

map scale The ratio of the distance mea-sured on the map to the distance on theground between the same points. On a mapwith a scale of 1:50 000, for example, oneunit on the map is equal to 50 000 of thesame unit (e.g. centimeters) on the ground.Most maps now are metric and thereforehave ratios such as 1:50 000, 1:100 000.The larger the ratio, the smaller the scale ofthe map.

The scale used is largely determined bythe purpose of the map: for index mapsand wall charts of the world a small-scalemap would normally be used; for townplans the larger the scale, the more accu-rate and detailed the plan can be.

map series A collection of maps, usuallyat the same scale, using the same specifica-

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tions and generally identified by a seriesnumber. Map series normally cover indi-vidual countries but can cover the entireEarth’s surface, e.g. the International Mapof the World 1:1 000 000.

maquis /mah-kee/ A type of scrublandthat occurs in the Mediterranean districtsof France. The vegetation consists mainlyof deep-rooted low-growing evergreenbushes and trees, whose leaves are able tosurvive the hot dry summers.

marble A rock composed largely of cal-cite or dolomite, produced by the regionalor contact metamorphism of limestones.During the metamorphism of a pure lime-stone, calcite recrystallizes to producecoarser interlocking grains, often with apreferred orientation imparting a weakschistosity. A variety of minerals may formwhen impure limestones are metamor-phosed, including quartz, diopside,forsterite, grossular, tremolite, micas, andepidote.

marcasite /mar-kă-sÿt/ A pale bronze-yellow iron sulfide mineral, FeS2, polymor-phous with pyrite but having ortho-rhombic symmetry. Marcasite is foundmainly as a replacement mineral in sedi-mentary rocks and as radiating nodules, es-pecially in chalk. It is used as asemiprecious gemstone.

mare’s tails See cirrus.

margarite /mar-gă-rÿt/ A pearly pinkmineral form of calcium aluminum silicate,(Ca,Al)Si2O10(OH)2. It crystallizes in themonoclinic system and is a member of theMICA group of minerals.

margin See plate boundary.

marginal sea (adjacent sea) A sea areathat is significantly open to the adjacentocean. Marginal seas may be of the deeptype or the shallow (shelf sea) type. Thedeep marginal seas usually lie between thecontinental blocks and offshore submarineridges, which may or may not have islands,and often display depths well in excess of

200 m. The shallow marginal seas occupythe continental shelf areas and are thusgenerally less than 200 m deep. There is anobvious contrast between the open oceanand those sea areas juxtaposed betweenmainland and groups of offshore islands.Compare inland sea.

marigram /ma-ră-gram/ The graphicrecord obtained from the use of an instru-ment that monitors the rise and fall ofwater level throughout a tide cycle. Fromthis, an average water-level curve, knownas a tide curve, can be plotted. Such curvesor records enable oceanographers to iden-tify the types of tidal motion affecting par-ticular areas.

marine borer An organism that is capa-ble of performing marine erosion, usuallyby a boring process. Such erosion may belimited to fairly local areas but is signifi-cant not only because of the way in whichrocks become riddled with holes but alsobecause many boulders, smaller stones,shells, and flakes of rock may be affected inthis way. Particularly susceptible are softlimestones and sandstones. Borers aboundin the sea and are particularly active in theshallow littoral zone. Boring algae are ef-fective, and certain sponges attack rocks;some worms, chiefly the annelids, togetherwith a variety of mollusks, excavate pitsand tubes in rocks. Crustaceans and fishsuch as the eel often enlarge the cavitiesthey tend to occupy. Serious damage mayeven be caused to timber and concretestructures, for example groins.

marine ecosystem The littoral and sub-littoral zones are characterized by an enor-mous diversity of plant and animal lifewhich, taken with their physical environ-ment, constitute the marine ecosystem. Thesystem is highly dynamic. Most sublittoralplant life occurs as plankton but there arealso a number of bottom-living seaweeds.The range of species of sublittoral animalsis enormous. The food chain of sublittorallife begins with plant plankton (phyto-plankton), which consume nutrient saltspresent in the sea water and also carbondioxide. Animal plankton (zooplankton)

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feed actively on the phytoplankton and arethemselves eaten by the smaller species offish. The small fish are consumed by largerfish, and so on. Decomposition of deadfauna and flora refurbishes the supply ofnutrients. Thus, the individual organismsin the marine ecosystem are interdepen-dent. The marine ecosystem is highly sus-ceptible to damage from marine pollutionand other forms of human intervention.

marine geology The scientific study ofthe floor of the ocean and its various phys-ical features, rocks, and sediments.

maritime air mass A large mass of airthat has its source region over the ocean, oris modified after passage over an expanseof ocean. Such air masses have significanteffects on weather, although the effects of,say, a tropical air mass and a polar air masswill be very different.

maritime climate The generalized char-acteristic climate of the temperate parts ofthe Earth’s surface that are affected by thesea. The sea acts as a vast storehouse of theSun’s energy and changes its temperatureonly slightly from winter to summer.Hence areas experiencing a maritime cli-mate have low annual ranges of tempera-ture (cool summers and mild winters) andan even distribution of precipitationthroughout the year with no dry season.Some coastal areas may not experience amaritime climate if the prevailing windsare offshore.

marker bed (marker horizon) A bed ofrock having some distinctive characteristicof lithology or contained fossils that per-mits its easy recognition wherever it oc-curs. Such a bed can often be inferred torepresent a very short period of time or, inthe case of beds resulting from certainkinds of volcanic eruption, to be virtuallyinstantaneous in its formation. Markerbeds thus have great use in STRATIGRAPHY

and in the CORRELATION of strata.

markfieldite /mark-feeld-ÿt/ A por-phyritic microdiorite. See diorite.

marl An ARGILLACEOUS sedimentary rockin the form of MUDSTONE that has a highproportion of calcareous material in itscomposition.

marsh An area of soft wet land that haspoor drainage and frequently becomes wa-terlogged. The dominant vegetation con-sists of grasses, reeds, and sedges. Marshesoccur in depressions left by retreating glac-iers and in estuaries, especially where thereis a DELTA. On flat low-lying land they gen-erally overlie impermeable rock where thewater table is near the surface. If an estuar-ine marsh is periodically covered by thetide, it can become a salt marsh. The drain-ing of marshes to create arable land com-pletely changes the ECOSYSTEM. See alsobog; swamp.

Marsupialia /mar-soo-pee-ay-lee-ă/Mammals whose offspring are born at animmature stage and continue their devel-opment within a special pouch (mar-supium) on the belly of the mother. Thisand other (skeletal) differences distinguishthem from the placental mammals (see Eu-theria). Fossils show that the marsupialsdiverged from the placentals in the Creta-ceous, when they appear to have beenmuch more abundant than they are today.They are known from the Tertiary of Eu-rope and America, but as the placentals be-came the more successful group, themarsupials gradually became confined toisolated areas such as Australasia andSouth America. In these regions marsupialspecies evolved to resemble placentalspecies elsewhere (see parallel evolution).

massif /ma-seef/ An area of high groundor mountain landscape with no level areasand features that distinguish it from thesurrounding terrain. The constituent rocksare generally more rigid and older thanthose around the massif.

mass movement (mass wasting) Thedownslope movement of rock and soil ma-terial on unstable slopes, that is slopeswhose angle is greater than the naturalANGLE OF REST of the constituent materialand whose SHEAR strength is not capable of

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maintaining this angle. Gravity, togetherwith other agents such as wind and water,may act to produce a mass movement ofsome kind, whereby slope angle becomesreduced to a more stable value. Massmovements constitute flows, slides, andfalls of rock material from a slope; theymay be sudden and short-lived, as in thecase of ROCKFALLS, or very slow and long-lived, as in the case of CREEP. Slides andfalls act along a shear plane within therock. Flows, on the other hand, involve in-ternal deformation and movement by over-turning of the material. Commonly massmovements are compound: flows on gen-tler slopes often start on steeper slopesabove as a slide or fall. See also basal sap-ping; earthflow; earthslide; landslide.

mass transport current The slow driftof water beneath waves, in the direction ofwave propagation, that arises from the na-ture of ORBITAL MOTION beneath waves. Be-cause, in the case of deepwater waves, theorbits of water particles are almost circularbut not quite closed 1oops, each particle ofwater in motion not only orbits but alsomoves slowly forward in the direction ofwave propagation. This asymmetry or or-bital motion in deep water becomes evenmore pronounced in shallow water, forhere the orbits become increasingly ellipti-cal with decreasing depth. Theoretically acork placed on the sea surface in deepwater should both bob up and down andshift slowly in the direction of wave travel.

mass wasting See mass movement.

matrix /may-triks/ The fine-grainedcomponent of a rock into which thecoarser components are set. In sedimentaryrocks, the matrix is often the material thatcements the larger grains or pebbles to-gether. In igneous rocks the GROUNDMASS isequivalent to the matrix.

maximum temperature The highestvalue recorded by a thermometer in a spec-ified time. The normal period between ob-servations is the day, but the mean monthlymaximum and mean annual maximum areoften extracted from the daily data. Themaximum temperature is recorded by amercury thermometer, which has a con-striction near its bulb. As the temperaturerises, the force of expansion is strong andpushes the thread of mercury up the col-umn. If the temperature then falls, the con-traction force is weaker and theconstriction prevents the mercury return-ing to the bulb. It remains at the position ofthe highest temperature. On a world scale,the highest maximum air temperaturerecorded, 57.8°C (136°F), was reportedfrom Azizia, Libya, on September 13,1922.

meadow soil See gley soil.

mean The arithmetic mean or average isa statistical method of representing themagnitude of a data set. It is obtained by

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Fig. 1: Slope angle becomes too steep to be stable

Fig. 2: Shear plane develops Fig. 3: Slide/fall lowers slope angle

shea

r pla

ne

new profilenew profilenew profileslid

eor

fallold

profile

debrisdebrisdebrispilepilepile

Mass movement

summing the individual values and divid-ing by the number of individuals. It givesmeaningful results only when the data havea normal distribution. For example, themean rainfall in desert areas is often calcu-lated on the basis of many years withoutrain interspersed with exceptionally heavyfalls and may indicate an annual total thathas never been experienced.

meander /mee-an-der/ A sinuous curveor loop in a river channel. The Americangeologist William Morris Davis (1850–1934) stated that meanders originatedwhen a river used up excess energy by side-cutting when it could no longer cut down,but since Davis other causes have been putforward: 1. Local erosion and depositionpatterns will lead to the development ofone meander, which will then influence thestream-flow pattern, propagating the sys-tem of bends down river. The initiation oflocal erosion and depositions is due to achance abrasion in the channel, e.g. a fallentree or stream-bed shoals. 2. Long profilesof rivers characteristically develop POOLS

AND RIFFLES. Shallowness over the rifflesleads to increased velocity and bed rough-ness, and hence greater energy loss over theriffles than the pools. Since rivers try to dis-tribute energy loss as evenly as possiblethroughout their lengths, it may be thatmeander development in the pools wouldbe one way of equalizing energy loss. Oth-ers have said the riffles are the site of initi-ation. 3. Irrespective of the cause, thedivergence of primary downstream flowwill set up a secondary HELICAL FLOW con-

centrating erosion on the outside of bendswith deposition on the inside, and there-fore continuing meander developmentdownstream. Individual meanders willtend to migrate downstream by this pro-cess of erosion of the outside of meanderbends, and deposition on the insides. If theerosion and deposition exactly comple-ment each other, the meander train retainsits form but migrates down valley. How-ever, erosion rates typically exceed deposi-tion rates in some meanders, leading tomodifications.

The distribution of velocity of streamflow in a meandering reach is not even: be-cause of the pattern of helical flow, veloc-ity is highest on the outside of meanderbends, least on the inside. There is there-fore active erosion in the former areas, de-position in the latter. As the outsides erodeand move downstream, so the insides ofthe bends sediment and move downstreamas well. Thus the whole reach migratesdown valley. See also entrenched meander;incised meander; ingrown meander.

mean sea level The level of the surfaceof the sea averaged over all tide states,being actually determined from the averageof hourly heights. The periods over whichreadings are recorded vary; computationshave been based on a month, a year, andeven a 19-year period. All levels, includingthose of the sea surface, coastal land eleva-tions, and the levels of coastal structuresare referred to a chosen datum planeknown as the tidal datum. See also sealevel.

mechanical weathering (physical weath-ering) The breakdown of solid rock ei-ther through the development of cracksproduced by strain as a result of internal orexternal stresses, or by the movement awayfrom each other of the individual grainswithin a rock. The cause of stresses withinrocks are many and mechanical weatheringcan operate at a number of different scales.The major types of mechanical weatheringare: FREEZE-THAW weathering, EXFOLIA-TION, GRANULAR DISINTEGRATION, UNLOAD-ING phenomena, WETTING-AND-DRYING

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mechanical weathering

streamlineof maximumflowmigration of

meander

depositionas point bar

maximumerosion

depositionas point bar

Meander

WEATHERING, FLAKING, and INSOLATION

WEATHERING.

medial moraine A ridge or spread ofangular material extending lengthwisealong the center of a glacier. Moraines maybe formed as a result of the joining of twoLATERAL MORAINES as two glaciers con-verge, or of material originating at a spurbetween two confluent glaciers. Beadedforms occur, reflecting a greater availabil-ity of rock waste in summer periods. Onthe melting of the glacier the medialmoraine may be left as a ridge runningdown the middle of the valley. See alsomoraine.

median ridge See mid-ocean ridge.

median valley (median rift; axial riftzone) A deep cleft that follows the axis ofa MID-OCEAN RIDGE, marking the site of aconstructive PLATE BOUNDARY.

Mediterranean climate In Köppen’sCLIMATIC CLASSIFICATION the distinctive cli-mate of hot summers and mild wet wintersof the Mediterranean basin, which he des-ignated a Cs climate. Similar climaticregimes are found in California, Chile, SAfrica, and W and S Australia.

meerschaum /meer-shăm, -shawm/ (sepio-lite) A mineral form of hydrated magne-sium silicate, Mg2Si3O6(OH)4. It is yel-lowish, pink, or white and claylike, and isused as a building stone and for making to-bacco pipes.

megabreccia /meg-ă-brech-ee-ă, -bresh-/A type of very coarse BRECCIA, with thelargest fragments more than 1 km across,often the result of rockfalls or landslides.Compare microbreccia.

megaripple /meg-ă-rip-ăl/ See sand wave.

melange /may-lahnzh/ A sedimentaryrock consisting of a jumbled mass of vari-ous rock fragments including schists, lime-stones, cherts, quartzites, graywackes, anda wide range of other rock types. They arethought to have been deposited as a result

of the slumping of large masses of unstablerock debris, associated with destructivePLATE BOUNDARIES.

melanite /mel-ă-nÿt/ A black or darkbrown member of the GARNET group ofminerals, containing significant amountsof titanium.

melanocratic /mel-ă-noh-krat-ik/ Seecolor index.

melilite /mel-ă-lÿt/ A member of a groupof minerals with composition (Ca,Na)2-(Mg,Fe2+,Al,Si)3O7 found mostly in ther-mally metamorphosed limestones andsome strongly undersaturated basic ig-neous rocks such as NEPHELINITES.

melilitite /me-lil-ă-tÿt/ A member of theNEPHELINITE group of minerals.

melteigite /mel-tee-ă-jÿt/ See ijolite.

meltwater Melted ice from a glacier,which may travel over the surface of the ice(supraglacial), within it (englacial), or be-neath it (subglacial), carrying material withit and achieving a certain amount of sort-ing and rounding of rock fragments in theprocess. At ice margins meltwater is re-sponsible for the formation of distinctivelandforms, such as KAMES and ESKERS,while the presence of meltwater at the ice-rock interface is essential for successfulglacial erosion in the form of GLACIAL

PLUCKING. See also meltwater channel.

meltwater channel (glacial drainagechannel) A channel cut by glacial melt-water in either solid rock or drift deposits,the most favorable locations being beneaththe ice, where water restricted to tunnelscan move at high velocity, carrying an ero-sively powerful load, and marginal to theice, where large volumes of water derivedfrom the ice surface can accumulate andflow. After the disappearance of the ice,these channels bear little similarity to nor-mal fluvially-developed ones. They maystart abruptly, frequently in unlikely loca-tions, crossing present-day drainage di-vides; they may be unusually straight and

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steep-sided and may contain only a smallstream compared with the channel size, orsometimes no stream at all.

member A division in the lithostrati-graphic classification of bodies of rock (seelithostratigraphy; stratigraphy). It is partof a FORMATION, distinguished by someparticular lithological characteristic.

Mercalli scale /mer-kah-lee/ A descrip-tive scale devised in 1902 by the Italian vol-canologist Giuseppe Mercalli (1850–1914)for measuring the intensity of earthquakesin terms of the damage they cause. Thescale underwent several modifications inEurope and in 1931 was adapted and pub-lished in the USA by the American seismol-ogists H. O . Wood and Frank Neumann asthe Modified Mercalli scale (MM scale).This scale, following further modificationby the American seismologist Charles

Francis Richter in 1956, is the one cur-rently used in the USA. It ranges from I(not felt except by a very few) up to XII(damage total; objects thrown into the air).Compare Richter scale.

Mercator projection /mer-kay-ter/ Anorthomorphic cylindrical MAP PROJECTION

that is most commonly used for navigationcharts. Although there is great exaggera-tion of area in the higher latitudes, thestraight-line bearings are correct. It istherefore used for portraying informationof a directional nature, e.g. ocean currents.Similar to the Mercator is the TRANSVERSE

MERCATOR PROJECTION. The projection isnamed for the Dutch cartographer andseismologist Gerardus Mercator (1512–94). See also orthomorphic projection.

meridian /mĕ-rid-ee-ăn/ A line of longi-tude passing through any given point and

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poles cannot be shown

rhumb line or loxodrome

180° 150° 120° 90° 60° 30° 0° 30° 60° 90° 120° 150° 180°

80°

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60°

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20°

0°

20°

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Mercator map projection

the North and South Poles. Meridians aresemicircles of equal length and cut theEquator and all other parallels at right an-gles. The Greenwich meridian is taken tobe the prime meridian from which all oth-ers are measured. Distance between twoadjacent meridians decreases with distancefrom the Equator. See also magnetic merid-ian.

meridional circulation /mĕ-rid-ee-ŏ-năl/Any large-scale atmospheric movementfrom south to north or vice versa. Comparezonal circulation.

mesa /may-să/ An isolated flat-toppedhill with steep sides most frequently foundin old-established landscapes based on hor-izontally bedded strata. This often exhibitsalternate layers of comparatively hard andsoft rocks, which on dissection producesteep and flat slopes respectively. Mesasoften have almost vertical upper slopes, de-veloped on the resistant cap rock, followedby much flatter slopes extending down to ageneral plain. The cap rock may be a lavaflow or the same effect can be producedwhere a resistant soil horizon has been cre-ated.

meseta /may-set-ă/ The high plateau re-gion in central Spain that tilts to the west.

mesocratic /mes-ŏ-krat-ik, mee-sŏ-/ Seecolor index.

mesolite /mes-ŏ-lÿt, mee-sŏ/ A white fi-brous calcium-containing ZEOLITE mineral,whose composition is intermediate be-tween those of natrolite and scolecite.

mesometeorology /mes-ŏ-mee-tee-ŏ-rol-ŏ-jee, mee-sŏ-/ The study of the atmos-pheric processes that give rise to weatherphenomena having scales that are interme-diate in size between those of MACROMETE-OROLOGY and MICROMETEOROLOGY. Theactual size of systems in mesometeorologyis a matter of some dispute but it is some-where from approximately 10 to 150 km.Mesosystems can be subdivided into thosetopographically induced, free-atmosphereconvective, and free-atmosphere noncon-

vective, depending upon their mode of ori-gin. A denser network of observing sta-tions is required to obtain adequate datafor this scale of investigation than wouldbe needed in macrometeorology.

mesopause /mes-ŏ-pawz, mee-sŏ-/ Theupper limit of the mesosphere at a height ofabout 80 km. It represents a temperatureminimum (–80°C) between the mesosphereand the ionosphere.

mesosphere /mes-ŏ-sfeer, mee-sŏ-/ 1.The layer of the upper atmosphere between50 km and 80 km above the Earth’s sur-face. Through the layer temperatures de-crease from values of about 0°C at thestratopause, its lower boundary, to about–80°C at the mesopause, its upper bound-ary. It is not thought to affect weather con-ditions in the troposphere.2. That part of the Earth’s interior lying be-neath the ASTHENOSPHERE.

mesostasis /mes-ŏ-stay-siss, -stass-iss,mee-sŏ-/ Glassy or late-stage crystalliza-tion minerals, such as quartz, alkalifeldspar, zeolites, or calcite, which infill IN-TERSTICES in igneous rocks.

mesothermal deposit /mes-ŏ-th’er-măl,mee-sŏ-/ See hydrothermal process.

Mesozoic /mes-ŏ-zoh-ik, mee-sŏ-/ Theera of PHANEROZOIC time that followed thePALEOZOIC and preceded the CENOZOIC,comprising three periods: the TRIASSIC,JURASSIC, and CRETACEOUS. It lasted about179 million years, from 245 million yearsago to the beginning of the Paleocene, 66.4million years ago. It is often known as theage of reptiles, for it was during this timethat the Reptilia underwent their majorevolutionary radiation, producing such di-verse forms as the dinosaurs, pterosaurs,and ichthyosaurs. Most reptiles became ex-tinct before the end of the era, and themammals, birds, and flowering plants firstappeared. Invertebrate groups importantduring the Mesozoic include the Am-monoidea and Belemnoidea. The Alpineepisode of orogenic activity began at theend of this era. See also geologic timescale.

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metabasite /mĕ-tab-ă-sÿt/ Any meta-morphosed basic rock.

metamorphic differentiation /met-ă-mor-fik/ In medium- to high-grademetamorphic rocks some constituents, es-pecially quartz and feldspar, migrate by adiffusion process into preferred zones,leaving behind micas and amphiboles. Lay-ers of contrasting mineralogical and chem-ical compositions result and constitute aFOLIATION parallel to the SCHISTOSITY.

metamorphic facies A method of clas-sifying METAMORPHIC ROCKS in terms of thechemical composition of their constituentminerals, which is thought to depend onthe temperatures and pressures at whichthey were formed.

metamorphic rock One of the threemain types of rock, formed from one of theother types (sedimentary or igneous) byMETAMORPHISM. Well-known metamorphicrocks include amphibolite, gneiss, hornfels,phyllite, marble, migmatite, quartzite,schist, shale, and slate.

metamorphism /met-ă-mor-fiz-ăm/ Theprocess by which mineralogical and chem-ical changes take place in the solid state inpreexisting sedimentary or igneous rocksas a response to the imposition of newphysical or chemical conditions. Metamor-phic changes take place at temperaturesranging from 100°C to those in excess of600°C, when rocks begin to melt. The min-eral assemblages produced when rockshave attained chemical equilibrium are de-pendent upon the chemical compositionsof the original rocks and the temperatureand pressure conditions prevailing, pro-vided that the metamorphism was iso-chemical and involved no METASOMATISM.

Different types of metamorphism arerecognized: 1. CONTACT METAMORPHISM,i.e. the thermal metamorphism developedin rocks that are intruded by hot magma. 2. Regional or dynamothermal metamor-phism, developed over large areas thathave suffered intense deformation and theregional emplacement of granitic bodies.The temperatures and pressures associated

with regional metamorphism are thoseincurred at depth within the Earth’s crust. 3. Dislocation metamorphism, developedas a result of the mechanical deformationor CATACLASIS of preexisting rocks and lo-calized along shear belts, fault planes, andthrust planes.

Prograde metamorphic assemblages areformed in response to an increase in inten-sity or grade of metamorphism. Retro-grade mineral assemblages are formed inmetamorphic rocks that have become sub-jected to a lower grade as the intensity ofthe metamorphism wanes. See also facies(def. 1); zone (def. 1).

metasomatism /met-ă-soh-mă-tiz-ăm/ Ametamorphic process by which mineralog-ical and chemical changes occur in rocks asa result of interaction with migrating fluidsintroduced from an external source. Meta-somatic changes should be distinguishedfrom those produced by PNEUMATOLYSIS.See fenitization; granitization.

Metazoa /met-ă-zoh-ă/ A group of ani-mals whose bodies are formed of manycells. The Metazoa includes all animals ex-cept the sponges (PORIFERA).

meteoric dust Extraterrestrial fine dustthat reaches the Earth’s surface throughentering the Earth’s gravitational field. Itcontributes to a small degree to marine sed-iments. Some deep-sea cores have beenfound to contain meteoric dust, often in theform of magnetic spherules, perhaps only0.1 to 0.5 mm in diameter. Meteoric dustmay be transported in the atmosphere forconsiderable distances across the oceans,and, if present in significant amounts, maylater be identifiable in certain seabed sam-ples. One estimate suggests that some fivemillion tonnes of meteoric dust fall ontothe Earth’s surface annually. Meteoric dustis probably a small though important partof some deep-sea RED CLAY deposits, andmeteoric spherules have been found to beplentiful in some slowly deposited deep-sealutites.

meteorite A solid body that has fallen tothe Earth’s surface from an extraterrestrial

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meteorite

source. Meteorites are composed of anickel-iron alloy (typically 90% iron and10% nickel) and silicate minerals, mainlyolivine and orthopyroxene. Three maintypes are distinguished: 1. Stony meteoritescontaining mainly silicate minerals plussome nickel-iron alloy. They are dividedinto chondrites and achondrites dependingupon the presence or absence of smallrounded bodies consisting of olivine or or-thopyroxene, termed chondrules. 2. Stony-iron meteorites contain about equalproportions of silicate minerals and nickel-iron alloy. 3. Iron meteorites consistlargely of nickel-iron with only accessorysilicates.Certain nonvolcanic glasses, termed tek-tites, are also thought to be of extraterres-trial origin. These are small bodiescomposed of acid glass and containinglechatelierite. No tektite fall has been wit-nessed and their origin remains controver-sial. See also SNC meteorite.

meteorology The science of the atmos-phere. It is concerned with the physics,chemistry, and movement of the atmos-phere and its interactions with the groundsurface. Some meteorological interest has also developed in studying the atmos-pheres of other planets by applying theprinciples obtained from the Earth’s at-mosphere to different environmental con-ditions. Because the main emphasis ofmeteorology has been on explaining andforecasting surface weather, meteorologyis primarily concerned with the tropos-phere and stratosphere. Higher levels ofthe atmosphere, where different techniquesof data collections are needed, tend to beregarded as part of geophysics. See alsoclimatology; macrometeorology; meso-meteorology; micrometeorology; synopticmeteorology.

miarolitic cavity /mÿ-ă-rŏ-lit-ik/ An ir-regular cavity within a plutonic rock intowhich large euhedral crystals project.Compare druse; vugh.

mica A mineral that has a layered struc-ture in which cations are sandwiched be-

tween sheets of (Si,Al)O4 tetrahedra andhydroxyl ions. The general formula is

X2Y4–6,Z8O20(OH,F)4,

where X = K,Na,Ca; Y = Al,Mg,Fe,Li, andZ = Si,Al. The main mica minerals are:

muscovite K2Al4(Si6Al2O20)(OH,F)4paragonite Na2Al4(Si6Al2O20)(OH)4margarite Ca2Al4(Si4Al4O20)(OH)4phlogopiteK2(Mg,Fe2+)6(Si6Al2O20)(OH,F)4biotite K2( Mg,Fe2+)6–4(Fe3+,Al,Ti)0–2-(Si6–5 Al2–3O20)(OH,F)4

zinnwalditeK2(Fe2+,Li,Al)6(Si6–7Al2–1O20)(OH,F)4lepidoliteK2(Li,Al)5–6(Si6–7Al2–1O20)(OH,F)4

Clintonite and xanthophyllite are relatedto phlogopite by the substitution of cal-cium for potassium.

All micas have a perfect basal cleavage,which reflects their layered structure.‘Books’ of mica, found in pegmatites, areso called because they have the appearanceof piles of pages. The cleavage flakes ofmicas are flexible and elastic. Muscoviteand paragonite are colorless whereas bi-otite and phlogopite are usually darkshades of red, brown, and black. Mus-covite and biotite are common in schists,gneisses, and granitic rocks. Biotite is alsofound in basic and intermediate igneousrocks. Phlogopite occurs in some peri-dotites and metamorphosed limestones.The lithium micas are found almost exclu-sively in pegmatites.

micrite /mÿ-krÿt/ Chemically precipi-tated CALCITE having microscopic grains ofless than 0.01 mm in diameter, present insome LIMESTONES; the terminology used inthe petrographic description and classifica-tion of limestones is based on the presenceof micrite, SPARITE and ALLOCHEMS.

microbiome /mÿ-kroh-bÿ-ohm/ A COM-MUNITY of living organisms that occupies acomparatively small area. Examples in-clude a hedgerow, pond, or wood. See alsobiome; habitat.

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microbreccia /mÿ-kroh-brech-ee-ă, -bresh-/A type of rock consisting of very small an-gular fragments in a finer matrix. Typicalparticle size is less than 0.2 mm. Comparemegabreccia.

microclimate /mÿ-kroh-klÿ-mit/ Theclimate within a few meters of the groundsurface, resulting from the interaction be-tween soil, atmosphere, and vegetation.

microcline /mÿ-kroh-klÿn/ A variety ofalkali FELDSPAR.

microfossil /mÿ-kroh-foss-ăl/ A fossil sosmall that a microscope is required for itsstudy. Microfossils include bacteria, di-atoms, protozoa, some Crustacea, the lar-vae of certain animals, and the isolatedskeletal parts or fragments of organisms.Apart from their taxonomic and evolution-ary interest, microfossils are important inthe correlation of rocks of which onlysmall samples are available, such as thecores from boreholes. The study of micro-fossils is known as micropaleontology.

microgabbro /mÿ-kroh-gab-roh/ See di-abase.

microlite /mÿ-krŏ-lÿt/ A small incipientcrystal that is found in glassy rocks. UnlikeCRYSTALLITES, microlites are sufficientlylarge for their nature to be determinedunder the microscope.

micrometeorology /mÿ-kroh-mee-tee-ŏ-rol-ŏ-jee/ The study of small-scale atmos-pheric processes, usually operating nearthe ground surface. Experiments and re-search in this subject require large numbersof accurate instruments, which result inmost studies being limited to small areasand short duration under ideal conditions.The enormous variety of surface condi-tions results in a complex interaction be-tween the atmosphere and the ground togive multitudinous microclimates; in facteach instrumentation site could be classedas unique. Compare macrometeorology;mesometeorology.

micropaleontology /mÿ-kroh-pay-lee-on-tol-ŏ-jee, -pal-ee-/ See microfossils.

microseism /mÿ-krŏ-sÿ-zăm/ A small ir-regular earth tremor and quavering, whichmakes up the background pattern recordedon seismometers. Such tremors can resultfrom natural sources such as waves break-ing on a beach, or from the passing ofheavy trucks.

Mid-Atlantic Ridge A very large relieffeature forming part of the MID-OCEAN

RIDGE system, which trends north–souththrough the Atlantic Ocean, connected byKerguelen Island and St Paul Island withthe Indian Ocean Ridge. The relief of theelevation above the adjacent ocean-basinfloor is 1 to 3 km, and the width in mostplaces is more than 1000 km. It is essen-tially a wide fractured arch taking up thecenter third of the Atlantic Ocean. Alongthe crest region are the rift mountains andhigh fractured plateaus, jointly making upa strip 80 to 300 km wide. Some of thehigher parts of the ridge rise above sea levelto form islands.

mid-latitude cyclone See depression.

mid-ocean ridge (median ridge) A mas-sive system of submarine elevations thatextends, in global terms, over a distance ofabout 60 000 km, and traverses all of themajor oceans. Its general form is that of abroad arch that stands, on average, 1 to 3km above the adjacent deep-sea floor, andwhich has experienced fracturing along itscrest region. Along much of its length, theridge is more than 1000 km wide. Parts ofthe ridge crest region are gashed by a steep-sided rift feature (see median valley),whereas other fracture zones (some ofthem known as TRANSFORM FAULTS) tend tobe perpendicular to the main axis of theridge. The rift is coincident with a belt ofshallow earthquake epicenters. The floorbeneath the ridge is generally hot and thebasaltic rocks of the ridge-crest and ridge-flank region appear to be quite young ingeologic terms.

In terms of the PLATE TECTONICS theory,such mid-ocean ridges mark the sites of

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constructive PLATE BOUNDARIES where thelithospheric plates are gradually movingapart. It is at these boundaries that basalticmagma from the Earth’s mantle reaches thesurface to emerge as lava and be added tothe oceanic crust in the form of pillowlavas and dikes. Because these eruptionsare relatively frequent by geologic scalesthe area does not have time to adjust im-mediately to regain isostatic balance butsinks as the sea floor spreads away fromthe ridge. See also sea-floor spreading.

migmatite /mig-mă-tÿt/ A coarse hetero-geneous gneissose rock consisting of bandsand patches of quartz-feldspar granitic ma-terial and mafic material consisting mainlyof biotite or hornblende. Migmatites havethe appearance of mixed rocks, comprisingportions that resemble granitic igneousmaterial and mafic portions that are meta-morphic and possess a SCHISTOSITY. Theyare associated with metamorphic rocksthat have formed at the highest grades.Under such conditions, partial meltingtakes place producing mobile granitic ma-terial, which invades and reacts with themetamorphic rocks.

Milankovitch cycles /mi-lank-ŏ-vich/ Atheory of cyclical changes in the Earth’sorbit in correlation with long-term climatechange. The Earth does not revolve in a cir-cular orbit round the Sun at a constant ve-locity, but over a period of many yearschanges take place affecting the amountand distribution of solar radiation receivedby the Earth. First, the tilt of the Earth’saxis of rotation relative to the plane of itsorbit varies between 21.8° and 24.4° overa period of 40 000 years. This affects theseasonality or thermal range between sum-mer and winter. Secondly, the ellipticity oreccentricity of the Earth’s orbit varies overa period of about 100 000 years. This re-sults in a greater seasonal range of radia-tion receipt. When the orbit is at its mosteccentric, there will be a 30% differencebetween aphelion and perihelion, com-pared with 7% at present and none whenthe orbit is circular. Finally, the seasonwhen the Earth is nearest the Sun changesover a period of about 21 000 years. At

present it is nearest in the N hemispherewinter (January 7) and farthest in N hemi-sphere summer (July 7). The effects of theseradiation changes vary from latitude to lat-itude. The variations likely to be producedwere calculated by the Serbian mathemati-cian Milutin Milankovitch (1879–1958) in 1940. It is believed that these cyclicchanges may have great importance for thecommencement of ice ages, but the associ-ation is not as close as might be expected.

millibar /mil-ă-bar/ (mb) One thou-sandth of a bar, a cgs unit of pressure. Be-cause 1 bar is equal to a pressure of 106

dynes per sq cm, one millibar is equal to1000 dynes/cm2 or 100 newtons/m2 (pas-cals). The millibar is still in use in meteo-rology.

milligal /mil-ă-gal/ See gal.

Millstone Grit The middle of the threelithological divisions of the CARBONIFEROUS

System in Britain. It corresponds to thelower half of the Upper Carboniferous (orNamurian) Stage.

mineral Any naturally occurring sub-stance having a definite chemical composi-tion. When a mineral of commercial valueis present in some rock body in economi-cally sufficient quantity to merit extrac-tion, the body constitutes a mineraldeposit.

mineralization /min-ĕ-ră-lă-zay-shŏn/The process by which minerals are formedwithin a rock, resulting in a vein (lode) orother deposit. There are various mecha-nisms by which this may take place, in-cluding replacement (of one mineral byanother), impregnation (of rock by min-eral-laden gases or liquids), and the fillingof preexisting fissures.

mineralogy /min-ĕ-ral-ŏ-jee/ The scien-tific study of minerals. It includes theirclassification and deals with their composi-tion, formation, and physical and chemicalproperties.

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minette /mă-net/ A type of LAMPRO-PHYRE consisting of biotite and orthoclase.It occurs in DIKES and other intrusions.

minimum temperature The lowesttemperature recorded during a given pe-riod either in a Stevenson screen (minimumair temperature) or on the ground (grassminimum temperature). Both temperaturesare normally measured by an alcohol ther-mometer in which the meniscus drags ametal index to the lowest temperature thenleaves it as the column rises when tempera-tures increase. Global minimum tempera-tures are found on the Antarctic ice caps.

Miocene /mÿ-ŏ-seen/ An epoch of theNEOGENE Period extending from the end ofthe OLIGOCENE about 23.7 million yearsago, for 18.4 million years to the beginningof the PLIOCENE. Grasses evolved andspread and, by their effects on the environ-ment, may have contributed to changes inthe mammals, which became more modernin appearance. Miocene mammals in-cluded pigs, deer, horses, rhinoceroses, ele-phants, monkeys, apes, and possiblyhominids. Alpine orogenic movementstook place during this epoch.

miogeosyncline /mÿ-ŏ-jee-oh-sing-klÿn/A geosyncline in which there are no vol-canic products or processes associated withits sedimentation.

mirabilite /mă-rab-ă-lÿt/ (Glauber’ssalt) A yellow mineral form of hydratedsodium sulfate, Na2SO4.10H2O. It crystal-lizes in the monoclinic system, and occursin salt lakes and around hot springs. It isused as a source of sodium sulfate.

mirage An optical phenomenon result-ing from the refraction of light through lay-ers of air having very large temperaturegradients. The rapid change of temperaturevertically produces varying refractive in-dices of the air and so light appears totravel in curved paths. The most commonis the inferior mirage above a heatedground surface. Light is strongly refractedupward near the surface so that the appar-ent pools of water on the ground are, in re-

ality, refractions from the clear sky. A su-perior mirage may occur above a flat sur-face of much lower temperature than theair above, as over ice or a cold sea in sum-mer. Light in this case is bent downwardfrom the object toward the viewer.

misfit stream A stream that is appar-ently too small for its valley. It is charac-terized by meander bends in its channel ofsmaller amplitude and greater intricacythan the meanders in the valley itself. Thelength of the valley meanders is ten timesthat of the current meanders. There is anestablished relation between meanderlength and discharge in streams, and onthat basis it has been calculated that thestreams that eroded the valley meandersmust have had a bankfull discharge80–100 times the current streams. Thisprobably occurred at the end of the ice age,when the glaciers thawed; the periglacialclimate of that period would have reducedvegetation and soils to a minimum, allow-ing all the meltwater to flow off the land tothe rivers, with none lost by infiltrationinto the soil. The resulting torrents cut thecurrent valley.

Misfits could also evolve through thecapture of a stream’s headwaters, reducingits volume, and leaving it a misfit in its val-ley. See river capture.

mispickel /miss-pik-ĕl/ An old name forARSENOPYRITE.

Mississippian /miss-ă-sip-ee-ăn/ A pe-riod within the American classification ofgeologic time that extends from the end ofthe DEVONIAN, about 360 million yearsago, for about 40 million years to the be-ginning of the PENNSYLVANIAN. The Missis-sippian System corresponds approximatelyto the Lower CARBONIFEROUS elsewhere. Itwas named for the Mississippi River Valleywhere the rocks associated with this periodare well exposed.

mist The state of atmospheric obscurity,composed of suspended microscopic waterdroplets and with an atmospheric relativehumidity of at least 95%, in which visibil-ity exceeds 1 km. Visibility is reduced to a

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lesser extent than FOG. The term is alsoused for dense but shallow condensationphenomena at ground level when theground surface may be totally obscured,but hedges or animals can be seen quiteclearly. Compare haze.

mistral /mee-strahl/ A strong cold drynortherly wind that is funneled down theRhône Valley in S France and then blowsacross the delta into the MediterraneanSea. Its frequency and strength have neces-sitated agricultural adaptations to preventcrop damage. These take the form ofhedges or screens oriented east–west toprotect plants. The winds are linked to in-tensification of depressions in the Gulf ofGenoa.

mixed tide Any tide that is not clearly ofthe diurnal or semidiurnal type. Such tidesmight be referred to as intermediate tides.The tidal wave is characterized by a signif-icant inequality in either the low-water orthe high-water levels and a double lowwater and a double high water occur dur-ing each tidal day.

mixing ratio The ratio of the mass of anatmospheric gas to the mass of air withwhich the gas is mixed. It is most fre-quently used for water vapor, the humiditymixing ratio, and is expressed in g kg–1.

mobile belt (orogenic belt; fold belt) Along linear area of the Earth’s crust under-going intense deformation, often accompa-nied by seismic and volcanic activity. Thesestructures originate as deep trenches,which fill up with a thick wedge of sedi-ment. As two continents later collide thiswedge between them is compressed and de-formed to form a mountain chain.

mobile dune A coastal sand dune at thestage when its vegetation (mainly marramgrass) is open, leaving sometimes 50% ormore bare sand subject to wind movementand hence mobile. Their appearance is of aconfused mass of sandy depressions andgrassy knolls superimposed on a profilethat shows a gentle windward slope and asteep lee slope.

mock sun (parhelion) An optical phe-nomenon seen in the sky as a result of therefraction of sunlight by hexagonal icecrystals with vertical axes. This is most fre-quent with cirrostratus clouds. Usually twomock suns are seen, equal distances on ei-ther side of the Sun, and appear as brighterareas of the cirrostratus clouds.

mode The percentage of each of thecomponent minerals contained in an ig-neous or metamorphic rock. Comparenorm.

model A theoretical representation of aprocess, system, or object developed toshow the essential features in a simplifiedcomprehensible form, or to enable predic-tions to be made. See also general circula-tion model.

moder humus HUMUS intermediate be-tween MULL and MOR HUMUS.

Modified Mercalli scale See Mercalliscale.

mofette /moh-fet/ A type of SOLFATARA

that also produces large amounts of carbondioxide. See also fumarole.

mogote /moh-goh-tay/ A large limestonehill that rises on a tropical or subtropicalKARST landscape.

Moho See Mohorovičić discontinuity.

Mohole project /moh-hohl/ An aban-doned project originally proposed in orderto obtain samples of the rocks of the uppermantle. It was hoped to drill down throughthe Earth’s crust to the Mohorovičić dis-continuity. A series of drillings werestarted in 1961 but the project was aban-doned in 1966.

Mohorovičić discontinuity /moh-hoh-roh-vee-chich/ (Moho) The seismicboundary that marks the junction betweenthe base of the Earth’s crust and the top ofthe mantle. This boundary varies in depthbelow the Earth’s surface from 5 to 10 kmbeneath the ocean and between 55 and 70

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km beneath the continents. The boundaryis marked by an increase in the primary (P)wave velocity from 6.9 to 8.2 km per sec-ond. It is named for the Croatian geologistAndrija Mohorovičić (1857–1936).

Mohs’ scale /mohz/ A scale of hardnessdevised to aid the identification of miner-als. Ten reference minerals were chosen,the softest, talc, being assigned a hardnessof 1 and the hardest, diamond, 10. Eachmineral on the scale can be scratched bythose of higher numbers: 1. talc. 2. gyp-sum. 3. calcite. 4. fluorite. 5. apatite. 6. or-thoclase. 7. quartz. 8. topaz. 9. corundum.10. diamond.

Other common substances are useful intesting hardness: finger nail (2.5), coppercoin (3), glass (5.5), and a penknife blade(6.5). The hardness of many mineralsvaries with direction. For example, thehardness of kyanite varies between 5.5 and7. The scale is named for the German min-eralogist Friedrich Mohs (1773–1839).

molasse /mŏ-lass/ A term originally usedby Alpine geologists for a particular se-quence of Miocene sediments. The termhas since been used to denote sequences ofsediments deposited under continental orfreshwater conditions after a period ofmountain-building. Compare flysch.

mold A fossil in which the original skele-tal parts have been dissolved away, leavinga space that preserves their shape. In somecases a mold may preserve features of onlythe outer or inner surfaces of such struc-tures as shells. These are known respec-tively as external and internal molds. Theinfilling of a mold with secondary materialproduces a CAST.

mollisol /mol-ă-sôl/ One of the twelvesoil orders of the US SOIL TAXONOMY. Mol-lisols are soft calcimorphic soils, developedin subhumid to semiarid conditions, char-acterized by a dark strongly structured sur-face horizon, often almost 1 m deep andmore than 50% saturated with bases(mainly calcium), which is termed a mollicepipedon. It includes the CHERNOZEM,CHESTNUT SOIL, PRAIRIE SOIL, RENDZINA,

BROWN EARTH, BROWN CALCAREOUS SOIL,and associated types such as humic gley,and solonetzic soil.

Mollusca /mŏ-lus-kă/ The phylum of in-vertebrate animals that includes snails,mussels, and octopuses. Mollusks are un-segmented creatures having a head, a vis-ceral region, a muscular ventral organ forlocomotion known as the foot, and a man-tle, which generally secretes a hard calcare-ous shell. The geologically importantclasses of this phylum are the GASTROPODA,the BIVALVIA, and the CEPHALOPODA, whichincludes important extinct forms such asthe AMMONOIDEA and BELEMNOIDEA. An-cient mollusks seem to have been marineanimals inhabiting shallow waters andtheir shells are known as fossils from theCambrian Period onward. Some, such asthe Bivalvia in the Carboniferous, havebeen used in stratigraphic CORRELATION.

Mollweide’s projection /mawl-vÿ-dĕz/A MAP PROJECTION showing the entireEarth’s surface on an ellipsoidal base. Themajor axis (Equator) is twice the length ofthe minor axis (central meridian). The geo-graphic parallels are straight lines, as is thecentral meridian; the other meridians arecurved, the curvature increasing towardthe outside limits of the projection. Thedistortion of shapes and directions is there-fore considerable, although the projectionis homolographic and consequently usedfor distributions. This projection is some-times interrupted (see diagram overleaf):each continental area has its own centralmeridian (C.M.), with different ones northand south of the Equator; the interruptionsare in the oceans. The projection is namedfor the German astronomer and mathe-matician Karl Brandan Mollweide (1774–1825).

molybdenite /mŏ-lib-dĕ-nÿt/ A silver-gray molybdenum sulfide mineral, MoS2,found as an accessory in acid igneous rocksand in hydrothermal veins.

monadnock /mŏ-nad-nok/ Any hillstanding up above the surface of a flat orgently undulating plain. These hills may be

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composed of rocks that are more resistantto weathering than those around them; al-ternatively they may be located in formerdrainage divide positions. In the latter casethey will have been subjected to active flu-vial erosion for a shorter period of timethan the rest of the area and it is for thisreason that they remain as upstandingmasses.

monazite /mon-ă-zÿt/ A rare-earth min-eral of composition (Ce,La,Y,Th)PO4

found as an accessory in acid igneous rocksand as a detrital mineral in certain beachsands.

monchiquite /mon-chă-kÿt/ An alkalinetype of LAMPROPHYRE.

monocline /mon-ŏ-klÿn/ A bending ofrock strata produced in sedimentary se-quences that have deformed under condi-tions favoring the development of a normalfault. Two areas of horizontally beddedsediments are left at different elevationsbut still connected by a steeply inclined se-ries of the same beds. See diagram at FOLD.

monoclinic /mon-ŏ-klin-ik/ See crystalsystem.

monomineralic /mon-ŏ-min-ĕ-ral-ik/ De-scribing a rock consisting almost wholly ofone mineral, e.g. dunite, anorthosite.

monsoon A large-scale reversal ofwinds in the tropics (from the Arabic wordfor season) due to differential heating ofcontinent and ocean, but because the circu-lation can be affected by many other fac-tors, the monsoon varies in character fromone area to another. It is best developed inIndia, SE Asia, and China but N Australiaand E and W Africa all show similar windreversals. The summer season is normallythe rainy period of the monsoon withsouthwesterly winds in the N hemisphere,the dry season having winds from the Nsector.

Monsoon Current A seasonal currentthat occurs in the N part of the IndianOcean where the surface currents reflectclosely the seasonal changes in the wind di-rection as the monsoon season develops.The North Equatorial Current is clearlyapparent during February and March, atthe time when the northwest monsoon isblowing. An Equatorial Countercurrent isalso strongly developed, with its axisaligned roughly along latitude 7°S. How-ever, when the southwest monsoon com-mences in August–September, the North

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pole North American C.M. pole Eurasian C.M.

60°

30°

0°

30°

60°

pole

Australian C.M.

pole

South African C.M.South American C.M.

pole

75°

45°

15°

0°

15°

45°

75°

150°150°150° 120°120°120° 90°90°90° 30°30°30° 0°0°0° 30°30°30° 60°60°60° 90°90°90° 120°120°120° 150°150°150°

Mollweide’s interrupted projection

Equatorial Current is replaced by the Mon-soon Current and flows eastward. Thesecurrents are generally located within theuppermost layers of water. See also equa-torial current.

montane /mon-tayn/ Describing a moun-tain or mountainous area, or something as-sociated with it, such as a montane tropicalforest.

monticellite /mon-tă-sell-ÿt/ A calcium-bearing variety of OLIVINE.

montmorillonite /mont-mŏ-ril-ŏ-nÿt/ Atype of CLAY MINERAL consisting of a hy-drated aluminum silicate with the unusualproperty of swelling when it absorbswater. It also undergoes ion exchange of itscalcium, potassium, and sodium ions. It isthe main component of bentonite andfuller’s earth.

Montreal Protocol (Montreal Protocolon Substances that Deplete the OzoneLayer) An international agreement de-signed to regulate ozone-depleting sub-stances and protect the atmospheric ozonelayer (see ozone). The treaty was drawn upin Montreal in 1987 by the United NationsEnvironment Program (UNEP), and cameinto force in 1989 when ratified by most ofthe industrialized nations. It sought a 50%reduction in the use of chlorofluorocar-bons (CFCs) by 1992, relative to 1986 lev-els. Subsequent amendments (London,1990; Copenhagen, 1992; Vienna, 1995;Montreal, 1997; and Beijing, 1999) haveintroduced other types of control measuresand added new controlled substances tothe original list.

monzonite /mon-zŏ-nÿt/ See syenodior-ite.

moonstone A bluish or silvery irides-cent type of orthoclase (an alkaliFELDSPAR), used as a semiprecious gem-stone. See also sunstone.

moraine /mŏ-rayn/ A depositional fea-ture composed of glacial TILL, which maybe in association with active ice or de-

posited by former glaciers and ice sheets.This material may be transported on thesurface of the ice (supraglacial), within theice (englacial), or beneath the ice (sub-glacial). See ground moraine; lateralmoraine; medial moraine; push moraine;terminal moraine.

mor humus Acid HUMUS, which formsin wet lime-deficient environments, e.g.podzolic conditions. Most of the organicbreakdown is achieved by the fungi in thesoil but it is a slow process with the resultthat the L, F, and H layers (see profile) areclearly differentiated. Compare mullhumus.

morphogenetic zone /mor-fŏ-jĕ-net-ik/ (in climatic geomorphology) An areaof the Earth in which climate and relief aredistinctive, through the control of climateon geomorphic process and the control ofprocess on landform. These zones runroughly parallel to major climates, whichconform to latitude. Early attempts to di-vide the Earth into such zones used verysimple climatic data (mean annual rainfalland mean annual temperature) but modernattempts use much more sophisticated pa-rameters. The French geographers JeanTricart and André Cailleux identified fourmajor world zones, running from a coldzone in the north, through a forested zoneand subarid zone to the equatorial tropicalzone; within each, climates and landformsare in sympathy through the link of pro-cess.

morphological species See species.

moss agate A type of CHALCEDONY thathas fernlike inclusions of black dendriticPYROLUSITE (manganese dioxide).

moss animal See Bryozoa.

mother-of-pearl See nacre.

mountain wind A form of KATABATIC

WIND in mountainous areas where, afternight-time radiational cooling, strongbreezes are concentrated in the valleys asthe cold air has subsided downslope.

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mud Sediment having a preponderanceof particles measuring less than 0.06 mm indiameter. Under natural conditions, mudsgenerally form very plastic masses com-prising sediment particles mixed withwater. The term applies to both silty andclaylike particles. Mud may be deposited,according to prevailing conditions, in bothshallow water and deep-sea areas. Shel-tered environments are particularly suscep-tible to mud deposition, as for example insheltered lagoons and estuaries, and alonglow-energy beaches and tidal flats. Oncepartly consolidated into mud banks, thematerial tends to be remarkably cohesiveand therefore resistant to scour; many mudbanks are eroded primarily through side-scour because of the gradual migration oftidal channels.

mud crack See desiccation crack.

mudflat A low area of fine silt that liesalong the shore of an estuary or on the leeside of an island. It supports no vegetationand is generally covered and uncovered bythe tide. Various types of arthropods, mol-lusks, and worms live in the mud, and as aresult mudflats are important feedinggrounds for several species of wadingbirds.

mudflow See earthflow.

mudrock See mudstone.

mudstone (mudrock) An ARGILLACEOUS

sedimentary rock that is less fissile alongbedding planes than SHALE.

mud volcano See sand volcano.

mugearite /moo-jee-ă-rÿt/ An alkalinebasalt. See trachybasalt.

mull humus Alkaline HUMUS, whichcommonly forms in a mild, moist, andbase-rich environment, e.g. chernozemconditions. It is rapidly incorporated intothe soil by the abundant soil fauna, partic-ularly earthworms. Compare mor humus.

mullion A tectonically produced struc-

ture in metamorphic and sedimentaryrocks that resembles a rod or column.

mullite /mul-ÿt/ A colorless mineral formof ALUMINUM SILICATE, approximate for-mula Al6Si2O13. It crystallizes in the ortho-rhombic system and is used as a refractory.

multiple intrusion An intrusion of ig-neous rock (see intrusive rock) consistingof several masses of similar composition.Most DIKES are multiple intrusions.

multiple reflection A seismic wave thathas been reflected more than once and as aresult may be recorded more than once ona seismic profile.

multispectral satellite imagery Seesatellite.

muscovite /mus-kŏ-vÿt/ One of themajor types of MICA, K2Al4(Si6Al2O20)-(OH,F)4. It crystallizes in the monoclinicsystem as colorless, green, or pale brownplates and occurs in various igneous rocks,mainly pegmatites. It is used as an electri-cal insulator and lubricant.

muskeg /mus-keg/ A type of BOG that oc-curs in TUNDRA areas. It forms when thesurface permafrost melts in summer, andgenerally supports lichens and mosses,such as sphagnum moss. The best-knownmuskegs are found in N Canada.

mylonite /mÿ-lŏ-nÿt/ A rock formedduring dislocation metamorphism by ex-treme granulation and shearing. Mylonitesare banded or streaky rocks and may con-tain augen of undestroyed crystals or rockfragments in a fine-grained matrix. Theyare commonly found at thrust planes alongwhich extensive movements have takenplace. See cataclasis; pseudotachylite.

myrmekite /mer-mĕ-kÿt/ Wormlike in-tergrowths of quartz in plagioclasefeldspars produced as a result of the re-placement of potassium feldspar bysodium feldspar, during which excess silicais liberated.

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nacre /nay-ker/ (mother-of-pearl) A lus-trous iridescent coating on the inside of theshells of various mollusks, such as oysters.It consists mainly of calcium carbonate,CaCO3, and is the substance that PEARLS

are made of.

NAO See North Atlantic Oscillation.

nappe A large tectonic feature that owesits origin to a combination of folding andthrusting. Nappes are basically large hori-zontal recumbent folds that have traveledfor many tens of kilometers along thrustplanes. They are well developed in theAlps, where they were first described.

native Denoting the state of occurrenceof certain elements or minerals when theyare uncombined with other elements orminerals.

natrolite /nat-rŏ-lÿt/ A colorless, white,or yellow mineral form of hydrated sodiumaluminum silicate, Na2(Al2Si3O10).2H2O.It crystallizes in the orthorhombic systemas needlelike crystals, usually in cavities inbasaltic rocks. It is a member of the ZEO-LITE group of minerals.

natural arch An archway on an ex-posed headland that results from erosionby wave action. Generally, a cave in chalkor limestone rock becomes enlarged so thatit cuts through to the other side of theheadland.

natural bridge A bridge of rock formedin limestone by erosion. It generally resultswhen the roofs of two underground cav-erns situated close to each other collapse,leaving a bridge of rock between the result-ing holes in the ground.

natural gas A mixture of flammable hy-drocarbon gases – mainly ethane andmethane – that occurs in underground de-posits (often below the sea bed), eitheralone or in association with oil. It isthought to have formed from the remainsof microscopic organisms that becameburied and compressed in undergroundsediments. It is an important FOSSIL FUEL.

natural selection The process proposedby the English naturalist Charles RobertDarwin (1809–82) in 1859 to explain theEVOLUTION of organisms from one speciesto another. It attempts to account for theorigin and diversity of all organisms. Plantand animal species produce relatively largenumbers of offspring that differ in variousways. If an organism possesses certain vari-ations that make it better suited to its envi-ronment, this organism is more likely tosurvive and reproduce. If these variationsare inheritable the likelihood of its off-spring also possessing them is increased. Inthis way, and over many generations, radi-cal changes may arise.

nautical mile A distance equal to 1minute of longitude along a GREAT CIRCLE,equivalent to about 1853 m. A speed of 1nautical mile an hour is a KNOT.

Nautiloidea /naw-yă-loi-dee-ă/ An al-most extinct subclass of mollusks of theclass CEPHALOPODA. The external shell ofthe nautiloids is similar to that of the re-lated AMMONOIDEA, except that in fossilforms it is straight or only slightly coiledand in all types the suture lines are simplecurves, showing none of the complex con-volutions characteristic of the Am-monoidea. The oldest fossils come fromupper Cambrian rocks and their shells

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N

sometimes reached a length of up to 4.5 m.These straight-shelled forms were extinctby the end of the Triassic Period. Thegroup as a whole declined from the Devon-ian Period onward; very few genera existedduring the Mesozoic and only one, Nau-tilus, has persisted to the present day.

neap tide A TIDE of relatively smallrange that occurs near the time of quadra-ture of the Moon (i.e. near the time of theMoon’s quarters). The tidal range fallsbelow the average range by about 10 to30%. Because of this low range, the tidalcurrent speeds during a neap tide are sig-nificantly less than those attained during aSPRING TIDE.

nearshore circulation system The sys-tem of currents, notably those induced di-rectly or indirectly through wave activity,that operate within and closely adjacent tothe SURF zone. These currents include theshoreward-directed MASS TRANSPORT CUR-RENTS beneath waves, the wave-inducedLONGSHORE CURRENTS, the powerful sea-ward-flowing RIP CURRENTS, and other cur-rents associated with the expandingrip-head zones. Superimposed on thesecurrents there is invariably a reversing tidalcurrent, and, on those occasions whenlocal winds are blowing, wind-inducedcurrents. In the vicinity of rivers and estu-aries, there may also be density-inducedcurrents. The relative importance of thevarious types of current varies both spa-tially and temporally.

nebular hypothesis An idea proposedby the Marquis de Laplace (1749–1827) in1796 in which he suggested that the ma-terial now forming the Sun and planetsoriginated as a disk-shaped nebula or gascloud, which contracted into discrete bod-ies.

neck (plug) An erosional remnant repre-senting the former conduit that fed a vol-cano. This conduit was filled with lava andpyroclastic material that was more resis-tant to erosion than that of the enclosingvolcanic cone, which has since been erodedaway.

needle ice Needles of ice, often severalcentimeters long, growing beneath the sur-face layer of debris and able to lift frost-shattered rock fragments and soil particlesperpendicularly from the ground surface.On melting of the needles (often called pip-krakes), the fragments are deposited a littleway downslope. Material can progressconsiderable distances and, as the processtends to be more effective in coarser ma-terial, it also achieves a certain amount ofsorting. Needle ice occurs in periglacial en-vironments, most effectively where there isdiurnal freezing and thawing.

negative gravity anomaly See gravityanomaly.

negative movement of sea level A riseof the land relative to the sea due either toan actual rise of the land through tectonicmovements, or a fall in sea level throughthe locking up of water in the form of ice,as happened in the Pleistocene Period.Whatever the cause, more land is created,and the relative relief of the land, from sealevel to mountain tops, is increased. Themajor result is the extension of river sys-tems out across former sea-bed areas andtheir REJUVENATION. Raised beaches, ma-rine beaches, knickpoints, river terraces,and incised meanders are all evidence ofpast negative movements. See also positivemovement of sea level.

nekton /nek-tonn/ (nektonic or-ganisms) Pelagic organisms that swimand move actively, such as fish. Compareplankton.

nematath /nem-ă-tath/ A series of volca-noes that form as one of the Earth’s LITHOS-PHERIC PLATES (tectonic plates) movesslowly across a thermal center (see hotspot). The farther a volcano is from thethermal center, the older it is. The youngestvolcanoes are generally still active, whereasthe older ones may be dormant.

Neogene /nee-ŏ-jeen/ The latter periodof the CENOZOIC Era, comprising theMIOCENE, PLIOCENE, Pleistocene, andHolocene Epochs. It began at the end of the

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OLIGOCENE Period, 23.7 million years ago,and extends to the present. Historically,the QUATERNARY has been widely consid-ered to be a period, equated by some au-thorities with the Pleistocene and HoloceneEpochs, but it had never been ratified as achronostratigraphic division within the in-ternational geologic time scale. It has sincebeen suggested that the Quaternary shouldbe considered as a sub-era.

nephanalysis /nef-ă-nal-ă-siss/ The analy-sis of cloud patterns on weather satellitephotographs. The photographs cover largeareas of the Earth’s atmosphere and sur-face and new techniques were required todescribe and analyze the larger features ofcloud patterns compared with the appar-ently random distribution seen by the sur-face observer. On a nephanalysis thefollowing information can be portrayed:cloud elements, cloud masses, cloud pat-terns, cloud systems, cloud bands, cloudtypes (stratiform, cumuliform, cirriform,cumulonimbus, or stratocumuliform), per-centage cloud cover, cloud boundaries, andsynoptic interpretations of these features.

nepheline /nef-ĕ-lin/ A major member ofthe FELDSPATHOID group of minerals.

nephelinite /nef-ĕ-lin-ÿt/ A strongly un-dersaturated basic to ultrabasic feldspar-free volcanic rock, having a plutonicequivalent within the IJOLITE-MELTEIGITE

series. Nephelinites consist of NEPHELINE

which may occur both as euhedral phe-nocrysts and as anhedral groundmassgrains, together with a variety of maficminerals, typically titanaugite, biotite,olivine, and magnetite.

Assemblages of LEUCITE plus mafic min-erals constitute the leucitites. Ugandite isan augite-rich olivine-leucitite. Melilititesare composed of melilite plus mafics andoften contain small amounts of nephelineand calcite in addition. Alnoite is a dikerock largely composed of melilite and bi-otite together with pyroxene, calcite, andolivine. Mafurite is an olivine-kalsiliterock.

Nephelinites and leucitites are composi-tionally related to phonolites and leucito-

phyres. These volcanics are restricted tostable continental areas and occur in asso-ciation with ijolites and carbonatites.

nephrite /nef-rÿt/ A tremolitic AMPHI-BOLE. See also jade.

neptunian dike A roughly vertical bodyof sediment, usually sandstone, that has in-filled a fissure in a preexisting rock bodyexposed at the Earth’s surface or on the seafloor.

neritic /ni-rit-ik/ Describing the shallow-water marine environment lying betweenlow-tide level and a depth of some 200 m.The environment rarely covers a distancefrom the shore in excess of several hundredkilometers; on a global scale, it coversroughly 30 million sq km or about 10% ofthe total ocean area. The environment ischaracterized by very diverse sediments (ofwhich fine to coarse terrestrially-derivedsediments mixed with what are mostly cal-careous organic remains tend to predomi-nate) and BENTHOS, and a wide array ofenvironmental conditions, as for examplein the degree of light penetration. Thewhole of the sea floor in the neritic zoneprobably experiences some measure ofwave disturbance and almost certainlytidal current action.

net radiation The difference in valuebetween all downward incoming short-wave radiation and upward outgoing long-wave radiation fluxes at the Earth–atmosphere interface.

neutron logging A subsurface loggingtechnique used in uncased boreholes torecord the presence of fluids. When used inconjunction with a gamma-ray log it can beused to calculate the porosities of the for-mations encountered. It measures the in-tensity of radiation in the borehole after ithas been exposed to a radioactive source.

névé /nay-vay/ 1. See firn.2. The area in which firn exists; firn field.

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New American Comprehensive SoilClassification See soil taxonomy.

New Red Sandstone A succession ofred sandstones forming the continental fa-cies of the PERMIAN and TRIASSIC Systems inBritain.

niccolite /nik-ŏ-lÿt/ (nickeline) A cop-per-colored mineral form of nickel ar-senide, NiAs. It crystallizes in thehexagonal system, and occurs associatedwith ores of copper and silver. It is used asa source of nickel.

niche /neesh/ (in ecology) The place orstatus of an organism in its COMMUNITY orECOSYSTEM. The niche can be defined interms of its moisture and temperature and,from the organism’s point of view, theavailability of food and the presence ofcompetitors, and how well adapted it is tolive there. No two species can occupy ex-actly the same niche, or they would be indirect competition all the time.

nickeline /nik-ĕ-lin/ See niccolite.

nimbostratus /nim-boh-strat-ŭs/ Themain rain-bearing cloud of temperate lati-tudes. It consists of a gray cloud layer ofappreciable depth beneath which are fre-quently low ragged clouds, called FRACTO-STRATUS clouds or scud. They are mostoften associated with low-pressure areasand precipitation may be prolonged, al-though it is not usually heavy.

niter /nÿ-ter/ (saltpeter) A colorless orwhite mineral form of potassium nitrate,KNO3. It is found as a surface deposit incaves and in dry regions. It has many uses:in curing meat; in making glass, pyrotech-nics, and explosives; and as a fertilizer. Seealso Chile saltpeter.

nitrification /nÿ-tră-fă-kay-shŏn/ Theconversion of nitrogen by bacteria to aform usable by higher plants. When ma-terial decays, ammonia is often liberatedand this is toxic to many plants. Some bac-teria, namely Nitrosomonas, can oxidizeammonia to nitrite and in turn it may fur-

ther be oxidized, by Nitrobacter, to nitrate,which can be used by plants. DENITRIFICA-TION, a conversion of nitrates back to ni-trogen gas, most of which is unusable, mayoccur when soils become poorly aeratedand acid. See nitrogen cycle.

nitrogen /nÿ-trŏ-gĕn/ The most abun-dant gas in the atmosphere and a criticalconstituent in the soil, which can only beused directly by a few specialized bacteria.To be of widespread value it has to be con-verted into the nitrate form (see nitrifica-tion). In nature, nitrogen is involved incyclic changes termed the NITROGEN CYCLE.

nitrogen cycle The series of chemicalreactions by means of which nitrogen cir-culates through the global ECOSYSTEM. Ni-trates and other inorganic nitrogencompounds in the soil are taken up byplants and turned into proteins and otherorganic nitrogen compounds. These areeaten by herbivores, which are in turneaten by carnivores. The excreta and deadbodies of all these animals returns nitrogento the soil, where decomposers (such asfungi and bacteria) reconvert it to an inor-ganic form. Some bacteria that live in thesoil cycle nitrogen between the atmosphereand plants. See nitrification; nitrogen fixa-tion.

nitrogen fixation The absorption of at-mospheric nitrogen by some heterotrophicbacteria. There are two groups: the non-symbiotic, e.g. Azotobacter, which makethe nitrogen part of their tissue so thatwhen they die it is made available by nitri-fication; and the symbiotic bacteria, e.g.Rhizobium, which form nodules on theroots of leguminous plants and pass ni-trogenous compounds on to their host.

nivation /nÿ-vay-shŏn/ (snow patch ero-sion) Any of a number of processes asso-ciated with a patchy snow cover overunconsolidated rocks, such as glacial de-posits. The most important erosional pro-cess is FREEZE-THAW, the role of the snowbeing to supply meltwater to assist in theprocess. The thickness of snow is an im-portant factor, because if it is too thick it

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will tend to protect surface materials fromthe effects of atmospheric freeze-thaw cy-cles. The most active nivation takes placearound the edges of snow patches. Thepresence of PERMAFROST greatly increasesefficiency since it stops the meltwater soak-ing away. In order to achieve any erosion,weathered material must be removed, andconsequently the process is most efficienton slopes, where SOLIFLUCTION and melt-water runoff are the principal transporta-tion agents. The characteristic formproduced by nivation is the NIVATION HOL-LOW.

nivation hollow A hemispherical hol-low of variable size produced by NIVATION

processes. These hollows can be describedas transverse, longitudinal, or circular, de-pending on the shape of the associatedsnow patch and its relationship with theslope. The downslope part of the hollow isgenerally flat or gently inclined, while theupslope part is steeper. It is considered thatthis backslope recedes gradually upslope,increasing in height, and that in this waythe hollows may develop into cirques, asmore and more snow can be accommo-dated, and FIRN begins to form.

noctilucent clouds /nok-tă-loo-sĕnt/Clouds observed in the high atmospherewhen it is almost dark at ground level.Sometimes they show brilliant colors, al-though they more usually have a bluish-white or yellow appearance. Almost allother clouds are limited to the troposphere,but the height of noctilucent clouds hasbeen shown to be about 80 km, sometimesreaching speeds of 300 knots. Their truenature and origins are not completely un-derstood, but they are believed to consist ofice particles, saturation being reachedthrough orographic wave development res-onated from the surface.

nodule 1. A rounded concretion in sedi-mentary rocks.2. See inclusion.3. A small lump containing nitrogen-fixingbacteria on the roots of certain plants, par-ticularly legumes. See nitrogen fixation.

nonconformity (heterolithic unconfor-mity) An unconformable contact be-tween overlying younger sedimentaryrocks, and underlying older igneous ormetamorphic rocks.

nondipole field /non-dÿ-pohl/ Thatsmall part of the Earth’s magnetic field thatis superimposed upon the main DIPOLE

FIELD. This small additional component isresponsible for the irregularities of theEarth’s magnetic field. It is irregular in itsdistribution and is different in each hemi-sphere; as a result the Earth’s magnetic dippoles are not antipodal, the north magneticdip pole being at 75°N, 101°W, while thesouth magnetic dip pole is at 67°S, 143°E.

nonsequence /non-see-kwĕns/ A shortbreak in sedimentation, causing a type ofunconformity detectable only by paleontol-ogical techniques.

nordmarkite /nord-mark-ÿt/ A memberof the SYENITE group of minerals.

norite /nor-ÿt/ A coarse-grained basic ig-neous rock consisting mainly of orthopy-roxene and plagioclase. See gabbro.

norm A chemical analysis of an igneousor metamorphic rock expressed in terms ofstandard minerals (the normative miner-als), the proportions of which form a basisfor comparison and classification. Thenorm is distinct from the MODE, which isthe actual mineral composition of a rock. Aquantitative classification for igneousrocks was devised in 1903, based upon anormative calculation by the Americanpetrologists W. Cross, J. P. Iddings, L. V.Pirsson, and H. S. Washington, and termedthe CIPW classification.

normal 1. (in climatology) Denoting theaverage value of any climatological ele-ment over a specified time period. Theusual time period is 30 years, which is be-lieved to be sufficiently long to average outyear-to-year variations but not longenough to contain trends in the value of theelements, such as temperature or precipita-tion.

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2. (in statistics) Denoting a distribution ofdata that corresponds to an accepted fre-quency distribution about a populationmean.

normal fault (gravity fault) A type offault in which the predominant displace-ment is downward in the same direction asthe inclination of the fault plane. This re-sults from a stress configuration in whichthe principal maximum stress is verticalwith the other two principal stresses (min-imum and intermediate) being horizontal.See fault.

North Atlantic Drift The portion ofthe major surface current in the North At-lantic that is an extension of the GULF

STREAM current. The flow of the North At-lantic Drift is generally in a northeasterlydirection. It tends to be more diffuse thanthe Gulf Stream, being characterized byseveral shifting bands of water. Each banddisplays large meander flows that appear,disappear, and reappear in a constantly al-tering pattern of flow patterns. Large ed-dies are often thrown off the main stream.In its more easterly portion, the North At-lantic Drift bifurcates, one stream movingnorth-northeastward toward the PolarSeas, the other stream (the Canary Cur-rent) moving east-southeastward. Being arelatively warm current, especially on ac-count of being partly replenished at depthby fairly warm water from the Mediter-ranean Sea, it has an important ameliorat-ing influence on the climate of northwestEurope’s ocean flank.

North Atlantic Oscillation (NAO) Alarge-scale pattern of fluctuations in thepressure systems over the North Atlanticthat is most marked in winter. It has signif-icant influence on the weather across theAtlantic and surrounding continents, espe-cially of Europe, but also affects the E coastof the USA. During what is known as thehigh (or positive) NAO index phase thereis a large difference in pressure between thelow-pressure system around Greenlandand Iceland (the Icelandic Low) and thehigh-pressure over the Azores (the AzoresHigh). The strong northerly orientation

takes depressions into NW Europe bring-ing mild wet conditions, while the Mediter-ranean has dry conditions. In a low (ornegative) NAO index phase there is a re-duced pressure gradient between a weakAzores High and Icelandic Low resulting inan east–west orientation in which depres-sions take a southerly route across the Iber-ian peninsula bringing wet conditions tothe Mediterranean; N Europe experiencescold dry conditions. The NAO changes be-tween high and low index phases at ran-dom, but usually shows either apredominantly positive or negative averagefor the year. It is closely related to the ARC-TIC OSCILLATION.

North Equatorial Current See equato-rial current.

northern lights See aurora.

northing /nor-thing, -thing/ Any of theeast–west grid lines on a map, quoted afterthe EASTING when coordinates are beinggiven, showing distance north from the ori-gin of the grid.

nosean /noh-see-ăn/ A member of thesodalite group of FELDSPATHOIDS.

nucleation /new-klee-ay-shŏn/ 1. (accre-tion) A process by which it has been sug-gested that continents grow, younger foldbelts being added to their margins. This isconsistent with the present theory of platetectonics because these fold belts originatein the deep trenches bordering continentsthat are overriding the adjacent oceanfloor.2. The first stage in precipitation and thegrowth of crystals in a melt.

nucleus (in meteorology) Any minutesolid particle suspended in the atmosphere.Nuclei have been classified on the basis ofsize into Aitken nuclei (smaller than 0.2µm), large nuclei with radii between 0.2and 1 µm, and giant nuclei with radiigreater than 1 µm. The Aitken nuclei aremost abundant in the atmosphere, reach-ing about 10 000 per cm3, but there are farmore in industrial areas than over the

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oceans. The nuclei are named for the Scot-tish physicist and meteorologist JohnAitken (1839–1919). Identification of thenuclei is very difficult, even with electronmicroscopes, unless they possess a distinctcrystalline form.

Some nuclei have an attraction forwater and are known as hygroscopic; oth-ers are nonhygroscopic. This is very impor-tant in enabling nuclei to act as favorablepoints for CONDENSATION as part of theprecipitation process. There are undoubt-edly sufficient such nuclei in the atmos-phere because condensation always takesplace as soon as saturation is reached.Compare freezing nucleus.

nuée ardente /new-ay ar-dahnt/ An in-candescent cloud of gas and volcanic frag-ments emitted during certain types ofvolcanic eruptions. See pyroclastic rock.

numerical weather prediction (NWP;numerical forecasting) The productionof weather forecasts through the use ofcomplex numerical models. Numericalweather forecasting was first propoundedin the early 20th century by the Norwegianmeteorologist Vilhelm Bjerknes (1862–1951) and his associates, and developed bythe British mathematician Lewis FryRichardson (1881–1953). With the adventof computers, especially from the 1950s,computer models were developed thatcould be applied to the production ofweather forecasts. Modern numericalweather prediction features feeding obser-vations of the current state of the atmos-phere (obtained from sources that includenetworks of land stations, buoys, ships,aircraft, and satellites) into powerful com-puters and using sophisticated computermodels in order to calculate the weather

forecast as accurately and quickly as possi-ble.

nummulites /num-yŭ-lÿt/ Benthonic ma-rine protozoa forming a family of theFORAMINIFERA. Their skeleton, up to 80mm in diameter in some species, consists ofa smooth flattened disk-shaped spiral di-vided into many separate chambers. Num-mulites are known from the CretaceousPeriod but were most abundant in theEocene and Oligocene Epochs; they areused locally in stratigraphic correlation.

nunatak /noo-nă-tak/ A rock peak stick-ing out above the surface of an ice sheet. Inmany instances these peaks were formerlycovered with ice and only subsequent re-duction in the extent of ice cover hasbrought about their emergence.

nutation /new-tay-shŏn/ The small os-cillation superimposed upon the preces-sional motion of the Earth. This resultsfrom the gravitational attraction on theEarth’s equatorial bulge from the Sun andMoon. This attraction varies continuouslyas the Sun and Moon change their posi-tions with respect to the Earth.

nutrient cycle (in ecology) The transferof nutrients from one stage in an ECOSYS-TEM to another stage. For example, withina forest leaves that fall to the ground rotand are broken down by bacteria to pro-vide nutrients in the soil, which are neededfor the growth of plants. Green plants usethe energy of sunlight (in PHOTOSYNTHESIS)to make more tissue, which provides foodfor herbivores (primary consumers). Theherbivores, in turn, provide food for carni-vores (secondary consumers). See also bio-geochemical cycle.

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nutrient cycle

oasis A place in a dry landscape (such asa desert) that has water and so can supportvegetation. The water may come from ariver that flows across the desert on its wayto the sea, or there may be a well for rais-ing groundwater to the surface. Naturaloases have long been sites of human settle-ment.

obduction In plate tectonics, the pro-cess by which OCEANIC CRUST is forced overthe edge of and incorporated into CONTI-NENTAL CRUST at a destructive PLATE

BOUNDARY. See also subduction zone.

objective analysis (in meteorology) Amethod of interpreting a two-dimensionalscalar field of some meteorological ele-ment, usually pressure, so that the resultsfrom a particular data set are independentof human subjectivity. The usual way is byfitting a two-dimensional surface to thedata by means of a least-squares method,which is a statistical process in which thesum of the squares of the difference be-tween actual and predicted values is re-duced to a minimum.

obsequent stream A stream that flowsin the opposite direction to a CONSEQUENT

STREAM, often against the direction of dip.In scarpland areas, the obsequent streamsdevelop after the consequent and subse-quent pattern has dissected the strata toproduce the characteristic sequence oflimestone and sandstone scarps, with clayvales; obsequent streams will flow downthe scarp faces, tributary to the subsequentmaster streams.

obsidian /ob-sid-ee-ăn/ A black, brown,or red lustrous type of volcanic glass witha conchoidal fracture, derived from rapidly

cooled rhyolitic lava (see rhyolite). It issometimes used as a semiprecious gem-stone.

occlusion (occluded front) A front thatdevelops during the later states of the evo-lution of a DEPRESSION when the air of thewarm sector is no longer at the ground sur-face. As the cold front normally travelsmore quickly than the warm front, itslowly reduces the area of the warm sectoruntil it merges with the preceding front tocomplete the occlusion process. The occlu-sion is therefore a compound zone withwarm and cold front characteristics. If theair behind the original cold front wascolder than that ahead of the originalwarm front it is known as a cold occlusion,and if warmer, as a warm occlusion. Thefrontal structure of the occluded system isshown in the diagram. In reality individualocclusions are more complex. Sometimes,as the depression moves eastward alongthe occlusion, the line of airmass contrastto the north of the depression swings coun-terclockwise giving a further period of

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subsequentstream

clay

vale

obsequentstream

scarp

consequent stream

dip slope

Obsequent, subsequent, and consequentstreams

rain. This is known as a back-bent occlu-sion.

ocean The water mass (excluding lakesand seas completely surrounded by land)that covers part of the surface of the Earth.The water mass as a whole constitutes theHYDROSPHERE. The interface between thehydrosphere and the atmosphere is thusthe sea surface, while the interface betweenthe hydrosphere and the lithosphere is thesea bottom. The DEEP OCEAN lies beyondthe shelf edge or shelf break.

ocean basin floor That part of theocean floor that extends from the CONTI-NENTAL MARGIN on the continental flank tothe MID-OCEAN RIDGE on the oceanic flank;together these constitute the three majormorphological divisions of the ocean floor.The ocean basin floor has been further sub-divided into a number of physiographicprovinces, two of which (the elevatedoceanic rises and the very deep abyssalfloor zones), are of particular importance.The ocean basin floor occupies very exten-sive areas: approximately a third of the At-lantic Ocean floor, a third of the IndianOcean floor, and three quarters of the Pa-cific Ocean floor. The ocean basin floor is

generally quite flat but also has volcanicfeatures that include SEAMOUNTS (either oc-curring individually or in large groups),and its linear-type archipelagos. It also fea-tures the deep-sea TRENCHES, the deepestparts of the ocean.

ocean current A distinct flow of waterin the ocean. Almost all the water in theocean is in a constant state of movement,true stagnation occurring only at the bot-tom of certain deep basins hemmed in byrock sills. Generally, currents in the oceansoccur at fairly low speeds, especially withinthe deep water masses, but there are alsowell-defined currents (such as part of theGulf Stream) that may flow as rapidly andstrongly as some of the largest rivers onland. Neglecting those currents that flowwithin the surf zone, ocean currents resultfrom three particular sets of circumstances:wind stresses acting on the sea surface,tidal motion, and differences in the densityof seawater. The density differences arisebecause of differential heating and cooling,differential salinity (perhaps because ofevaporation, ice melting, river discharge,and so on), and changes in turbidity levels.The wind-induced ocean currents may bedirectly or indirectly driven.

ocean floor The whole of the seabedseaward of lowest low-water mark, includ-ing the continental shelf, continental slope,and deep-sea floor. It is characterized bydiverse relief forms, some of them of im-mense size. Significant tracts of the deep-sea floor are remarkably flat and smooth.In spite of considerable depths of water inthe ABYSSAL ZONE, most of the ocean floorexperiences some current action. In globalterms, the ocean floor is vast: some 360million sq km out of a total of some 510million sq km for the Earth’s surface as awhole. Ocean floor occupies about 61% ofthe N hemisphere and 81% of the S hemi-sphere. In general, the ocean floor archesupward rather than downward. Truly con-cave basins are rare.

oceanic crust That part of the Earthabove the Mohorovičić discontinuity form-ing the floor of the ocean basins. It is usu-

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cool air

cold air

warm air aloft

warm air aloft

cold air

cool air

Fig. 1: Warm occlusion

Fig. 2: Cold occlusion

Occlusions

ally up to 15 km thick and consists of threelayers: layer one composed of young andpoorly consolidated sediments, layer twocomposed of basalt pillow lavas and dol-erite dikes, and layer three composed ofdolerite dikes and gabbro. The ocean crustis generated at the constructive plateboundaries and destroyed at the destruc-tive plate boundaries. See also continentalcrust.

oceanicity /oh-shă-niss-ă-tee/ The de-gree of oceanic influence on the climate ofan area. This is indicated by low annualand diurnal ranges of temperature, andabundant precipitation throughout theyear.

oceanite /oh-shă-nÿt/ A basaltic rockcontaining a high proportion of olivinephenocrysts. Compare ankaramite.

oceanography /oh-shă-nog-ră-fee/ Thestudy of the oceans, with particular refer-ence to their overall form, the nature of thesea floor and associated sediments, thecharacteristics of the ocean waters, and thetypes of fauna and flora living within theoceans. This field has widened its scope toembrace aspects of human intervention inthe ocean environment since the oceanshave assumed such importance economi-cally and strategically. It is a multidiscipli-nary subject, embracing physics,mathematics, biology, and several others.

ocean trench See trench.

ocean wave A wave affecting ocean wa-ters. Ocean waves of some kind are operat-ing all the time, some affecting the surfaceof the sea, others the internal water masses.Some waves are generated by wind blow-ing over the sea’s surface, often travelingfar beyond the generation area as SWELL.Large waves with long periods may spreadfrom storm centers and travel thousands ofkilometers before affecting distant coasts.Some waves or wave energy may be re-flected, or generated, in a seaward direc-tion, as with SURF BEATS or the occasionalcatastrophic landslide surges (see tsunami).tsunami waves are relatively rare and result

from submarine earthquakes or explosivesubmarine volcanic eruptions. Like land-slide surges and very big storm waves, theymay cause severe damage and loss of lifealong coasts. Oceans also experience tidalwaves: these waves operate as standing os-cillations that are modified by the Earth’srotation and move around nodal centersknown as amphidromic points (see am-phidromic system). Within the watermasses of the oceans, there occur inter-facial waves known as INTERNAL WAVES.See also wave.

ocellar /oh-sell-er/ Describing a textureof some volcanic rocks in which PHE-NOCRYSTS are enveloped by small radiallyor tangentially arranged crystals of anothermineral.

ocelli /oh-sell-ee/ Small spherical ordropletlike bodies found in certain alkalineigneous rocks. Ocelli contain mineral as-semblages supposed to have crystallizedfrom a liquid that was immiscible with thehost magma.

ocher /oh-ker/ A yellow, brown, or redtype of mineral iron oxide – or claystrongly colored by iron oxide – which ispowdered and used as a pigment. Redocher usually contains HEMATITE, whereasthe yellow and brown forms are generallybased on LIMONITE.

offlap /off-lap/ The disposition of sedi-ments laid down as a sea regresses, the pro-gressively younger sediments beingdeposited seaward of the shoreline thatmarked the maximum extent of the formermarine transgression. In this way, insteadof older rocks being covered by youngerdeposits, they are exposed. The beds thatmark the maximum extent of the formertransgression have the largest areal devel-opment. See also overlap.

offset See chaining.

offshore bar A bar that is partly orcompletely submerged beyond the SURF

ZONE. Considerable littoral transport ofsediment, especially sand, may occur along

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the crest of nearshore bars and offshorebars that lie in fairly shallow water becauseof wave and tidal action. See also barrierbeach.

offshore zone The zone extending fromthe outer limit of the SURF ZONE to thedepth at which waves cease to influence theseabed. But because experimental evidencesuggests that such wave influence occursalmost everywhere over continental shelves(with the exception, possibly, of shelvesthat have been abnormally deepened), theoffshore zone effectively extends out to theSHELF-EDGE zone.

ogive /oh-jÿv, oh-jÿv/ 1. (in statistics) Acumulative frequency curve. It constitutesone of the most effective ways of repre-senting a frequency series, because unevenclass intervals do not distort the curve andinterpolation is easy. Calculation and plot-ting are also easy. The accumulated valuesfor each class interval are determined as apercentage of the grand total, or simplyplotted as frequency against the elementconcerned.2. (in glaciology) A broad-banded surfacepattern on a glacier. In general, ogivescurve down-glacier as a result of relativelyfaster ice movement at the center of theglacier. A number of different types ofogive have been recognized: the banding insome cases is of alternating white ice,which contains many air bubbles, and darkice, which contains few; others may relateto longitudinal pressures in ice flow.

oil sand Any porous rock that is im-pregnated with liquid hydrocarbons orcontains deposits of oil (petroleum). Com-pare gas sand; tar sand.

oil shale A type of fine-grained shalethat can be strongly heated to produce oil(petroleum). The resulting shale oil is richin unsaturated hydrocarbons.

oil trap See trap.

okta (in meteorology) A measure ofcloud cover equal to one-eighth cover.

Cover of 8 oktas corresponds to a com-pletely overcast sky.

Old Red Sandstone A succession ofconglomerates, red shales, and sandstonesforming the continental facies of the DE-VONIAN System in NW Europe. These rockscontain abundant remains of ostracodermand gnathostome (jawed) fish.

Oligocene /ol-ă-goh-seen/ The epoch ofthe PALEOGENE Period that followed theEOCENE and preceded the MIOCENE. Itbegan about 36.6 million years ago andlasted for some 13 million years. Manymammalian groups common in the Eocenebecame extinct but others continued toflourish, including camels, rhinoceroses,and the first pigs. The main phase of Alpinefolding began at the end of the Oligocene.

oligoclase /ol-ă-goh-klayss/ A sodic pla-gioclase FELDSPAR.

oligotrophic /ol-ă-goh-troff-ik/ Describ-ing a lake or other body of water that is de-ficient in plant nutrients and so has veryclear water (because of the scarcity ofplankton). The lowest cold levels of waterare rich in dissolved oxygen, but the de-posits on the bottom contain little organicmaterial. See also eutrophic.

olivenite /o-liv-ĕ-nÿt, ol-ă-vĕ-/ A raregreen mineral form of hydrated basic cop-per arsenate, Cu2(AsO4)(OH). It crystal-lizes in the orthorhombic system andoccurs in deposits of copper.

olivine /ol-ă-veen, ol-ă-veen/ A memberof a group of orthorhombic rock-formingminerals consisting of (SiO4) tetrahedralinked by divalent metal cations. The gen-eral formula is R2SiO4, where R = Mg,Fe2+, Mn, or Ca (in part). A complete gra-dation in chemical and physical propertiesexists between the two end-members,forsterite (Mg2SiO4) and fayalite(Fe2SiO4).

Olivines are green, brown-green, andyellow-green and show little or no cleavagebut possess a chonchoidal fracture. Peridotis a pale green semiprecious gemstone vari-

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ety. Magnesium-rich olivine is common inbasic and ultrabasic rocks whereas fayaliteoccurs in acid igneous rocks. Forsterite andmonticellite (CaMgSiO4) are found inmetamorphosed limestones.

omnivore /om-nă-vor, -vohr/ An animalthat feeds on both plants and other ani-mals. Typical omnivores include cock-roaches, bears, chimpanzees, hogs, andhuman beings. See also Carnivora; herbi-vore.

omphacite /om-fă-sÿt/ A member of thePYROXENE group of minerals.

oncolite /onk-ŏ-lÿt/ A rounded, chalkybody that originated as algae. Most oncol-ites are up to 2 cm across, although someare five times that size. They are thought tohave been formed when sediments becametrapped round an algal growth that wasthen rolled around by currents in the shal-low sea.

onion weathering (onion-skin weather-ing) Small-scale expansive EXFOLIATION

in which the splitting away of layers ofrock resembles that of concentric onionskins.

onlap /on-lap/ See overlap.

onyx A type of CHALCEDONY that has al-ternating straight bands of various colors(typically whitish with black or brown). Itis used for making ornaments and cameos.See also banded agate.

oolite /oh-ŏ-lÿt/ An allochemical LIME-STONE that is formed predominantly fromooliths. These are more or less sphericalbodies formed by the precipitation ofcarbonate in concentric layers around anucleus. They are usually small, up toabout 2 mm in diameter. Ooliths largerthan this are often known as PISOLITHS. Oo-lites are common in the JURASSIC system ofBritain. Some noncalcareous oolites areknown, such as those formed from ironminerals.

ooze See pelagic ooze.

opal A cryptocrystalline or colloidal va-riety of hydrous silica, SiO2.nH2O (see sil-ica minerals). It usually contains between 3and 9% water, and occurs in various col-ors, from transparent to multicolored; thereddish type is called fire opal. Found in ig-neous rocks and near hot springs, it ishighly valued as a gemstone.

open fold A fold in which the interlimbangle is greater than 70°. See diagram atFOLD.

ophiolite /off-ee-ŏ-lÿt, oh-fee-/ The se-quential association of ultramafic rocks,gabbros, dolerite dikes, pillow lavas, andcherts occurring within eugeosynclinal en-vironments. Ophiolites are thought to rep-resent slices of the basaltic ocean crust thathave been tectonically emplaced onto con-tinental margins. See also spilite; Stein-mann trinity.

ophitic /oh-fit-ik/ Describing a textureof common occurrence in basalts and do-lerites in which large plates of clinopyrox-ene completely envelop earlier-formedeuhedral plagioclase plates. The term sub-ophitic may be used when the pyroxenesonly partly enclose the plagioclases.Ophitic texture is a specific example ofPOIKILITIC texture.

orbicular /or-bik-yŭ-ler/ Describing cer-tain plutonic rocks that contain large ovoidbodies made up of concentric shells of al-ternately light and dark minerals. The OR-BICULES are the result of rhythmiccrystallization around XENOLITHS, whichhave acted as nuclei. Compare rapakivistructure.

orbicule /or-bă-kyool/ A large ovoid orspherical rocky mass consisting of concen-tric shells of minerals. Most orbicules areup to 15 cm in diameter, although somemay be as large as 3 m across. See orbicu-lar.

orbital motion Waves on the sea’s sur-face experience orbital motion of the water

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particles beneath each wave; concurrently,the wave form is propagated along thesea’s surface, in the direction of wave ad-vance, at speeds that are dependent on thelength of the waves. Beneath the waves, theorbital motion of water particles takesplace at much lower speeds. The orbits ofthe water particles are nearly circular andhave their greatest diameter just below thesea’s surface, but the diameters decreaserapidly with increasing depth. The orbitalpaths are referred to as open, because thecircular orbits are not completely closedloops; with each wave period, a water par-ticle moves slightly in the direction of wavepropagation. When the waves enter shal-low water, orbital motion becomes dis-torted, the orbital paths becomingincreasingly elliptic as waves run into in-creasingly shallow water. Beneath a givenshallow water wave, the long diameter ofthe ellipses decreases with increasingdepth, while at the bed the water particlesno longer orbit but merely oscillate to andfro along a straight line, although againwith a slight asymmetry shoreward. Seealso mass transport current.

order A group in the taxonomic classifi-cation of organisms. Several related ordersconstitute a CLASS, and an order itself iscomposed of one or more FAMILIES. For ex-ample, the Primates and Carnivora are twoorders of the class Mammalia. See taxon-omy.

Ordnance Datum See datum.

Ordovician /or-dŏ-vish-ăn/ The periodin the PALEOZOIC Era that followed theCAMBRIAN and preceded the SILURIAN. Itbegan about 505 million years ago andlasted for about 67 million years. Thename was proposed for rocks in the ArenigMountains of N Wales, part of an area in-habited by a Celtic tribe, the Ordovices.Sedimentary rocks containing fossils of theOrdovician occur in all present-day conti-nents; the graptolites, an abundant andrapidly evolving group, are used as the basefor the Ordovician geologic timescale. TheEarly Ordovician consists of the Tremar-doc and Arenig series, the Middle Ordovi-

cian the Llanvirn and Llandeilo series, andthe Late Ordovician the Caradoc andAshgill series. Fossil evidence suggests thatthe landmasses were in very different loca-tions to those of today. During the periodthe major landmass Gondwanaland ex-tended from the South Pole north to thetropics; the landmasses of Laurentia andBaltica began to move toward each othernarrowing the Iapetus Ocean betweenthem. There is evidence that tectonic andvolcanic activity was extensive and intense,with the beginning of mountain building(the Caledonian orogeny) along the sub-duction zone of the E margin of what isnow North America. Sedimentary rocks ofthe period are deep-water graptolitic shalesand mudstones and calcareous sandstones,mudstones, and limestones. Common fos-sils include brachiopods (articulate formswere dominant), trilobites, and gas-tropods. The first corals, echinoids,crinoids, and Bryozoa appeared and thestraight-shelled nautiloids reached theirpeak. The first fossils of ostracoderm ver-tebrates are found in Ordovician rocks.

ore A naturally occurring usually rockymaterial from which a useful product (suchas a metal or one of its minerals) can be ex-tracted. The commercial worth of a metal-lic ore depends on the value of the metaland the percentage of it in the ore, bearingin mind how difficult it is to extract the oreand, subsequently, the metal itself. See alsomineral.

organic acid (in soil science) Simpleproducts such as oxalic acid, (COOH)2.2H2O, acetic acid, CH3COOH, and lacticacid, CH3CH(OH)COOH that are re-leased when organic matter is attacked bythe soil microorganisms. These acidsgreatly increase the power of leaching insoils. See podzolization.

organic horizons The L, F, and H lay-ers in a soil PROFILE, which lie above themineral soil. However, humified organicmatter is also usually found intermixedwith the mineral matter in the A horizonand may also be present lower down thesoil profile in the B horizon.

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organic matter (in soil science) Ma-terial that forms as a result of the incorpo-ration of plant and animal remains in thesoil and their breakdown by the microor-ganisms present. Some of this material isbroken down completely into soluble sub-stances but the more resistant materialtends to remain, and with the new resyn-thesized compounds this forms the soilHUMUS. Organic matter rarely forms morethan 5% by weight of the soil yet it ishighly significant in promoting a produc-tive soil because it improves the structureand provides a source of important mineralnutrients.

organic reef See reef.

organic rock Any sedimentary rockthat consists essentially of the remains ofplants or animals.

organic soil (bog soil) A soil with an ac-cumulation of peat more than about 70 cmin depth. Such soils form in water-satu-rated environments where the decomposi-tion is slow. In upland regions with highrainfall, rapid leaching of bases results inthe formation of an acid peat called blan-ket bog. Similar acid soils, called raisedbogs, may occur in lowland sites whererainfall is sufficient. The most characteris-tic lowland organic soil, however, is fensoil (or peat), which is neutral to alkaline inreaction as a result of saturation by base-rich waters. When this black peat isdrained, rich agricultural soils are formed.

organic weathering The breakdownand decomposition of rocks caused byplants and animals. Both mechanical andchemical breakdown are involved. Bur-rowing animals and plant roots can physi-cally separate rock particles and can mix ortransfer material elsewhere. Through therespiration of vegetation, the carbon diox-ide content of the soil may be substantiallyincreased, having a marked effect onweathering, while soil bacteria can also bevery important, especially in chemically re-ducing conditions. Lichens can exist onbare rock surfaces and are frequently re-sponsible for the initial decomposition of

rocks by their removal of nutrients. Largerplants can affect the surface atmospherethrough shading while even in areas of onlyshallow soils, weathering has been seensurrounding roots at great depths, owingto their localized ability to alter the con-stituent minerals chemically.

Ornithischia /or-nă-th’iss-kee-ă/ An orderof dinosaurs characterized by a pelvis sim-ilar to that of birds, although they were notbird ancestors nor very closely related tothem. They were all herbivorous and hadcorrespondingly modified dentitions; someornithischians had a horny beak. Foravoiding saurischian predators some devel-oped rapid bipedal locomotion, while themore slow and bulky forms evolved hornsand other types of armor. Ornithischiansbecame extinct at the end of the CretaceousPeriod (see K/T boundary event). CompareSaurischia.

orocline /ô-rŏ-klÿn/ An orogenic beltthat has a marked change in trend. This isthought to have resulted from horizontalforces operating within the Earth’s crust.

orogenesis /ô-rŏ-jen-ĕ-sis/ The processby which mountains are formed, i.e. the de-formational processes such as thrusting,folding, and faulting, which result from thecollision of two continental plates. Thesediments between the continents are thenstrongly deformed, causing them to spillout onto the older stable continents and becompressed into long linear mountainchains. There is frequently associated ig-neous activity. Compare epeirogenesis.

orogenic belt /ô-rŏ-jen-ik/ See mobilebelt.

orogeny /ô-roj-ĕ-nee/ A mountain-building period. Major orogenic phasessince the Precambrian include the Caledon-ian, Variscan, and Alpine orogenies.

orographic cloud /ô-rŏ-graf-ik/ A cloudthat owes its formation to rising air result-ing from airflow over mountains. Becauseair is forced to rise over high ground therewill always be some upward component in

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the wind field. If the air is already moist, orvertical motion extensive, the condensa-tion level may be reached and orographiccloud formed. If there are stable layerswithin the airstream, LEE-WAVE clouds maybe seen. These are the most easily recogniz-able type of orographic clouds.

orographic precipitation Rainfall andother forms of precipitation resulting fromthe vertical motion of an airstream, causedby the presence of uplands. However, it isfairly rare for orographic precipitation tofall without either convective or cyclonicprocesses also acting. The orographic com-ponent is normally weak and merely acts asan enhancement to the other two mecha-nisms.

orpiment /or-pă-mĕnt/ A yellow to or-ange mineral form of arsenic sulfide, As2S3.It crystallizes in the monoclinic system, andoccurs in association with CINNABAR andREALGAR, often in lodes containing gold,lead, or sulfur. It may also be found in de-posits around hot springs. Despite itshighly toxic nature, it has long been used asa pigment; it is also used as a source of ar-senic.

orthite /or-th’ÿt/ See allanite.

orthoclase /or-th’ŏ-klayss/ A potassicalkali FELDSPAR.

orthoferrosilite /or-th’oh-fĕ-ross-ă-lÿt/One of the PYROXENE group of minerals.

orthogeosyncline /or-th’oh-jee-oh-sing-klÿn/ A GEOSYNCLINE that is associatedwith both continental and oceanic environ-ments, and contains both volcanic andnonvolcanic areas in association with theaccumulating sediments.

orthogonal /or-th’og-ŏ-năl/ (wave ray) Aline that is perpendicular to a series ofwave crests. This is usually done on thebasis of a wave refraction diagram, whichshows a series of wave crests in the vicinityof the coast or a harbor. In theory, thewave energy between any two orthogonalsremains roughly the same as they are

traced shoreward into decreasing depths.For this reason, where orthogonals bunchtogether at the coast, or conversely whenthey tend to spread out, one can delimitzones of wave energy concentration (zonesof CONVERGENCE), and zones of energy dis-sipation (zones of DIVERGENCE) respec-tively.

orthomagmatic /or-th’oh-mag-mat-ik/Describing the stage in the crystallizationof a magma during which the early-formedprimary minerals having high crystalliza-tion temperatures are formed. The or-thomagmatic stage is followed by periodsof pegmatitic, pneumatolytic, and hy-drothermal crystallization.

orthomorphic projection /or-th’ŏ-mor-fik/ (conformal projection) Any MAP PRO-JECTION on which the shape of an area ofthe map is the same as the correspondingarea of the Earth’s surface.

orthopyroxene /or-th’oh-pÿ-roks-een/One of the PYROXENE group of minerals.

orthoquartzite /or-th’oh-kwort-sÿt/ Seequartzarenite.

orthorhombic /or-th’ŏ-rom-bik/ Seecrystal system.

oscillation (in meteorology) Any cyclicvariation of a climatic element above andbelow its mean value or position. Its mostfrequent use is as the Southern Oscillation(see El Niño) where an oscillation with aperiod of 2.33 years was found in the sea-sonal distribution of pressure and to alesser extent temperature and rainfall overthe oceanic areas of tropical latitudes. Seealso North Atlantic Oscillation.

oscillatory current The type of currentthat waves induce at the seabed arisingfrom the ORBITAL MOTION of the water par-ticles beneath waves. The larger the waves,the deeper such currents operate. Aswaves, especially long waves, enter shallowwater, the orbital motion of the water par-ticles tends to be elliptic, while at theseabed the particles oscillate to and fro

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about a mean position along a straight line.This gives rise to an oscillatory current atthe bed. Beneath the wave crest there oc-curs a short sharp acceleration shoreward,while beneath the flattish trough of thewave there is a more protracted seawardmovement of water. Such an oscillatingtype of flow beneath waves often has a pro-found influence on the movement of bedmaterials (although far less an influence onmaterials moving continually in suspen-sion), and may cause pebbles to moverapidly shoreward.

oscillatory wave A wave, often of theprogressive oscillatory type, in which onlythe waveform on the sea’s surface advancesin the direction of wave propagation, whilethe water particles beneath the wave orbitin almost closed loops. Whereas the wave-form may be propagated across the sea’ssurface quite rapidly, at a speed that islargely dependent on the length of thewave, the gradual shift of the water parti-cles in the direction of wave travel may bequite slow.

oscula See Porifera.

Osteichthyes /oss-tee-ik-th’ee-eez/ Theclass constituting the bony fish. It includesthe ACTINOPTERYGII (teleosts and moreprimitive ray-finned fish) as well as thefleshy-finned CROSSOPTERYGII. Some earlygroups had a body armor of closely fittingscales of complex construction; these werereduced and simplified in later forms. Theclass is characterized by the developmentof some kind of lung, which may be con-verted into a swim bladder in advancedforms for maintaining hydrostatic equilib-rium. Fossil fish first appeared in the lateSilurian, since when the class has contin-ued to expand in diversity and abundance.Compare Chondrichthyes.

Ostracoda /oss-tră-koh-dă/ A group ofaquatic arthropod animals that belong tothe class CRUSTACEA. The body is com-pletely enclosed within two calcareousvalves forming a carapace. Unlike theshells of the BIVALVIA, these have no growthlines; they are periodically completely re-

placed as the animal increases in size. Theyare usually very small, less than 1 mm inlength, but some species grow up to 20 mmacross. Varied ornamentation of the shellsis of use in taxonomy. Ostracods rangefrom the Cambrian Period to the Holoceneand because of their small size, distinctiveforms, and widespread distribution, theyare valuable in the calibration and correla-tion of the rocks in which they occur, espe-cially in the cores from boreholes.

Ostracodermi /oss-tră-koh-der-mÿ/ Fos-sil jawless fish (see Agnatha), having anouter protective covering of bony platesand scales. Usually not more than 30 cmlong, they were common inhabitants of Sil-urian and Devonian ponds and rivers and afew fossil fragments are known from theOrdovician. This group probably gave riseto the jawed fishes.

outburst flood See glacial outburstflood.

outcrop The area on the Earth’s surfacewhere a particular rock type or body is pre-sent. This includes both exposed areas andothers covered by drift.

outer core The layer of the Earth be-tween 2900 km and 5000 km beneath itssurface. It is bounded at 2900 km by a seis-mic discontinuity (see Gutenberg disconti-nuity) that separates it from the overlyingmantle. On geophysical evidence it isthought to have a composition of iron alloyand nickel and to behave essentially as aliquid, unlike the INNER CORE, which issolid.

outgassing The action of heat in remov-ing occluded gases from rocks. The out-gassing of water vapor and other gasesfrom molten rocks in the primeval Earth isbelieved to be the source of the atmos-phere.

outlier /owt-lÿ-er/ An area of exposedyounger sediments completely surroundedby older rocks, usually as a result of fold-ing, faulting, and erosion. Compare inlier.

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outwash plain (sandur) A widespreadarea of fluvioglacial deposits produced bydeposition from meltwater streams emerg-ing from the margins of an ice mass. Thematerial decreases in size with distancefrom the ice and is frequently well bedded.Some unstratified till may be found amongthe gravels, indicating a period of re-advance of the ice margin. These outwashplains may exhibit irregular surfaces, re-sulting from the presence of kettle holes,and are frequently found in associationwith other fluvioglacial landforms, such asKAMES, KAME TERRACES, and ESKERS.

overflow channel A valley carved outby water that overflows from a lake. Oftenmeltwater from a glacier or ice cap over-flows from the lakes that form when the icemelts. Some overflow channels becomedeep gorges.

overfold (overturned fold) A type ofFOLD in which very strong compression hascaused the rock strata to be pushed rightover.

overland flow (wash; rain wash; sheeterosion) The movement of water as ashallow unchanneled sheet over the soilsurface. Overland flow, which is interme-diate between flow of water through thesoil and surface channeled flow, occursonce the infiltration capacity, or ability tohold and store water, of the soil is ex-ceeded. Water then collects in depressionson the surface and begins to flow as abroad shallow layer, often giving a glisten-ing sheen to the surface. The flow is noteven but turbulent, moving as a series ofwaves, although LAMINAR FLOW in sheets 8cm deep has been recorded. Speeds anddepth of flow are limited but soil particlescan be entrained and moved. The thicknessof the sheet determines the size of materialmoved. Overland flow is most devastatingon bare soils of limited permeability, espe-cially on long steep slopes. The depth offlow increases downslope from nil at thehill crest until at the downslope end thesheet becomes channeled into rills andceases to exist. It tends to erode a hydraulicconcave depression, and as such is impor-

tant in PEDIMENT evolution. Most authori-ties attribute to it a major capacity to shapethe landscape.

overlap (onlap) The disposition of bedslying above an unconformity, where theywere deposited by a transgressing sea. Eachsuccessively younger bed extends fartheronto the previous land surface than its pre-decessor. See also offlap.

oversaturated rock An igneous rockthat contains free quartz. Examples includegranites and rhyolites. See silica saturation.

overstep The disposition of strata asso-ciated with an unconformity whereyounger transgressive marine beds en-croach onto progressively older beds of theunderlying sequence.

overturned fold See overfold.

oxbow lake (cutoff) A crescent-shapedlake occupying the abandoned channel(oxbow) that was formerly part of a streammeander. Oxbow lakes are typical featuresof meandering streams, and result from thestream’s ability to erode laterally beingconcentrated on the outsides of meanderbends. Once created the oxbow rapidlysilts up. (See diagram overleaf.)

An alternative origin of cutoffs has beensuggested by laboratory experiments,which showed that cutoffs develop whenstream gradient falls below a critical limit,because the stream needs a minimum gra-dient to flow, and meanders effectively de-crease gradient by increasing length over agiven fall. If the meanders develop to suchan extent that they rob the stream of itsminimum necessary gradient, cutting offthat meander will restore the gradientneeded. See also meander.

oxidation (in geology) The chemicalweathering process involving the reactionbetween rocks and atmospheric oxygen,the oxygen usually being dissolved inwater. The products of oxidation are ox-ides and hydroxides, iron being the mineralmost frequently affected and its oxidationproducts give many weathered rocks their

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reddish or yellowish color. A certainamount of oxidation may also result frombacteriological activity. Compare reduc-tion.

oxide A chemical compound of oxygenand one or more other elements. In miner-alogy, the non-oxygen component is ametal or metals; metal oxides make up alarge percentage of all minerals.

oxisol /oks-ă-sôl/ One of the twelve soilorders of the US SOIL TAXONOMY character-ized by the presence of free iron and alu-minum oxides (Fe2O3 and Al2O3) andpossibly other oxides of titanium andchromium, as well as a high proportion ofsimple clays (especially kaolin) and somequartz. This includes the currently devel-oping ferrallitic soils and the relic lateritesof tropical areas. Oxisols are highly weath-

ered and leached, generally lacking in fer-tility, and in the case of the relic lateritesmay well include deposits of a more geo-logic than pedological origin.

oxygen One of the main gases of the at-mosphere (about 20% by volume). Fromthe meteorological point of view, oxygen ismost important as an absorber of radiationin the ultraviolet wavelengths (0.13–0.17µm). It dissociates at high levels of the at-mosphere into monatomic oxygen (O),which combines with oxygen molecules(O2) to give OZONE (O3).

oyster reef A REEF largely formed fromthe consolidation of oyster shells. Themaximum development of oysters tends tobe in the brackish water areas skirtingcoasts, as within certain estuaries, or inbays with fairly narrow inlets. A reason-able supply of fresh water from rivers, toreduce salinity levels, is also an importantfactor. Oysters do also occur in the opensea, although in lesser numbers. The reefsthey give rise to are common to many la-goonal areas in the humid zone. The reefsmay attain a significant thickness, for in-stance, commonly 10 m or more alongparts of the coast of Texas.

ozone Triatomic oxygen (O3), whichforms in the upper atmosphere especiallyat about 20–25 km level (the ozone layer orozonosphere). It results from the dissocia-tion of molecular oxygen into single atomsof oxygen, which then combine with otheroxygen molecules to give ozone. Below thislevel of formation, it gradually dispersestoward the ground surface, but being achemically reactive gas it is quickly re-duced to oxygen. Ozone absorbs stronglyin the 0.23–0.32 µm waveband, preventingpotentially damaging ultraviolet radiationfrom reaching the ground surface. It alsoabsorbs at other wavelengths, but these areof less importance.

The distribution of ozone would be ex-pected to correspond to its rate of produc-tion under conditions of prolongedexposure to ultraviolet radiation. How-ever, it seems to reflect vertical and hori-zontal movements in the stratosphere

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Fig. 2: As a result of the pattern of erosion and deposition, meander migrates as shown

Fig. 3: In a storm, the narrow neck is breached leaving the oxbow

Fig. 1: Pattern of erosion and deposition in a meandering reach

erosionerosionerosion

erosion

depositiondeposition

oxbowoxbowoxbow

Oxbow lake formation

rather better. High latitudes have the max-imum levels of ozone in spring with a min-imum in the fall.

Since the 1980s scientists have detectedozone holes, particularly near the SouthPole, in which significant depletions ofstratospheric ozone occur at certain timesof the year. They are thought to be causedmainly by chlorofluorocarbons (CFCs),used as aerosol propellents, which enterthe atmosphere and break down the ozone.

As a result, more damaging ultraviolet ra-diation can reach the Earth’s surface, pos-sibly also leading to climate change. In1987 the signing of the Montreal Protocolestablished the first global agreement to re-strict CFCS.

Ozone also occurs within the lower tro-posphere as a major constituent of photo-chemical SMOG where it is a pollutantpotentially harmful to plants and to humanlung tissue.

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Pacific ring of fire The circum-Pacificsystem of earthquakes, volcanoes, andocean trenches that surrounds the Pacificbasin’s oceanic plates where they are sub-ducting below other plates. It extendsaround the Philippines, Japan, and the Wcoast of North and South America. See alsoplate tectonics.

packing The arrangement in space ofthe atoms or ions in a CRYSTAL or the grainsin a rock. The individual particles can bethought of as tiny spheres, and they can bepacked in various ways. The closest ishexagonal close packing, in which in anyplane each sphere is centered on a corner ofa regular hexagon. In cubic packing theyare located at the corners of a square. In arock, the packing of the grains determinesits porosity.

pahoehoe /pah-hoh-ee-hoh-ee/ A lavahaving a surface that resembles coils oftwisted rope. Compare aa; block lava.

palagonite /pă-lag-ŏ-nÿt/ Hydrated bas-altic glass, usually yellow or orange,formed when basalts are erupted beneathwater or ice. Palagonite breccias or tuffsproduced by the fragmentation of theglassy material are termed hyaloclasticrocks.

paleobotany /pay-lee-oh-bot-ă-nee, pal-ee-/ The branch of PALEONTOLOGY thatdeals with the nature and evolution ofplants through geologic time, as shown byfossil remains.

Paleocene /pay-lee-ŏ-seen, pal-ee-/ Thefirst epoch of geologic time in the PALEO-GENE Period. Beginning about 66 millionyears ago and lasting for 8.6 million years,

it followed the CRETACEOUS Period and wassucceeded by the EOCENE Epoch. Occasion-ally the time represented by this epoch isincluded within the Eocene. Paleocenerocks are of both marine and nonmarinefacies. Following the extinction of the largereptiles at the end of the Cretaceous (seeK/T boundary event), mammals becameabundant and diversified into a variety ofprimitive and now mainly extinct groups.Insectivores were common and by the endof the epoch primates and rodents hadevolved.

paleoclimatology /pay-lee-oh-klÿ-mă-tol-ŏ-jee, pal-ee-/ The study of the climatesof earlier geologic periods. The evidencefor the former climate is obtained fromsediments and fossils. Complications arisefrom the fact that the continents have beenchanging their latitudinal positionsthrough time, so the time period and loca-tion of deposition have to be determinedbefore conclusions can be deduced.

paleoecology /pay-lee-oh-ee-kol-ŏ-jee,pal-ee-/ The study of the ecology of fossilorganisms, i.e. the relationships of organ-isms in past geologic ages with their non-living environment and with other animalsand plants. Paleoecology involves the in-vestigation of the rocks in which the or-ganisms are found to determinepaleoenvironmental information and de-tails of TAPHONOMY. TRACE FOSSILS oftenprovide information regarding the behav-ior of an animal.

Paleogene /pay-lee-ŏ-jeen, pal-ee/ Thelower period of the CENOZOIC Era, com-prising the PALEOCENE, EOCENE, andOLIGOCENE Epochs. It lasted for about 42.5million years from approximately 66.5 to

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24 million years ago, when it was followedby the MIOCENE Epoch of the NEOGENE.

paleomagnetic correlation /pay-lee-oh-mag-net-ik, pal-ee-/ The correlation ofthe residual magnetism in rocks. The direc-tion of polarization of the magnetic field ofthe Earth changes periodically (see mag-netic reversal). It is possible to measure theresidual magnetization of suitable rocksand to discover the direction of polarity atthe time of their formation. By this meansa sequence can be established showing nor-mal and reversed periods in the Earth’smagnetic field. It may be calibrated bysome means, such as radiometric dating(see calibration), to provide a valuable toolfor the CORRELATION of stratigraphic se-quences of rock. See also magnetic divi-sion; magnetic interval.

paleomagnetism The study of the RE-MANENT MAGNETISM preserved in rocks andchanges in the Earth’s magnetic fieldthrough geological time. During the timeof their formation those rocks in whichmagnetic iron-bearing minerals are presentacquire a permanent record of the Earth’smagnetic field at that time; such rocks in-clude mafic igneous rocks and lake andmarine sediments. During the cooling pro-cess of mafic lava small crystals of mag-netite form and while the lava is still fluid,are aligned with the Earth’s magnetic field.Once cooled and solidified the alignmentof the crystals is frozen in place. The align-ment also occurs when magnetite particlessettle in the sediments on the floor of abody of calm water, e.g. a lake. The recordof reversals in the Earth’s magnetic field isparticularly evident on the ocean floorwhere there is little erosion to disturb thesediments. The magnetic reversals re-corded in the eruptions of basaltic lavaalong mid-ocean ridges have provided evi-dence for SEA-FLOOR SPREADING. Paleomag-netism has also supported plate tectonicstheory and has been used to locate the po-sitions of continents in relation to eachother in the past. See also magnetic rever-sal.

paleontology /pay-lee-on-tol-ŏ-jee, pal-ee-/ The study of all aspects of ancient or-ganisms, including their TAXONOMY,anatomy, ECOLOGY, and EVOLUTION. Theevidence for this comes from FOSSILS pre-served in rocks, which are either the re-mains of the organisms themselves orTRACE FOSSILS. Much paleontological workis applied to the elucidation of the strati-graphical relationships between bodies ofrock by means of BIOSTRATIGRAPHY. Thebranch of the subject that deals with mi-croscopic organisms is known as micropa-leontology; fossils of such organisms areMICROFOSSILS.

Paleozoic /pay-lee-ŏ-zoh-ik, pal-ee-/ Thefirst era into which PHANEROZOIC time isdivided. It followed the PRECAMBRIAN andconsists of the following periods: the CAM-BRIAN, ORDOVICIAN, SILURIAN, DEVONIAN,CARBONIFEROUS, and PERMIAN. Systems ofthe first three periods constitute the LowerPaleozoic, and the second three the UpperPaleozoic. It lasted for about 325 millionyears from 570 to 245 million years ago,when it was followed by the MESOZOIC Era.The fauna of the Paleozoic is characterizedby an abundance of invertebrate types,many of which, such as the rugose corals(see Anthozoa), trilobites, graptolites, andproductid brachiopods, became extinct atthe end of the era. In the course of the Pa-leozoic fish evolved and the first amphib-ians appeared, flourished, and declined.Two episodes of major orogenic activityoccurred, giving rise to mountain rangesand associated tectonic activity. The CALE-DONIAN OROGENY was the earliest, occur-ring at the end of the Silurian and thebeginning of the Devonian; the VARISCAN

orogeny took place toward the end of theera. See also geologic timescale.

paleozoology /pay-lee-oh-zoh-ol-ŏ-jee,pal-ee-/ The branch of PALEONTOLOGY

concerned with the nature and evolution ofanimals through geologic time by the in-vestigation of their fossil remains.

pallasite /pal-ă-sÿt/ A stony-iron METE-ORITE that has grains of OLIVINE in a nickel-iron matrix.

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paludal /pă-loo-dăl, pal-yŭ-/ Describingsomething that lives in, is related to, or isproduced by a marsh.

palustrine /pă-lus-trin/ Describing plantsthat grow in or deposits that accumulate ina marsh.

palygorskite /pal-ă-gorsk-ÿt/ Any of anumber of claylike minerals, hydratedmagnesium aluminum silicates, (Mg,Al)2-Si4O10(OH).4H2O. They form thin fibrouscrystals that resemble cardboard and occurin desert soils.

palynology /pal-ă-nol-ŏ-jee/ The studyof fossil spores, especially POLLEN. Becausespores are usually adapted to resist de-struction and to be dispersed over large dis-tances, they are valuable for use in theCORRELATION of the rocks in which theyoccur. They are also important environ-mental indicators and have been used inmonitoring climatic change during theQuaternary Period. Palynology forms partof micropaleontology (see paleontology).

pampas A region of temperate grasslandin South America, found in Argentina andUruguay near the Plate River estuary. Usedoriginally as pasture for cattle and sheep,today much of it has been plowed forgrowing crops such as alfalfa, corn, andwheat.

Pangaea /pan-jee-ă/ A hypothetical su-percontinent that is thought to have ex-isted until near the close of the TriassicPeriod. This single continent was com-posed of all the present-day continents,which have since been derived from it as aresult of repeated episodes of SEA-FLOOR

SPREADING. This supercontinent is thoughtto have fragmented initially into two seg-ments, LAURASIA to the north and GOND-WANALAND to the south.

parabolic dune A U-shaped type ofDUNE in which the curve of the crest bowsoutward in the downwind direction, andelongated arms extend upwind (oppositeto barchan dunes in which the arms extenddownwind). They are common in coastal

desert areas. The coastal blowout dune is atype of parabolic dune that occurs adjacentto a beach (see blowout).

paragonite /pa-ră-gŏ-nÿt/ One of themajor types of MICA, a hydrated sodiumaluminum silicate, Na2Al4(Si6Al2O20)-(OH)4. It forms colorless, yellowish, orgreen crystals and occurs mainly in variousmetamorphic rocks.

parallel A line of latitude, running par-allel to the Equator round the Earththrough any given point. The plane of eachparallel is at right angles to the Earth’saxis. Parallels range in length from a pointat each pole to the circumference of theEarth at the Equator, which is the only par-allel that is a GREAT CIRCLE. Parallels areuniformly spaced along the meridians.

parallel drainage A type of drainagepattern that develops where there is strongstructural control in one direction, streamsbeing strictly guided in that direction. Auniform regional slope, rigid alternation ofhard and soft lithologies, or parallel foldswill produce parallel streams; recentlyglaciated areas may also have paralleldrainage nets if the surface is covered influted ground moraine or drumlin fields.

parallel evolution The CONVERGENT

EVOLUTION of closely related and anatomi-cally similar organisms, which undergonearly identical evolutionary changesthrough time.

parallel retreat A form of SLOPE EVOLU-TION in which the original shape and angleof the slope is maintained as it erodes(compare slope decline). Hence it remainsparallel in new positions to its previous po-sitions. Walter Penck and later Lester Kingused this principle as the basis for theirscheme of the CYCLE OF EROSION. As amethod of slope evolution it is strongly re-lated to geomorphic processes, in that aslope will remain parallel only if the debrisbeing eroded in its retreat is removed at anequal rate from its base. If the debris accu-mulates, the slope declines.

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The idea was first formulated in rela-tion to the tropical scheme, with the origi-nal work being in Africa; since then, G. H.Dury claims to have identified it as a viableprocess in the humid temperate lands, andit can be seen in areas of BASAL SAPPING, e.g.active sea cliffs. S. A. Schumm has sug-gested it operates preferentially on certainlithologies through their control on pro-cess: on sandstones, dominated by over-land flow, parallel retreat operates,whereas on clays, dominated by creep, theslope declines. It seems that a number ofsituations can favor parallel retreat, notmerely certain climates; lithology andprocesses, in various permutations, arealso vital.

paramagnetic /pa-ră-mag-net-ik/ De-scribing a material that has a magnetic per-meability slightly greater than 1. In amagnetic field, a paramagnetic mineral ismagnetized in proportion to the fieldstrength. Platinum is a paramagneticmetal.

parameterization /pă-ram-ĕ-tĕ-ri-zay-shŏn/ (in numerical weather prediction)The inclusion of the effects of meteorolog-ical systems that are smaller than the gridon which the model is based. It involves ap-proximating the statistical effects of thesesmall-scale processes in terms of the large-scale factors.

parasite An organism that lives on or in-side another organism (the host) on whichit is totally dependent for food and energy.This may or may not harm the host. Exter-nal parasites are called ectoparasites; inter-nal ones are endoparasites.

parasitic cone A small cone on theflank of a major volcanic cone. These conesdevelop frequently and indicate that themagma trying to reach the surface hasfound an easier route than via the mainvent. They are usually several tens of me-ters beneath the main vent.

parataxitic /pa-ră-taks-it-ik/ See eu-taxitic.

paratectonics /pa-ră-tek-tonn-iks/ Thetectonics of stable areas, such as old foldbelts and cratons.

paraunconformity /pa-ră-un-kŏn-form-ă-tee/ See disconformity.

parautochthonous /pa-raw-tok-th’ŏ-nŭs/Describing rock bodies that are midwaybetween ALLOCHTHONOUS and AUTOCH-THONOUS, i.e. that have been displaced onlyshort distances.

parent material The material fromwhich the soil forms, which may not be thesame as the underlying bedrock. Before theimportance of climate was recognized, par-ent material was thought to be the domi-nant soil-forming factor. Certain featuresof the parent material, such as texture,mineral composition, and porosity, have aconsiderable influence on the soil-formingprocesses, e.g. a limestone soil rich in cal-cium will delay the process of acidificationin a humid climate. Parent material is likelyto have a greater influence in temperate re-gions where soils have been forming for arelatively short time, e.g. in Scotlandbrown earths have been recorded on basicigneous rocks and podzols on acid igneousrocks. See also soil formation.

parhelion /par-hee-lee-ŏn, -hel-yŏn/ Seemock sun.

partial melting The geologic process inwhich a rock is subjected to extremely hightemperature and pressure so that it partlymelts and the liquid flows away. The liquidthen solidifies to form of a different type ofrock.

partial pressure In a mixture of gases,such as the atmosphere, the partial pres-sure of one constituent is the pressure itwould exert on its own while occupyingthe same volume as the whole mixture. Thesum of all the individual partial pressuresof the components is equal to the pressureof the whole mixture.

particle size The effective diameter of aparticle. Particle size analysis is the tech-

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nique of determining the proportion of thedifferent particle sizes present (each parti-cle size is termed a separate) in a sample.There are several classifications in use; the main classes in the British standardclassification are shown in the table. Thereare three main techniques of analyzing par-ticle size: by sieving in a stack of sieves ofdecreasing mesh size, by allowing particlesto settle in a tube of water, the rate of set-tling being proportional to size, and bymicrometric methods. It is a techniquecommonly used in soil science and geo-morphology, e.g. to describe textureclasses in soil, or to infer the origin of asedimentary deposit in geomorphology.See also phi scale; Udden–Wentworthscale.

passive continental margin (passivemargin) A margin of a continent thatdoes not coincide with a plate boundary(or margin) and where there is minimal tec-tonic activity. Examples include the At-lantic margin of North and South America.Compare active continental margin.

passive glacier A GLACIER that accumu-lates little material and loses little material;technically, its rates of alimentation andablation are about equal. This occurs whenthere is not much winter snow and a coolsummer with little melting of the ice. Suchglaciers move very slowly, causing littleerosion and transporting few materials.

patina /pat-ă-nă/ A film or skin on thesurface of a boulder, produced by chemical

weathering. Patinas are distinctive in colorand physical properties, their thicknessesoften reflecting age.

patterned ground A distinctive geo-morphological feature mostly found inperiglacial areas. Where the ground surfaceis flat, STONE POLYGONS or CIRCLES may de-velop, either in groups or in isolation,which may grade into STONE STRIPES evenon very gentle slopes. In nonperiglacial cli-matic areas the most likely cause of pat-terning is cracking due to heat shrinkage(see desiccation crack).

pays A low plateau of land in northernFrance.

peacock ore See bornite.

pearl An accumulation of white orcream-colored, usually spherical, NACRE in-side the shell of a mollusk, such as a clamor oyster. It forms when layers of nacregradually build up on a foreign body, suchas a grain of sand. Pearls are valued as pre-cious gemstones; most prized are blackpearls.

peat A mass of fibrous plant debris,which is only partly decomposed, and isoften dark brown in color. This is a resultof wet anaerobic conditions, which retardmicroorganism activity. Certain soils suchas peaty gleys and peaty gleyed podzolshave peat as one of their main horizons;other soils may have a thicker accumula-tion of peat. (See organic soil.)

peaty gley podzol A hydromorphicvariant of PODZOL, characterized by a blue-gray waterlogged area above the imperme-able iron pan, capped by a thick peathorizon. Above and below the gleyed areathe color is orange-brown because of thefree drainage and oxidation, as contrastedwith the reducing conditions in the gleyedarea. Such soils are most probably polyge-netic, being originally BROWN EARTHS thatbecame degraded to podzols with forestclearance, and subsequently became gleyedwhen the iron pan developed to the pointwhere it became impermeable. Once gley-

passive continental margin

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CLASSIFICATION OF PARTICLE SIZES

Class Size range (mm)

boulder > 200cobble 60–200gravel 2–60coarse sand 0.6–2medium sand 0.2–0.6fine sand 0.06–0.2silt 0.002–0.06clay < 0.002

ing had set in, microbiological activity wasinhibited and so litter was not brokendown but accumulated as peat. It is char-acteristic of many gently sloping uplands.

pebble A rounded rock fragment from 5mm to 65 mm across. It is thus larger thana granule but smaller than a cobble.

ped An aggregate of soil particles, form-ing a crumb of soil.

pedalfer /pĕ-dal-fer/ A leached soil inwhich iron and aluminum increase relativeto silica during pedogenesis. Pedalfers aresoils of humid climates, including PODZOLS,BROWN EARTHS, LATERITES, and PRAIRIE

SOILS. See also soil formation.

pedestal rock A rock that has beenshaped like a mushroom by the erosive ac-tion of windblown sand (corrasion). Indesert regions, the corrasive action of sandis strongest about a meter above theground and most pedestal rocks have‘stems’ of this height.

pediment A surface of low relief thatslopes away from residual uplands at an-

gles of between 9° and 0.5° or less, beingbounded on their upslope ends by a sharpbreak of slope, the PIEDMONT ANGLE, at thepoint where they meet rock residuals. Suchfeatures are said to be typical of arid andsemiarid regions, although some would at-tribute this type of surface with a muchwider occurrence.

Pediments have a concave long profile,with the angle of slope decreasing awayfrom the piedmont angle. They are cut inbedrock, often with a capping of gravel oralluvium, and display great regularity ofform over wide areas. Pediments have beenidentified at various scales, ranging fromvalley-side types to the continental-scaleforms that the South African geologistLester King identified in Africa.

Pediments evolve by PARALLEL RETREAT

of the upstanding residuals, leaving abroad area of low relief as they retreat.King and Walther Penck, who see this asthe usual tendency for landscape evolution,therefore see pediments as the end prod-ucts of denudation. The processes respon-sible are not certain, but much discussionhas centered on erosion by running water.Because the pediments have a concave pro-file, analogies have been drawn betweentheir evolution and that of the long profilesof rivers: both are hydraulic curves. Theform of this water erosion is disputed:some authorities consider that the scarp re-treats by headward erosion of streamsemerging from the upland, leaving the low-level surface as it retreats; others considerthat pediments are due to lateral planationof streams, while a major school of thoughthas stressed the importance of sheet wash,which has been recorded up to a meterdeep on pediments. King states that pedi-ments begin on weak rocks and then ex-tend into harder rocks as the scarp retreatsby stream erosion at the base. Several dif-ferent processes may be responsible, withdifferent emphases in different areas.

pediplain /ped-ă-playn/ A broad land-scape of low relief broken by isolated resid-ual uplands, which meet the pediplain in asharp PIEDMONT ANGLE. It is formedthrough the coalescence of a number ofPEDIMENTS. According to the South African

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peatpeatpeat

mineral and organicmineral and organicmineral and organichorizon–ungleyedhorizon–ungleyedhorizon–ungleyed

gleyed B horizon–gleyed B horizon–gleyed B horizon–iron pan impermeableiron pan impermeableiron pan impermeable

pppooossssssiiibbbllleee hhhuuummmuuusss ssstttaaaiiinnniiinnnggg

ungleyed B–ungleyed B–ungleyed B–free drainagefree drainagefree drainage

parent materialparent materialparent material

A horizon

B horizon

iron pan

C horizon

Peaty gley podzol

geologist Lester King and German geolo-gist Walther Penck, a pediplain is the logi-cal final product of subaerial denudation inan area of PARALLEL RETREAT (comparedwith a PENEPLAIN in areas of SLOPE DE-CLINE). Pediments tend to be smooth withgentle concave profiles, broken by fewstream lines. It is difficult to be categoricalas to whether a surface of low relief consti-tutes a true pediment, owing to the factthat landscapes have not evolved underconstant climates and hence a single cycle,but are polycyclic in origin (see polycycliclandscape).

pedocal /ped-ŏ-kăl/ Unleached soil inwhich lime accumulates during pedogene-sis, e.g. CHERNOZEM, CHESTNUT SOIL, andSIEROZEM.

pedogenesis /ped-ŏ-jen-ĕ-sis/ See soilformation.

pedology /pĕ-dol-ŏ-jee/ The scientificstudy of soil, including its classification,origins, and composition. See soil; soil for-mation.

pedon /ped-on, pee-don/ A three-dimen-sional body of soil, including the fullSOLUM and the PARENT MATERIAL, with acircular or hexagonal cross section, rang-ing in size from 1 to 10 square meters. In-troduced by the US SOIL TAXONOMY methodof classification, it replaces the earlier useof the two-dimensional profile as the low-est unit of classification. It includes all theproperties formerly noted from the profile,plus notes on interfingering of horizons.Similar contiguous pedons are groupedinto polypedons, which can then be amal-gamated into soil series, and hence map-ping of soil is dovetailed into classification.

pegmatite /peg-mă-tÿt/ A very coarse-grained igneous rock occurring as segrega-tions, veins, and dikes within andemanating from granite bodies. Pegmatitesconsist largely of alkali feldspar andquartz, often graphically intergrown, andcontain abundant accessory minerals suchas micas, tourmaline, topaz, spodumene,beryl, cassiterite, and wolframite. The

largest crystals occur in pegmatites, occa-sionally measuring meters in length, andbecause of the concentration of rare ele-ments, pegmatites may be of economic im-portance. Coarse-grained facies of igneousrocks other than granite may be termedpegmatitic. Pegmatites represent a stage inthe crystallization of a magma duringwhich a residual silicate melt and an aque-ous gas phase coexist over the temperaturerange 500–700°C. The growth of largecrystals may be ascribed to conditions ofslow cooling (and therefore slow crystal-lization), rapid diffusion, and low viscositydue to the high concentration of volatiles.See also aplites.

pelagic /pĕ-laj-ik/ 1. Denoting the envi-ronment of the open ocean. A classificationof ocean sediments, based on distance fromthe coast, might include beach, shelf,hemipelagic, and pelagic sediments. Thelatter have been precipitated from the openocean or have accumulated from the or-ganic remains of planktonic marine ani-mals. The main types of pelagic sedimentare the PELAGIC OOZES and RED CLAY. Thehemipelagic-abyssal environment is takento lie within several hundred kilometers ofthe coast, at depths of over 1000 m. Thepelagic-abyssal environment also occurs indepths greater than 1000 m but is fartherfrom the coast. Similar sediments occur inthe hemipelagic environment but in thiscase they are often mixed with land-de-rived material.2. Denoting communities of marine organ-isms (free-swimming nekton and floatingplankton) that survive within the oceanwaters quite independent of shore orseabed environments. Compare benthos.

pelagic ooze Organic deposits made upof shells and other hard parts of marine or-ganisms, which cover large tracts of thedeep-sea floor. The nature of the skeletonsof the various organisms determines thekind of deposit ultimately formed, the de-posits being named after the most promi-nent organisms present. They include thecalcareous GLOBIGERINA and PTEROPOD

OOZES, and the siliceous RADIOLARIAN andDIATOM OOZES. Nearly all the oozes are soft

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and are easily disturbed by crawling orburrowing organisms and deep-sea cur-rents, a fact confirmed by deep-sea pho-tographs. The radiolarian and pteropodoozes occupy very limited areas of deep-seafloor; diatom ooze, a cold-water deposit,occupies some 9% of the total ocean floor,while globigerina ooze, the most wide-spread of the oozes, globally occupies some130 million sq km.

Pelean See volcano.

Pelecypoda /pel-ĕ-sip-ŏ-dă/ See Bivalvia.

Pele’s hair, Pele’s tears /pel-ee/ See py-roclastic rock.

pelite /pel-ÿt/ A metamorphosed ARGILLA-CEOUS rock. Compare psammite; psephiticrock.

pellet An ALLOCHEM that is well roundedbut has no internal structure. Pellets maybe produced by algae or they may be thefeces of invertebrates.

pendant See roof pendant.

penecontemporaneous /pen-ĕ-kŏn-tem-pŏ-ray-nee-ŭs/ Describing geologic eventsthat appear to have occurred at almost thesame time. For example, the depositionand erosion of a bed of sediment are de-scribed as penecontemporaneous if the bedunderwent erosion during, or just after, itsdeposition.

peneplain /pen-ĕ-playn/ An extensivearea of low relief dominated by broadfloodplains and gentle interfluves, with iso-lated residuals (monadnocks) left upstand-ing in areas of resistant rocks.

According to the American geologistWilliam Morris Davis’s original CYCLE OF

EROSION scheme, the peneplain is the endproduct of subaerial denudation. Hisscheme being formulated mostly in thehumid temperate climate of New England,he regarded this as the normal cycle, withthe PEDIMENT and INSELBERG landscapes ofarid and semiarid areas being deviationsdue to climatic controls.

There are no large peneplains on theEarth, because no landscape has evolvedunder just one cycle. All have polycyclicorigins, interrupted by base-level move-ments and process variations. The pene-plain concept is developed from the theorythat slopes evolve by decline, such that asthe cycle nears its end the remaining inter-fluves gradually become inconspicuous asweathering and rill wash continue to erodethem, with the slope foot zones in the val-leys being areas of little activity, mostly ac-cretional in nature. Relative reliefdiminishes, therefore leaving an almost flatplain. This contrasts with the ideas of theSouth African geologist Lester King andGerman geologist Walther Penck, whostated that slopes evolve by PARALLEL RE-TREAT, leaving broad PEDIPLAINS and smallupstanding residuals as the final products.

Penman formula A method of estimat-ing POTENTIAL EVAPOTRANSPIRATION. Evap-oration loss is expressed in terms of thesolar radiation, mean air temperature,mean atmospheric relative humidity, andmean wind speed. It was devised in the1940s by the British physicist Dr HowardLatimer Penman (1909–1984) and the for-mula and subsequent modifications havebeen applied in climatology and in agricul-tural and soil studies.

Pennsylvanian A period within theAmerican classification of geologic timethat followed the MISSISSIPPIAN, and pre-ceded the Permian. It began about 320 mil-lion years ago and lasted for about 34million years. The Pennsylvanian Systemcorresponds approximately to the UpperCARBONIFEROUS elsewhere. It was namedfor the State of Pennsylvania, where rocksfrom this period are widespread and haveproduced much coal.

pentad /pen-tad/ A period of five succes-sive days. It is often used in climatologicalwork because the period divides exactlyinto the number of days in a year, leapyears excepted.

pentlandite /pent-lănd-ÿt/ A yellow ororange mineral, an iron-nickel sulfide,

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(FeNi)9S8. It crystallizes in the cubic sys-tem, and occurs as masses, generally asso-ciated with PYRRHOTITE. It is used as asource of nickel.

percentile The percentage division of aranked data series. For example, the upperfive percentile indicates the value that is ex-ceeded by 5% of the data, and the lowerfive percentile is the value above which95% of the values occur.

percolation /per-kŏ-lay-shŏn/ The pro-cess by which water moves downward,under the influence of gravity, throughpores in soil or cracks and joints in rock.The water may carry dissolved chemicals,or may dissolve minerals from the soil (seeleaching).

percoline /per-kŏ-lin/ A line just belowthe surface that denotes the extent of waterseepage through soil. It generally followsthe slope of the surface, but may be af-fected by plant roots and the activities ofburrowing animals.

percussion mark A circular chip out ofa pebble, caused by a sharp blow againstanother pebble when they were movingrapidly, probably transported by water.

perennial stream A stream or river thatflows permanently throughout the year.Compare intermittent stream.

perfect elasticity The property of a ma-terial that can regain its original form afteran applied force has been removed.

perfect plasticity The property of a ma-terial that retains a new form after an ap-plied force is removed.

periclase /pe-ră-klayss/ A pale-coloredcubic magnesium oxide mineral of compo-sition MgO, found in metamorphoseddolomites and limestones.

pericline /pe-ră-klÿn/ A fold structure,such as a basin or dome, in which the bedsdip around a central point. In a basin theydip toward the center (CENTROCLINAL); in a

dome they dip away from the center(quaquaversal).

peridot /pe-ră-dot/ An olive-green gemvariety of OLIVINE.

peridotite /pe-ră-dot-ÿt/ An ultramaficrock consisting wholly or largely of olivine,together with other ferromagnesian miner-als, and devoid of feldspar.

periglacial /pe-ră-glay-shăl/ Describingthe climate, physical processes (see altipla-nation; congelifraction; congeliturbation;freeze-thaw; frost-shattering; needle ice;solifluction), and resultant landforms (seeinvolution; patterned ground; permafrost;pingo; stone polygons or circles; stonestripes) characteristic of an area borderingan ice sheet or glacier. Although ice sheetsand glaciers do not occur in the periglacialzone, it is still extremely cold, temperaturesremaining below freezing throughout thewinter and rising slightly above only dur-ing the summer. Periglacial zones are re-stricted to high latitudes today but were farmore widespread during the PLEISTOCENE

glacial periods.

perihelion /pe-ră-hee-lee-ŏn/ The pointon the Earth’s orbit that is nearest the Sun.It is also applied to the nearest point to thefocus of any orbiting body. Compare aphe-lion.

period An interval of geologic time inthe Chronomeric Standard scale of chrono-stratigraphic classification (see chrono-stratigraphy). The equivalent StratomericStandard term, indicating the body of rockformed during this time, is the SYSTEM. Pe-riod and system are usually known by thesame name; for example, rocks constitut-ing the Jurassic System were formed duringthe Jurassic Period. Periods are formed of anumber of EPOCHS grouped together andare themselves compounded to form ERAS.Thus the Triassic, Jurassic, and CretaceousPeriods together constitute the MesozoicEra.

periodicity /peer-ee-ŏ-diss-ă-tee/ An os-cillation in a time series of a climatic ele-

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ment that is found to recur at approxi-mately equal time intervals. There havebeen numerous searches for periodicities inclimatic data because they would be ofgreat assistance in long-range forecasting.Unfortunately, although periodicities canbe found for short time periods, they arerarely persistent.

Perissodactyla /pĕ-riss-ŏ-dak-tă-lă/ Theorder consisting of hoofed mammals hav-ing an odd number of toes, such as thehorse and rhinoceros. Horses evolved fromhyracotherium, a small dog-sized five-toedanimal of the Eocene, by increasing in sizeand, as an adaptation to fast locomotion ofthe plains, by gradually losing all but onetoe on each foot. They acquired high-crowned teeth with a complicated enamelpattern, well suited for chewing a diet ofcoarse grass. Horses originated in NorthAmerica and spread to Europe, Asia, andAfrica. Other, now extinct, types of peris-sodactyl lived in the early Tertiary, somereaching a large size, but the order has notachieved the success of the Artiodactyla.

perlitic /pĕ-rit-ik/ Describing glassyrocks that contain concentric onion-likecracks caused by contraction during cool-ing.

permafrost /per-mă-frôst/ Permanentlyfrozen soil occurring in PERIGLACIAL areas,where the winter temperatures rarely ex-ceed freezing point and the soil is frozen toconsiderable depths. In summer only thetop meter or so of soil thaws, that beneathremaining frozen and acting as an imper-meable barrier to percolating water. Thistop layer, which is subject to freezing andthawing, is known as the active layer andwhen melting occurs it rapidly becomessaturated and hence mobile (see muskeg;solifluction). Pedogenic and cryoturbationprocesses are confined to this layer.

permanent hardness See hard water.

permeability /per-mee-ă-bil-ă-tee/ Theability of a rock, sediment, or soil to allowpore fluids and gases to pass through it.This is governed by the extent to which the

pore spaces within the material are con-nected.

Permian /per-mee-ăn/ The final periodof the PALEOZOIC Era. Beginning about 286million years ago and lasting some 40 mil-lion years, it followed the CARBONIFEROUS

and was succeeded by the TRIASSIC (whichmarks the start of the Mesozoic Era). It isnamed for the Perm region of the RussianUrals. The lower part of the Permian Sys-tem, the Cisuralian Epoch, consists of fourages: the Asselian, Sakmarian, Artinskian,and Kungurian. The next epoch, theGuadalupian, consists of the Roadian,Wordian, and Capitanian Ages. This wasfollowed by the Lopingian Epoch compris-ing the Wuchiapingian and ChanghsingianAges.

Large-scale movements of the crustalplates continued into the Permian; the con-tinent of Laurasia in the N hemisphere be-came linked to Gondwanaland in the Shemisphere and, as a result, by the middleof the period the vast supercontinent, Pan-gaea, was created. The continuation of theAllegheny orogeny in E North Americasaw the culmination of the formation ofthe Appalachian fold belt; in Europe andAsia the corresponding Variscan orogenyof the Hercynian Mountains was also di-minishing in intensity by the end of the pe-riod. Characteristic rocks of the Permianare red continental sediments and evapor-ites. They are widespread in North Amer-ica with great thicknesses of depositsoccurring in Texas, New Mexico, Nevada,and Utah.

The period saw the extinction of manyanimal groups, including the tabulate andrugose corals (see Anthozoa), productidbrachiopods, trilobites, and blastoids, anda great reduction in others. Reptiles be-came abundant and pteridophyte plantswere superseded by gymnosperms. See alsoPermo-Triassic.

Permo-Triassic /per-moh-trÿ-ass-ik/ Therocks of the PERMIAN and TRIASSIC Periodsin Britain, when these are considered as asingle unit. Because continental conditionsprevailed for much of the Permian and Tri-assic Periods in Britain recognition of any

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boundary between the two systems is diffi-cult.

perovskite /pĕ-rof-skÿt/ A black orbrown cubic mineral form of calcium ti-tanate, CaTiO3, but usually with somesubstitution of rare earth elements for cal-cium and niobium for titanium. It is foundas an accessory mineral in some highly un-dersaturated alkaline igneous rocks and inthermally metamorphosed limestones.

persistence The continuance of any syn-optic event or climatic element beyond itsexpected duration; the term is frequentlyused for sea-surface temperatures or pres-sure anomalies. In a time series of climaticdata, persistence also indicates a nonran-dom tendency for relatively high or lowvalues to occur in succession.

perthite /perth-ÿt/ An intergrowth of or-thoclase and albite feldspar in which the al-bite occurs as patches or lenticles in theorthoclase host. Perthite may be producedby the reaction of potassium feldspar withsodic fluids or by the unmixing of an orig-inally homogeneous alkali feldspar. See an-tiperthite; exsolution.

perthosite /per-thŏ-sÿt/ A member ofthe SYENITE group of minerals.

Peru Current See Humboldt Current.

pervious Describing a rock throughwhich water can pass along cracks and fis-sures. The water can in this way percolateas far as the water table and form part ofthe supply of groundwater.

petrogenesis /pet-rŏ-jen-ĕ-sis/ The ori-gin or mode of formation of rocks.

petrography /pi-trog-ră-fee/ The studyof the mineralogical and textural relation-ships within rocks revealed by observationof thin sections and hand specimens.

petroleum /pĕ-troh-lee-ŭm/ A naturallyoccurring complex mixture of flammablehydrocarbons, known also as oil or crudeoil. It is a green to black liquid, often of

high viscosity, and occurs seeping out ofthe ground or in underground deposits(from which it is extracted by drilling). Thecrude extract is distilled to produce variousfuels and starting materials for the petro-chemical industry.

petrology /pi-trol-ŏ-jee/ The scientificstudy of rocks, which deals with their clas-sification, origins, distribution, and com-position. Petrology involves mineralogy,chemistry, petrography, and petrogenesis.

pH A measure of the acidity or alkalinityof a solution. It is the negative logarithm ofthe hydrogen ion concentration. A neutralsolution has a pH of 7; a pH of more than7 is acidic, less than 7 is alkaline.

phacolith /fak-ŏ-lith/ A minor concor-dant igneous intrusion, usually lensoid (theconcave surface facing downward) andpresent in folded strata.

phanerocrystalline /fan-ĕ-roh-kris-tă-lin,-lÿn/ (phaneritic) Describing an igneousrock in which the constituent crystals canbe seen with the naked eye. Compareaphanitic.

Phanerozoic /fan-ĕ-rŏ-zoh-ik/ The geo-logic time that has elapsed since the end ofthe PRECAMBRIAN, comprising the PALEO-ZOIC, MESOZOIC, and CENOZOIC Eras. It haslasted for some 570 million years, from theCAMBRIAN Period to the present. The name,meaning visible life, refers to the fact thatclearly recognizable fossils are found inrocks laid down during these periods. InPrecambrian rocks fossils are extremelyrare and often of obscure biological affini-ties.

phase boundary (boundary line) (ingeophysics) The line along which two con-stituents meet where only two are present.In a more complex system it is the linealong which any two liquid phases meet.

phenakite /fen-ă-kÿt/ (phenacite) Awhite or colorless quartzlike mineral formof beryllium silicate, Be2SiO4. It crystallizesin the hexagonal system, and occurs in

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veins in granite. When cut it has a brilliantluster and it is used as a semiprecious gem-stone.

phenoclast /fen-ŏ-klast/ A large frag-ment or clast set in a finer-grained matrixof a sedimentary rock.

phenocryst /fen-ŏ-krist/ A large gener-ally euhedral crystal contained in many ig-neous rocks and set in a fine-grainedmatrix or groundmass. Such rocks are saidto be porphyritic. The phenocrysts are thefirst crystals to form as a melt cools and arethus minerals having the highest crystal-lization temperatures.

phillipsite /fil-ips-ÿt/ A fibrous mineralform of a hydrated aluminosilicate of cal-cium, sodium, and potassium, (Ca,Na,K)3-(Al3Si5O16).6H2O. It crystallizes in theorthorhombic system, and is one of the ZE-OLITE group of minerals.

phi scale A logarithmic scale of particleor grain size in sediments or sedimentaryrocks. The scale is a logarithmic-basedmodification of the UDDEN–WENTWORTH

SCALE. The conversion formula is used:φ = –log2d

where φ is the size expressed in phi unitsand d is the grain diameter (expressed inmillimeters). The scale was devised by theAmerican geologist William ChristianKrumbein. See also particle size.

phlogopite /flog-ŏ-pÿt/ A brownishmineral form of potassium magnesiumaluminum silicate, with some iron, K2-(Mg,Fe)6(Si6Al2O20(OH,F)4. It crystallizesin the monoclinic system, and occursmainly in ultrabasic igneous rocks andmetamorphosed limestones. It is a memberof the MICA group of minerals.

phonolite /foh-nŏ-lÿt/ A strongly under-saturated lava, the volcanic equivalent ofnepheline-syenite. Phonolites are leuco-cratic rocks containing a high proportionof sanidine or anorthoclase feldspar andnepheline. Mafic minerals include sodapyriboles and biotite. Most phonolites areporphyritic and possess phenocrysts of

feldspar, nepheline, and aegirine–augite.The typically aphanitic groundmass has aflinty appearance and the rocks ring whenstruck with a hammer and fracture sub-conchoidally. Trachytes containing acces-sory nepheline up to 10% are termedphonolitic trachytes. In many phonolites,analcite, sodalite, or leucite accompanynepheline. With an increase in the contentof leucite and the disappearance ofnepheline, leucite-phonolites pass intoleucitophyres.

Tinguaite is an obsolete term forphonolite when it occurs as a medium-grained dike rock.

Phonolites often occur with stronglyundersaturated basic lavas, nephelinites,and in association with nepheline-syenitesin oceanic islands and continental rift envi-ronments.

phosphate /fos-fayt/ A salt of phos-phoric acid, containing the ion PO4

3–.Many minerals are composed of phos-phates, although few are common, exceptfor APATITE.

phosphatic nodule /fos-fat-ik/ Arounded mass that occurs in sedimentaryrocks that have been laid down on the seafloor. Up to 30 cm across, they consist ofpieces of corals, mica flakes, sand grains,and shell fragments.

phosphorite /fos-fŏ-rÿt/ (phosphaterock) A calcic phosphate deposit that oc-curs as a marine deposit in roughly nodu-lar form and (according to sampling andphotographs) sometimes in quite large con-centrations. It is mainly used for the manu-facture of fertilizers. It is found on someshelf areas, and over parts of the continen-tal slope. It is currently dredged at a num-ber of offshore sites, for example, off the Ecoast of the USA.

photochemical smog See smog.

photocontour map /foh-toh-kon-toor/A topographic map produced by aerialphotography. All the detail normally ap-parent on topographic maps is shown, theinformation being extracted from the pho-

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tographs using stereoscopic equipment.This technique has been extensively usedfor 1:50 000 series mapping of level under-developed areas of the world, e.g. part ofArabia and Botswana, especially if a fairlyrapid result is required.

photogrammetry /foh-tŏ-gram-ĕ-tree/The technique of producing maps andcharts using stereoscopic equipment to ob-tain reliable measurements from aerialphotography. See photomap.

photomap /foh-toh-map/ A map pre-pared by adding grid information, names,boundaries, and other map data to a re-production of photographs or photomo-saics. It is a quick method of producingmaps, and is often used in areas of little re-lief, e.g. deserts, or for towns. Contourscan be added to these maps for use in areasof relief difference, although this greatlylengthens the time taken to produce themap.

photoperiodism /foh-toh-peer-ee-ŏ-diz-ăm/ The response of plants to variationsin day length. Some plants flower onlywhen the day length (photoperiod) exceedsa specific time; others flower only in pho-toperiods of less than ten hours. This isobviously important in the natural dis-tribution of plants.

photosynthesis /foh-tŏ-sin-th’ĕ-sis/ Thechemical processes by which green plants,algae, and other chlorophyll-containing or-ganisms use the energy of sunlight to makecomplex organic compounds from carbondioxide (from the atmosphere) and water.The main reaction is catalyzed by chloro-phyll, and oxygen is released into the at-mosphere as a by-product. It is a vitalprocess because nearly all plants and ani-mals rely either directly or indirectly on itfor their existence.

phototroph /foh-tŏ-troff/ A living or-ganism that obtains energy from sunlight,such as all green plants. See photosynthe-sis.

phreatic activity /free-at-ik/ The violentreaction that results from hot lava cominginto contact with cold water. The surfaceof the lava chills and forms a glassy skin.The pressure of gasses such as water vaporpresent within the lava then cause this sur-face to fracture. Small fragments are hurledinto the air, the reaction continuing until atuff ring has built up, which separates thelava and water.

phreatic eruption A volcanic eruptioncaused by escaping steam generated whena lava flow comes into contact withgroundwater.

phreatic water GROUNDWATER, espe-cially that occurring below the water table,i.e. in the ZONE OF SATURATION (phreaticzone) where all fissures and voids are filledwith groundwater. Phreatic water maysupply springs and wells. Compare vadosewater.

phyllite /fil-ÿt/ A metamorphosed rockresembling slate but of a coarser grain size.The cleavage or schistosity surfaces have alustrous sheen caused by muscovite andchlorite. Phyllites may possess incipientbanding due to the segregation of quartzand feldspar into layers parallel to thecleavage. The characteristics of phyllitesare essentially intermediate between thoseof slates and schists.

phylum /fÿ-lŭm/ (pl. phyla) A majorgroup in the taxonomic classification of or-ganisms. Animal phyla include the Mol-lusca, Cnidaria, and Chordata. Eachphylum is composed of one or moreCLASSES. In traditional plant classificationsystems phyla are known as divisions. Seetaxonomy.

physical geography The branch of ge-ography that includes certain aspects ofgeomorphology, hydrology, climatology,meteorology, oceanography, and pedol-ogy.

physical weathering See mechanicalweathering.

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phytoplankton /fÿ-toh-plank-tŏn/ Seeplankton.

picotite /pic-ŏ-tÿt/ A dark brown varietyof SPINEL that is rich in chromium.

picrite /pik-rÿt/ An ultramafic rock con-sisting of FERROMAGNESIAN MINERALS andaccessory calcic plagioclase feldspar.

piedmont /peed-mont/ A gentle slopeleading from the foot of a mountain rangedown to comparatively flat land. In aridand semiarid areas, where these featuresmost frequently occur, they consist of aneroded upper segment (known as a PEDI-MENT), which makes an abrupt angle withthe mountain front, followed by an accu-mulation form or BAJADA, consisting oftransported debris from the mountains.This merges into a flat INLAND BASIN orplaya.

piedmont angle The sharp inflexion be-tween lowlands and uplands in pedimentand inselberg landscapes. Characteristi-cally, the scarp of the upland slopes at25–35°, passing to the slope of the pedi-ment at 9° or less over a very short dis-tance, leaving an abrupt angle. This anglecan be very sharply defined, but usually itis a concavity, with the break of slopespread over a considerable horizontal dis-tance.

Its origin is closely linked with theprocesses of PARALLEL RETREAT of the up-land and beveling of the pediment. Somegeologists have found that the angle ismaintained by intense weathering in thescarp foot zone, caused by runoff from thepediment and through flow emerging atthe angle; in other cases it coincides with atectonic or geologic boundary. Others con-sider it to be a product of contrastedprocesses, e.g. unconfined wash on theresidual with channeled water on the pedi-ment (Bryan), or turbulent wash on theresidual with laminar wash on the pedi-ment (King); still others have explained itby slope process, the increased volume ofwater in the scarp foot area undercuttingthe slope of the residual, leading to massmovements which keep the angle sharp. It

is almost definitely produced by differentmechanisms in different areas. See alsopediment.

piedmont glacier A large fan-like lobeof ice spreading out over lower reliefformed when a valley glacier extends be-yond the valley walls that have containedand confined it. It is associated with theterminus or snout of the glacier. TheMalaspina Glacier in SE Alaska, consid-ered the classic example of a piedmontglacier, is formed from the merger of sev-eral valley glaciers and is also known as acompound glacier.

piemontite /peed-mont-ÿt/ A member ofthe EPIDOTE group of minerals.

pigeonite /pij-ŏn-ÿt/ A calcium-poormonoclinic PYROXENE.

pillow lava A lava extruded underwater and having the appearance of pil-lows piled one upon another. The outerskin of the lava is chilled on extrusion anda bubble of lava grows, flattening under itsown weight and producing the characteris-tic pillow shape.

pilotaxitic /pil-ŏ-taks-it-ik/ Describing aclose-packed felted arrangement of acicu-lar MICROLITES. In many trachytes and an-desites, feldspar laths exhibit parallelismdue to flow and are deflected around phe-nocrysts in the direction of flow, this tex-ture being called trachytic.

pinch and swell A deformation featuredeveloped when competent rocks aresqueezed. The more competent beds ex-tend and thin toward their margins andeventually break. The less competent bedsdeform plastically to fill any space. Seeboudinage.

pingo /ping-goh/ A dome-shaped hillfound in PERMAFROST areas. Pingos vary insize but seldom exceed 60 m in height and300 m in diameter. The top may be broken,so that it resembles a crater, and often con-tains a lake. Internally they frequently con-sist of outward dipping beds of stratified

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sand or silt. Many have a visible or sup-posed core of ice. They are believed to havebeen formed as a result of bulging, pro-duced by subsurface pressure build-upwithin isolated groundwaters before thecomplete extension of permafrost over thearea.

pipkrake /pip-krayk/ See needle ice.

pisolith /pis-ŏ-lith/ A pea-sized accretionthat occurs in some sedimentary rocks(such as pisolite). It consists of concentriclayers of calcium carbonate, possibly re-sulting from the biochemical encrustationof algae. See also oolite.

pitch The direction of dip of the axis ofa fold, measured by the angle between theaxis and the horizontal on the axial plane.

pitchblende /pich-blend/ A massiveform of URANINITE.

pitchstone See rhyolite.

pivot fault See hinge fault.

placentals /plă-sen-tălz/ Placental mam-mals (see Eutheria).

placer A surface deposit of sand orgravel that contains significant quantitiesof valuable minerals, such as chromite, di-amonds, gold, platinum, or tin. Smallamounts of the minerals can be removed bypanning; large-scale extraction usually in-volves dredging and concentration of themineral by various processes.

Placodermi /plak-ŏ-der-mÿ/ A class ofextinct jawed fish comprising a number ofprimitive but varied groups common in theDevonian. They had paired fins and, in thearthrodires and antiarchs, an armor of pro-tective bony plates. Some placoderms wereof great size, reaching a length of up to 10 m.

plagioclase feldspar /play-jee-ŏ-klayss/Any member of a series of minerals withcompositions varying between two end-

members, albite (NaAlSi3O8) and anorthite(CaAl2Si2O8). See feldspar.

plain An extensive region of low-lyingland, which is generally flat or gently un-dulating. Most plains are formed by depo-sition of eroded sediments; others arecreated by the wearing away of higher land(DENUDATION), forming a peneplain.

planation surface /plă-nay-shŏn/ A sur-face of low relief, the end product of a cycleof subaerial or marine erosion. Character-istically such surfaces bevel indiscrimi-nately across structures; they may bePEDIPLAINS, PENEPLAINS, or surfaces of ma-rine erosion. In order to develop, a plana-tion surface needs a long period ofbase-level and climatic stability: such con-ditions have not existed since the end of theNeogene Period, because of the oscillationsin the base level and climate of the Pleis-tocene and the post-Pleistocene. Those pla-nation surfaces that do exist therefore arerelic and much dissected. They have beenreconstructed by analysis of relief, whichshows in general concordance of summitlevels at the level of former planation sur-faces.

plane tabling A rapid surveying methodfor fixing detail in fairly open areas. Theplane table is a flat board with an attachedsheet of paper onto which a baseline isdrawn to scale between two known points.The whole stands on a tripod, which is setup over one end of the baseline, with thedrawn line aligned with the actual line.Points of detail are then sighted using anALIDADE and rays to them drawn directlyonto the paper. Once enough points havebeen covered, the plane table is set up atthe other end of the baseline, aligned, andrays drawn to the same detail points. Theiractual positions on the map occur at thepoints of ray intersection on the paper.More accurate fixing can be achieved bydrawing rays from three known points.

planetary boundary-layer See bound-ary layer.

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planetary wind Any of the world’smajor winds, affecting large areas of theglobe. They include the TRADE WINDS of thetropics, the south-westerlies of northerntemperate regions, and the north-westerliesor ROARING FORTIES in the S hemisphere.

planimetric map /plan-ă-met-rik/ Amap on which, unlike topographic maps,no vertical information such as contouringis shown.

plankton /plank-tŏn/ Aquatic and usu-ally microscopic organisms that float anddrift passively in the sea and other bodiesof water (compare nekton). Plankton con-sists mainly of animal larvae, PROTOZOA,and algae, such as DIATOMS. Phytoplanktonrefers to plant and algal forms, on whichall other marine organisms depend, di-rectly or indirectly, for their survival; zoo-plankton refers to the immense variety ofanimal forms. The distribution of bothphytoplankton and zooplankton through-out the oceans is very patchy; plankton-rich areas form highly fertile zones in theoceans, for example where UPWELLING cur-rents occur.

Plankton is of considerable geologic im-portance as it contributes to rock-building;for example, much of the chalk (see Creta-ceous) consists of COCCOLITHS, the skeletalremains of past planktonic organisms. Fos-sil plankton is studied in micropaleontol-ogy (see microfossil), which has importantstratigraphical applications.

planosol /plan-ŏ-sôl/ An INTRAZONAL

SOIL developed in flat areas, principallyunder humid continental warm summerclimates. Planosols are characterized by awhitened A2 horizon passing abruptly to aB horizon of high clay content, either dueto in-situ weathering or LESSIVAGE from theA horizon. Periodic waterlogging resultsfrom the poor drainage due to the high claycontent in the B horizon and lack of relief.They intergrade with a number of soils, in-cluding GRAY-BROWN PODZOLIC SOILS,PRAIRIE SOILS, and surface-water GLEY SOILS.In the US SOIL TAXONOMY they are split be-tween a number of orders, including mol-lisols, alfisols, and ultisols.

plant cover The vegetation of a particu-lar area. The geomorphological signifi-cance of the plant cover is as a balancebetween the soil and weathered rock be-neath and the processes acting on the sur-face from above. Its loss greatly acceleratesrates of erosion, e.g. in areas of cultivation,overgrazing, human trampling, or destruc-tion of vegetation by burning or pollution.The beneficial effects of vegetation includethe binding action of the roots, the divert-ing action of its mass on water flowing onthe surface, shelter from wind action, andthe increase in organic matter on the soilsurface, which improves aggregation andhence the ability of the soil to hold water,thereby reducing runoff. Vegetation canhold soil on a slope that would otherwisebe too steep for stability and hence beeroded; it also intercepts rainfall, prevent-ing RAINSPLASH erosion. In ecologicalterms, plant cover provides HABITATS forvarious organisms.

plastic deformation of ice A processoperating in glacier flow. Glaciers do notmove down their valleys as rigid massessimply sliding across the bedrock. Ice is acrystalline solid and as such changes shape(or deforms) at temperatures near its melt-ing point. Movements between adjacentcrystals are limited but individual crystalscan deform internally, although littlechange in shape appears to occur over a pe-riod of time: this indicates that recrystal-lization accompanies the deformation.Individual crystal deformation is achievedby relative movements of layers, one aboveanother, parallel to the basal plane. Fieldevidence has shown that ice is molded as itmoves across its bed, but that resultant fea-tures are maintained within the ice beyondthe location of molding. This suggests thatthe ice acts plastically under pressure, butthat it remains rigid on removal of the pres-sure.

plastic relief map A three-dimensionalmap produced by printing a topographicmap on plastic and then molding the plas-tic to fit the relief. It is generally used onlyfor demonstration purposes because there

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are still certain inaccuracies involved inthis method.

plastic shading See hill shading.

plate See lithospheric plate.

plateau A fairly flat elevated area ofland. A dissected plateau is broken up byriver valleys or canyons, an intermontaneplateau is surrounded by mountains, andsome plateaus may incorporate the moun-tains themselves.

plateau basalt See flood basalt.

plate boundary (plate margin) Theedge of a LITHOSPHERIC PLATE (tectonicplate). There are three types of boundary:1. Constructive, where new sea floor is

added to the plates on each side of theboundary, e.g. at the Mid-AtlanticRidge.

2. Destructive, where sea floor is destroyedthrough subduction. This is usuallymarked by a deep ocean TRENCH and re-sults from one plate overriding another.See also subduction zone.

3. Conservative, where plates slip passivelypast each other without destroying oradding sea floor. This occurs alongTRANSFORM FAULTS.

plate tectonics A theory arising from aseries of ideas developed in the early 1960sproposing that the surface of the Earth iscomposed of a number of relatively thinplates of rigid material. These LITHOS-PHERIC (or tectonic) PLATES extend down tothe low-velocity zone of the upper mantle.They are all in motion relative to one an-other and it is through these movementsand the consequent collisions between theplates that the present distribution of al-most all volcanic, seismic, and orogenic ac-tivity is controlled. The plates are boundedby PLATE BOUNDARIES. The plates them-selves are composed of either oceanic orcontinental crustal types, or a combinationof both. Only the oceanic parts of theplates grow or are destroyed; the conti-nents ride along on these plates and growonly slowly by the addition of volcanic ma-

terial and sediment deposition along theirmargins. These sediments are compressedand folded when two continental regionscollide by orogenic processes, which resultsin the formation of fold mountain chains.These have a central nucleus of older rocks,frequently cratons.

playa /play-yă/ A flat-bottomed en-closed basin in a desert, sometimes occu-pied by an ephemeral playa lake. Theremay be deposits of EVAPORITES just belowthe surface, and similar deposits on the sur-face when any lake dries up. See also inlandbasin.

Pleistocene /plÿ-stŏ-seen/ The earlierepoch of the QUATERNARY, extending fromthe end of the PLIOCENE about 1.6 millionyears ago, until the beginning of theHOLOCENE Epoch. During this period, oftenreferred to as the ICE AGE, the world expe-rienced great fluctuations in temperature,resulting in cold periods (GLACIALS), sepa-rated by warmer periods (INTERGLACIALS).The Earth’s climate cooled by about5–10°C during the glacials. In NorthAmerica ice sheets developed in Canadaand advanced S into the N USA at leastfour times before retreating. In the Alps,four main glacial episodes can be recog-nized: the Gunz, Mindel, Riss, and Würm.

pleochroism /plee-ŏ-kroh-iz-ăm/ A prop-erty of some crystals that display differentcolors when viewed from different direc-tions under transmitted plane-polarizedlight. It is caused by selective absorption ofsome wavelengths along the different crys-tal axes.

Plesiosauria /plee-see-ŏ-sor-ee-ă/ A groupof Mesozoic reptiles that were secondarilyadapted to an aquatic life but, unlike theICHTHYOSAURIA, were not fishlike. Theyhad a rigid flattened trunk and well-devel-oped powerful paddles for propulsion. Along neck provided the necessary rapidflexibility for catching the fish on whichthey fed. Some plesiosaurs attained alength of 17 m. The group became extinctby the beginning of the Cenozoic.

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Plinian See volcano.

Pliocene /plÿ-ŏ-seen/ The second epochof the PALEOGENE Period, preceded by theMIOCENE and followed by the PLEISTOCENE

Epoch. It began about 5.3 million yearsago and lasted some 3.7 million years.Mammals similar to modern forms existed,and there were definite hominids, includ-ing species of Australopithecus and Homo(see man).

plucking See glacial plucking.

plug 1. Any roughly cylindrical verticalbody of intrusive igneous rock, usually rel-atively small.2. See neck.

plumbago /plum-bay-goh/ See graphite.

plunge The tilt of the axis of a fold fromthe horizontal, the value of plunge beingthe angle between the axis and the hori-zontal lying in the same vertical plane. Seediagram at FOLD.

plunging breaker See breaker.

pluton /ploo-tonn/ A deep-seated majorintrusive body of coarse-grained igneousrock, generally of granitic composition.

plutonic rock /ploo-tonn-ik/ An igneousrock that crystallizes at depth and coolsslowly, resulting in a coarse grain size.Granite is an example. Igneous rocks of thesame composition may show striking tex-tural differences depending on their modeof occurrence. Those that are extruded onthe Earth’s surface as lava flows and coolrapidly, producing a fine grain size, arecalled VOLCANIC ROCKS. Hypabyssal rocksare those that have crystallized at levels in-termediate between those of plutonic andvolcanic conditions. They are medium-grained and occur mostly as sills and dikes.Gabbro and dolerite are, respectively, theplutonic and hypabyssal equivalents ofbasalt.

pluvial period /ploo-vee-ăl/ A period oftime experiencing greater rainfall than pre-

ceding or succeeding periods, usually on ageologic timescale. Many of the semidesertareas of the tropics experienced pluvialsduring full glaciation in polar regions. Thiswas a response to the southward move-ment of the circulation belts and depres-sion tracks. Evidence for these wetterperiods comes from greater water erosion,increased plant growth, and animal re-mains found in areas in which they wouldnot be found under present conditions.However, this has been disputed becauselower temperatures could have reducedevaporation, making the same rainfallamounts more effective.

pneumatolysis /new-mă-tol-ă-sis/ A pro-cess occurring during the final stages of thecrystallization of acid igneous rocks whenresidual borofluoric gas escapes alongjoints and fissures and brings about miner-alogical changes in the crystalline parentrock (compare metasomatism). Threekinds of pneumatolysis are recognized: 1.Greisening. Alteration at the margins ofgranites to assemblages of muscovite,quartz, and topaz (greisen) is the result ofthe action of fluorine-bearing vapors.Original feldspars are pseudomorphed byaggregates of mica, often varieties rich inlithium such as zinnwaldite. The end prod-uct of the process is a quartz-topaz rockknown as topazfels. 2. Tourmalinization.The boron pneumatolysis of granitic rocksresults in the growth of TOURMALINE at theexpense of feldspar and biotite. The rock,luxullianite, contains radiating aggregatesof black tourmaline but some feldspar sur-vives. The final stage is a quartz-tourma-line assemblage known as schorl rock. 3.Kaolinization. Feldspars in granitic rocksmay be altered to aggregates of kaoliniteand sericite under the action of high-tem-perature aqueous fluids. The end productof this pneumatolytic/hydrothermal pro-cess is KAOLIN.

podzol /pod-zol/ (podsol) A soil charac-terized by an ashen-colored acid eluviatedA horizon and a B horizon illuviated withiron or humus, possibly in the form of acompact pan. The A horizon is depleted ofbases and sesquioxides by LEACHING

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processes, especially CHELUVIATION, whilethe B horizon is divided into an upper part(Bh) dominated by the redeposited humusand a lower part (Bs or Bfe) dominated bythe redeposited iron, and to a lesser extent,aluminum. If both the Bh and Bs horizonsare present, the soil is an iron-humus pod-zol; if the Bh only is present, a humus pod-zol; if Bs only, an iron podzol.

Podzols are zonally developed soils inthe TAIGA zones of Russia and North Amer-ica, where they form a mosaic with reg-osols, gleys, and peats, and develop frominitiation directly on the parent material.Elsewhere podzol is intrazonal, e.g. oncoarse deposits such as sands and gravelsor human-induced by forest clearance. Asthe profile matures, the Bh-s horizons maybecome impermeable and lead to GLEYING

producing hydromorphic variants such asPEATY GLEY PODZOLS.

podzolic soil /pod-zol-ik/ (podsolic soil)A soil with certain features of morphologyand genesis akin to true podzols. There is alarge number of varieties: in North Amer-ica there are gray-brown podzolics in a beltbelow the true podzols, which developslowly by increased leaching, destructionof the Bt horizon, and graying of the A2horizon to brown podzolics. In the south,in the subtropical areas, are red-yellowpodzolics, which have free aluminum andferric oxides like other tropical soils butalso clay skins in the B horizon.

podzolization /pod-zŏ-lă-zay-shŏn/ (pod-solization) The movement of clays andsesquioxides down the soil profile andtheir deposition in the B horizon. The dom-inant process is CHELUVIATION by organicacids, but opinions vary as to the otherprocesses involved. Some pedologists be-lieve that podzolization operates best inacidic and anaerobic conditions, so thatthe cheluviation is preceded by DECALCIFI-CATION, LESSIVAGE, and clay destruction byweathering. Others have suggested thatacidic conditions are not necessary, and de-calcification is therefore not a necessarypreliminary; it has also been argued thatanaerobic conditions (i.e. waterlogging)are not necessary because the organic acids

can reduce the sesquioxides themselves,and do not rely on the anaerobic reductionof iron from its insoluble ferric to its solu-ble ferrous state. Podzolization is a zonalprocess in areas of natural LEACHING andconiferous vegetation, which togetherfavor intense cheluviation, the chelates ofpine litter being among the most powerful.Intrazonally, it is the dominant process oncoarse-textured parent materials, or whereforest clearance has increased leaching.

poikilitic /poi-kă-lit-ik/ Describing ig-neous rocks in which large crystals of amineral completely enclose crystals of ear-lier-formed minerals. Poikiloblastic de-notes a similar texture in metamorphicrocks. See also ophitic.

poikiloblastic /poi-kă-lŏ-blas-tik/ Seepoikilitic.

point bar A depositional feature thatdevelops on the inside of meanders, com-plementing the erosion that occurs on theoutside of meander bends. Point bars ex-tend downstream from the point of maxi-mum curvature of the meander bend,leaving a trough between themselves andthe bank, which eventually becomes filledup by the sedimentation of fine material.See meander.

Poisson’s ratio /pwah-sawnz/ The ratiobetween the fractional longitudinal strainand the fractional lateral strain in a de-formed material. It is equal to the ratio ofchange in diameter divided by change inlength. It is named for the French mathe-matician and physicist Siméon Denis Pois-son (1781–1840).

polar Describing the climate that ischaracteristic of the regions within the Arc-tic and Antarctic Circles (around the Northand South Poles). They have permanentlylow temperatures, a short growing season,and no trees.

polar air depression See polar low.

polar easterlies The easterly wind beltbetween the weak polar anticyclone and

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the westerly depression tracks. Because ofthe variability of the intensity and tracks ofthe mid-latitude depressions and the weak-ness of the polar anticyclone, the polareasterlies are rarely strong or persistent.

polar front The major frontal system,situated in the N Pacific and N Atlantic,separating tropical and polar air masses.Depressions are often initiated on this pro-nounced thermal gradient and the Norwe-gian meteorologist Vilhelm Frimann KorenBjerknes (1862–1951) based his theory offrontal evolution on it. In summer the ther-mal contrast across the front is less and itsposition is much more variable than inwinter.

polar-front jet stream See jet stream.

polar glacier See cold glacier.

polar high (polar anticyclone) An ex-tensive area of semi-permanent high pres-sure in the polar latitudes. The anticycloneover Antarctica (the Antarctic high) issemi-permanent whereas that over the Arc-tic (the Arctic high) is more seasonal. Thepolar high is a source of very cold and dryair.

polarity reversal See magnetic reversal.

polarization colors See birefringence.

polarized light In ordinary light theelectromagnetic vibrations take place in alldirections in a plane at right angles to thedirection of propagation of the ray of light.When vibrations are confined to a singledirection in this plane, the light is said to beplane-polarized. Certain crystals constrainordinary light to vibrate in only two direc-tions at right angles to one another; i.e.they polarize it. See anisotropic; birefrin-gence.

polar low (polar air depression) A smallshadow depression or cyclone that formsover the ocean, poleward of the polar front(or other major baroclinic zone), and ex-tends horizontally for several hundredkilometers. It brings severe weather with

strong near-gale or gale force winds andheavy showers of precipitation, much assnow, but is short-lived and rapidly dissi-pates over land. Intense polar lows aresometimes known as Arctic hurricanes andthe cloud pattern in satellite images has asimilar appearance to that of a tropical cy-clone.

polar night jet stream A very steepthermal gradient in winter giving corre-spondingly very strong westerly winds, sit-uated around the stratospheric cold pole atlevels of 20–30 km. During summer thestratosphere at this level warms up becauseof absorption of ultraviolet radiation, anda reversal of the wind system takes place togive stratospheric easterlies.

polar wandering curve A theoreticalline produced for a particular point on theEarth’s surface by joining successive paleo-magnetic pole positions through time.

polar zenithal gnomonic projection AMAP PROJECTION in which the center of theprojection is the pole, from which thestraight lines representing the great circlesradiate (see diagram). The parallels areportrayed as concentric circles. It is com-monly used to show the polar areas, the ex-aggeration becoming too great in areasfarther away from the center.

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polar zenithal gnomonic projection

90°90°90°

60°

30°0°

30°

60°

90°60°

45°

15°

Equator

projected viewon tangent plane

Polar zenithal

polder A low-lying tract of land that hasbeen reclaimed from a body of water suchas sea or lake and is enclosed by embank-ments or dikes. Polders are frequentlybelow the level of the sea or lake fromwhich they have been reclaimed. They areespecially common in the Netherlands onthe North Sea coast but also occur else-where.

pole of rotation (Euler pole) A poleabout which a lithospheric plate rotatesduring sea-floor spreading and continentaldrift.

polje /pol-yay/ The largest type of solu-tion depression found in limestone areas,extending up to 250 sq km. The floors sup-port a certain amount of surface drainage,are alluvium-covered, and often uneven,reflecting the collapse of former cave sys-tems. Poljes are frequently surrounded bysteep marginal walls, up to 100 m inheight, while considerable numbers showsigns of being tectonically bounded basins.

pollen Microscopic spores of the higherplants (gymnosperms and angiosperms).Because pollen is very resistant to destruc-tion and may be carried for large distancesby the wind, fossilized pollen provides avaluable means of correlating the rocks inwhich it occurs. Pollen may also be used asa sensitive environmental indicator and hasproved particularly useful in monitoringclimatic change during the Quaternary.The study of pollen and other spores isknown as PALYNOLOGY.

pollen analysis A technique of both rel-ative dating and environmental reconstruc-tion, consisting of the identification andcounting of pollen types preserved in peatsand lake beds. It is based on the assump-tion that the vegetation in an environmentis in equilibrium with it, and therefore re-flects its conditions; dispersal of pollenleads to some of it being preserved in re-ducing environments, such as peat bedsand lake bottoms. Subsequent careful closesampling and analysis allows the structureof the vegetation at each particular horizonin the deposit to be worked out. Reference

to the present-day vegetation in differentconditions allows deduction, by analogy,of the environment that the vegetationlived in. This method has shown well-de-fined sequences of changes for each of theinterglacials and the postglacial period,which allowed relative correlation betweendifferent sites, and hence dating.

pollution dome An accumulation ofpolluted air in a low dome-shaped layerover a large urban or industrial area. Thepollutants are trapped in the warm airbelow a temperature inversion (see inver-sion layer) when conditions are calm orwinds are light and there is persistent highpressure. See also heat island.

pollution plume 1. A trail of pollutedair that extends downstream from a pollu-tion source, such as a large urban or indus-trial area.2. In a stream or in groundwater, an areaof pollutants that moves along the flowfrom the point of origin.

polyconic map projection /pol-ee-kon-ik/ A modified CONICAL PROJECTION inwhich each parallel is treated as a standardparallel on a normal conical projection. Allthe parallels and the central meridian aretruly divided, the meridians being con-structed by joining the divisions on the par-allels. This projection is neither anORTHOMORPHIC PROJECTION nor a HO-MOLOGRAPHIC PROJECTION and is thereforerestricted to showing small areas. See alsomap projection.

polycyclic landscape /pol-ee-sÿ-klik/ Alandscape that has evolved under a numberof geomorphic cycles (see cycle of erosion)or part cycles of different types. The inde-pendent variables of geomorphology areclimate, geology, and base level: a changein any one starts a new cycle. Regionalstudies of landscape evolution emphasizethat most if not all landscapes have experi-enced changes in these factors and havehence evolved under several different cy-cles. This realization has led to the aban-donment of the youthful, mature, andsenile labels for landscapes, because they

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assume that each landscape is a product ofonly the current cycle, variation being dueto the STAGE of that cycle. Instead eachlandscape can be seen as composed of ele-ments from several different cycles, onlysome of which are related to current condi-tions.

polygenetic /pol-ee-gĕ-net-ik/ (in geo-morphology) Describing a soil or land-scape that has evolved under a number ofdiffering conditions that occurred in suc-cessive phases of its history. Each of theseconditions has played a part in influencingits current appearance. With landscapes, achange in base level, climate, or geologywill institute a new cycle and hence super-imposition of varying conditions in its evo-lution; with soil, a change in climate,vegetation, topography, or drainage willinstitute a new set of soil-forming condi-tions and hence change the direction of thesoil’s evolution. Some authorities considerthis term to be synonymous with POLY-CYCLIC when applied to landscapes; othersconsider that polygenetic should be used todescribe only the influence of minorchanges in conditions, leaving polycyclicfor the major changes.

polymorphic transition /pol-ee-mor-fik/A change in minerals that involves achange in their atomic structure but nottheir chemical composition, e.g. aragonite(CaCO3) to calcite (also CaCO3).

polymorphism /pol-ee-mor-fiz-ăm/ Theability of some substances to exist in twoor more structurally distinct forms, eachhaving its own characteristic properties buthaving identical chemical compositions.Calcium carbonate, CaCO3 occurs as arag-onite and calcite. Andalusite, kyanite, andsillimanite are polymorphs of the alu-minum silicate, Al2SiO5. Graphite and dia-mond are mineral polymorphs of theelement carbon.

polypedon See pedon.

polyphase deformation Folding andfaulting occurring in several periods duringa single orogeny.

pools and riffles /pol-ee-fayz/ The alter-nating gravel bars (riffles) and pools thatoccur along a stream: these features areregularly spaced, usually such that the dis-tance between successive pools is 5–7 timesthe channel width. In meandering streams,lengths of meanders are about twice thedistance between pools, and there may be arelationship between the pool and riffleformations and MEANDER creation.

Porifera /pŏ-riff-ĕ-ră/ The sponges: aphylum of simple sessile aquatic (mostlymarine) multicellular animals with asaclike body, often containing small cal-careous or siliceous skeletal elementsknown as spicules. Water is drawn into thebody through small holes and expelledfrom an opening, the oscula, during whichfood particles are filtered off. The spiculesmay become fossilized. Fossil sponges areuseful environmental indicators and havecontributed to the growth of bioherms andformation of rocks. They are known tohave extended from the Cambrian Periodto the present day, and structures found inPrecambrian rocks have been attributed tothis group.

porosity The extent to which a body ofsoil, rock, or sediment is permeated withcavities between grains, usually expressedas a percentage of the volume. These poresare filled by air and water (air is mainly inthe larger macropores and water in the mi-cropores), which impede air movementand allow water to move by CAPILLARITY

only. In sands and compact soils, where theparticles lie close together, porosity is low(25–30% in compacted soils); conversely,where organic matter content is high, pro-moting good aggregation, porosity is high,reaching possibly 60%. Cropping, whichremoves organic matter, eventually lowersporosity, and hence aeration.

porphyritic /por-fă-rit-ik/ Describing anigneous rock that possesses large crystalscalled PHENOCRYSTS set in a finer-grainedgroundmass.

porphyroblastic /por-fă-roh-blas-tik/ De-scribing a metamorphosed rock containing

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large crystals set in a fine-grained matrix.The large crystals are called porphyrob-lasts by analogy with the corresponding ig-neous term, PORPHYRITIC.

porphyry /por-fă-ree/ A medium- orfine-grained igneous rock containing nu-merous PHENOCRYSTS. The porphyriticmineral may be indicated by the prefix of amineral name, e.g. quartz-porphyry. Arhomb-porphyry is so called on account ofthe distinctive shape of the feldspar phe-nocrysts.

positive area A large area of the Earth’scrust that has remained above sea level fora long period of geologic time.

positive gravity anomaly See gravityanomaly.

positive movement of sea level A fallof the land relative to the sea, usually dueto sea-level rise, as has happened since theend of the last ice age. The results are basi-cally twofold: flooding, and deposition ofmaterial creating broad low plains.

Flooding produces estuaries or riasfrom inundated river valleys; estuariescharacterize lower-lying areas (e.g. theChesapeake and Delaware bays on the At-lantic Coast of the USA), rias the morerugged areas (e.g. in SW Britain, SW Ire-land). If the valleys inundated are glacial inorigin, fiords result (e.g. in Norway). Low-lying parts of the land may be floodedwholesale, creating new seas and severingislands from the mainland (the British Isleswere severed from the rest of Europe by thepostglacial sea-level rise).

The effect on rivers is that CAPACITY andCOMPETENCE are reduced, leading toAGGRADATION in the lower reaches, whichfills up the newly created estuaries and cre-ates broad alluvial floodplains. The lengthsof drainage systems are reduced, leavingburied portions beneath the elevated sealevel. See also negative movement of sealevel.

postkinematic /pohst-kin-ĕ-mat-ik/ De-scribing minerals developed after a period

of deformation (usually metamorphic min-erals).

potassium feldspar A type of alkaliFELDSPAR that contains potassium alumi-nosilicate, KAlSi3O8. The two principalcrystal forms are the monoclinic ORTHO-CLASE and triclinic MICROCLINE. Adulariaand sandine are also alkaline feldspars.

potential energy The energy possessedby a body as a result of its position. It ismeasured by the amount of work requiredto move that body from a position of zeropotential energy, usually sea level, to itsnew position. Stationary air at high levelsof the troposphere therefore has a high po-tential energy.

potential evapotranspiration (PE) Theevaporation from an extended surface of ashort green crop, actively growing, com-pletely shading the ground, of uniformheight, and possessing an adequate supplyof soil moisture. It is essentially a functionof climate rather than the nature of the veg-etation, and as such was used by Thornth-waite as a method of climatic classification(see Thornthwaite classification). It can beestimated or measured more easily and ac-curately than ACTUAL EVAPOTRANSPIRATION

and so has achieved more universal accept-ability in studies of evaporation.

potential temperature The tempera-ture of a parcel of air if it is uplifted or sub-sides at the DRY ADIABATIC LAPSE RATE to astandard pressure, usually 1000 mb. Use isalso made of the wet-bulb potential tem-perature by changing the temperature ofthe parcel of air at the SATURATED ADIA-BATIC LAPSE RATE from the wet-bulb tem-perature of the original level to 1000 mb.This is useful because the wet-bulb poten-tial temperature is a conservative propertyin such processes as evaporation and con-densation, and both dry and saturated adi-abatic temperature changes.

pothole /pot-hohl/ 1. A circular holeworn in rock in an eddy of a stream orriver, often at the foot of a waterfall, bymoving pebbles and gravel.

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2. See sinkhole.

prairie A region of flat treeless temper-ate grassland of Canada and the USA, ex-tending from Alberta to Texas. Theprairies have summer rain and fairly drywinters. Once home to large herds ofbison, cattle, and sheep, much of the east-ern part has been plowed for growing ce-real crops, particularly wheat. Repeatedplowing in the drier western prairies com-bined with drought conditions resulted inerosion and the formation of a DUST BOWL

during the 1930s.

prairie soil (brunizem) Dark soil of sub-humid grasslands in which the profile hasbeen decarbonated but leaching has notproduced any movement of sesquioxides.The carbonate appears as flecks in the Chorizon. These soils develop under tallgrass vegetation and have a brown or gray-brown A horizon grading to a yellow-brown B horizon, with blocky structuresand a light parent material at around onemeter. Their exact status is not certain.They may be degraded CHERNOZEMS ormore related to the podzolic group, withwhich they intergrade at their boundary, ortheir distinctive profile may be caused by awater table standing in the profile produc-ing GLEYING. In the US SOIL TAXONOMY theyfall in the order of MOLLISOLS.

Pratt’s hypothesis A proposed mecha-nism of hydrostatic support for the Earth’scrust (named for the British physicist JohnHenry Pratt (1809–71). This hypothesis re-lies upon crustal density being greaterunder mountain chains than under oceans.If this were true at a datum level, called theCOMPENSATION LEVEL by Pratt, rockcolumns of equal diameter would haveequal mass.

Precambrian /pree-kam-bree-ăn/ Thegeologic time prior to the Cambrian Periodor the rocks stratigraphically below theCambrian System. Because the age of theEarth’s crust has been estimated at about4600 million years, and the beginning ofthe Cambrian is dated around 570 millionyears ago, the Precambrian represents

most of geologic time. Precambrian rockscontain rare FOSSIL evidence and, be-cause of their extreme age, have often beensubjected to a great deal of subsequent al-teration. The original stratigraphical rela-tionships are therefore frequently obscure.The SHIELD areas represent the largest areasof exposed Precambrian rocks.

The Precambrian is often now dividedinto three eons. The earliest time span isthe Hadean (‘beneath the Earth’), fromwhich time virtually no rocks or fossils re-main. During the Archean eon (about 3.8to 2.5 billion years ago) evidence for theearliest known life forms (early bacteria)appears in rocks (see stromatolite). Rocksof the Proterozoic eon contain fossil evi-dence of the first multicelled organisms, in-cluding jellyfish and soft marine worms.Calibration and correlation of isolated suc-cessions is achieved mainly by radiometricdating.

precession /pree-sesh-ŏn/ A form of mo-tion of a rotating body that results when acouple, having its axis at right angles to theaxis of rotation, is applied to the body sothat it then turns about a third mutuallyperpendicular axis. As a result of the grav-itational attraction of the Sun and Moon,the Earth’s axis precesses and traces out aconical figure in space.

precipitable water /pri-sip-ă-tă-băl/ Theamount of water that could be obtained ifall the vapor in a standard column of airwas condensed onto a horizontal surface ofunit area. It is a useful index of the mois-ture content of air above a specified point,although precipitation processes are neverso efficient that all the water would be pre-cipitated and it neglects the effects of vaporadvection. It is calculated by the formula:

Mw = (1/g)∫p1p2

rdp

where Mw is the precipitable water, g is theacceleration of free fall, p1 and p2 are thepressures (mb) at the top and bottom of thelayer, r is the mixing ratio, and dp is thedepth of the individual layer of mixingratio.

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precipitation /pri-sip-ă-tay-shŏn/ Thedeposition of water in solid or liquid formfrom the atmosphere. It therefore covers awide range of particles including rain, driz-zle, sleet, snow, hail, and dew. Precipita-tion is initiated within clouds by theBergeron–Findeisen process (see Berg-eron–Findeisen theory), by the collision–coalescence process, or by a combinationof both operating together. For precipita-tion to reach the ground a number of otherconditions must be satisfied. The dropletor ice crystal produced by one of the abovemechanisms must be sufficiently heavy toovercome upward vertical motion andlarge enough to withstand evaporation be-neath the cloud base. Thus, some cloudscan be seen to be giving precipitation thatdoes not reach the ground.

precipitation effectiveness A measureof the usefulness of the annual rainfall totalfor agricultural purposes or for hydrology.For example, 600 mm of rain in temperatelatitudes with low evaporation maintainshumid vegetation and surplus runoff forriver flow. In the tropics, such an annualtotal would give semidesert. Precipitationeffectiveness was used by Thornthwaite asa basis for climatic classification (seeThornthwaite classification). However, thesparsity of data of both measured and cal-culated POTENTIAL EVAPOTRANSPIRATION,from which precipitation effectiveness isobtained, has prevented its use on a globalscale.

precipitation variability The annualvariation in rainfall totals, althoughshorter time periods can be used. It is as-sessed by many statistical methods but themost common is the coefficient of varia-tion, which is the standard deviation of theannual totals divided by the mean annualvalue, multiplied by 100 to be expressed asa percentage. In Britain, values range frombelow 10% in the NW to 20% in S Eng-land, but these are low compared withmany other parts of the world. Malden Is-land in the central Pacific is believed tohave the highest rainfall variability withannual rainfall totals varying between 30mm and 1422 mm in a thirty-year period.

predator An animal that kills other ani-mals (the prey) for food. It is a secondary(and sometimes tertiary) CONSUMER in aFOOD CHAIN.

prehnite /pray-nÿt, pren-ÿt/ An ortho-rhombic mineral of composition Ca2A12-Si3O10(OH)2 found chiefly in basic lavas asa secondary mineral associated with ZEO-LITES. It is also found with pumpellyite inlow-grade regionally metamorphosedrocks.

pre-kinematic /pree=kin-ĕ-mat-ik/ De-scribing minerals that are formed beforedeformation.

pressure (in meteorology) The weight ofair vertically above a unit area centered ona point. It is measured by balancing theforce exerted by the atmosphere with adense liquid, usually mercury. As a resultof this, atmospheric pressure was formerlyquoted as a number of millimeters of mer-cury representing the length of the mercurycolumn required to balance air pressure. Itis now measured in MILLIBARS. Sea-levelvalues range from extremes of about 890mb in hurricanes to 1060 mb in strong an-ticyclones.

pressure gradient force The force thatacts on an air molecule by virtue of the spa-tial variations in pressure at any horizontallevel. It is the primary motivating force ofair movement in the atmosphere.

pressure release See unloading.

pressure system A pattern of isobarsthat exhibits coherent weather characteris-tics. It normally denotes low-pressure areasor anticyclones.

pressure tendency The local rate ofchange of surface pressure. It is recorded atmost observing stations with a 10 pointcode to distinguish various trends of thepressure trace in the preceding time period,usually 3 hours. It is very useful for indi-cating future movements of pressure sys-tems.

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prevailing wind The most frequentlyoccurring wind at any site. In most parts ofthe globe this will correspond with the cli-matic zone. Occasionally local factors,such as wind funneling in a valley, becomedominant. Also for shorter time periods,the prevailing wind may be different fromits annual value, as in monsoon areas.

primary consumer (in ecology) An or-ganism in the second level of a FOOD CHAIN,usually a HERBIVORE. It provides food forsecondary consumers (carnivores).

primary magma A magma whose com-position has remained unchanged since itwas first formed.

primary mineral A mineral that wasformed at the same time as the rockcontaining it and which has remained un-changed in composition. Compare sec-ondary mineral.

primary producer (in ecology) An or-ganism at the lowest level of a FOOD CHAIN.It ‘feeds’ on inorganic substances. Greenplants and photosynthetic algae are exam-ples; they produce carbohydrates from car-bon dioxide and water using the energy ofsunlight in PHOTOSYNTHESIS. They can thenbecome the food for consumers in the nextlevel of the food chain.

primary wave (P wave) A type of SEIS-MIC WAVE, the first to be recorded on aseismogram of an earthquake. These longi-tudinal waves travel by a series of com-pressions and rarefactions (pushes andpulls) in the direction of propagation, i.e.each particle vibrates backward and for-ward along the direction in which the waveis traveling. P waves can pass throughsolids, liquids, and gases. See also sec-ondary wave.

Primates /prÿ-may-teez/ The order ofmammals that includes man, apes, mon-keys, and lemurs. They are characterizedby adaptations for an arboreal habitat,such as flexible limbs and hands, anteriorlydirected eyes providing stereoscopic vision,often a long tail for balance, a relatively

large brain, and an unspecialized dentition.Fossil primates are known from the begin-ning of the TERTIARY onward; they proba-bly evolved from the INSECTIVORA.Monkeys appeared in the Oligocene andthe remains of humanoid forms have beenfound in Pliocene strata (see man).

prime meridian See meridian.

primitive equations The physical equa-tions governing momentum in the atmos-phere. They have been used mostextensively in models simulating the gen-eral circulation of the atmosphere.

principal shock The suite of waves pro-duced by the main movement in an earth-quake. See also aftershocks; foreshock;seismic wave.

prismatic Describing the habit of somecrystals that are elongated and show well-developed prism faces. See crystal habit.

prismatic compass A magnetic com-pass within which the needle, or the zero ofa 0° to 360° scale, always points towardmagnetic north. Also incorporated is asighting arrangement, consisting of a verti-cal hairline at the far side and a prism witha sight at the observer’s side, which allowsthe object and the graduated scale to beviewed simultaneously. Readings takenwith such a compass are true bearings frommagnetic north.

Proboscidea /proh-bŏ-sid-ee-ă/ The orderof mammals that includes the modern ele-phants and the extinct mammoths. Fossilproboscideans are known from theOligocene. They were small creatures withlong jaws, both upper and lower bearingtusks, and short trunks. Evolutionary mod-ifications associated with the subsequentincrease in size have occurred: solid limbsand flat feet support the great weight; thetrunk has evolved into an efficient food-gathering organ; the teeth have lengthened,and the jaws have shortened, in modernforms accommodating only one largegrinding tooth at a time, which is replacedthrough life. Formerly a widespread and

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diverse group, the Proboscidea are todayconfined to Africa and S Asia.

producer See primary producer.

profile 1. (soil profile) The arrangementof soil HORIZONS found between theground surface and the parent material.The normal depth of a profile in temperatelatitudes is about a meter. Pedologists, no-tably Dokuchaev (see soil formation), haveassigned letters to certain horizons. Thesurface horizon is known by the letter Aand is usually rich in organic matter andplant nutrients. Because material is oftenleached from this horizon down the profileto the subsoil B horizon, it is known as theeluvial horizon (see eluviation). The B hori-zon, known as the illuvial horizon (see illu-viation), contains less organic matter andmore closely resembles the parent materialowing to the lesser effect of the soil-form-ing processes. This horizon usually mergesinto the C horizon, which is the weatheredparent material. Well-developed soils con-sist of more than these three horizons andsubdivision occurs. Characteristics of cer-tain horizons are symbolized by a suffix orprefix, e.g. Bh = humus-rich B horizon; Ap= plowed A horizon. Superficial organichorizons are denoted by the letters L (lit-ter), F (fermenting), and H (humus).Groups of soils having a similar suite ofhorizons are linked in broad soil zones.2. See river.

progressive wave A wave that can berecognized by the progressive movement ofthe waveform at the sea surface. For exam-ple, such a wave would be propagatedalong a channel of infinite length, havingbeen generated at one or other end of it bya wave paddle. In progressive waves, thespeed of propagation of the waveform willlargely depend upon the depth of water,one example being a tide wave that may bevery long in relation to the water depth. Aswith wind-generated waves, the speed ofwave propagation far exceeds the speed atwhich the water particles themselves ad-vance. The wave-induced currents associ-ated with progressive waves are at theirmaximum at the highest point of the wave

crest and the lowest point of the wavetrough, although the direction of the cur-rent as between crest and trough is re-versed.

projection (in cartography) See mapprojection.

promontory /prom-ŏn-tŏ-ree/ A smallpeninsula or headland that projects intothe sea. Wave action can cause erosion onboth sides, which are likely to have cliffsand possibly caves.

Proterozoic /prot-ĕ-rŏ-zoh-ik/ See Pre-cambrian.

proto-Atlantic Ocean (Iapetus Ocean)An ocean that existed between the late Pre-cambrian and early Devonian times.Within this ocean, which reached its maxi-mum extent during the Ordovician, a seriesof sediments were deposited on an oceanfloor of pillow lavas. When the two conti-nents on each side of the ocean closed,these sediments were subjected to orogenicprocesses that resulted in the formation ofthe Caledonian mountains of Norway andScotland, and the Appalachian mountainsof North America. Evidence for the closureof this ocean can be found in Anglesey,Wales, where typical rocks associated witha destructive PLATE BOUNDARY are presentin fairly close association with pillowlavas.

protozoa /proh-tŏ-zoh-ă/ Small, usuallymicroscopic, unicellular organisms, for-merly classified as animals but now usuallyincluded with certain other organisms inthe kingdom Protista (or Protoctista). Pro-tozoa of the orders FORAMINIFERA and RA-DIOLARIA possess skeletons, either secretedor of agglutinated material, which may befossilized. They are present in large num-bers in marine plankton, and someForaminifera are benthonic. Fossils areknown from the Cambrian Period onward,perhaps even the Precambrian; they are im-portant in micropaleontology, and instratigraphic correlation, especially ofrocks from borehole cores. Under certainconditions their remains accumulate in suf-

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ficient numbers to contribute to rock for-mation.

provenance The source area fromwhich the particles composing sedimentsare derived.

province (faunal province) A large re-gion characterized by a particular assem-blage of animal species, which differs fromcontemporaneous assemblages in similarenvironments elsewhere. The differencesbetween faunal provinces have arisen be-cause the communities have been geo-graphically isolated from each other overlong periods of time and have evolved indifferent ways. The detection of ancientfaunal provinces is valuable in reconstruct-ing paleogeography.

psammite /sam-ÿt/ A metamorphosedARENACEOUS rock (see sandstone). Com-pare pelite; psephitic rock.

psephitic rock /sĕ-fit-ik/ A metamor-phosed RUDACEOUS rock. Compare pelite;psammite.

pseudo-bedding See unloading.

pseudo-karren /soo-doh-ka-rĕn/ KAR-REN (grooves) found on the surfaces ofrock types not normally associated withthe process of solution.

pseudomorph /soo-doh-morf/ 1. A min-eral that has assumed the external form ofanother earlier mineral. Pseudomorphsmay be produced as a result of:(a) Replacement or alteration of one min-eral by another. In certain cases thepseudomorph may be a polymorph of theoriginal mineral, e.g. calcite after aragonite(see polymorphism). Other common exam-ples of pseudomorphism include gypsumafter anhydrite, iddingsite after olivine,and kaolinite after feldspar.(b) Encrustation or investment when amineral is deposited as a crust on crystalsof another, e.g. quartz on fluorite. Some-times the first mineral is removed, leaving anegative pseudomorph.

(c) Infiltration by a different mineral orsubstance of a cavity or mold previouslyoccupied by a soluble crystal, e.g. claypseudomorphs after halite.2. A fossil in which the original skeletalsubstance of the organism has been re-placed by a secondary material in thecourse of fossilization, which preserves itsshape. See also cast; mold.

pseudotachylite /soo-doh-tak-ă-lÿt/ Aglassy material produced by the fusion ofcrushed rock by frictional heating, andfound as narrow veinlets and streaks inmylonite zones.

psychrometer An instrument that mea-sures the RELATIVE HUMIDITY of the atmos-phere. It is a type of HYGROMETER andconsists, in its basic form, of a wet-bulband a dry-bulb thermometer that aremounted side-by-side.

pteridophytes /te-rŏ-dŏ-fÿts/ Plants thathave a vascular system, leaves, stems, androots but reproduce by spores (rather thanseeds). They are predominantly terrestrialand include the ferns and horsetails. Pteri-dophytes were especially abundant in thelate Paleozoic but declined during theMesozoic. These plants formed much ofthe coal-measure forests in the Carbonifer-ous Period, when many grew to the size ofmodern trees.

pterodactyl /te-rŏ-dak-tăl/ A colloquialterm for one of the PTEROSAURIA, takenfrom the name of one of the genera of thisorder (Pterodactylus).

pteropod ooze /te-rŏ-pod/ A calcareousdeep-sea ooze deposit (see pelagic ooze)containing more than 30% organisms. Itcomprises the shells of pelagic mollusksand may also include the dead bodies oftiny swimming snails and marine butter-flies, especially in the vicinity of the Equa-tor. It is limited to a depth range of 1500 to3000 m, i.e. it lies in generally shallowerwater than GLOBIGERINA OOZE, being par-ticularly common in the vicinity of coral is-lands and on submarine elevations that liewell offshore. Viewed on a global scale, it

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is very limited in extent, occupying a fairlysmall north-south strip in the center of theS Atlantic and several deep-water sites offBrazil and in the N Atlantic. Collectively,these areas amount to only 1% of the totalocean floor.

Pterosauria /te-rŏ-sor-ee-ă/ An order ofextinct flying reptiles (subclass AR-CHOSAURIA), prominent in the Mesozoic.The body was small in relation to the sizeof the wings, which were formed of a thinmembrane of skin supported by an elonga-tion of the fourth finger. This arrangementdiffers from that found in the birds (seeAves). The early Jurassic species weresuited to flapping flight, but by the lateCretaceous Period pterosaurs were charac-teristically adapted for gliding, and wereamong the most efficient gliders known.They ranged from sparrow-sized creaturesto the enormous Pteranodon, which had awing span of up to 9 m. Some had teeth;others were toothless.

ptygmatic structure /tig-mat-ik/ Anyone of a series of highly contorted parallelfolds, as commonly shown by veins of PEG-MATITE in MIGMATITE or other high-grademetamorphic rock.

puddingstone A type of CONGLOMER-ATE consisting of small rock fragments orpebbles in a sandy matrix, so called be-cause of its supposed resemblance to a fruitpudding.

pulaskite /pûl-ăss-kÿt/ A member of theSYENITE group of minerals.

pumice /pum-is/ Highly vesicular, usu-ally acid, volcanic rock. See pyroclasticrock.

pumpellyite /pum-pel-ee-ÿt/ A hydrouscalcium-bearing mineral found in low-grade regionally metamorphosed rocks.

puna /poo-nă/ A region of high, flat, andbleak land in the South American Andes,up to 4000 m above sea level. Because ofthe altitude, the air is thin and tempera-tures are low, even in summer; they fall

below freezing point at night. In the Boli-vian and Peruvian puna there are some richdeposits of minerals.

push moraine A ridge of material accu-mulated by the bulldozing action of anadvancing glacier or ice sheet. These mor-aines tend to be more convex in profilethan TERMINAL MORAINES and can easily bedifferentiated by considering the internalstructures, which show signs of faultingand thrusting. If really distinct faults occurin the material, then it was almost certainlyfrozen when incorporated into the pushmoraine. See also moraine.

puy /pwee/ A plug of volcanic rock,sometimes left standing when the sur-rounding rock has been eroded away.

P wave See primary wave.

pycnometer /pic-nom-ĕ-ter/ (density bot-tle) An instrument for measuring specificgravity. It consists of a small glass bottle ofknown volume which is filled and weighed(to find the specific gravity of the liquid).Alternatively, it is filled with a dense liquidand a weighed sample of mineral grains in-troduced, which displaces some liquid. Thespecific gravity of the mineral can then befound.

pyralspite /pÿ-răl-spÿt/ A chemical se-ries of GARNET minerals.

pyramidal peak (horn; horn peak) Anindividual steep-sided mountain peakformed in a similar way to an ARÊTE but inthis case involving three or more converg-ing CIRQUE headwalls, thereby isolating asingle residual rock mass rather than a lin-ear divide.

pyranometer /pÿ-ră-nom-ĕ-ter/ Any in-strument for measuring scattered andglobal radiation on a horizontal surface.Instruments that measure global (directand diffuse) radiation are also known assolarimeters.

pyrargyrite /pÿ-rar-gă-rÿt/ A dark redto black mineral form of silver antimony

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sulfide, Ag3SbS3. It crystallizes in the trigo-nal system and commonly occurs in associ-ation with other silver-bearing minerals. Itis an important source of silver.

pyrheliometer A radiation instrumentfor measuring direct solar radiation only.Diffuse radiation is excluded by having a5° aperture continuously facing the Sun. Itrequires sophisticated equipment to main-tain the aperture in this position and istherefore used only at a few special sites.

pyriboles /pÿ-rib-ŏ-leez/ A group ofminerals consisting of PYROXENES and AM-PHIBOLES.

pyrite /pÿ-rÿt/ (iron pyrites; fool’sgold) A pale brass-yellow mineral formof iron sulfide, FeS2, commonly occurringas cubes and octahedra. It is the most wide-spread and abundant sulfide mineral,found as an accessory in igneous rocks, inhydrothermal and replacement deposits, incontact metamorphic rocks, and in sedi-ments laid down under reducing condi-tions. It is used as a source of sulfur. Themineral’s common name of fool’s gold de-rives from its bright golden color when firstexposed to air.

pyroclast /pÿ-rŏ-klast/ Any material,from fine dust and ash to large blocks ofrock, ejected from the vent of an eruptingvolcano.

pyroclastic flow /pÿ-rŏ-klass-tik/ Amixture of hot gases and PYROCLASTS thatmoves under gravity along the surfacefrom the vent of an erupting volcano. Suchflows can move quickly and be extremelydangerous, destroying anything in theirpath. See also pyroclastic rock.

pyroclastic rock A rock formed by theaccumulation of fragmental materialsthrown out by volcanic explosions (liter-ally, fire-broken). Such material is knownas TEPHRA or ejectamenta and may be ex-pelled as solid fragments or in the moltenstate, chilling in the air and producing vit-reous material. All volcanic eruptions arethe result of the release of gas that has been

confined under pressure. This gas may bederived from the magma or from the trans-formation to steam of water from the seaor a crater lake coming into contact withmagma. Phreatic eruptions are caused bysteam produced when lava flows come intocontact with groundwater.

The main kinds of pyroclastic materialsand rocks are as follows:Bombs and blocks. Bombs are large frag-ments with a rounded to subangular shapeand erupted in a plastic condition. Duringflight, aerodynamically modified shapesare produced, the most common being aspindle form. Blocks are large angular frag-ments ejected in a solid condition. Accu-mulations of blocks and bombs, known asagglomerate, occur near volcanic vents.Showers of hot blebs of magma that flattenand weld on impact with the ground arecalled spatter or agglutinate.Lapilli. Round to angular fragments of di-ameter between 64 and 2 mm. Most of theirregular vesicular fragments known as cin-ders or scoriae are of lapilli size. A specialform of lapilli,known as Pele’s tears(named for the Hawaiian goddess of fire,lightning, and volcanoes), are droplets oflava that solidified in the air to pear-shapedpieces of glass. The droplets trail behindthem threads of liquid that solidify to glassfilaments, known as Pele’s hair.Ash. Tephra fragments less than 2 mm indiameter. Consolidated ash is known astuff. The most common variety, vitric ash,is formed by the disruption of magma byexpanding gas. The gas causes a frothing ofthe magma, which becomes torn apart.Some highly vesicular masses survive aspumice. Most is completely disintegratedto shards. These small glass septa, whichseparated individual bubbles, have charac-teristically curved and forked shapes. Ashmay be carried a great distance from thevolcanic source by the prevailing windsand is deposited in layers, often graded,draping over the underlying topography.Consolidated ash with a high proportionof larger fragments is sometimes calledlapilli tuff. Crystal tuff contains a high pro-portion of broken crystals, which representthe PHENOCRYSTS present in the magma be-fore eruption.

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Ignimbrites (welded tuffs). Fragmentalflows having some of the characteristics oflava and some of air-fall pyroclastic ma-terial. Ignimbrites typically have a streakyor banded appearance known as eutaxiticstructure. The glossy streaks, usuallydarker in color than the surrounding ma-trix, are laterally discontinuous, unlikeflow banding. Individual streaks called fi-amme are arcuate or elliptical in plan andare thought to represent pumice fragmentsor lava blebs flattened by the weight of theignimbrite and welded together as the flowdeflates. The shards that compose the ma-trix are also flattened and drawn out.

The flows producing these deposits arecalled ash flows and the rocks producedare ignimbrites, ash-flow tuffs, or weldedtuffs. Not all ash flows become welded andin many flows welded eutaxitic materialpasses up into unwelded material. Ashflows are mostly of acid composition andare the result of deposition from NUÉES AR-DENTES, incandescent clouds of gas and ashproduced by the explosive vesiculation ofmagma.

pyrolusite /pÿ-rol-yŭ-sÿt/ A soft pow-dery or fibrous black mineral form of man-ganese dioxide, MnO2. It crystallizes in thetetragonal system, and occurs in associa-tion with other manganese minerals. It isused a source of manganese, and as a de-colorizer and oxidizing agent.

pyrope /pÿ-rohp/ A deep yellow-red toblack member of the GARNET group of min-erals, Mg3Al2Si3O12. It crystallizes in thecubic system, and occurs in kimberlite andother ultrabasic igneous rocks. Transpar-ent examples are valued as semipreciousstones.

pyrophyllite /pÿ-roff-ă-lÿt/ A soft cream-colored, gray, or green mineral form of hy-drated aluminum silicate, AlSi2O5(OH),which resembles talc. It crystallizes in themonoclinic system, and occurs in meta-morphic rocks such as schist as thin flakes.

pyroxene /pÿ-roks-een/ A member of agroup of ferromagnesian rock-formingminerals. Pyroxenes are orthorhombic or

monoclinic and have a continuous chainstructure of SiO4 tetrahedra linked by shar-ing two of the four corners. Cations linkthe chains laterally. The general formula ofpyroxenes is X1–pY1+pZ2O6, where X =Ca,Na; Y = Mg,Fe2+,Mn,Li,Al,Fe3+,Ti, andZ = Si,Al. In the orthopyroxenes (ortho-rhombic pyroxenes), p = 1 and the contentof trivalent ions is small. In clinopyroxenes(monoclinic pyroxenes), p varies from 0 asin diopside (CaMgSi2O6) to 1 as in spo-dumene (LiAlSi2O6).Orthorhombic pyroxenesThe orthopyroxenes have a composition(Mg,Fe2+)2Si2O6 and form a series fromenstatite (MgSiO3) to orthoferrosilite(FeSiO3) that is produced by the replace-ment Mg↔Fe2+. Intermediate composi-tions in the series include bronzite andhypersthene. Orthopyroxenes are found inbasic and ultrabasic igneous rocks andhigh-grade metamorphic rocks.Monoclinic pyroxenesMost clinopyroxenes can be considered tobe members of the four-component system,CaMgSi2O6–CaFeSi2O6–MgSiO3–FeSiO3.Within the system, three series are recog-nized: 1. diopside–salite–hedenbergite

Ca(Mg,Fe)Si2O62. augite–ferroaugite

(Ca,Na,Mg,Fe2+,MnFe3+Al,Ti)2-(Si,Al)2O6

3. pigeonite (Mg,Fe2+,Ca)(Mg,Fe2+)Si2O6.The pigeonite series are calcium-poor

clinopyroxenes. The diopside series havehigher Ca/Mg + Fe ratios than the augites.Clinopyroxenes rich in titanium are calledtitanaugites. Pigeonite is found in basiclavas but under the slow cooling condi-tions associated with plutonic crystalliza-tion, pigeonite expels calcium-rich ions aslamellae of augite and inverts to an or-thorhombic structure. Diopside and heden-bergite are found in calcium-rich andiron-rich thermally metamorphosed sedi-ments respectively. Hedenbergite also oc-curs in intermediate and acid igneousrocks. Pyroxenes of the augite-ferroaugiteseries are found in basic igneous rocks.Omphacite, a high-pressure pyroxene re-sembling augite but with some CaMg re-placed by NaAl, is found in ECLOGITES.

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Sodic pyroxenes may be considered to be members of the system CaMgSi2O6–CaFeSi2O6–NaFe3+Si2O6, in which the re-placements Ca(Mg,Fe2+)↔ NaFe3+ occur.The sodic end-member is aegirine (or ac-mite), NaFe3+Si2O6, and as the nameimplies, aegirine-augite, (Na,Ca)(Fe3+,Fe2+,Mg)Si2O6 is of intermediate composi-tion. Sodic pyroxenes are found in inter-mediate and acid alkaline igneous rocksand glaucophane schists.

Other pyroxenes include spodumene(LiAlSi2O6), found in lithium-rich peg-matites, and jadeite (NaAlSi2O6), found inhigh-pressure metamorphic rocks. Com-pare amphibole.

pyroxenite /pÿ-roks-ĕn-ÿt/ An ultra-mafic rock consisting wholly of PYROX-ENES. Pyroxenites may be monomineralicor bimineralic.

pyrrhotite /pi-rŏ-tÿt/ A yellow-brownmagnetic mineral form of iron sulfide witha variable amount of sulfur, average for-mula Fe7S8. It crystallizes in the hexagonalsystem, and often occurs associated withnickel sulfide, when it is a major source ofnickel.

quaquaversal /kwah-kwă-ver-săl/ Seepericline.

quartile /kwor-tÿl. -tăl/ If a data series isarranged in order of magnitude, a quartileindicates a quarter of the values. It is morefrequently quoted as upper and lower quar-tile, the former separating the highest quar-ter of the values and the latter the lowestquarter. It can therefore be used as a mea-sure of the dispersion or range of the data.

quartz One of the most important rock-forming minerals, consisting of silicondioxide, SiO2 (see silica minerals). It crys-tallizes in the trigonal system, and occursas sand and as deposits of highly transpar-ent colorless crystals (rock crystal), some-times tinted by impurities. These give riseto the varieties known as rose quartz andsmoky quartz, as well as to amethyst andcitrine. Quartz is also the basis of cryp-tocrystalline minerals such as agate, chal-

cedony, jasper, and opal. Many of these areused as gemstones. Quartz is a componentof granite and other acid igneous rocks; italso occurs in metamorphic rocks(quartzite) and some sedimentary rocks(sandstone). It has many uses, mainly in themanufacture of abrasives and glass.

quartzarenite /kwort-sa-rĕ-nÿt/ (ortho-quartzite) An ARENACEOUS rock having acomposition including more than 95%QUARTZ.

quartz diorite A coarse-grained igneousrock consisting mainly of PLAGIOCLASE

FELDSPAR, sometimes also with some OR-THOCLASE. The minerals biotite and horn-blende occur as the dark constituents, withup to 30% quartz as the light component.

quartzite /kwort-sÿt/ A tough massiverock consisting almost wholly of QUARTZ

and usually having a pale color. A quartziteis the product of the metamorphism of apure sandstone, during which process thequartz grains recrystallize and become in-terlocking. See also psammite.

quasi-geostrophic motion Atmosphericmotion that approximates to the GEO-STROPHIC WIND. It is one of the assumptionsused in the preparation of numericalweather forecasts. Because it is not quitegeostrophic flow, it cannot be used for de-riving other quantities based on thegeostrophic wind.

Quaternary /kwă-ter-nă-ree/ A subdivi-sion of the CENOZOIC Era, formed of twoepochs, the PLEISTOCENE and HOLOCENE.The Quaternary has been considered to bea period (comprised of the Pleistocene andHolocene Epochs) within the CenozoicEra, following the TERTIARY Period but al-though the term has been widely used in-formally it has never been formallyrecognized. Recent proposals are for theQuaternary to be formally recognized as asubera of the Cenozoic, and for it to spanthe past 2.6 million years. During the Qua-ternary parts of Europe were subjected tofour major advances of the ice sheets,which were separated by warmer inter-

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glacial episodes, and most British Pleis-tocene rocks consist of glacial and associ-ated fluvial deposits. Fossil animals andplants were essentially modern. The landfauna alternated between forms adapted tocold conditions, such as the mammoth andwoolly rhinoceros, and species now re-stricted to the tropics. Pollen and the re-mains of beetles have proved valuable inmonitoring climatic change. The Quater-

nary is the time when humans became thedominant terrestrial species.

quicksand An area of mud and sandcontaining a large amount of water andthus almost liquid in composition. It canoccur on the shore or near a river, wherethe water table is near the surface anddrainage is poor.

Q-wave See Love wave.

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radar (radio detection and ranging) Amethod of active REMOTE SENSING in whichelectromagnetic energy in the range of mi-crowave wavelengths is used to detect ob-jects by their ability to reflect the beams.Radar systems emit pulses of microwaveradiation and record the returned mi-crowave echo. The resulting image is pro-duced from analysis of the time andstrength of the returned pulse. Radar useswavelengths that are able to penetrateclouds so can provide an image of theEarth’s surface in areas normally obscuredby cloud cover. Side-looking airborneradar (SLAR) is an airborne instrumentthat emits beams of microwaves to theground at an angle perpendicular to theaircraft’s flight path to produce a three-dimensional image of the land surface. Theground surface is illuminated at an obliqueangle that enhances features such as foldsand faults. Synthetic aperture radar (SAR)carried on aircraft or satellites has wide-spread applications, e.g. in land-use andvegetation studies, studies of the cryo-sphere, geology, oceanography, and hy-drology. The technique of radar inter-ferometry using SAR (InSAR) has particu-lar potential for the solid earth sciences asit enables the measurement of motions ofthe ground surface with considerable preci-sion. Areas of potential use include thestudy of crustal deformation, earthquakes,and volcanoes. See also radar meteorology.

radar meteorology The investigationor determination of aspects and propertiesof the atmosphere by radar. Using suitablewavelengths, radar offers great possibilitiesof investigating the areal distribution ofmany meteorological elements, which oth-erwise would be unobserved. By scanningin the horizontal field (Plan Position Indi-

cator), radar can measure the distributionand intensity of surface precipitation anddistinguish particularly intense storms suchas tornadoes, hurricanes, thunderstorms,and hailstorms. Scanning about the vertical(Range-Height Indicator) enables theheight and structure of precipitating cloudsto be obtained as well as the altitude of themelting level, if present. Operation over aperiod of time clarifies the evolution ofmany of these atmospheric systems and it isat the subsynoptic scale that most advancesin radar meteorology have occurred.

radial dike A dike that radiates from acentral-vent volcano. This radial fracturepattern is the result of the volcano’s super-structure swelling prior to an eruption aspressure builds up.

radial drainage A drainage pattern thatdevelops when structural control is in theform of a volcanic dome or cone, or someother sort of dome. Slope will causestreams to radiate out from their commoncenter at the crest of the dome like thespokes of a wheel, e.g. in the English LakeDistrict.

radiation 1. (in meteorology) The portionof the ELECTROMAGNETIC RADIATION spec-trum that is emitted by the Earth and theSun. In terms of wavelength, this is radia-tion between 0.1 and 70 µm,encompassingpart of the ultraviolet, all the visible, andpart of the infrared spectrum. This type ofwave differs from most other waveforms inthat it does not require an interveningmedium for its propagation; it can travelthrough a vacuum.

The behavior of electromagnetic radia-tion is described by certain physical laws.All substances above the absolute zero of

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temperature (–273°C) emit radiation inamounts and wavelengths dependent ontheir temperature. Some bodies emit andabsorb radiation in certain wavelengthsonly; this is particularly true of gases (seeatmospheric window). The ideal radiatingbody is a black body (see black-body radi-ation), which emits the maximum amountof radiation for its temperature, theamount being proportional to the fourthpower of its temperature on the Kelvinscale. Absorption and emission take placeat the same wavelengths for any substance.The hotter a body, the shorter will be thewavelengths at which the maximumamount of emission will take place. Theselaws determine the way in which bodiesreact to radiation and enable the utilizationof solar energy by the Earth and atmos-phere to be explained. The electromagneticradiation emitted from the Sun is termedsolar radiation (or solar energy). It repre-sents nearly all the energy available to Earth.

The Sun has an emission temperature of6000 K, giving an energy maximum in thevisible light wavelength. This reaches thetop of our atmosphere at the rate of about2 calories per sq cm per minute or 1.35kW m–2. On its passage through the at-mosphere, some of this short-wave radia-tion is reflected back to space by cloudsand dust, some is scattered by gas mol-ecules and dust particles to give diffuse ra-diation, and some (about 18%) is absorbedby water vapor, carbon dioxide, and dust.The remainder reaches the ground surfacewhere some is reflected and the rest is ab-sorbed. On a global average, this amountsto approximately 47% of the radiationreaching the top of the atmosphere.

The Earth’s surface has a mean emis-sion temperature of about 290 K as a resultof solar heating. Thus terrestrial radiationis in the longer wavelengths with a maxi-mum about 10 µm. The gases of the at-mosphere have a very different response tothis radiation and much of it is absorbed(see absorption). Certain wavelengths areunaffected, especially those between 8 and12 µm (radiation windows). The atmos-phere warms as a result of this absorptionand atmospheric counterradiation is re-turned to the surface and helps to maintain

higher temperatures than would otherwisebe expected.2. A type of PLANE TABLING in which pointsof detail are fixed by ascertaining directionusing an ALIDADE and distance by measur-ing on the ground with a tape. The methodmay be used over short distances, dispens-ing with the necessity to set up the planetable at two stations, as in INTERSECTION

methods.

radiation balance The net effect of thedifference between incoming and outgoingradiation at any point. These two factorsrarely balance. Normally there is a surplusof radiation on the ground surface duringthe day, which helps to warm the surfaceand atmosphere, and a deficit at nightwhen cooling takes place. The atmospherehas a negative radiation balance at alltimes. Taking the Earth and atmospheretogether, the areas equatorward of 38°have a radiation surplus and polewardthere is a radiation deficit.

radiation fog Nocturnal cooling result-ing from terrestrial radiation losses canlead to the air near the ground surfacereaching the saturation point so that con-densation takes place, causing radiationfog. It is most likely to occur during long,clear, and calm nights with a moist atmos-phere. In industrial areas, the abundance ofcondensation and hygroscopic nuclei facil-itates fog formation and it may even occurwith relative humidities less than 100%.However, urban warmth frequently coun-teracts this factor and so city centers maybe almost clear of fog while it is still quitedense in the cooler suburbs.

radioactive decay The spontaneouschange of one atomic nucleus into anotherthrough the emission of a photon (gammaray) or a particle (alpha ray or beta ray), orby electron capture. It is the basis of RA-DIOMETRIC DATING. See also half-life; iso-tope.

radiocarbon A radioactive ISOTOPE ofcarbon, commonly one of mass 14. Its de-tection is the basis of radiocarbon dating oforganic remains. See also half-life.

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radiocarbon dating (carbon-14 dating)A method of RADIOMETRIC DATING in whichthe age of an organic material is deter-mined by measuring the proportion of ra-diocarbon (carbon-14) within the carboncontent of the material. Carbon-14 is pro-duced in the upper atmosphere from theaction of cosmic waves on nitrogen-14.Living organisms continuously absorb car-bon-14 in the form of carbon dioxide fromthe atmosphere, or through the food chain,during their lives, but on death no more istaken in and the amount present decreasesthrough radioactive decay at the knownhalf-life rate. Radioactive carbon-14 has a5730-year half-life and can be used for dat-ing materials up to about 70 000 years.The method is used for dating archaeolog-ical artifacts and for materials from theHolocene and Pleistocene. It was pioneeredby the American physical chemist WillardF Libby (1908–80) in the 1950s.

Radiolaria /ray-dee-oh-lair-ee-ă/ An orderof PROTOZOA whose members have an in-ternal skeleton composed of silica. Theyare one of the constituents of marineplankton. In certain conditions their re-mains accumulate to form RADIOLARIAN

OOZE, and analogous ancient radiolariancherts are known. Fossils from rocks as oldas the Precambrian have been attributed tothis group.

radiolarian ooze A siliceous ooze (seepelagic ooze) deposit containing more than30% organisms. It is reddish or brownishin color and results from the deposition ofminute siliceous skeletal remains of radio-larians. The deposits are very limited in ex-tent when viewed on a global scale, beingalmost confined to a narrow tract of deep-sea floor in the Pacific Ocean, located justnorth of the Equator.

radiolarite /ray-dee-oh-lair-ÿt/ A hardfine-grained siliceous rock formed predom-inantly from the skeletal material of RADIO-LARIA.

radiometric dating /ray-dee-oh-met-rik/A method of dating rocks by determiningthe relative proportions present of parent

and daughter isotopes of a radioactive el-ement. Because these decay reactionsprogress at a known rate, the age of therock is calculated from the ratio present.

radiosonde /ray-dee-oh-sond/ A packageof small electronic instruments for measur-ing pressure, winds, temperature, and hu-midity in the upper atmosphere, togetherwith a small radio transmitter that is at-tached to a helium-filled meteorologicalballoon, which lifts the radiosonde to alti-tudes of about 30 000 m. Observations ofpressure, temperature, and humidity aretransmitted to a ground station. The bal-loon is tracked by radar and from its posi-tion wind velocities and direction can becalculated.

rain PRECIPITATION in the form of liquidwater drops ranging in size from about 0.5mm to 5 mm in diameter.

rainbow An optical phenomenon con-sisting of an arc of light across the sky bro-ken up into the spectral colors. It resultsfrom the refraction and internal reflectionof sunlight through falling water drops.The intensity of the rainbow depends uponthe size of the drops, larger sizes producingbrighter colors. Some of the light may bereflected twice within the raindrop to pro-duce a double rainbow effect.

rain day In Britain, a climatological DAY

from 9 am GMT within which 0.2 mm ormore of rainfall is recorded. See also wetday.

raindrop size spectrum The range ofraindrop sizes within or beneath a precipi-tating cloud. The range of sizes varies withthe rate of rainfall. At low rates (0.1mm hr–1) most drops will have a diameterof less than 0.75 mm and there will be veryfew above 1.25 mm. With heavier rain, theupper range of drop sizes increases and thedensity of drops rises markedly.

rainfall The water equivalent of all formsof precipitation from the atmosphere, re-ceived in a rain gauge. This includes dew,hoar frost, and rime on the collecting area

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of the gauge, because on melting it is indis-tinguishable from the rest of the catch. It isnow recorded in millimeters in most coun-tries of the world. The value quoted repre-sents the amount of water that has fallenon a horizontal surface assuming norunoff, percolation, or evaporation.

Rainfall has been classified into threetypes depending on its mode of formation:convective, frontal (or cyclonic), or oro-graphic rainfall.

rainforest A type of dense forest thatgrows in regions that have heavy rainfallall the year round. Most rainforests occurin the tropics, although there are some inwarm temperate regions. They are amongthe most complex ECOSYSTEMS in theworld, with a great diversity of plants andanimals. Their plants are also an importantsource of atmospheric oxygen (from PHO-TOSYNTHESIS). See also deforestation.

rain gauge An instrument for measuringrainfall. In its simplest form it is essentiallya funnel and a collecting vessel placed ver-tically into the ground. Different types ofrain gauge are used throughout the worldbut most are set about 30 cm above theground surface. This is to avoid splashingduring heavy rain, to retain snow, and toreduce the effects of wind eddying aroundthe gauge. It is widely accepted that a raingauge does not catch the precise amount ofrain falling on a surface, largely because ofthe wind eddying effect, but as long as allsites are standardized there is comparabil-ity between records. The rainfall is meas-ured by emptying the contents of thecollecting vessel into a special cylindricalflask, which is graduated in relation to thediameter of the gauge. The rainfall can beread off directly from the water level in theflask. Recording versions, e.g. the tipping-bucket rain gauge, can be linked to auto-matic weather stations. More sophisticatedversions also record rainfall intensity.

rain pit A sedimentary structure formedin subaerially exposed soft fine-grainedsediments as a result of the impact of rain-drops upon the sediment’s surface.

rain shadow An area on the leewardside of a range of hills where rainfall totalsare less than would be expected from theirlatitudinal position. It is the result of a dry-ing-out of the winds through precipitationon the upwind side of the hills.

rainsplash The impact of raindrops onthe soil. It influences slope evolution in twoways: firstly, as the drop hits the groundand rebounds, it brings up with it particlesof soil, which tend to fall downslope; sec-ondly, this process will, if continued, com-pact the soil surface, often breaking up soilstructures, and reduce the infiltration ca-pacity (ability to hold and store water) ofthe soil. This promotes surface flow, eitheras an OVERLAND FLOW or as RILLS, whichthen erode the surface.

rain wash See overland flow.

raised beach An inland terrace of de-posits that marks the location of a formershoreline, but is now well above the pre-sent sea level. There may be cliffs inland ofthe raised beach. Most of these features re-sult from uplift of the land following glacia-tion, although some were produced by themovement of lithospheric (tectonic) plates.

raised bog See organic soil.

range 1. A line of mountains that formsa continuous barrier, perhaps with somegaps or passes.2. An open area of pasture, as in the west-ern USA.3. The difference between the highest andlowest values of a variable, such as temper-atures or the heights of the tides.

rapakivi structure /rap-ă-kee-vee/ AnORBICULAR structure exhibited by somegranites in which large oval pink ortho-clase crystals are mantled by white oligo-clase.

rapids A stretch of fast-flowing turbu-lent broken water at a break in the longprofile of a river that is not a vertical drop.Rapids often mark KNICKPOINTS, some-times in the form of the remains of an

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eroded waterfall. If the fall in base levelcausing rejuvenation results in a retreat ofthe shoreline some distance offshore, be-cause the coastal area is gently sloping, theknickpoint may be spread over a consider-able horizontal distance and rapids will becreated rather than a waterfall. Rapids canalso occur where a river crosses a hard rockband, not yet graded to fit a smooth longprofile. Rapids and waterfalls tend to befairly quickly eliminated owing to the in-creased turbulence and erosion they them-selves create.

rarefaction wave After the impact of aMETEORITE with the ground, a wave of de-compression that follows the initial com-pressional shock wave through the rocks atthe impact site. It is the rarefaction wavethat causes the ejection of debris from thecrater (see ejecta).

raster In geographical information sys-tems (GIS), a method of representing spa-tial data as a matrix of cells (or pixels).Geographic space is divided into cells in agrid to which properties or attributes canbe assigned. Remote-sensing satellites cap-ture data in this form and it is transmittedto ground stations where it can be analyzedand distributed.

ravine A small narrow deep-sided valleycarved by the erosive action of a river orproduced by faulting.

raw sienna See sienna.

raw soil Soil with an (A)C profile, pro-duced by incipient soil-forming processeson fresh rock.

raw umber See umber.

Rayleigh wave /ray-lee/ One of twotypes of surface wave produced either nat-urally by earthquakes or volcanic erup-tions, or artifically by an explosion (namedfor the British physicist John WilliamStrutt, 3rd Baron Rayleigh (1842–1919)).In Rayleigh waves the motion of surfaceparticles is elliptical in the vertical plane

containing the direction of wave propaga-tion. See also Love wave.

reaction rim A CORONA STRUCTURE con-sisting of a concentric shell (or shells) of amineral often arranged in a radial or fi-brous manner around a primary mineral.Such rims, in igneous rocks, may be the re-sult of a reaction between an early-formedmafic mineral and the liquid with which itis no longer in equilibrium. Thus a latermember of the REACTION SERIES is pro-duced, e.g. olivine may be mantled by py-roxene or amphibole. Kelyphitic rims aresecondary coronas formed by late-stage ormetasomatic fluids reacting with primarycrystals, e.g. iddingsite rimming olivine.

reaction series During magmatic crys-tallization, in order to maintain equilib-rium between crystals and liquid, mineralsinteract chemically with the liquid. The re-action may be progressive so that a contin-uous series of homogeneous solid solutionsis produced. In the plagioclase feldspars,the first-formed crystals are richest in cal-

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discontinuousreaction series

continuousreaction series

olivine

orthopyroxene

clinopyroxene

hornblende

biotite

quartzalkali feldspar

anorthite

bytownite

labradorite

andesine

oligoclase

albite

(iron:magnesiumratio increasing)

Reaction series

cium and as the temperature falls thesereact with the liquid to become progres-sively more sodic. The reaction series in-volving the ferromagnesian minerals isdiscontinuous, each reaction taking placeonly over discrete temperature intervalsand corresponding to the transformationof one mineral to another of different crys-tal structure. At high temperatures, magne-sium-rich olivine crystals are the first toappear in a basic magma but as coolingproceeds, olivine is no longer stable andmagnesium-rich orthopyroxene crystal-lizes. Likewise orthopyroxene is stableover a limited temperature range and re-acts to produce clinopyroxene. Ferromag-nesian minerals have both high- andlow-temperature variants, those rich inmagnesium preceding those rich in iron.The common rock-forming minerals havebeen arranged by the Canadian geologistNorman Levi Bowen (1887–1956) in theorder in which the reactions take place andconstitute two series.

The early high-temperature members ofboth series generally crystallize together;hence gabbros contain magnesium-richolivine and pyroxenes together with calcicplagioclases. The low-temperature miner-als, alkali feldspar, mica, and quartz, areassociated in granitic rocks. When the re-action between crystals and the liquid isunable to go to completion because of toorapid cooling, early-formed members ofthe reaction series persist as relics in thefinal rock and zoned crystals (see crystalzoning) and REACTION RIMS are often ob-served.

realgar /ree-al-ger/ A rare orange-redmineral form of arsenic sulfide, As2S2. Itcrystallizes in the monoclinic system ascompact aggregates in hydrothermal veinsand hot springs; it may be found in associ-ation with ORPIMENT. It is used as a pig-ment and as a source of arsenic.

Recent See Holocene.

recrystallization /ree-kris-tă-li-zay-shŏn/The formation of new mineral grains in arock while it is still in the solid state. Thecomposition of the new material may be

the same as or different from the original(primary) grains, and they may be signifi-cantly larger.

rectangular drainage A drainage pat-tern that is characterized by streams flow-ing in two directions at right angles to eachother. Neither direction is subordinate,both being equally developed. The mostusual control is right-angled faulting in anarea of little other structural guidance.

rectilinear slope /rek-tă-lin-ee-er/ Aslope or individual facet of a slope that isstraight, i.e. has a single angle of slope.

recumbent fold A fold type in whichthe axial plane is almost horizontal. See di-agram at FOLD.

recurrence interval (return period) Theaverage interval of time between occur-rences of a particular event. In hydrology itis used for events such as floods or particu-lar amounts or intensity of rainfall (e.g. aflood of a certain magnitude or above mayhave a recurrence interval of 100 years);such information is used in flood forecast-ing. It is also used in geology for the aver-age time interval between earthquakes of agiven magnitude in a particular area.

red beds Layers of sedimentary rocksconsisting mainly of sandstone, siltstone,and shale colored red by iron oxide(HEMATITE). An example is OLD RED SAND-STONE.

red clay (brown clay) A pelagic deposit

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Rectangular drainage

that covers large parts of the abyssal floor,actually about a quarter of the Atlantic andIndian Oceans respectively, and somethingapproaching a half of the Pacific Ocean, orcollectively some 28% (approximately 130million sq km) of the total ocean floor. Thedeposit is characterized by a low silica andcarbonate content. The clay particles maybe mixed in places with manganese nod-ules, whale bones, sharks’ teeth, pumice,and other assorted materials.

red earth See ferrallitic soil.

red rain Rain colored red by dust parti-cles, usually fine sand. A phenomenon ofmid-latitudes, an example is the red rainthat sometimes falls on S Europe contain-ing dust picked up in the Sahara.

reduction /ri-duk-shŏn/ A chemical pro-cess that occurs in rocks and in the gleyedand leached parts of soil, whereby oxygenis removed from the weathering material.The continuous presence of water rendersoxygen scarce, leading to its displacement,for example from ferric iron oxide (Fe2O3)to give ferrous iron oxide (FeO), in whichstate it is rendered more soluble and hencepotentially more mobile. The characteristicred and yellow colors of OXIDATION are ab-sent, being replaced by greens and grays.Bacteria are important in this process.

reef A ridge, island chain, or area ofrocks that projects above the surface of thesea for all or part of the tide or it may bepermanently submerged. Reefs constitutedistinct navigational hazards. A reef maybe formed of solid rock or pebbles, but theterm is more commonly applied to organicreefs (BIOHERMS). They are sometimes at-tached to the coast and act as groins by col-lecting debris against them (see fringingreef), sometimes parallel to the coast andseparated from it by a lagoon too deep forcoral growth (see barrier reef), sometimesin tabular sheetlike masses (apron reefs),and sometimes in circular form (see atoll).Coral, a hard calcareous material, dead oralive, may be predominant in all of these(corals are at present the commonest reef-building organisms). CORAL REEFS are com-

mon in the tropics. Other organic reefs in-clude OYSTER REEFS and serpulid reefs(which result from the cementing actionbrought about by certain types of marineworms). The presence of a freshwater out-flow from reefs locally inhibits organicgrowth, causing gaps through the reefs.

reentrant A small valley or area of low-land that stretches into higher ground. Itgenerally results from erosion by water,but may be caused by FREEZE-THAW weath-ering.

reflection profile A seismic profile pro-duced and recorded by equipment designedto reflect energy from layered rock bodies.

reflux The process by means of whichdense concentrated salt solutions movedownward through the EVAPORITE on thefloor of a PLAYA or other basin. It is proba-bly one of the mechanisms that leads to theincreasing magnesium content in some sed-imentary rocks.

refolded fold A fold that has been sub-jected to more than one period of folding.The folds produced in the first period offolding have further fold trends over-printed on them by subsequent periods offolding.

reforestation The planned replanting oftrees in an area where they have been cutdown or destroyed (see deforestation). It isparticularly important as a method of pre-venting soil erosion and the formation ofinfertile land.

refractory inclusion /ri-frak-tŏ-ree/ Atype of irregular mineral grain found,along with carbonaceous chondrules, insome chondrite METEORITES. Up to 2 mmacross, the inclusions consist of high tem-perature-resistant oxides of aluminum, cal-cium, magnesium, and titanium.

reg A type of arid desert plain consistingof gravel, especially in the Sahara. See alsoerg; hammada.

regelation /ree-jĕ-lay-shŏn/ A process of

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thawing and refreezing within a GLACIER,which contributes toward the down-valleymovement of ice. It is believed that pres-sure within the ice of a glacier causes themelting of some ice crystals. The resultantmeltwater will move to locations at whichpressure is less, i.e. normally downslope,and then refreeze. The process has beenclassed as the primary cause of ice motionbut many authorities on the subject con-sider it to be only a secondary factor aris-ing from flow.

regeneration (in ecology) The naturalregrowth of plants that follows destruction(such as of grassland after fires or of treesafter forest fires or felling). It is an impor-tant factor in the long-term stability ofmany ECOSYSTEMS.

regime The total economy or habit of anatural system, e.g. fluvial regime, estuar-ine regime, coastal regime. See also equilib-rium regime.

regional metamorphism See metamor-phism.

regolith /reg-ŏ-lith/ (waste mantle) Un-consolidated weathered material betweenthe ground surface and the bedrock, whichmay reach thicknesses of 60 m in the trop-ics. It is formed either in situ, by the weath-ering of the underlying bedrock, or it istransported into the area by water, wind,or ice. The upper biochemically weatheredportion of the regolith is the soil.

regosol /reg-oV-sôl/ A thin azonal soilthat may develop where one soil-formingfactor delays the soil-forming processes.Such soils are young in development andlack a B horizon, resulting in an AC profilewith the A horizon, because of organicmatter, being darker than the C. Regosolsare typically developed in unconsolidateddeposits such as loess, recent till, or sanddunes, and the parent material dominatesthe characteristics of the profile. They areclassified in the ENTISOL order of the USSOIL TAXONOMY.

regression The retreat of the sea from aland area.

rejuvenation The revitalizing of streamsby an increase in their erosive capacities re-sulting from a fall in BASE LEVEL. Thiscauses oversteepening of the lower reaches,and increased erosion cuts out a new longprofile which will intersect the originallong profile at a KNICKPOINT. Tributary val-leys meeting the main stream seaward ofthe knickpoint will also be rejuvenated andbegin to cut down; if they do not keep pacewith the incision of the master stream,there will be a marked break of slope at thejunction, leaving them ‘perched’. Below theknickpoint, the former valley floor will beleft hanging above the stream as it cutsdown to a new level, becoming a RIVER TER-RACE. The knickpoint will advance up-stream at a rate dependent on the lithologyof the bed material and the nature of theflow over it; if it meets a hard rock band itmay be halted, and so the effects of the re-juvenation will not be felt upstream.

Valley-side slopes will also be rejuve-nated, because the incision of streams willoversteepen their lower portions, and tem-porarily create a valley-side facet adjacentto the stream.

rejuvenation head See knickpoint.

relative dating The ordering of rocks orfossils in terms of the GEOLOGIC TIMESCALE,without taking into account their absoluteages.

relative humidity The most frequentlyused index of atmospheric HUMIDITY. It isthe actual vapor pressure of the air ex-pressed as a percentage of the saturationvapor pressure at the same temperature. Itcan be measured directly by changes inlength of a hair or a skin in a hygrometer,or indirectly using wet- and dry-bulb tem-peratures. Because the value of relative hu-midity changes inversely with temperature,even for the same moisture content of theatmosphere, it is not a very precise unit.

relative plate motion The motion ofone LITHOSPHERIC PLATE in relation to an-

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other. Compare absolute plate motion. Seealso plate tectonics.

relative relief The degree of dissectionof a landscape, shown by the difference inheight between the tops of drainage dividesand the bottoms of adjacent valley floors.Cartographically it is approached by calcu-lating the difference between the highestand lowest points in each grid square of therelief map, and then expressing the resultsas a series of isolines joining areas of equalrelative relief. According to the normalCYCLE OF EROSION, relative relief increasesto a maximum in maturity and thereafterdecreases. One geomorphologist has baseda scheme for landscape attractiveness onrelative relief, high relative relief producinghigh scenic appeal. See also relief.

relative vorticity See vorticity.

relic sediment (in oceanography) A ma-rine deposit, usually present on shelf areas,that is incompatible with contemporarymarine environments. For example, muchrelic sediment on the continental shelf sur-rounding Britain was initially depositedthere under glacial or interglacial condi-tions. Much of the material lying on theshelf off S California is relic, especially thesands that contain extinct shallow-waterforaminifera, and which currently lie onouter parts of the shelf. Some sands offLong Beach, California, have been foundto include Pleistocene fauna. Many relicshelf sediments are now undergoing sort-ing and transport, under the action ofwaves and tidal flow, gradually shifting tomore compatible sedimentary environ-ments. Modern sediments are those beingderived under contemporaneous condi-tions.

relief The variation in elevation or phys-ical outline of a landscape, shown on mapsby the use of contours, spot heights, hyp-sometric tinting, and hill-shading. Relief isalso used synonymously with RELATIVE RE-LIEF. Positive relief indicates land risingabove the general level, i.e. hills.

relief map A map that uses contours,

coloring, or shading to indicate the differ-ent heights of various features. See con-tour; hachures.

remnant magnetism The magnetism‘frozen’ into a mineral or rock as it coolsthrough its Curie point.

remote sensing The process of obtain-ing information about the surface of theEarth and the atmosphere by using meth-ods that involve no physical contact. Itincludes the use of orbiting satellites,spacecraft, and aircraft, which transmit theinformation down to a ground station. Themost commonly employed techniques areaerial photography, infrared photography,and radar, and usually computers are em-ployed in interpreting the data. See alsoLandsat; satellite.

rendzina /rend-zee-nă/ Soil developedon soft calcareous parent material with anAC profile (no illuvial B horizon). The A isusually dark brown or black, rich in car-bonates and humus, with a good crumb orgranular structure, and a pH value aboveneutral. There are no free sesquioxides asin the related TERRA ROSSA soils, which alsodevelop on calcareous material. It may bethat the rendzinas are less mature than theterra rossa soils, or that the terra rossas de-velop on hard limestone, whereas rendzi-nas develop on soft. Rendzinas occur inhumid to semiarid climates, under grass orgrass-and-tree vegetation, and are perhapsbest known as the soils of chalk downland.

reniform /ren-ă-form/ Denoting the mas-sive kidney-shaped form in which someminerals occur. Reniform hematite iscalled kidney iron ore. Compare botry-oidal.

replacement (in geology) The naturalsubstitution of one mineral for another,generally by the action of gases or solu-tions. See mineralization; reflux.

representative fraction (RF) The ratiobetween distance on a map and distance onthe ground, expressed as a fraction.

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Reptilia /rep-til-ee-ă/ A class of cold-blooded terrestrial vertebrates whoseyoung complete their development withina tough-shelled egg; they are therefore pro-tected from desiccation and totally inde-pendent of an aquatic environment.Reptiles evolved from the AMPHIBIA in theCarboniferous Period and their main radi-ation began at the end of the Permian. Dur-ing the Jurassic and Cretaceous a greatvariety of reptiles came to dominate life onland. These ranged from the giant DI-NOSAURS to the flying PTEROSAURIA and thesecondarily aquatic ICHTHYOSAURIA andPLESIOSAURIA. All these groups became ex-tinct at the beginning of the Cenozoic,when they were replaced by the mammals(see K/T boundary event). Modern reptilesinclude crocodiles, lizards, and snakes.

reservoir (in geology) A rock that has ahigh porosity and good permeability and isable to store and transmit fluids or gases.

residual clay A type of clay that isformed where it lies by the weathering ofrock. It may result from the removal ofnonclay minerals from the rock, or fromchemical changes to FELDSPAR.

residual deposit Rock fragments thatare left behind after weathering and ero-sion of the preexisting rock. SCREE, for ex-ample, consists of such rock fragments lefton a hillside. Rockfalls or transport by iceor water may move some residual depositsfrom their original site. See also residualclay.

residual hill An isolated small hill, allthat is left of a larger mass of high groundthat has suffered erosion or land move-ment. See monadnock; puy.

resistance thermometer See thermom-eter.

resistant rock Any rock that has re-sisted weathering and erosion better thansurrounding rocks because it is relativelyharder. Such rocks may form higherground (see residual hill).

resorption The remelting of crystals dueto reaction with the magma from whichthey originally crystallized. Early-formedphenocrysts frequently fail to maintainequilibrium with the melt and are partly re-sorbed. Markedly anhedral crystal formsare produced and a REACTION RIM of a newmineral may result.

resultant wind The vector average ofall wind velocities and directions for agiven level at a given place over a given pe-riod of time. A wind velocity may be di-vided into zonal (east) and meridional(north) components. To determine the re-sultant wind of a series of wind velocities,the individual components of the actualwind are summed, squared, added to-gether, and the square root taken. That is:resultant wind = √(ΣVN)2 + (ΣVE)2

resurgence The emergence of an under-ground stream, usually where it encountersimpermeable rock after having flowedthrough permeable strata.

reticulated Describing something, suchas a lode or vein, that has a netlike appear-ance or structure.

return period See recurrence interval.

reverse fault A type of fault in whichthe movement along the inclined faultplane has been up-dip. This results from aprincipal stress configuration in which themaximum principal stress is vertical,whereas the intermediate and minimumprincipal stress directions are horizontal.(See diagram at FAULT.)

Reynolds number /ren-ŏldz/ (Re) Theextent to which viscosity modifies a fluidflow pattern depends upon the speed offlow (u), the width of an obstacle placedacross the flow or of the flow passage (D),and a value for the kinematic viscosity (ν).The relationship, uD/ν gives the Reynoldsnumber (named for the Irish engineer andphysicist Osborne Reynolds (1842–1912).

It is an important number in laboratorysimulations of prototype flow conditions,because ideally the Reynolds number for

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scale hydraulic model flow should be thesame as for the prototype, i.e. to maintainscale relationships, the small model sizehas to be compensated by increasing theflow rates in the model.

RF See representative fraction..

rheidity /ree-id-ă-tee/ Deformation of asubstance by plastic flow as a result of anapplied stress over a long period.

rheomorphism /ree-ŏ-mor-fiz-ăm/ Theprocess in which a rock changes composi-tion while it flows. It may or may not resultfrom intense heat or pressure.

rhodochrosite A pink, gray, or brownmineral form of manganese carbonate,MnCO3. It crystallizes in the hexagonalsystem, and occurs in masses in hydrother-mal veins. It is used as a source of man-ganese and as a semiprecious gemstone.

rhodonite /roh-dŏ-nÿt/ A pink orbrownish translucent mineral form ofmanganese calcium silicate, (Mn,Ca)SiO3.It crystallizes in the triclinic system, and oc-curs in metamorphic rocks as granularmasses containing veins of black man-ganese dioxide. It is sometimes used formaking ornaments.

rhombochasm /rom-bŏ-klaz-ăm/ A par-allel-sided break in the Earth’s continentalcrust, the break being filled by oceaniccrust. The fracture is thought to be the re-sult of SEA-FLOOR SPREADING.

rhumb line /rum/ (loxodrome) A line ofconstant bearing, used in navigation, cut-ting all meridians at the same angle. Com-pare great circle.

rhyodacite /rÿ-od-ă-sÿt/ See rhyolite.

rhyolite /rÿ-ŏ-lÿt/ A fine-grained volcanicrock. Rhyolites, rhyodacites, and dacitesare the volcanic equivalents of GRANITES,adamellites, and granodiorites respectively.Rhyolites usually contain sanidine, thehigh-temperature form of potassium feld-spar, both as phenocrysts and microlites in

the groundmass. Phenocrysts of sodic pla-gioclase and quartz are also common, thelatter often markedly anhedral owing to re-sorption. The dominant mafic mineral isbiotite, although hornblende and clinopy-roxene also occur.

In dacites, sodic plagioclase is dominantover sanidine and, together with quartz,occurs as phenocrysts. With a decrease inthe amount of quartz, rhyodacites anddacites pass into LATITES and ANDESITES butthe relative proportions of minerals are dif-ficult to assess in fine-grained volcanicrocks and classification is often based uponchemical analyses. Rhyolitic and andesiticlavas constitute the calc-alkaline volcanicsuite, which is characteristic of island arcsand orogenic regions.

Strongly alkaline rhyolites, rich insodium and poor in aluminum, are foundin continental areas, particularly those sub-jected to rift movements, and are the vol-canic equivalents of alkali granites. Theselavas contain anorthoclase feldspar andsoda PYRIBOLES. An arbitrary division at12.5% femic constituents separates pantel-lerites from the more leucocratic comen-dites. With a decrease in the amount ofquartz, alkali rhyolites pass via quartz-tra-chytes into TRACHYTES.

Many lavas of rhyolitic compositionoccur in the glassy state, as OBSIDIAN andpitchstone. Obsidian is a typically blacknatural glass with a conchoidal fracturebut pitchstone has a dull appearance.Whereas obsidian contains very littlewater, pitchstone may contain up to 10%and owes its appearance to secondary hy-dration and devitrification. Most volcanicglasses contain crystallites; many containspherulites and exhibit perlitic cracking.

Rhyolitic lavas are often flow-bandedand it is sometimes difficult to distinguishsuch rocks from ignimbrites with para-taxitic structure. See pyroclastic rock.

rhythmite /rith-mÿt/ A sedimentary de-posit that has alternate layers of coarse-grained light and fine-textured darkmaterial. Most were formed on the beds ofcoldwater lakes. See varve.

ria /ree-ă/ A DROWNED VALLEY eroded by

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subaerial (nonglacial) processes at a timewhen the sea level was lower than it now is.Like river valleys of the present, rias have aV-shaped cross section, which deepens sea-ward and narrows inland, possibly bifur-cating. Former MONADNOCKS may remainas small islands. Rias are produced wher-ever a dissected area of hills and valleys,perpendicular to the coastline, has beensubmerged as a result of a postglacial risein sea level. Erosion of the projecting inter-fluves eventually reduces the indentation ofsuch coasts.

richterite /rik-ter-ÿt/ A monoclinic AM-PHIBOLE.

Richter scale /rik-ter/ A logarithmicscale that was devised by the Americanseismologist Charles Francis Richter (1900–85) in 1935 for comparing the magnitudeof EARTHQUAKES. Although originally de-vised for Californian earthquakes, it hassince been adopted for worldwide use. It isbased on the amplitude of seismic wavesrecorded by seismographs at a number ofstations, with adjustments made for dis-tances between the seismographs and theearthquake epicenter. The scale has noupper limit but earthquakes of magnitude9 and above are rare and extremely de-structive.

ridge See mid-ocean ridge.

ridge of high pressure A long narrowregion of high atmospheric pressure thatextends from an ANTICYCLONE. It is associ-ated with short-lived anticyclonic weather,with generally short dry sunny spells. It issometimes known as a wedge.

riebeckite /ree-bek-ÿt/ A monoclinicAMPHIBOLE.

rift valley (taphrogeosyncline) A struc-tural and topographical feature formedwithin the Earth’s crust consisting of asteep-sided flat-bottomed valley, the rocksof the valley floor having subsided betweentwo parallel faults or two parallel series ofstep faults. There are conflicting views asto the origin of rift valleys, the main ones

favoring tension in the Earth’s crust, com-pression, or the cracking of a crustal domealong the crest. There is often associatedvolcanic activity along the sides of the val-ley floor. Famous rift valleys include theEast African system, extending more than4000 km from Syria to the Zambezi, andthe Rhine rift valley between Mainz andBasle. See also graben.

rigidity modulus The ratio of stress tostrain when the stress is a shear. This is cal-culated by dividing the tangential force perunit area by the angular deformation.

rill A minute ephemeral channel at thehead of drainage systems, forming at thepoint where unconfined sheet wash be-comes concentrated into definite channels.This usually occurs on the lower concaveportions of slopes, because there the vol-ume of sheet flow has built up to a suffi-cient amount for channeling to occur. Rillsare said to be responsible for shaping theconcavity of the basal part of the slope.Downslope they run into more permanentgullies, which constitute the smallest tribu-taries of drainage systems. Rills carrywater only in storms. See also gully ero-sion; overland flow.

rillerkarren See karren.

rill mark A small channel found onbeaches, aligned more or less up and downthe beach. The channels develop because ofthe seaward flow of water that has perco-lated into the upper parts of the foreshoreduring the period around mid- to high-water or just after, or during the action ofSTORM waves with powerful swash actionbetween low- to mid-water. As the tide re-treats, or as the waves subside, the infil-trated water may flow out on to the beachface and cut the rills. Once developed, therills tend to channel this flow seaward andpossibly enlarge themselves. The seawatermay be reinforced by freshwater seepagesfrom landward, for instance where springsissue from a rock platform beneath thebeach.

rime A deposit of white ice crystals

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formed by the freezing of supercooledwater droplets onto surfaces below 0°C. Itgrows on the windward side of the surface,especially on sharp edges, by impaction asthe droplets drift past in the wind.

ring complex A circular igneous intru-sive body of rock including both concentricdikes and cone sheets.

ring dike See concentric dike.

rip A turbulent area of water where tidalcurrents meet head-on, where waves meet acurrent flowing in the opposite direction,where a tidal current suddenly enters shal-low water, or in a RIP CURRENT.

rip current A strong well-defined cur-rent that flows away from the shore. Suchcurrents are wave-induced, providing themechanism whereby water piled up at thecoast because of mass transport, and possi-bly wind, is returned seaward. The currentspeeds are usually fairly high. They areusually manifested by a fairly narrow zoneof agitated water and by an accentuatedamount of sandy material in suspension.They consist of feeder currents that flowinshore of the breakers and more or lessparallel to the shore as longshore currents,the neck, where the current is concentratedinto a RIP and flows seaward through thesurf zone, and the expanding rip-headwhere the rip loses impetus within or be-yond the surf zone. They account for manydrownings.

ripple bedding Small-scale cross-bed-ding resulting from the rapid deposition ofsand in the form of ripples.

ripple mark The most common minorbeach morphological form, consisting offairly regular and generally small ridgesformed in sediment (usually sand, but pos-sibly shell and fine gravel) on a river bed, inthe inter-tidal zone, or on the seabed sea-ward of low-water mark. Fossil ripples orripples formed by the wind may occurabove high-water mark. Ripples are causedby water or wind flow, and are alignedmore or less perpendicularly to the flow di-

rection. They can be used, therefore, to in-dicate the existence of and something ofthe nature of wind and water currents. Rip-ples tend to be larger in more exposedareas and in deeper water offshore. Verylarge ripples are usually termed megarip-ples or SAND WAVES. Dune areas abovehigh-water mark may display numerousripple features.

rise A broad elevated area of the seafloor, similar to a MID-OCEAN RIDGE butlacking a median rift valley.

river A channeled flow of water runningdownhill under the force of gravity. Mostrivers flow into the sea, although someenter lakes and a few flow into desertareas, where they dry up. The course of ariver from start to finish is called its profile.The source is usually on high ground, at aspring or marsh, or where the water tablereaches the surface. It then generally flowsthrough mountains, typically throughrocky areas as a turbulent flow along nar-row, steep-sided valleys. There may be wa-terfalls or cascades. The river thenbroadens along its middle course throughan upland area. As the valley widens, me-anders may form; there may also be bluffs.Finally the lower course of the river crossesa wide floodplain, where large meandersmay become cut off as oxbow lakes. Thewidest part of a large river as it enters thesea is an estuary, which may include adelta.

river bar A build-up of waterborne silt(alluvium) in the channel, along the banksor at the mouth of a river. It is generallyexposed at low tide. See also braidedstream.

river basin The area from which wateris collected by a river. See catchment area;drainage basin.

river capture The diversion of the head-streams of a river into an adjacent riverwith more powerful headward erosion. Itcuts back its valley into that of the weakerriver, thus enlarging its drainage basin atthe expense of the other. Rivers whose

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headstreams have been captured often flowas MISFIT RIVERS in valleys that are too largeto have been eroded by the present capac-ity of the river. The ELBOW OF CAPTURE isthe bend at which the captured head-streams are diverted.

river cliff See bluff.

river profile A section of a river alongits length from source to mouth. See river.

river regime The variations in the vol-ume of water in a river that take place withthe changing seasons, which can be plottedas a graph. In temperate regions, the great-est flow is usually in the winter (corre-sponding to the greatest precipitation). Inmonsoon and savanna regions, it is usuallyin the summer (the rainy season); a rivermay even dry up in the winter dry season.

river terrace (stream terrace) A bench-like landform bordering many rivers, ele-vated above the current streams andending in scarps suspended on the valleyside. These terraces constitute all that re-mains today of past abandoned valleyfloors formed when the river was flowingat a higher level than today. Most com-monly they are paired on each side of thevalley, the product of REJUVENATION of theriver or climatic changes (via their influ-ence on stream discharge, base level, andsediment load). They consist of a rock-cutbench, with or without a thin veneer of al-luvium.

Roaring Forties The region between40°S and 50°S where the westerlies blowwith great regularity across the openocean. The frequent depressions andstrong winds produce a stormy climate un-favorable to shipping.

roche moutonnée /rosh-moo-tonn-ay/ A glacial erosion form found in areas oflow relief and consisting of a resistant rockmass, which has been eroded to comprise agently sloping up-valley face and a steepdown-valley face. The up-valley face issmoothed, polished, and striated, theeffects of glacial abrasion, while the down-

valley face has a broken shattered appear-ance as a result of freeze-thaw processes,which are assisted by increased pressure-melting induced by the presence of the rockmass.

The term is extremely old and is widelyused for similar landforms of greatly con-trasting size. As a more suitable alterna-tive, the term stoss and lee topography hasbeen suggested.

rock An assemblage of MINERALS, gener-ally cemented or consolidated together. Itmay contain only one type of mineral, ormany. The surface of the Earth is made upof rocks. They are classified into threemajor groups: IGNEOUS ROCK, SEDIMENTARY

ROCK, and METAMORPHIC ROCK.

rock crystal A colorless transparent va-riety of QUARTZ. See silica minerals.

rock cycle See geochemical cycle.

rockfall A type of landslide involvingpurely dry materials. Basal sapping orsome other type of trigger mechanism willinstitute an instantaneous downslopemovement, leaving a pock-marked slope atthe source and a confused pile of talus atthe foot. See also mass movement.

rock flour Fine powdery rock producedby the abrading action of a glacier, whichgrinds particles off the rock it flows past.Most of the rock flour is transported bysubglacial streams, which have a milky orinky appearance as a result.

rock pedestal See pedestal rock.

rock salt See halite.

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polished, smoothed,

Roche moutonée

roller A large wave rolling in onto an ex-posed coast, often following a storm, in-creasing its height before breaking in arather destructive manner. Such wavesmake up a series of long-crested forms.They characterize such coasts as the SouthAtlantic, the South Indian Ocean, and theWest Indian islands.

roof pendant A small downward pro-jection of country rock into an underlyingigneous body. See also cupola.

ropy lava Volcanic lava that has solidi-fied in long strands. A thin crust forms onthe molten lava while it is still moving; theliquid center continues to move and formthe linear structure. See also pahoehoe.

rose quartz A pink QUARTZ sometimesused as a semiprecious gemstone.

Rossby wave /ross-bee/ See long wave.

rotational slip The semicircular move-ment of a mass of loose material, such asice, rock, or soil, down a concave slope.

rotation of the Earth The turning ofthe Earth on its axis, which it completesevery 24 hours. As different parts of theEarth face the Sun, day and night occur,whose lengths vary because of the 2.5° tiltof the Earth’s axis. At the EQUINOXES, thereare 12 hours daylight and 12 hours dark-ness everywhere.

roughness length A measure used inmicrometeorological studies as an indica-tor of degree of roughness of a surface toairflow. It is determined by extrapolatingthe observed relationship between windspeed and height to the point where thewind speed becomes zero. Its value is aboutone tenth that of the true height of theroughness elements, i.e. for a lawn of thick-ness 1 cm it is about 0.1 cm.

rubellite /roo-bell-ÿt/ A pink or red formof TOURMALINE, used as a semipreciousgemstone.

ruby A deep red transparent gem variety

of CORUNDUM (the color is caused bychromium impurities). It crystallizes in thetrigonal system, and has been synthesized.

ruby copper See cuprite.

rudaceous /roo-day-shŭs/ Describing aCLASTIC sedimentary deposit or rock inwhich the constituent fragments are of rel-atively large size. It is formed of GRAVEL,i.e. the clasts are greater than 2 mm in di-ameter.

rudite /roo-dÿt/ A type of sedimentaryrock that consists of coarse grains (morethan 2 mm across). The category includesBRECCIA and CONGLOMERATE.

rundkarren See karren.

runnel A depression extending along abeach perpendicular to the direction ofwave approach. Runnels separate and lieparallel with BEACH RIDGES.

runoff That part of total precipitationleft to flow into rivers after evaporationand transpiration by plants have takenplace. It has several components: rainfalling into the channel of the river, surfacerunoff, rainfall that soaks into the soilmoving laterally toward the river to reachit as ‘interflow’, and water that percolatesthrough the soil to the water table, feedingsteadily all the year round to the stream asgroundwater. Over the world as a whole,around one fifth of rainfall becomesrunoff, but regional variations are verygreat, dependent on climate, the nature ofvegetation and soil, whether the rain comesin severe storms or widespread gentleshowers, etc.

rutile /roo-teel, -tÿl/ A brown, violet, orblack tetragonal mineral. It is the com-monest polymorph of titanium dioxide,TiO2, and is widespread as an accessorymineral in igneous and metamorphicrocks. It also occurs in high-temperatureveins with quartz and apatite and as an al-teration product of sphene and ilmenite. Itis an important source of titanium. See alsoanatase; brookite.

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sabkha /sab-kă/ A salt flat that has a sur-face encrusted with HALITE; there are alsocalcium and magnesium salts in the de-posit. It runs along the coast just inlandand is periodically flooded by the sea.

saccharoidal /sak-ă-roi-dăl/ Describinga fine- to medium-grained granular rockresembling sugar.

saddle A broad shallow depresion in amountain ridge forming a pass.

saddle reef A deposit of minerals thatoccurs at the top of an anticline fold. Insome places in Australia and Canada, sad-dle reefs in folded slate beds contain gold.

saeter /see-ter/ An area of pasture in theNorwegian mountains, above the treeline,used for the summer grazing of cattle.

Saffir–Simpson Hurricane Scale Ascale of one to five based on the intensity ofa hurricane at a particular time that can beused to estimate the potential damage andflooding if the hurricane makes landfall.All winds in the scale use the US one-minute average. During 2005, the most ac-tive Atlantic hurricane season on record,three hurricanes (Katrina, Rita, andWilma) reached category five intensity.Category five hurricanes to have madelandfall in the USA are the Labor Day Hur-ricane of 1935, Camille, in 1969, and An-drew, in 1992. The scale was developed inthe early 1970s by the Americans HerbertSaffir (1917–), a consulting engineer, andRobert Simpson (1912–), then director ofthe US National Hurricane Center.

Sahel /să-hel/ A large region to the southof the Sahara, extending from Mauritania

and Senegal in the west to Ethiopia andSudan in the east. Overgrazing has re-moved much of the vegetation, with conse-quent soil erosion. DESERTIFICATION (fromthe Sahara) is occurring in the north.

salic /sal-ik/ Denoting the silicon- andaluminum-rich minerals of the CIPW nor-mative classification (see norm). Comparefemic.

salina /să-lÿ-nă/ A saltpan, named afterthose that occur in semiarid areas of Spain.See playa.

saline /say-lÿn/ Describing any solutionthat contains a salt, particularly commonsalt, sodium chloride, NaCl. The salinity ofseawater, for example, is 35 000 parts permillion (of which 85% is sodium chloride).

salinity The extent to which water con-tains dissolved salts. Together with tem-perature and oxygen content, it is one ofthe fundamental properties of sea water. Awide range of salinity occurs in the variousseas of the world: normal sea water has asalinity of approximately 34.33 grams perthousand grams (3.433%), although itmay be as high as 4.0% (in the Red Sea andPersian Gulf) or less than 3.3% (at high lat-itudes). The differences arise largely as a re-sult of variations in the rate of evaporation.Increased salinity raises the relative den-sity, lowers the freezing point, and greatlyinfluences marine ecosystems to the extentthat fauna and flora have to adapt to salineconditions. A knowledge of salinity is alsoimportant in understanding the circulationof ocean water.

salinization /sal-ă-nă-zay-shŏn/ The ac-cumulation in water or soil of salts of mag-

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nesium, potassium, and sodium. It oftenoccurs in arid and semiarid regions, wherethe rate of evaporation is greater than therate of precipitation. Plants cannot grow inthe salty soil and in this way salinizationcontributes to DESERTIFICATION.

salite /say-lÿt, sal-ÿt/ A monoclinic PY-ROXENE.

salt 1. Chemically, a compound formedwhen an acid is neutralized by a base. Mostsalts are crystalline ionic compounds, andmost minerals are composed of salts.2. See halite.

saltation /sal-tay-shŏn/ A mechanism ofsediment transport, whereby individualgrains move by bouncing off the surface ofthe land or bed. It is the most commonform of transport by wind. It is much lesssignificant in rivers because of the viscosityof water as compared with air; the ‘sticki-ness’ of water slows down the rise of thegrain and cushions its fall, so the height ofsaltation is but a few grain diameters ascompared with several thousand grain di-ameters in air. The initial rise of the grainis near vertical, followed by a gentle down-ward flight in the direction of wind orstream flow, terminated by impact againwith the surface at an angle of 10–16°. Inair, this impact may cause the grain itself torebound, or dislodge another grain on thesurface, which will then saltate. In water,the lifting force is eddy turbulence, whichtemporarily overcomes the weight of thegrain, imparting an upward movement.

salt dome A circular domelike structureformed by the upward movement of a col-umn of salt (HALITE), generally below strataof sedimentary rock. It may be capped by a

layer of anhydrite or gypsum (calcium sul-fate) or calcite (calcium carbonate), possi-bly containing some sulfur. The rocksbelow the dome are deformed, and mayhold deposits of oil and natural gas.

salt-earth podzol See solod.

salt flat A level salt surface constitutingthe bed of a former salt lake.

salt lake A highly saline lake formed inan INLAND BASIN.

salt marsh See marsh.

saltpeter /sawlt-pee-ter/ See niter.

San Andreas fault /san-an-dray-ăs/ A1125-km transcurrent fault that runsthrough California. It is on the boundarybetween two lithospheric plates (the Pacificand the N American plates) that intermit-tently move laterally, producing frictionthat causes earthquakes.

sand Particles of mineral or rock with asize range of 0.06–2.00 mm in diameter(i.e. between silt and gravel):

0.6–2.0 mm coarse sand.0.2–0.6 mm medium sand.0.06–0.2 mm fine sand.

Rocks formed from sediment in this rangeare known as sandstones.

According to the British soldier, ex-plorer, and scientist Ralph A. Bagnold(1896–1990) in 1941, sand is that materiallying above a lower limit of material capa-ble of carriage by suspension (mostly dust)and below an upper limit of material inca-pable of movement either by direct windforce or by the impact of falling grains.Windblown sands are said to be mostly 0.3to 0.15 mm in diameter.

Most sands are predominantly made ofQUARTZ, other material being too easilyeroded to survive for long. The quartz isderived from the weathering of quartz-bearing rocks, subsequently reduced in sizeby water or ice abrasion. Desert sandgrains are more rounded than fluvial ormarine types.

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sand bank A bank of sand and othersediments that occurs in estuaries andalong the open coast where the tidalstreams tend to flow in a rectilinear fashionand where transportable sediments arefairly abundant. The form of the bank andthe pattern of a group of banks often re-flect the nature of tidal flow. The sur-rounding or included channels may be ofthe ebb or the flood type, depending on therelative dominance of one or other tidalstream. The banks tend either to be linearor parabolic in shape.

sand dune See dune.

sand ribbon A submarine morphologi-cal form of linear shape that, like a sandwave, occurs where tidal streams are suit-able for its formation and where trans-portable sediment is sufficiently abundant.The ribbons occur within certain estuariesand along parts of the open coast, seawardof low-water mark. They are often only afew centimeters thick, but may be up to100 m wide and extend for several kilome-ters. The general trend of sand ribbons offthe S coasts of Britain is parallel to both thecoast and the directions of the strongesttidal streams. Where surface streams areless than 1 knot, the boundaries of the rib-bons tend to be somewhat obscure; in con-trast, where surface tidal streams attainmore than 1 knot, the boundaries becomemore strongly defined. Such morphologicalinformation is clear from high-precisionecho sounding.

sand shadow See eolian form.

sand sheet See eolian form.

sandstone A lithified ARENACEOUS de-posit. It is a CLASTIC sedimentary rock com-posed of grains between 0.0625 mm and 2mm in diameter. The particles may bebound by a secondary cement such as CAL-CITE or various iron minerals, or be weldedtogether by pressure. A great variety ofsandstones are known, and they can beclassified by both mineralogical and tex-tural characteristics.

sand stream A broad tract of movingsediment (largely of sand grade) seawardof low-water mark, possibly tens of kilo-meters in width. Sand streams tend to ex-tend over considerable distances, roughlyparallel to the coast. The streams are evi-dent from various field data, includingmorphological forms such as SAND RIB-BONS, SAND BANKS, and SAND WAVES, infor-mation relating to sediment grain-sizedistribution, and a knowledge of wavesand tidal streams. Sand streams may occuranywhere on shelf areas, but are especiallydeveloped near to coasts when the tidalcurrents flow in rectilinear fashion. Onesuch stream moves northward out of thesouthern bight of the North Sea, towardthe Netherlands and Germany.

sandur See outwash plain.

sand volcano (mud volcano) A sedi-mentary structure produced in poorly con-solidated sands and clays that have a highwater content. As a result of shaking, alower layer of sediment will be injected upand through the overlying bed. When thisprocess is completed the lower bed buildsup as a small cone above the upper bed.Where the process does not reach comple-tion or the overlying bed is very thick,FLAME STRUCTURES are produced.

sand wave (megaripple) A morphologi-cal form that resembles a desert dune andhas been called an underwater dune. Thesediment is usually of sand grade. Thestructures are created by tidal flow, and, iflarge, may have many smaller sand wavessuperimposed upon them. They oftenoccur on the surface of SAND RIBBONS. Theytend to be orientated perpendicular to themain tidal flow direction. Most display ameasure of asymmetry, which enables thenet direction of sediment transport to bedetermined, the steepest face indicating thedirection of travel. A small difference, per-haps only 0.1 knot, in the flood and ebbpeak velocities, is sufficient to cause asym-metry of form and to make the sand wavesmigrate in one or other direction. Thecrests of sand waves may be straight or sin-uous; their height, as in the case of the

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southern North Sea, may be as much as 15m. In contrast, off Florida, many of thesand waves are less than 1 m in height.Some sand waves are formed in the rela-tively deep water near the shelf edge, forexample off SW England, where theirheights vary between 8 and 12 m, and theiraverage length is some 850 m.

sanidine /san-ă-deen/ A potassic alkaliFELDSPAR.

Sanson–Flamsteed projection /san-sŏnflam-steed/ A modified CONICAL PROJEC-TION and a special case of Bonne’s projec-tion. The projection is named for theFrench cartographer Nicolas Sanson(1600–67) and the English astronomerJohn Flamsteed (1646–1719). Both thecentral meridian and the Equator (stan-dard parallel) are straight lines that aretruly divided and at right angles to eachother. All other parallels are horizontalstraight lines, equally divided and equaldistances apart. This is a sinusoidal mapprojection because the meridians are sinecurves drawn through predetermined pointson the parallels. Although the shapes aredistorted away from the center of the pro-jection, it is an equal-area projection andtherefore commonly used in atlases forareas astride the Equator, e.g. Africa.

saponite /sap-ŏ-nÿt/ A magnesium-richclaylike mineral related to montmoril-lonite. See clay minerals.

sapphire A colored transparent varietyof CORUNDUM (the color is caused by tracesof impurities such as oxides of chromium,cobalt, and titanium), valued as a preciousgemstone. It may be deep blue (the mostprized), pale blue, green, or yellow. It crys-tallizes in the trigonal system, and has beensynthesized.

saprolite /sap-rŏ-lÿt/ The part of theweathering profile that, although weath-ered, remains in situ and retains such orig-inal structures of the parent rock asjointing, banding, and veining.

sapropel /sap-rŏ-pel/ A loose deposit

made up mainly of the remains of algae,with some mineral fragments. Sapropelsform in anaerobic conditions at the bot-toms of lakes or shallow seas, and whencompacted change into shale containingcoal, bitumen, or oil (see oil shale).

saprotroph /sap-rŏ-troff/ (saprobe; sapro-phyte) An organism that feeds on deador decaying organic matter (plant or ani-mal). The best-known saprotrophs arefungi.

SAR See radar.

sardonyx /sar-don-iks/ A type of CHAL-CEDONY that has alternate parallel bands ofreddish-brown and white. It is used as asemiprecious gemstone.

satellite (artificial satellite) A man-made spacecaft that orbits the Earth,moon, or another planet. Since 1957, nu-merous satellites have been placed in Earthorbit. Some of these have had specific me-teorological purposes and tremendousamounts of new information have ap-peared as a result of these instruments.Continuous records are now received ofcloud patterns, and infrared photographsand experiments have shown that it is pos-sible to obtain vertical temperature pro-files. Geostationary satellites, which areorbiting at the same rate as the Earth andare therefore providing a continuousrecord of the same portion of the atmos-phere and surface, can determine the windfield at cloud level from the varying cloudpatterns, in addition to the other facilitiesoffered by the normal satellite. The move-ments of tropical cyclones (hurricanes, ty-phoons) can be tracked from theirformation enabling advisories and warn-ings to be issued to lands in their predictedpath. See Landsat.

Developments in satellite imagery anddata have contributed greatly to the obser-vation of features on, above, and below theEarth’s surface and the information thusobtained can be incorporated into GEO-GRAPHICAL INFORMATION SYSTEMS (GIS).Originally restricted to simple photo-graphs, satellite imagery can now provide

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widely diverse imaging at a variety of reso-lutions. High-resolution satellite imagery,in which images with 1-meter resolutionsare now available in the public domain, haswidespread applications. For example, incartography it is used in updating and cre-ating maps and navigation charts; otheruses include locating potential sites in min-eral exploration, showing areas of defor-estation, and locating pollution and itssources. Multispectral satellite imagery canbe used to reveal subtle topographic andvegetation features. Infrared satellite im-agery, which depicts the intensity of in-frared radiation emitted from Earth, can beapplied in the study of volcanoes. Satelliteimagery has been used to study variationsin ocean characteristics over wide areas;for example, data from AVHRR instru-ments (high-resolution visible and infraredlight radiometers) on such satellites as theNOAA Polar Orbiting EnvironmentalSatellite series enable sea surface tempera-ture and cloud patterns to be mapped. In1999 the first of a series of TERRA satel-lites to monitor effects of human activityon the global environment was launched;measurements recorded are to include thatof pollution in the lower atmosphere. Seealso Galileo; Global Positioning System.

satin spar A type of GYPSUM (calciumsulfate) that has translucent white fibrouscrystals. The same name is also sometimesgiven to a type of calcite (calcium carbon-ate) of similar appearance.

saturated adiabatic lapse rate Therate of cooling of a parcel of saturated airas it rises. Because the amount of latentheat released depends upon air tempera-ture, the lapse rate is not a constant. Athigh temperatures the rate is slow becausemuch latent heat is released on condensa-tion, whereas when it is very cold little heatis released and its value approaches the DRY

ADIABATIC LAPSE RATE. See also environ-mental lapse rate.

saturated rock An igneous rock thatcontains neither quartz nor feldspathoids.See silica saturation.

saturation 1. The condition in which asample of moist air is in equilibrium withan open water surface at the same temper-ature and pressure. In this state there is abalance between the number of moleculesleaving the water surface and the numberreturning from air to water. The point ofsaturation can be reached by adding mois-ture to a system or cooling it. The capacityof air to hold water is largely determinedby temperature as shown in the diagram ofthe saturation vapor pressure curve.2. See silica saturation.

Saurischia /sor-iss-kee-ă/ An order ofdinosaurs distinguished from the ornithis-chians by having a pelvis constructed onthe typical reptilian plan. These dinosaurswere primarily carnivorous and some grewto a very large size: Tyrannosaurus, nearly7 m high, was the largest carnivorous landanimal known and the herbivorous specieswere even larger, Diplodocus, for example,reaching a length of nearly 28 m. Likeother dinosaurs the group became extinctat the end of the Cretaceous period. SeeK/T boundary event. Compare Ornithis-chia.

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saussuritization /saw-sŭ-ră-tă-zay-shŏn/The alteration of plagioclase to a fine-grained assemblage of albite, epidote,calcite, and sericite during low-grade meta-morphism and hydrothermal processes.

savanna A region of tropical grassland,occurring mostly in a band between thetropical forests and the deserts (between 5°and 20° north and south of the Equator).There are few trees, except for solitarydrought-resistant acacias, baobabs, andeucalyptuses. See also savanna climate.

savanna climate A tropical climatewith a seasonal rainfall regime. It has sim-ilar properties to the monsoon climate, therain being associated with the INTERTROPI-CAL CONVERGENCE ZONE, although the sea-sonal reversal of winds is not necessarily aswell developed. The name is derived fromsavanna vegetation characterized by tallgrasses.

scablands A type of highly eroded ter-rain that was formed by sudden extensiveflooding after the melting of glaciers andice sheets. There is little or no soil on thebare rock surface, and so very few plantsgrow. A well-known example occurs in thenorthwestern USA.

scapolite /skap-ŏ-lÿt/ A member of a range of minerals that vary in composi-tion between Na4(Al3Si9O24)Cl andCa4(Al6Si6O24)CO3, mainly by the substi-tution NaSi↔CaAl. They are found in awide variety of metamorphic rocks.

scar A short steep slope of generally barecraggy rock, usually in a limestone region.

scarp (escarpment) A steep clifflikeslope often of considerable size, which risesabove the surrounding land surface. Suchstructures result from faulting (see faultscarp; fault-line scarp) or the differentialerosion of gently inclined strata (seecuesta).

scattering In the atmosphere, a processin which the electromagnetic radiationfrom the Sun is affected by air, dust, and

water vapor molecules, which break up orscatter its unidirectional beam to give dif-fuse radiation. This scattering is selective inthat blue wavelengths are scattered moreeasily than the longer red wavelengths andso diffuse radiation from the sky appearsblue. The reduction of blue light from thedirect beam results in the Sun’s disk ap-pearing red or yellow.

Larger dust particles scatter light with-out any wavelength dependence. Hazyconditions with high dust concentrationstherefore give a whitish hue to the sky.

scheelite /shay-lÿt, shee-/ A white, yel-low, green, or brown mineral form of cal-cium tungstate, CaWO4. It crystallizes inthe tetragonal system, and occurs in quartzveins and as deposits in contact metamor-phic rocks. It is used as a source of tung-sten.

schiller /shil-er/ A striking play of colorsexhibited by some minerals, particularlyfeldspars and pyroxenes, in hand speci-men. This is due to very small inclusions ofiron ore, orientated along certain cleavagesor partings, which reflect the light whenthe crystal is rotated. See diallage.

schist /shist/ A strongly foliated coarse-grained rock in which mica minerals areabundant and their subparallel orientationproduces a marked SCHISTOSITY. The schis-tosity is accentuated by the segregation ofminerals into thin layers alternately rich inmicaceous minerals and quartz/feldspar.Schists in general are the product of re-gional metamorphism.

schistosity /shis-toss-ă-tee/ A variety ofSLATY CLEAVAGE that is well developed inSCHISTS and to a lesser extent in GNEISSES.Platy minerals such as micas, amphiboles,and chlorites recrystallize or crystallizeperpendicular to the direction of maxi-mum stress. All traces of the original bed-ding are destroyed and the rock readilysplits along schistosity planes. See also foli-ation.

schlieren /shleer-ĕn/ Streaked-out patchesfound in igneous rocks, representing soft-

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ened xenolithic material drawn out alongthe direction of flow.

schorl /shorl/ A black variety of TOUR-MALINE.

scirocco /să-rok-oh/ See sirocco.

scolecite /skol-ĕ-sÿt, skoh-lĕ-/ A whitemineral form of hydrated calcium alu-minum silicate, Ca(Al2Si3O10).3H2O. Itcrystallizes in the monoclinic system asgroups of radiating crystals. It is a memberof the ZEOLITE group of minerals.

scoria cone See cinder cone.

scoriae /skor-ee-ee, skoh-ree-ee/ See py-roclastic rock.

Scotch mist A mixture of thick cloudand heavy drizzle, which derives its namefrom frequent occurrences in the hillierparts of Scotland. It is formed in maritimetropical air when uplift over the hills pro-duces large amounts of condensation andcloud droplets, some of which are largeenough to give drizzle. It can occur any-where that the conditions of weak uplift ofalmost saturated but stable air are fulfilled.

scour Localized erosion, for exampletidal scour in estuaries, removing sedimentperiodically and depositing it again at an-other stage; in rivers periodic bed scour oc-curs during periods of high flow,compensated at low flow by infill of de-pressions, a process sometimes calledscour-and-fill. In connection with glacialerosion, scour refers to the etching and pol-ishing of solid rocks by rock material in-corporated in the ice.

scree An accumulation of loose rockfragments on a hillside or at the base of aweathered cliff. Scree that slips downhill(when it is wet) becomes slightly sorted,with the larger pieces at the bottom.

screw dislocation A type of defect in acrystal lattice that results from the twistingof one part of the lattice relative to theother.

scrub A type of vegetation consisting oflow shrubs and small trees that grows in re-gions that have unreliable rainfall, such asthe margins of deserts. Examples includethe chaco of Paraguay, CHAPARRAL ofNorth America, the MALLEE of Australia,and the MAQUIS of the Mediterranean area.

scud See fractostratus.

Scyphozoa /sÿ-fo-zoh-ă/ The class of thephylum CNIDARIA that includes the jelly-fish. Because they possess no hard partsthey are rare as fossils, although remainsattributable to them have been found inrocks as old as the Precambrian.

sea breeze A local wind resulting fromdifferential surface heating between landand sea. It blows only during the day whenland areas heat up relative to the sea toproduce a weak thermal pressure gradientfrom sea to land. Initially the wind blowsdown the pressure gradient force at rightangles to the coast, but as the system con-tinues the Coriolis effect comes into play,causing a veering of the wind until eventu-ally the breeze may be almost parallel tothe coast. It can extend up to 80 km oreven more inland and is separated from thewarmer air by a sea-breeze front, ratherlike a cold front. Above the surface seabreeze is a returning circulation from landto sea maintaining continuity. See also landbreeze.

sea-floor spreading A concept first for-mulated in the 1960s by the American geo-physicist Harry H. Hess (1906–69), bywhich a satisfactory mechanism for CONTI-NENTAL DRIFT was found. This theoryshowed that the ocean floor is one of theyoungest and most active parts of theEarth’s surface. Magma rising from theEarth’s mantle reaches the surface alongthe MID-OCEAN RIDGES. The magma coolsand becomes part of the Earth’s crust. Re-peated rifting of the area accompanied bythe addition of further magma causes theolder material to be displaced sideways.Because the Earth is not expanding in di-ameter, the generation of new sea floor iscompensated by its destruction and reab-

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sorption into the mantle at destructiveplate boundaries. As the oceans spread thecontinents are moved across the surface ofthe Earth as part of a large LITHOSPHERIC

PLATE. The ocean continues to expand untilthe movement of other plates causes its rateof destruction to exceed its rate of con-struction. At that time the ocean begins toclose. Examples of spreading oceans arethe Red Sea and Atlantic Ocean; an exam-ple of a closing ocean is the MediterraneanSea. Spreading rates are quite low, being inthe order of 4 cm per year but varying fromplace to place along a mid-ocean ridge, andfrom ocean to ocean. See also plate tecton-ics.

sea fog Any fog over the sea, whateverits origin. It is most frequent over cold seasurfaces where contact cooling can lead tosaturation of the overlying moist air. Anexception to this is ARCTIC SEA SMOKE,which is an evaporational fog.

sea-fret See haar.

sea level The average level of the seasurface at a given time and in relation to achosen datum (see mean sea level). Becauseof the numerous irregularities and slopespresent on the sea surface, an accuratemeasure of sea level is rather difficult.Apart from day-to-day variations, sea levelhas changed with time, sometimes showinga net upward trend, at other times showingthe reverse. This has happened for manydifferent reasons; among these are isostaticreadjustment, geosynclinal sinking, localtectonic instability, geodetic changes, andglacioeustatism. Hence, superimposed onthe short-term fluctuations of sea level dueto tides, winds, etc., are longer-term fluctu-ations. Global sea level is closely related toclimate change; it has been estimated thatsea levels are rising by about 1–2 mm peryear as a result of the reduction in icesheets and the thermal expansion of theoceans. See also base level.

seamount A submerged and isolated el-evation on the deep-sea floor. Seamountsstand over 1000 m above the surroundingsea floor. Their slopes are comparatively

steep and they display relatively small cir-cular or elliptic summit areas. On a globalscale, seamounts are very numerous andwidespread, being especially numerous inthe Pacific Ocean. Their summits lie indepths of between 200 and 2500 m, al-though the majority of them lie between1000 and 2000 m. Many stand on the CON-TINENTAL RISE. Although some seamountsare found clustered in roughly lineargroups, many others rise from the sea floorquite independently of their immediateneighbors, the floor between them oftenbeing relatively flat. Three or moreseamounts in a line comprise a seamountchain, three or more not in a line make upa seamount group, and three or more sitedon a rise or ridge comprise a seamountrange. See also guyot.

sea roke See haar.

season A subdivision of the year consist-ing of a period of supposedly uniform orsimilar climatic characteristics. The lengthand properties of the seasons vary acrossthe globe. In tropical areas, temperaturesare fairly uniform throughout the year anddistinction is made between the wet anddry seasons only. In temperate latitudes,the seasons are based on the equinoxes andsolstices, but for climatological purposesthe divisions are on a monthly basis: (in theN hemisphere) spring – March, April, May;summer – June, July, August; fall – Sep-tember, October, November; winter – De-cember, January, February. These periodsdo have some average significance, but in-dividual days may have weather conditionsappropriate to any season. In polar re-gions, the changeover from winter to sum-mer and vice versa is very sudden and thereis thus a two-season year as in the tropics.

sea surge A large movement of water inthe oceans, which generally causes largewaves and high tides. Sea surges may becaused by strong winds associated withdeep DEPRESSIONS, and can cause extensiveflooding of low-lying areas, especially ifthey travel up an estuary.

sea valley An elongated depression cut

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in the sea floor and having a valley-likeform. Such features display a variety offorms: SUBMARINE CANYONS are deeply in-cised and possess steep walls, whereasmany others have a broad cross-sectionalvalley form, with relatively gently slopingsides that contrast markedly with the steepcanyon walls. Some valleys have a trough-like form, like that off the Mississippi Deltaor that off the Ganges Delta. Their floorsare broad and flat. Some resemble land val-leys by having levée features along one orboth flanks (see levéed channel). Some aremerely short gullies, sometimes arranged ina subparallel series. Eroded submarine val-leys have been grouped into shelf channelsand submarine canyons. The former groupmay be further subdivided into glacialtroughs, tidal scour channels, and drownedriver valleys.

sea wave A wave in the generating area,i.e. the area in which waves are being ac-tively formed. As the waves travel beyondthis area they become longer-crested andmore regular, being known as SWELL. Be-cause there may be present at any one timewaves of quite different lengths (perhapsranging from a few centimeters to severalhundred meters) and different heights, su-perimposed one upon the other, the sea’ssurface may present a very confused pic-ture. The size of sea waves depends largelyon three variables: the speed of the wind,wind duration, and available fetch. Seealso wave.

secondary depression A DEPRESSION

that forms along a front within the generalcirculation of a primary depression. Ini-tially it is less intense than the parent low,but on many occasions it intensifies andeventually incorporates the original de-pression.

secondary enrichment The concentra-tion of valuable metals in the lower part ofa vein as a result of the weathering of theupper part. Dissolved metals seep down-ward and react with the lower-grade com-pounds, enriching them with metal content.

secondary mineral A mineral present

in a rock as a result of the chemical alter-ation or breakdown of the original min-eral. Compare primary mineral.

secondary reflection A signal receivedat a receiver such as a geophone (hy-drophone) from a particular reflecting rockhorizon after it has been reversed in its di-rection of propagation at least twice byother reflecting horizons, and therefore ar-rives considerably later than a primary re-flection from that horizon which has had adirect path of propagation.

secondary vegetation Plants that growin an area that has been cleared of its pri-mary vegetation, for example following afire or human activity.

secondary wave (S wave; shear wave) Atype of SEISMIC WAVE that travels throughthe solid body of the Earth. It is propagatedby the oscillation of the particles at rightangles to its direction of propagation. Thistype of wave cannot be transmitted by liq-uids. See also primary wave.

secular trend A climatic variation thatcan be observed during the period of in-strumental records and indicates a persis-tent tendency for the mean value toincrease or decrease. It is most easily deter-mined by smoothing the data series byRUNNING MEANS.

secular variation Changes in the prop-erties of the Earth’s magnetic field over along period of time.

sediment Particulate material that hasbeen deposited in a fluid medium. The fluidconcerned is mostly water, but eolian sedi-ments are not uncommon.

sedimentary rock A rock formed bylithification (consolidation and compres-sion) of sediments laid down by wind orwater. The sediments may consist of fineparticles of preexisting rock, which may beigneous, sedimentary, or metamorphic, orbe derived from plant or animal remains.The resulting rock strata may subsequentlybe uplifted, tilted, folded or faulted. Sedi-

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mentary rocks include CLASTIC rocksformed of land sediments, including clay,silt, sand, and gravel; consolidated typesare sandstone and shale. Organic sedimen-tary rocks, usually lacking land sediment,include LIMESTONE and DIATOMITE (com-posed of the fragmentary remains of seacreatures), COAL (derived from plants), andcoralline (derived from coral). Chemicalsedimentary rocks are those formed bychemical processes, and include the EVAP-ORITES and some ironstones.

sedimentary structure A structureformed within sedimentary rocks duringtheir deposition (primary structure) orafter deposition (secondary structure).Such structures include CROSS-BEDDING,FLUTE casts, BIOTURBATION and RAIN PITS.

sedimentation /sed-ă-men-tay-shŏn/ Theprocess of sediment deposition. Sedimenta-tion of any particular particle size takesplace when the velocity of flow in thetransporting medium falls below its termi-nal, or settling, velocity.

sedimentology /sed-ă-men-tol-ŏ-jee/ Thestudy of sediments and sedimentary rocks.

sediment transport The movement ofmineral grains by wind, water, or ice. Theforce of gravity also moves sediments inrockfalls and on scree slopes. See eoliantransport; saltation; suspended load.

sediment trap A natural or artificial de-vice that serves to trap part of the sedi-ments moving across or above the seafloor. Artificial traps have been success-fully used for experimental purposes in cer-tain limited situations, for example in someAlpine lakes, and in scale hydraulic modelexperiments. In nature, certain types ofseaweed, mangrove, spartina, marramgrass, and so on perform similar functions.Soft algae, which in temperate seas may at-tain lengths of up to 10 m, not only breakthe force of waves but may reduce the or-bital velocity beneath waves to the extentthat sediments being carried in suspensionmay settle out on the seabed. Off S France,poseidonia beds in depths of between 5 and

50 m have led to deposition rates of up to1 m per century. Synthetic seaweed, whichis made of a special plastic material, hasbeen used in experiments to test its effec-tiveness in building up beaches. Spartinatraps marsh muds, marram grass trapsdune sands, and mangroves collect estuar-ine and tidal lagoon muds.

seiche /saysh/ A fluctuation in waterlevel sometimes occurring in the open sea,in semienclosed bays, harbors, lakes, andother water bodies. The period of oscilla-tion generally ranges from a few minutes toseveral hours. Small seiches may be a per-manent characteristic of many harborbasins. The period of oscillation is deter-mined by the resonance characteristics ofthe water body in question, usually being afunction of the actual dimensions andshape of the water body. The more severeseiches often result from rapid changes inwind fields and barometric pressure. Har-bor basin seiches may result from wavepenetration.

seif dune /sÿf/ (longitudinal dune) Asand dune occurring as a long chain run-ning parallel to the prevailing wind direc-tion. Such dunes tend to form in areaswhere the wind regime is two-directional,with a gentle prevailing wind that suppliesthe sand and short-term cross winds, oftenof greater strength, that help build thedune, producing curved faces. They canoriginate ab initio, or from BARCHANS whenthe wind regime changes. Their character-istic form is of lines of ridges, running par-allel over long distances, rising to summitsperiodically. The dunes grow downwind ofthe prevailing wind, with little lateralmovement. Individual chains reach 100 kmin length, but several distinct chains cancontinue one line for much greater dis-tances. Heights up to 210 m have beenrecorded, while the distance between suc-cessive lines varies from a maximum of 500m to a minimum of 20 m.

seismic array /sÿz-mik/ A series of seis-mometers laid out in an L- or T-shapedpattern.

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seismic discontinuity A boundary be-tween rocks at which seismic waves sud-denly change speed, because of thedifferent densities of the rocks on each sideof the boundary. Major examples includethe GUTENBERG DISCONTINUITY and the MO-HOROVIčIć DISCONTINUITY.

seismic event A short-lived event thatacts as a source of energy for the formationof SEISMIC WAVES, e.g. an earthquake or ex-plosion.

seismic noise See microseism.

seismic reflection The technique of de-termining the structure of a rock body bymeasuring the time taken for a pulse froma source at the surface to travel to thatbody, be reflected from a seismic disconti-nuity within that rock body, and return tothe surface.

seismic refraction The technique of de-termining the velocity and attitude of asubsurface rock body by measuring theshortest-time travel path from a source to aset of receivers distributed around the rockbody.

seismic shooting A technique used ingeologic surveys in which explosives aredetonated underground and the resultingseismic waves detected at various locations(using seismometers).

seismic wave A wave generated by anexplosion or earthquake within the Earthor on its surface. There are four main typesof seismic wave: PRIMARY WAVE, SEC-ONDARY WAVE, RAYLEIGH WAVE, and LOVE

WAVE.

seismic zone (earthquake zone) A nar-row well-defined belt in which the majorityof earthquakes occur. These zones usuallycoincide with the junctions between lithos-pheric plates, especially in association withisland arcs, mid-oceanic ridges, major frac-ture zones, and young orogenic belts.There are two main continental seismiczones, one bordering the Pacific and theother corresponding to the Alpine–Hi-

malayan chain. There are larger areas thatexperience virtually no seismicity.

seismogram /sÿz-mŏ-gram/ A record ofthe frequency and magnitude of the oscilla-tions produced during an earthquake,recorded on a time-correlating device.

seismograph /sÿz-mŏ-graf, -grahf/ Aninstrument that detects and records theseismic waves generated by earthquakes orother tectonic movements and explosions.The epicenter of the earthquake can be cal-culated from the recordings of a number ofseismograph stations.

seismology /sÿz-mol-ŏ-jee/ The scien-tific study of earthquakes, including theirorigins and manifestations. One branch ofthe science concentrates on seeking meth-ods for predicting earthquakes.

seismometer /sÿz-mom-ĕ-ter/ An elec-tronic or mechanical device that detects,amplifies, filters, and records the motionsof the Earth in a particular direction. Aseismic set is a series of three seismometersorientated at right angles to each other.

selenite /sell-ĕ-nÿt/ A colorless transpar-ent mineral form of calcium sulfate,CaSO4, a type of GYPSUM. It crystallizes inthe monoclinic system, as separate largecrystals or in crystalline masses.

selva /sel-vă/ A type of dense tropicalrainforest, as in the Amazon basin andparts of central Africa. See tropical rainfor-est.

sepiolite /see-pee-ŏ-lÿt/ See meerschaum.

septarium /sep-tair-ee-ŭm/ A largerounded nodule of clay ironstone or lime-stone, with a characteristic network of ra-diating cracks that contain calcite or otherminerals.

séracs /say-raks/ An extremely irregularice surface, usually at the foot of an ICEFALL

on a glacier. It is formed when the slowlyflowing ice decreases speed, and crevassed

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ice piles up into impenetrable pillars andpinnacles.

sericite /se-ră-sÿt/ Secondary muscoviteoccurring as fine-grained flaky aggregatesformed from the alteration of FELDSPARS.

series A division of rock in theStratomeric Standard scheme of strati-graphic nomenclature (see chronostratigra-phy). It indicates the body of rock that hasformed during one EPOCH. A series consistsof several STAGES grouped together and sev-eral series may be combined to form a SYS-TEM. In the past the term has been appliedloosely to bodies of rock characterized onthe basis of gross lithology and containedfossils, but a continuation of this informalpractice is not generally recommended.

serozem /se-rŏ-zem/ See sierozem.

serpentine One of a group of mineralshaving the general composition Mg3-Si2O5(OH)4 and a layered structure. Theyare monoclinic, typically green or white,and occur in two main forms: CHRYSOTILE

and ANTIGORITE. Chrysotile is fibrous andis used in the manufacture of asbestos,whereas antigorite has a platy habit. Ser-pentines are found in altered basic andultrabasic rocks where they have formedfrom the breakdown of olivines and pyrox-enes.

serpentinite /ser-pĕn-tee-nÿt/ A rock con-sisting largely of SERPENTINE, formed by thehydrothermal alteration of ultramaficrocks such as dunite and peridotite inwhich olivine and pyroxene have been con-verted to serpentine.

sessile benthos See benthos.

Seventh Approximation See Soil Tax-onomy.

shade temperature (in meteorology)The temperature of the air measured in theshade, usually by a thermometer in an in-strument shelter. In this way, direct sun-light, breezes, and variations in shade donot affect the measurement, which is the

basis of all official meteorological read-ings.

shadow zone An area on the Earth’ssurface whose EPICENTRAL ANGLE is greaterthan 103° and less than 143° from anearthquake focus. Within this area seismicwaves are received only after they havebeen reflected at the Earth’s surface.

shale A well-laminated ARGILLACEOUS

sedimentary rock that is fissile, and splitseasily along bedding planes (compare mud-stone). The fissility is related to the dispo-sition of clay minerals within the rock.

shallow inland sea See inland sea.

shard See pyroclastic rock.

shatter cone A striped conical feature inrock formed by compressive shock wavesthat follow the impact of a meteorite. Thecones, which may be up to several metersacross, are aligned with their tops orientedtoward the center of impact. They arefound in most sedimentary rocks and ingranite.

shear (shearing stress) A force tending todeform a rock mass through the movementof one part of it relative to another, as forexample at a fault or thrust plane. See alsodifferential shear.

shear joint A shear fracture, constitut-ing a site of possible future shearing.

shear wave See secondary wave.

sheet erosion See overland flow.

sheeting See unloading.

shelf edge (shelf break) The break ofslope between the CONTINENTAL SHELF andthe CONTINENTAL SLOPE, where a markedslope increase occurs. The shelf-edge depthvaries considerably from place to place,ranging between about 100 and 350 m, butthe norm is taken as roughly 200 m.

shell The hard casing of certain marine

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organisms. On the death of the organismsthe shells are deposited on beaches or onthe sea floor. Many remain virtually intactfor long periods, especially in environ-ments devoid of marked wave and tidal ac-tion. Others rapidly become pulverizedinto minute particles or broken into severalfragments. In tropical areas, beaches oftenconsist of shell sands, perhaps mixed withBryozoa, Foraminifera, and echinoid frag-ments. Outside tropical areas, shellbeaches are much less common. They areusually of rather coarse material and there-fore tend to be steep. Shelly material mayfigure prominently in reef-building, for in-stance in the case of OYSTER REEFS.

shield A very large rigid area of theEarth’s crust made up of Precambrianrocks, which have been unaffected by laterorogenic episodes. Shields represent areasof the Earth’s earliest formed continentalcrust. Examples include the Baltic Shieldand the CANADIAN SHIELD.

shield volcano A VOLCANO with gentlysloping sides and a wide base. It is built upby successive flows of highly liquid basalticlava, usually issuing from many fissures.See also flood basalt.

shingle Pebbles that make up a beach ona seashore. They have been rounded byrolling back and forth up the sloping beachwith the tides (attrition). See also gravel.

shock wave A wave of compression cre-ated when the speed of something exceedsthe speed of sound in the medium in whichit is traveling. Within a rock, for example,a shock wave (as from a meteorite impact)may deform, melt, or even vaporize thematerial, perhaps altering its compositionat the same time (termed shock metamor-phism).

shonkinite /shonkă-nÿt/ A type of alka-line SYENITE containing orthoclase feldsparand various mafic minerals.

shoreface The narrow zone that lies toseaward of low-water mark and which is,therefore, always covered by water but

which is shallow enough to permit activemovement of sea-floor sediments under theinfluence of waves. Some authorities usethe term to mean that part of the beachthat is periodically covered then uncoveredby the tide, i.e. the FORESHORE zone.

shoreline The line that is produced bythe intersection of a sea or lake surfacewith the sloping beach-face or shoreface.With regard to US National Geodetic Sur-vey marine charts and surveys, the shore-line approximates to the average line ofhigh tides. However, the term is often usedloosely to indicate coastline.

shore platform See wave-cut platform.

shoshonite /shoh-shoh-nÿt/ An extru-sive igneous rock with a groundmass ofpotassium-rich ORTHOCLASE and augite,olivine, and sometimes biotite mica.

sial /sÿ-al/ The Earth’s CONTINENTAL

CRUST, which is composed of granitic rocktypes that are rich in silica (Si) and alu-minum (Al).

siallitic soil /sÿ-ă-lit-ik/ A soil with amedium to high silica to aluminum ratio.Such soils are typical of the temperatezone, because tropical weathering removessilica and lowers the ratio, producing a soilin which the aluminum content is high.

Siberian anticyclone (Siberian high) Alarge area of high pressure that dominatesthe mean pressure field over Siberia, Rus-sia, in winter. It is emphasized by extensiveradiational cooling of the Eurasian land-mass.

side-looking airborne radar See radar.

siderite /sid-ĕ-rÿt/ (chalybite) A yellowto brownish-black mineral form of ironcarbonate, FeCO3. It crystallizes in thehexagonal system, and occurs mainly asveins in sedimentary rocks. It is used as asource of iron. See also carbonate minerals.

siderophile /sid-ĕ-rŏ-fÿl/ An elementthat has an affinity for iron, such as cobalt

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or nickel. Siderophile elements are found inmetallic meteorites and are probably con-centrated in the Earth’s core. See also at-mophile; chalcophile; lithophile.

sienna (raw sienna) /see-en-ă/ A yellow-brown earthy pigment containing ironoxide. When it is roasted, its color changesto reddish brown (called burnt sienna).

sierozem /sye-rŏ-zem/ (serozem; graydesert soil) A soil occurring in semidesertareas in more arid zones than the CHESTNUT

SOILS, and characterized by low organicmatter contents, a lack of leaching, and ac-cumulations of lime at the top of the Chorizon or lower B, typically within 30 cmof the surface, and possibly reaching up tothe surface. Vegetation cover is scanty:short grass and scattered brush. These soilsare included in the aridisol order of the USSOIL TAXONOMY.

significant wave A wave that has theaverage period and height of the highest33.3% of the waves in the WAVE SPECTRUM.Calculation of significant wave character-istics requires careful wave measurement atsea and subsequent analysis of this infor-mation, or the use of a wave analyzer in ascale model that involves wave action.

silcrete /sil-kreet/ A surface deposit, thatoccurs in semiarid regions, consisting ofgravel and sand cemented together bychert, opal, and quartz.

silica /sil-ă-kă/ Silicon dioxide, SiO2. It is the chief mineral in chert, diatomite, and sand, and in crystalline form it makesup all the various varieties of QUARTZ

and other SILICA MINERALS. Opal consists ofhydrated noncrystalline (amorphous) sil-ica.

silica concentration For an igneousrock, the percentage of SILICA it contains. Itis a common way of classifying such rocks.Those with more than 66% silica aretermed acid, 55–66% silica are intermedi-ate, 45–55% are basic, and less than 45%are ultrabasic.

silica minerals Quartz, tridymite, andcristobalite are the three commonly occur-ring polymorphs of SILICA (SiO2).Tridymite and cristobalite are stable at in-termediate and high temperatures, respec-tively, but neither form persists at highpressures and both are absent from plu-tonic rocks. The polymorphs have distinctcrystal structures built from SiO4 tetrahe-dra but the pattern of linking is different ineach case. In addition, each polymorph hasa low- and high-temperature modification,the transitions taking place rapidly at theparticular inversion temperature involved.Changes from one polymorphic form toanother are very sluggish and tridymiteand cristobalite both exist metastably atordinary temperatures. Coesite and stish-ovite are high-pressure high-density formsof silica found near meteorite impactcraters. Chalcedony is the cryptocrystallineform of silica containing minute crystals ofquartz with submicroscopic pores. Opal isa hydrous cryptocrystalline or amorphousform of silica. Lechatelierite is a raremetastable silica glass.

Quartz is stable over a wide range ofphysical conditions and because silica isthe most abundant oxide in the Earth’scrust, it is a very common mineral. The dif-ferent colored varieties are: colorless – rockcrystal, yellow – citrine, gray-brown toblack – smoky quartz, pink – rose quartz,and violet – amethyst. Quartz is trigonaland the absence of cleavage is characteris-tic. On Mohs’ scale it is the standard min-eral of hardness 7. Quartz is an essentialconstituent of silica-rich igneous rocks,such as granites, rhyolites, and pegmatites,and is found in oversaturated, intermedi-ate, and basic rocks. Quartz often occursintergrown with alkali feldspar in graphicgranite. Vein-quartz, precipitated from hy-drothermal solutions, is often associatedwith ore minerals. Because of its hardnessand resistance to chemical weathering,quartz is the most abundant detrital min-eral and is concentrated to give rise tosands and gravels of various types, which,on lithification, constitute the arenaceousrocks. Authigenic quartz is often depositedaround detrital grains, cementing them to-gether. During metamorphism, the quartz

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of sediments and igneous rocks recrystal-lizes to a coarser grain size. Quartz is alsoproduced by the released silica during themetamorphic breakdown of preexistingminerals. At high grades, segregation andmobilization take place giving rise toquartz veins and pegmatites.

Tridymite and cristobalite are typicallyfound in cavities in acid volcanic rocks,such as rhyolites and andesites, and may beproduced as a result of the passage of hotgases.

Chalcedony occurs in numerous vari-eties. Carnelian is red. The banded variety,agate, is formed by intermittent depositionin cavities. Jasper is a red-brown color.Chert and flint are opaque, dull-colored, orblack varieties, the former being the mas-sive or stratified form and the latter beingfound as nodules in chalk. Chalcedony isfound in hydrothermal veins, amygdales,and sediments.

Opal occurs in many color varieties, theprecious variety being iridescent. It is de-posited at low temperatures from silica-bearing waters and is found around geysersand hot springs. Diatomite is a rock madeup almost entirely of the accumulated opa-line skeletons of diatoms. Radiolarian anddiatomaceous deposits may recrystallize toform chert.

silica saturation The extent to which arock contains SILICA, this measure consti-tuting a convenient method of classifyingigneous rocks into three groups: 1. oversat-urated – rocks containing free silica asquartz. 2. saturated – rocks in which all thesilica is combined and in sufficient quantityto exclude feldspathoids. 3. undersaturated– rocks containing feldspathoids.

silicates /sil-ă-kayts, -kits/ A group ofminerals constituting about one third of allminerals and approximately 90% of theEarth’s crust, in which feldspar (60%) andquartz (12%) are the most abundant. Thebasic structural unit common to all sili-cates is the SiO4 tetrahedron. Silicates areclassified on a structural basis according tohow the tetrahedra are linked together.The six classes of silicates are as follows:nesosilicates (olivine), sorosilicates (hemi-

morphite, melilite), cyclosilicates (beryl,axinite), inosilicates (amphiboles and py-roxenes), phyllosilicates (micas and clayminerals), and tektosilicates (quartz, zeo-lites, feldspars).

silicification /să-liss-ă-fă-kay-shŏn/ Theprocess by which SILICA (usually in theform of chalcedony or quartz) enters poresor replaces other minerals in a rock.

sill A near-horizontal tabular intrusivebody of igneous rock, usually dolerite, ofroughly uniform thickness but thin relativeto its area. It is concordant with the planarstructures of the rock types into which itintrudes. See also dike.

sillimanite /sil-ă-mă-nÿt/ A pale green,gray, or brown mineral form of aluminumsilicate, Al2SiO5. It crystallizes in the or-thorhombic system, generally as fibrousaggregates (fibrolite) in metamorphicrocks. It is used as a refractory. See alsoaluminum silicates.

silt A fine-grained deposit that ranges insediment particle size from 0.06 mm to0.002 mm, and is therefore significantlyfiner than sand. Like clays, silts may in-clude clay minerals, and also hydroxidesand oxides of iron, silica dioxide, and nu-merous other fine material particles. Clayand silt together form the ARGILLACEOUS di-vision of sediments. Silts result not onlyfrom decomposition of certain in-siturocks and rock particles but also fromgradual abrasion processes (impact, grind-ing, and rubbing). Fine silty material tendsto collect in sheltered quiescent marine en-vironments and can be a serious threat tonavigation. It is sometimes difficult todredge, often returns rapidly from spoil-grounds to the dredged areas, and can in-undate seaweed beds where fish and othermarine organisms normally abound.

siltstone A fine-grained layered sedi-mentary rock derived from SILT. It resem-bles shale but contains less clay. See alsomudstone.

Silurian /sa-loo-ree-ăn, sÿ-/ The last pe-

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riod in the Lower PALEOZOIC beginningabout 438 million years ago, at the end ofthe ORDOVICIAN Period, and lasting about30 million years, until the start of the DE-VONIAN. The Silurian System is divided intofour series: the Llandovery, Wenlock, Lud-low, and Pridoli, but there are many alter-natives throughout the world. It wasnamed by the British geologist Sir RoderickImpey Murchison (1792–1871) for theCeltic tribe, the Silures, that inhabitedmuch of SE and central Wales where Sil-urian rocks are well developed. Rocks ofthe Silurian occur throughout the world; inNorth America they are exposed in the Ap-palachians and across the Mid-West, no-tably around the Great Lakes. The NiagaraFalls are on an escarpment of Niagaranlimestones from the Silurian. Importantearth movements were taking place withinthe period: the later stages of the Taconianorogeny of North America and the Cale-donian orogeny in NW Europe. The Iape-tus Ocean, which separated Scotland onthe North American continent from therest of the British isles on the Europe andScandinavian continent, was continuing toclose. During the Upper Silurian extensiveevaporite deposits were laid down with thewidespread regression of the seas.

Similarity in stratigraphy between theOrdovician and Silurian is reflected in thefauna, which seems to be a continuation ofthe Ordovician fauna. Trilobites and grap-tolites became less common; the latter werealmost extinct by the end of the period.Brachiopods were very varied and crinoidsand corals increased in importance. Ostra-coderm fish became more abundant andthe first gnathostomes (jawed vertebrates)appeared. The first evidence of land plantsalso comes from Silurian rocks.

sima /sÿ-mă/ The Earth’s OCEANIC CRUST,which is composed of basaltic rock typesthat are rich in both silica (Si) and magne-sium (Mg). The term was formerly used forthat portion of the continental crust belowthe CONRAD DISCONTINUITY but this was in-accurate and led to confusion with theoceanic crust.

simatic crust /sÿ-mat-ik/ The denser

lower regions of the Earth’s oceanic crustthat consists of sima.

singularity /sing-gyŭ-la-ră-tee/ The an-nual recurrence of particular types ofweather at certain times of year as illus-trated by the BUCHAN SPELLS. Unfortunatelytheir reliability is low except in a very gen-eral sense.

sinistral fault A fault at which move-ment is to the left relative to the side of thefault plane on which the observer is stand-ing. Compare dextral fault.

sinkhole (sink; pothole) A saucer-likehollow, typical of limestone areas, varyingfrom 1 to 1000 m in diameter and from afew centimeters to 300 m in depth, pro-duced by solution (see limestone solution)or by rock collapse. The ratio of diameterto depth is usually about 3:1. They are usu-ally formed on flat or gently sloping sur-faces, because here water can accumulateand stand, thereby causing solution. Sink-holes may also be simply enlarged verticaljoints, but whatever their shapes they oftenact as channels down which surface waterseeps to underground drainage systems.See also swallow hole.

sinusoidal projection /sÿ-nă-soi-dăl/ SeeSanson–Flamsteed projection.

sirocco /să-rok-oh/ (scirocco) A wind ofsimilar origin to the Egyptian KHAMSIN oc-curring farther west between Algeria andLibya. It is a hot dry southerly wind on theNorth African coast, but on crossing theMediterranean it can pick up much mois-ture and bring extensive low stratus cloudto the Italian coast.

SI units (Système International d’Unités)A system of units used, by internationalagreement, for all scientific purposes. It is based on the meter-kilogram-second(MKS) system and replaces both the cen-timeter-gram-second (cgs) and Imperial(fps – foot, pound, second) systems. It con-sists of seven base units and two dimen-sionless units (see Table 1). Measurementsof all other physical quantities are made in

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derived units, which consist of combina-tions of two or more base units. Eighteenof these derived units have special names(see Table 2). Base units and derived unitswith special names have agreed symbols,which are used without a period.

Decimal multiples of both base and de-rived units are expressed using a set ofstandard prefixes with standard abbrevia-tions (see Table 3). Where possible a prefixrepresenting 10 raised to a power that is amultiple of three should be used (e.g. mm ispreferred to cm).

skarn A calc-silicate mineral assemblageproduced during the CONTACT METAMOR-PHISM and METASOMATISM of highly cal-careous rocks.

sky-view factor The proportion of thesky that is visible to a person. It is an im-portant parameter in urban climatology.

slack tide (slack water) The period orstate of the tide in which there is negligiblehorizontal flow of water or when the tidalcurrents are at virtually zero velocity. Thisperiod occurs, in the case of a rectilineartidal stream, when the currents are revers-ing their direction. Some authorities con-sider the period covers that time whenflood and ebb velocities are less than 0.1knot.

SLAR See radar.

slate A fine-grained rock in which theparallel orientation of platy crystals of micaand chlorite results in the production of aperfect slaty cleavage. The individual con-stituent minerals cannot be resolved by thenaked eye but some slates contain porphy-roblasts or are spotted, the spots represent-ing embryonic porphyroblasts. Slates areformed during the low-grade regional meta-morphism of mudstones, siltstones, andother fine-grained argillaceous sediments.

slaty cleavage A parallel alignment ofplaty minerals in fine-grained rocks, atright angles to the direction of compres-sion. Slaty cleavage is usually also a formof AXIAL PLANE CLEAVAGE.

sleet Precipitation comprising ice pelletswith a diameter of 5 mm or less formedwhen raindrops or melting snowflakesfreeze or refreeze. In Britain the term isused for melting snow or a mixture of rainand snow.

slickensides 1. Small parallel grooves orstriations formed on the surface of fault orjoint surfaces as a result of the movementof rocks against each other. Small platyminerals are developed on them, fromwhich the relative sense of movement ofthe rocks can be determined. The surface ofthe slickenside is smooth in the direction ofmovement, but rough in the opposite di-rection.2. (in soils) The polished and smoothedsurfaces formed in subsurface soils by twomasses sliding past each other. Slickensidesare common in swelling clays where thereare large changes in water content.

slip-off slope The gentle slope on the in-side of a MEANDER bend opposite the un-dercut slope that occurs on the outside of ameander bend owing to erosion. The depo-sition of POINT BARS and creation of slip-offslopes is mostly attributable to the HELICAL

FLOW in meandering reaches, which con-stantly transfers sediment from erodingareas on the outside of bends to the de-positing areas on the inside.

slip sheet A gravity collapse structuredeveloped in the beds on the limb of an an-ticline. If the beds at the base of the limbfracture, the overlying beds will slip down-ward under the influence of gravity andthen spill out onto the adjacent strata.

slope An inclined surface whose lengthis determined by the horizontal distance

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outside

undercutundercutundercutslopeslopeslope(erosion)(erosion)(erosion)

inside

slip-off slope(deposition ofpoint bar)

Section through a meander bend

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TABLE 1: BASE AND SUPPLEMENTARY SI UNITS

Physical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr

*supplementary units

TABLE 2: DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv

TABLE 3: DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- µ 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y

from crest to foot and whose angle is de-termined by the inclination of the slopefrom the horizontal. Usually a slope is saidto have one angle in broad terms, but mostslopes are composed of a number of differ-ent parts, called facets if straight and el-ements if rounded, each having its ownslope angle. See also slope concavity; slopeconvexity; slope decline; slope evolution.

slope concavity The bottom parts ofhill slopes are typically concave, except incertain areas where this basal concavityhas been buried by aggradation. Analogieshave been drawn between the profile of aconcave slope and the long profile of ariver; this similarity of form has also pro-voked claims of a similarity of origin, theconcavity being essentially a product ofrunning water action, especially rill andgully action. This type of slope form is pro-nounced on lower slopes because there is agreater catchment area for running waterthan on upper slopes, there is increasedweathering of debris downslope, produc-ing smaller particles that can move on amore gentle slope, and there is increasingimpermeability as debris becomes finerdownslope, leading to increased runningwater action. For running water action, animpermeable surface is a prerequisite andconsequently concave slopes are well de-veloped on the finer rocks, e.g. clays,shales, and argillites, and less well devel-oped on the permeable rocks, e.g. sand-stone, chalk, and limestone. Plentifulrainfall is also important and, other thingsbeing equal, the concave parts of slope pro-files tend to become more pronouncedwith increasing age, as weathering of de-bris advances, making it finer and less per-meable, and hence more able to supportrunning water. See also slope convexity;slope evolution.

slope convexity The upper parts of hillslopes are characteristically convex inshape, and a good deal of discussion onslope evolution is directed at suggestingcauses for this shape. Early workers in-cluded the American geomorphologist G.K. Gilbert, who said convexity was a prod-uct of CREEP; as the soil mantle moves from

upslope, the volume passing a given pointincreases downslope, so an increasinglysteep slope is needed to transmit the debris,hence a convex slope. Today, this idea isstill widely accepted, even though creep isnow thought to operate as a slide ratherthan a flow, so that the thickness of the de-bris does not increase downslope. The Ger-man geologist Walther Penck said convexslopes were a product of uplift acting fasterthan downcutting; Nevin M. Fenneman(1908) thought it might be due to the ac-tion of wash; S. Schumm found that creepdoes produce a convex profile, but becauseit operates only on clay lithologies, convexprofiles tend to form only in claylike ma-terials. It may also be that because theupper part of a hill is attacked by weather-ing on all sides, the tendency is to weatherout a convex slope. In reality a number offactors may be responsible.

slope correction The reduction of dis-tances measured on a slope in the field totheir correct horizontal value for presenta-tion on a map. The corrected figure is ob-tained by multiplying the measured lengthby the cosine of the slope angle.

slope decline The original CYCLE OF

EROSION, as formulated by the Americangeographer William Morris Davis, wasbased on a progressive lowering of slopeangles (slope decline) as contrasted to thelater schemes of the German geologistWalther Penck and the South African geol-ogist Lester King, emphasizing PARALLEL

RETREAT of slopes. Slopes will decline whenmore material is eroded upslope than is re-moved from the foot of the slope, leadingto a differential rate of evolution, the upperpart retreating faster and hence loweringthe overall slope. For material to accumu-late at the foot of a slope there must be noeffective BASAL SAPPING, e.g. in a sea cliff cutoff from the sea by the growth of a shinglebank or sand spit, protecting it from waveaction, it has been seen that parallel retreatgives way to slope decline. Others havefound that lithologies of certain types favorslope decline, e.g. S. Schumm found declinewas a feature of slope evolution in clays.Originally, this type of slope evolution was

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seen as characteristic of the normal cycle inhumid temperate areas, i.e. as a climati-cally controlled process. Today it is knownthat slopes in the same climate can undergovery different evolutions and slopes in dif-ferent climates take the same course.Whether or not a slope undergoes declinedepends on process and lithology as well asclimate.

slope evolution Studies of slope evolu-tion are concerned with the relation be-tween slope form and the processesoperating on that slope, the lithology of therock, and the stage of evolution. It is inti-mately linked to studies of the general ten-dencies of landscape form, i.e. whetherthey tend to retreat parallel to themselves(see parallel retreat), leaving INSELBERGS

and PEDIMENTS, or whether they tend to de-cline (see slope decline), ultimately produc-ing a PENEPLAIN.

Many different techniques and ideashave resulted from these studies. Some ge-ologists have studied the current evolutionof small-scale slopes of different rock typesto see the influence of lithology; othershave studied the role of climate, through itscontrol of geomorphic process, by compar-ing slopes in different latitudes, or on theshady and sunny sides of valleys; othershave related slope form to tectonic action,or studied the role of base-level changes inproducing polycyclic slopes. Early workersinferred a series of sequential stagesthrough which any slope might evolve withtime, whereas some modern workers havesuccessfully identified actual time series inthe form, for example, of a growing spit ata cliff foot progressively cutting that cliffoff from wave action and hence inducing achange of process. Finally, some have em-ployed a statistical approach, measuringthe angles of slopes and finding those val-ues around which angles tend to bunch(characteristic angles). Despite all these ef-forts, slope evolution is still a highly com-plex topic, although many general modelshave been produced. In any particularslope, its shape is determined by the bal-ance of losses of rock by erosion and gainsby deposition, but because the geomorphicprocesses operating may have changed

through time, there is no reason why cur-rent slope forms should have any strong re-lation to the current processes operating onthem.

slump A single mass movement of rockdebris and soil downhill. It most often oc-curs when water-holding permeable rocksoverlie impervious ones (such as shale).Rocks may shear away, leaving a scarredrockface. Slumping also occurs underwaterat the edges of continental shelves.

slurry A liquid mixture of mud andwater, which readily slips downhill. It mostcommonly occurs in clays and shales.

smectite /smek-tÿt/ A member of one ofthe major groups of CLAY MINERALS.

smithsonite /smith-sŏn-ÿt/ A white, yel-low, pink, pale green, or blue mineral formof zinc carbonate, ZnCO3, known also ascalamine in the UK. Its color is caused byimpurities. It crystallizes in the trigonalsystem, and occurs in beds and veins in cal-careous rocks, often associated with HEMI-MORPHITE.

smog The polluted air that occurs aboveurban and industrial areas. The term is de-rived from the words smoke and fog. Theburning of fossil fuels, especially coal, re-leases sulfur compounds and smoke parti-cles into the atmosphere; sulfurous smogforms if these particles are unable to be dis-persed because of a persistent INVERSION.The concentration of toxic chemicals maybuild up to harmful levels. Photochemicalsmog, a hazy brown blanket of smog, iscreated when sunlight reacts with pollu-tants in the air. It is most prevalent inurban areas with high concentrations ofmotor vehicles; heat from burning fossilfuels causes nitrogen and oxygen to chem-ically combine to form nitrous oxide andhydrocarbon vapors. Under sunlight theseundergo photochemical reactions, one ofthe substances formed being OZONE.

smoker See hydrothermal vent.

smoky quartz A yellow to dark grayish-

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brown type of QUARTZ, which owes itscolor to the presence of impurities. It issometimes used as a semiprecious gem-stone.

SNC meteorite Any of a few smallstony METEORITES (named after Shergotty,Nakhala, and Chassigny, the three mainones) that have significantly different com-positions and ages from usual meteorites.Some planetologists believe that they orig-inated from mantle material on Mars up to1.3 billion years ago.

Snell’s law A law stating that the angleof refraction R of a light ray is related tothe angle of incidence I by the equation: sinI/sin R = µ, where µ is a constant called therefractive index of the material. The law isnamed for the Dutch mathematician andphysicist Willebrord van Roijen Snell(1591–1626).

snout The down-valley margin of a glac-ier.

snow Solid precipitation composed ofindividual ice crystals or aggregations ofice crystals forming snowflakes. The for-mer occurs when temperatures are wellbelow freezing and the moisture content ofthe atmosphere is not high. As tempera-tures approach 0°C, snowflakes are foundand these increasingly aggregate into largerflakes at about freezing point. Snow is verydifficult to measure accurately. It tends toblock rain gauges or be blown out. It isnormally recorded by inserting a graduatedruler into a flat surface of undrifted snow.The water equivalent of 30 cm of snow isapproximately equal to 25 mm of rainfall.

snowfield A large accumulation ofsnow in a mountainous region. If it is thickenough, downward pressure graduallyconverts it to ice, which may form glaciers.

snow line The altitude of the lower limitof permanent snow. In tropical areas thesnow line is about 5000 to 7000 m, butgradually decreases poleward, until itreaches sea level in the Arctic Seas, Green-land, and Antarctica. In general terms, its

altitude is determined by mean summertemperatures, but local climate conditionssuch as aspect or shelter from snow-bear-ing winds can produce variations.

snow patch erosion See nivation.

soapstone (steatite) A rock composedlargely of TALC.

soda lake An inland area of water con-taining high concentrations of dissolvedsodium salts, particularly the carbonate,chloride, and sulfate. The highly alkalinewaters do not support any life.

sodalite /soh-dă-lÿt/ A major member ofthe FELDSPATHOID group of minerals.

soil The natural accumulation of uncon-solidated mineral particles (derived fromweathered rocks) and organic matter(humus) that covers much of the Earth’ssurface and forms the supporting mediumfor plant growth.

The exact classification of soils is diffi-cult because soil occurs as a graded contin-uum, breaks in this continuum beingchosen subjectively for classification. In thepast classification was based on the PRO-FILE, but in the US SOIL TAXONOMY classifi-cation the unit is the PEDON and rather thanbeing based purely on factors of soil for-mation (as formerly) it is concerned withmorphology, HORIZON arrangement, andother properties. A soil phase is the lowestunit in the hierarchy of soil classification,and it reflects the erosional state of the soil,e.g. its depth, stoniness, and degree of ero-sion. A soil association is a collection ofsoils grouped on the basis of geographicproximity even though they may differgreatly in profile characteristics (thisgrouping is usually used in the generaliza-tion of a detailed map in order to reduce itto a smaller scale). A soil series is a collec-tion of soils similar in all respects exceptfor the texture of the surface horizon andits erosional state. Series are the basic soilmapping units. A soil type is a subdivisionof a soil series on the basis of the texture ofthe A horizon, these textural variationsusually being related to slight changes of

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slope. A soil group is a group of soils shar-ing a similar type and sequence of hori-zons, occurring over wide areas withsimilar temperature and moisture regimes(differences in surface horizon due to agri-culture are discounted). A soil order is thehighest level of generalization in soil classi-fication (the US Soil Taxonomy has 12 or-ders) and is based on the type anddevelopment of horizons present.

Soil structure denotes the mode of bind-ing together of individual soil particles intosecondary units (aggregates or peds),mostly by organic matter. There are sevenmain structure types (see table) and thegrade of structure can be classified intostructureless, weak, moderate, or strong,depending on the observable degree of ag-gregation. Structure is important to soilfertility, because it influences POROSITY

(and hence air and water supply), bulk den-sity, heat transfer, etc. Cultivation tends tobreak down structures by removing or-ganic matter, the major binding agent.

Soil texture denotes the proportion ofthe various PARTICLE SIZES in a soil. Thereare four main texture classes (see table) andmany intermediate grades. Texture can bedetermined accurately by laboratory analy-sis and allocated to a specific class by a tex-ture triangle, or more approximately in thefield by rubbing a wet sample between thefingers and estimating the proportion ofsand (gritty feel), silt (silky feel), and clay(plastic). Loams are generally the best agri-cultural soils because they contain a goodmixture of all particle sizes.

Soil consistency denotes the degree ofcohesion and adhesion in a soil, hence itsability to resist deformation or rupture. Itis described by the following terms: wet,sticky, plastic, nonplastic, moist, loose, fri-able, firm, dry, and hard, with a number ofintermediate grades.

Soil color as defined by the MunsellColor System is composed of three vari-ables: hue, value, and chroma. Hue is de-termined by the wavelength of the lightreflected from it, chroma is the purity orstrength of the color, and value is the rela-tive lightness or intensity of the color.Using a color code, a soil sample is allo-cated to a particular class that is composed

of varying degrees of these three variables.color is an important factor in the recogni-tion and description of soils, particularly asit often reflects formation, e.g. blue-graycolors typify gleying, red the concentrationof ferric oxide, and gray or grayish brownoften shows the accumulation of calciumcarbonate.

Soil moisture is fundamental to plantgrowth and whether it becomes available

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TABLE 1: SOIL STRUCTURE TYPES

Type Characteristicsplaty thin leafy flaky

layersprismatic prismlike units with

flat tops; mainlysubsoil

columnar prismatic units with rounded tops

blocky rectangular blocks;common in humidzones

subangular blocky blocky withrounded edges

granular rounded andporous; mainlysurface soil

crumb soft, rounded, andvery porous;mainly surface soil

TABLE 2: SOIL TEXTURE CLASSES

Name Compositionsand > 85% sand

percentage of siltplus 1.5 times thepercentage of clay is not more than15%

loam 7–27% clay28–50% silt< 52% sand

silt 80% or more silt< 12% clay

clay < 40% clay with< 45% sand and< 40% silt

to plants depends to a large extent on thesize and distribution of the soil pores (seeporosity) within which the water is held. Ingeneral the finer the texture of the soil thegreater its capacity to store water. Water isheld by the soil with varying degrees oftenacity; the less the moisture content themore firmly it is held.

Each soil horizon is determined by theoriginal parent material and subsequentadditions, losses, and transformations ofmaterials within it. Especially important inthe processes accomplishing horizon differ-entiation are those that transfer materialfrom one horizon to another, because thesedifferentiate the eluvial A horizon from theilluvial B. See also abnormal erosion;cheluviation; decalcification; eluviation;erosion; gleying; humification; illuviation;laterization; leaching; lessivage; podzoliza-tion; soil formation.

soil creep The slow movement of soilparticles downhill. It occurs particularly inwet areas and is so slow that it cannot nor-mally be seen. Nevertheless, in time largequantities of rock fragments and soil slip tothe bottom of the slope.

soil erosion The removal of soil by theaction of wind or water. It occurs mostoften when vegetation has been removed,usually by fire or human activity. See bad-lands; Dust Bowl; erosion.

soil formation (pedogenesis) Theprocess by which soil forms. The early Rus-sians, particularly Vasily VasilievichDokuchaev (1840–1903), stated that soilforms as a result of five factors: local cli-mate, PARENT MATERIAL, plant and animalorganisms, relief and elevation, and the ageof the land surface. In 1941, the Swiss-American pedologist Hans Jenny (1899–1992) tried to formulate the Russian workin a mathematical equation, s = f(cl, o, r, p, t ....), where s = soil, cl = climate, o = or-ganisms (including man), r = relief, p = par-ent material, and t = time. However, all thefactors are interrelated and the equationwould be very difficult to solve in the field,but in certain cases it is possible to evaluatethe effect of one of the factors against the

background of the others where these areconstant.Climatic factor. This is thought to be themost important factor in soil formation. Itis very complex because it can considerablyinfluence the effect of the other factors. Pa-rameters of fundamental importance arerainfall (regime and intensity), temperature(maximum and minimum, soil tempera-tures, and duration of sunshine), and evap-otranspiration. The effects of these singlecomponents can be measured, e.g. thedepth to which calcium carbonate isleached can be broadly correlated with theamount of rainfall. The combined influ-ence of temperature and precipitation iswell illustrated in Russia, where the se-quence of soil zones changes from north tosouth in relation to an increase in tempera-ture and a decrease in precipitation. Thisrelationship between soils and climaticzones forms the basis of the zonal classifi-cation of soils.Organic factor. Organisms are responsiblefor the breakdown of plant tissue resultingin the accumulation of organic matter, therecycling of nutrients, structural stability,and profile mixing in the soil. Vegetationitself, by binding the soil together, preventserosion. Humans through their farmingpractices such as cropping, burning, and ir-rigation are playing an increasing role insoil formation. Organisms are often de-pendent on climate but they may act as in-dependent variables. For example, certainmajor soil types are associated with partic-ular vegetation types and when the vegeta-tion type changes in the same climaticregion a change in soil type may result.Time factor. The time a soil takes to de-velop is measured from when fresh rock isfirst exposed at the surface, e.g. when thePleistocene glaciation left the British Islessome 10 000 years ago. Soil develops grad-ually (the rate is partly dependent on par-ent rocks, e.g. soils on till will develop inless than 1000 years whereas those on re-sistant quartzite will take much longer),probably through an (A)C, AC, A(B)C,ABC horizon sequence, and eventually astage is reached in which the soil is in equi-librium with the environment and can becalled mature. However, because the sys-

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tem is dynamic and any environmentalchange will affect the soil, it is better to useabsolute or relative datings than termssuch as young, mature, and old.Topographic factor. Topography affectssoil moisture. Drier shallower soils occurat the top of a slope, where drainage ismore rapid. Soils at the slope foot tend tobe deeper because of an increase in subsur-face weathering due to the presence ofmore moisture and the accumulation ofmaterial eroded upslope. As a result of as-pect, soils on south-facing slopes (in the Nhemisphere) tend to be drier than those fac-ing north. Because vegetation type isclosely related to soil moisture, throughvegetation topography also has an indirectinfluence on soil development. See alsocatena.

soil horizon See horizon (def. 1).

soil moisture storage capacity Seefield capacity.

soil profile See profile (def. 1).

Soil Taxonomy The US soil classifica-tion system, published by the US Depart-ment of Agriculture (USDA). The systemwas originally published in 1960 as theSeventh Approximation (the name comingfrom the fact that it represented the seventhattempt by its authors to find an ‘ideal’classification) and subsequently revised in1975. It originally comprised 10 orders butsubsequent revisions since 1975 have seenthe addition of 2 new orders: the andisolsand, most recently, the gelisols. Unilke ear-lier classifications based on pedogenesis,the emphasis is placed on properties ofsoils and diagnostic horizons that affect pe-dogenesis or result from it. As a result thetwelve orders of soils have some unusualgroupings; for example, there is no sepa-rate group for gleys, and podzols occur inmore than one order. Although the nomen-clature is somewhat obtuse, and laboratoryanalysis to measure certain properties isnecessary before definite classification,there is the great advantage that the majorlevels are as tightly defined as the fieldmapping units, so that there is no awkward

gap between mapping and broad classifica-tion, as so typically happened with manyolder systems. To some extent, the SoilTaxonomy is now being used at a worldlevel, and the FAO has published a worldsoil map based on it. See alfisol; andisol;aridisol; entisol; gelisol; histosol; incepti-sol; mollisol; oxisol; spodosol; ultisol; ver-tisol.

soil temperature The temperaturerecorded at any level within the soil (stan-dard depths being 30 cm and 100 cm) witha soil thermometer, which consists of amercury thermometer that has a horizontalsection as usual, but is then bent through90° to a predetermined length where thebulb is located. The diurnal temperaturewave decreases very rapidly with depthand below 10 m there is usually little signof an annual wave.

soil texture The arrangement and sizesof the particles in SOIL. Soils based on clay,for example, have a fine texture but rela-tively poor drainage, whereas soils con-taining sand have a coarse texture andgood drainage. A mixture of the two typesis called LOAM.

solar constant The intensity of RADIA-TION received from the Sun on a unit areaof a horizontal surface at right angles to thesolar beam above the atmosphere, at theEarth’s mean distance from the Sun. Itsvalue is 1.35 kW m–2. There has been con-siderable debate as to whether this value isa constant or not. Before the use of satel-lites it had to be measured from high-alti-tude observatories where atmosphericinterference could be reduced but not elim-inated. It now appears that the value is al-most constant except in the shortest part ofthe spectrum where the amount of energyreceived is small.

solarimeter See pyranometer.

solar radiation See radiation.

sole mark A sedimentary structureformed at the base of sandstone and silt-stone beds that overlie softer sediments.

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solfatara /sohl-fah-tah-rah/ See fuma-role.

solid geology The geology of rocks un-derlying glacial and other superficial de-posits. See also drift.

solifluction /sol-ă-fluk-shŏn/ A processof mass movement on slopes, usually re-stricted in its application to areas ofperiglaciation (see periglacial). The majorprocess of solifluction concerns the pres-ence of PERMAFROST. In spring, meltingtakes place in the top layer of the soil, butthe water produced cannot drain away be-cause beneath this active layer the groundremains frozen and hence impermeable.Consequently, the upper soil becomeshighly saturated, and its cohesion may bereduced to such an extent that flowage cantake place and material be moved down-slope, often creating characteristic terraceforms and burying former vegetation layers, although the presence of well-developed vegetation usually hinders theprocess.

Solifluction sometimes also embracesthe effects of NEEDLE ICE and CONGELITUR-BATION.

solitary wave A water wave consistingof only one elevation which, once formed,may travel practically unchanged as a sin-gle hump for a considerable distance. Theactual water particles move forward ashort distance as the wave passes by, i.e.the passage of a solitary wave causes a netshift of the water particles beneath the soli-tary wave in the direction of wave propa-gation. The solitary wave is neitherpreceded nor followed by other waves, un-less these are generated quite indepen-dently. TSUNAMI waves, which may be ofvery limited height at sea, possess some ofthe characteristics of solitary waves.

sol lessivé /sohl-less-ee-vay/ A soil thatcorresponds to the gray-brown podzolicsoil of North America. The profile is char-acterized by a thin surface accumulation ofmoder humus. Below this the A and Ebhorizons have been depleted of clay, whichhas been eluviated down the profile to

form a textural Bt horizon (a process notpresent in a true brown earth). This is clearfrom a study of the structure of the soil asthe Bt horizon has a prismatic or blockystructure whereas the surface horizon oftenhas a weak crumb/blocky structure. Thesesoils are common in the humid climate ofNW Europe on freely drained fine-texturedparent materials. They fall into the alfisolorder of the US SOIL TAXONOMY.

solod /sol-od/ (salt-earth podzol) A soilresulting from the leaching of a SOLONETZ

soil, forming a white-gray A horizon fromthe upper B horizon of the solonetz. Thisbleached A horizon, as in true PODZOLS, isdeficient in iron and aluminum, has asandy texture and an acid reaction. The Bhorizon is gleyed, with mottlings and acolumnar and prismatic structure devel-oped in a compact clay. These soils repre-sent the leached end of the developmentcontinuum from SOLONCHAK throughSOLONETZ to solod in which salts are pro-gressively removed through leaching, ei-ther naturally, sometimes owing to afalling base level, or artificially throughleaching by irrigation water.

solonchak /so-lon-chak/ (white alkali soil)Saline soil in which sodium chloride andsodium sulfate form more than 0.3% of thetotal soil. During drought the salts dry outat the surface giving a white color. Salinegroundwater exists within two meters ofthe surface, and this supplies the salts forupward rise by CAPILLARITY to the surfacesoil; it also imparts gley features to the soil,usually just the lower part of the profile butpossibly the whole profile if the water tableis near the surface. These soils are charac-terized by a pH value of more than 8 (morethan 15% of exchangeable cations aresodium), a flocculated structure, no illuvialB horizon, and one or two salt layers; theyare friable and soft in a moist state, crustywhen dry. They are fairly common in arid,semiarid, and subhumid regions, wherethey may occur naturally in depressions oras a result of secondary salinization of thezonal soils, or they may be artificial due toexcessive irrigation or irrigation withwater dominated by sodium salts. In the US

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SOIL TAXONOMY they are classified in theARIDISOL order.

solonetz (black alkali soil) Soil domi-nated by sodium carbonate. Solonetz de-velops from SOLONCHAK by the leaching ofsome of the sodium, due to increased rain-fall, improved drainage, or irrigation,which deflocculates the soil mass. As a re-sult the structure is single-grain when wet,but on drying the illuvial horizon developsa massive columnar structure. It is thisstructure difference that originally dividedthe solonetz from the solonchak. Whereasthe solonchak has an AC profile, increasedleaching in the solonetz produces a darkgray or brown B1 horizon enriched withcolloids, and lighter gray B2. The C horizonis enriched with carbonates and salts, whileat the surface alkaline solutions of humicmaterial accumulate in wet periods whichon drying produce black crusts. They maybe partly zonal soils, being concentrated atclimatic transition zones at 15° and 40°N,but traditionally they are considered as in-trazonal. In the US SOIL TAXONOMY theyare classified partly as aridisols, partly asmollisols.

solstice /sol-stiss/ The time of maximumor minimum declination of the Sun. Thesummer solstice in the N hemisphere oc-curs on or about June 22 when the Sun isoverhead at the Tropic of Cancer, its far-thest north. The winter solstice is aboutDecember 22 when the sun reaches23°30′S, the Tropic of Capricorn. At thesummer solstice is the year’s longest dayand at the winter solstice, the shortest.

solum /soh-lŭm/ The surface soil and thesubsoil, excluding the parent material.

solution (in geology) A basic form ofchemical weathering in which solid ma-terials are dissolved by water. Rates of so-lution vary considerably from onesubstance to another; silica is fairly insolu-ble and its removal rate increases with tem-perature, whereas the most soluble rockminerals are calcium, sodium, magnesium,and potassium. Halite and gypsum are verysoluble, subsequent recrystallization some-

times causing mechanical breakdown ofrocks (see granular disintegration; flaking).The efficiency of the solution process de-pends largely on the volume of water flow-ing, but the presence of weak acids withinthe water may cause increased solution.See also limestone solution.

solution breccia A BRECCIA in which theoriginal rock has been fragmented by theremoval of soluble material.

sonic log A subsurface logging tech-nique run in a borehole, which measuresthe time taken for a compressional soundwave to travel through 30 cm of the adja-cent formation. In this way a continuousrecord of the properties of the rocksagainst depth is recorded.

sonoprobe /sonn-ŏ-prohb/ A continu-ously recording acoustic reflection instru-ment, which has made an importantcontribution to the study of the continentalterrace and some deeper areas. Its use pro-vides data on the approximate thickness,distribution, and character of marine sedi-ments and the surface morphology and in-ternal structure of exposed or buried rockareas. Various sound sources are employedto give varying degrees of penetration ofthe seabed. There are many applications ofthe sonoprobe technique to offshore min-eral exploration and engineering surveys,for example in the laying of submarinepipes and cables and in the siting of oil rigs.

sorting The process by which materialsforming a particular sediment are gradedaccording to size by natural processes. Ifthe size range is confined within narrowlimits, so that most of the material is siltsize (as in eolian brickearth) or fine sandsize (as in many beach sands), the deposit iswell sorted. If the deposit has many sizeranges (as in a slump deposit, for example)it is poorly sorted.

Sorting is best in sediments subject to along distance of transport, with repeatedreworking, as happens to beach sandsbeing washed along a coast by wave action.In slump deposits, transport is sharp andshort with no time for any sorting.

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The basis of sorting concerns the set-tling velocity of a particle; each size anddensity of grain has a minimum level of en-ergy necessary to overcome its weight andkeep it in suspension. If the energy envi-ronment falls below that critical level, thegrain settles out. Hence on a coast, wherewave energy shows a gradient from high onheadlands to low in bays, there is a corre-sponding sorting of sediment, the coarsestfractions in the high-energy areas, the finerfractions in the low-energy areas.

soufrière /soo-free-air/ A volcano thatemits sulfurous gases; a large solfatara. Seefumarole.

sound 1. A waterway somewhat widerthan a STRAIT linking two bodies of water,or a passage between an island and themainland.2. A relatively long sea arm or inlet.

sounding 1. A measure of the depth ofwater. Before the advent of modern echo-sounding techniques, a sounding line witha lead weight on the end was used, but thisoften gave very inaccurate readings be-cause of the effect on the sounding line ofwave and current action. Almost all sound-ings are now obtained using echosounders. These devices are usually set torecord at speeds of some 1600 m/s. Mod-ern sounding techniques largely stem fromthe use of acoustic equipment in submarinewarfare during World War II.2. (upper-air observation) (in meteorology)A vertical profile of the chemical and/orphysical properties of the atmosphere (e.g.relative humidity, dry-bulb temperature,wind direction, and wind speed). Observa-tions may be obtained directly, such asfrom a radiosonde, or indirectly throughsatellite imaging.

South Equatorial Current See equato-rial current.

southern lights See aurora.

southern oscillation See El Niño; oscil-lation.

spar A general term for any lustroustranslucent or transparent mineral thatcleaves easily. Examples include Icelandspar and various other forms of CALCITE.

sparite /spar-ÿt/ Coarse CALCITE of agrain size in excess of 0.01 mm in diameter,which may infill cavities within andaround ALLOCHEMS in LIMESTONE. The ter-minology used in the petrographic descrip-tion and classification of limestones isbased on the presence of sparite, MICRITE

and allochems.

spatter cone A volcanic structure con-sisting of a steep-sided mound of lava thathas issued from a central vent or along afissure. The lava takes the form of lumpsand cinders, adhering together, that havecome from sprays of frothing moltenmagma.

species The basic group in the taxo-nomic classification of organisms, which isusually defined as a collection of similar in-dividuals that are capable of interbreedingto produce fertile offspring. In classifyingfossil species this criterion is impossible tosatisfy, and these species are usually de-fined on the basis of structural similarities(which in practice are also used in classify-ing modern species). This type of species,defined solely on morphological character-istics, is known as a morphological species.Difficulties in defining fossil species arisebecause of the absence of soft parts, whosestructural characteristics provide an im-portant means of classifying modernspecies. In addition the boundaries be-tween the successive fossil species of a con-tinuously evolving line must be arbitrary,because the species tend to grade into eachother. Some fossil species do not show ob-vious affinities with any organisms alivetoday. Examples of such species are thosethat produced CONODONTS: different skel-etal elements belonging to this group aregiven different specific names, although inlife they may have formed part of a singleindividual.

A species is defined with reference to aTYPE SPECIMEN, which is selected as a stan-dard and formally described. The name of

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a species is always italicized and precededby the name of the GENUS to which it be-longs. For example, the species that in-cludes modern man is Homo sapiens, or H.sapiens. A species name is often followedby the name (in abbreviated form) of theperson who first formally described thespecies.

specific latent heat See latent heat.

specular iron ore /spek-yŭ-ler/ Seehematite.

speleology /spee-lee-ol-ŏ-jee/ The scien-tific study of caves and caverns, includingtheir exploration, geology, and mineral-ogy.

speleothem /spee-lee-ŏ-th’em/ A min-eral formation in a cave that results fromthe action of water. Most consist of CAL-CITE (calcium carbonate), and the most typ-ical are STALACTITES AND STALAGMITES.

spessartite /spess-er-tÿt/ (spessartine) Anorange to deep red member of the GARNET

group of minerals, Mn3Al2Si3O12. It occursin igneous and thermally metamorphosedrocks. It is used as a semiprecious gem-stone.

sphalerite /sfal-ĕ-rÿt/ (zinc blende, blende,black jack) A yellow to red-brown lus-trous mineral form of zinc sulfide, ZnS,sometimes containing some cadmium andiron. It crystallizes in the cubic system, andoccurs in hydrothermal veins and metaso-matic replacements. It is the chief source ofzinc.

sphene /sfeen/ (titanite) A monoclinicmineral of composition CaTiSiO5 but withsome replacement of O by OH and F.Sphene is a common ACCESSORY MINERAL inigneous rocks. It is also found in metamor-phosed calc-silicate rocks.

sphenochasm /sfee-nŏ-klaz-ăm/ A tri-angular block of oceanic crust betweentwo continental areas.

spheroid /sfeer-oid/ A figure resembling

a sphere, used as a reference surface for ge-odetic surveys. Depending on the area ofthe world in which the survey is being car-ried out, different spheroids are used as areference datum, e.g. in the British Isles, theAirey spheroid (named for the Britishmathematician George Biddell Airey(1801–92)) is used, whereas in much ofAfrica, Clarke’s spheroid (1880), intro-duced by the British geodesist AlexanderRoss Clarke (1828–1914), is used.

spheroidal weathering /sfeer-oi-dăl/ Aform of EXFOLIATION that takes place onboulders below ground level. Initially an-gular joint blocks are gradually roundedowing to preferential weathering on thecorners, a resultant block consisting of asolid rounded core surrounded by spher-oidal shells. The term is sometimes re-stricted to exfoliation involving no changein volume (any weathering products are re-moved in groundwater), in contrast toFLAKING. The layering within spheroidalblocks is due to the chemical migration ofelements within the rock, creating zones ofenrichment and depletion. Any totallyrounded blocks found above ground levelhave probably been exhumed (see exhuma-tion).

spherulitic /sfe-rû-lit-ik/ Describing atexture commonly found in glassy or cryp-tocrystalline acid rocks in which acicularcrystals of quartz and alkali feldspar are ra-dially arranged to form a sphere.Spherulitic growth is the result of DE-VITRIFICATION of glassy material or rapidcrystallization of viscous magma. See vari-olitic.

spicule See Porifera.

spilite /spil-ÿt/ Any of several basalticlavas containing albitic plagioclase and inwhich the primary mafic minerals havebeen altered to chlorite and epidote. Al-tered acid rocks associated with spilites arecalled KERATOPHYRES and may contain phe-nocrysts of albite and hornblende in agroundmass of albite, chlorite, epidote,and quartz. Spilitic rocks occur as PILLOW

LAVAS interbedded with marine sediments.

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They appear to be basalts and rhyolites ofthe ocean floor, which have suffered post-magmatic soda metasomatism due to inter-action with sea water. See also ophiolite;Steinmann trinity.

spilling breaker See breaker.

spinel /spi-nel/ One of a group of oxideminerals with cubic symmetry, often oc-curring as octahedral crystals. The generalformula is R2+R2

3+O4 where R2+= Mg, Fe,Zn, Mn, or Ni and R3+ = Al, Fe, or Cr.Most spinels fall into one of three series de-pendent upon the nature of the trivalentcation: spinel (Al), magnetite (Fe3+), andchromite (Cr). Ulvöspinel also has a spinelstructure.

There is a continuous chemical seriesbetween hercynite (Fe2+Al2O4) and truespinel (MgAl2O4) because of the replace-ment Fe2+↔Mg. Minerals of the spinelseries are a wide variety of colors includ-ing red, brown, blue, green, and black.Magnetite (Fe2+Fe2

3+O4) is black andforms a continuous series with ulvöspinel(Fe2

2+TiO4). Chromite is Fe2+Cr2O4 butwith some replacement Mg↔Fe2+. There is a complete chemical series betweenchromite and hercynite due to the replace-ment Cr↔Al. The brown variety, picotite,is a chromium-rich hercynite.

Spinels occur in high-temperature meta-morphosed rocks, especially limestones.Hercynite is found in metamorphosedpelites and in basic and ultrabasic rocks.Magnetite is ubiquitous in metamorphicand igneous rocks and is an important ironore. Chromite is the chief chromium oreand, like magnetite, often occurs inmonomineralic bands in layered basic ig-neous bodies.

SP interval (in seismology) The timethat elapses between the arrivals of the firstsecondary (S) and primary (P) waves fol-lowing an earthquake. It provides a meansof calculating the distance to the earth-quake’s focus. See also seismic wave.

spirit level structure See geopetal cav-ity.

spit An elongated accumulation of sandor shingle (or both) attached to the coast atone end and extending out to sea. Spitsoccur most frequently where the coastlinechanges direction abruptly. The longshoremovement of beach material is an essentialfactor in the formation of spits. The ma-terial is moved along one straight section ofthe coastline and, rather than rounding asharp headland, accumulates seaward inthe original direction of movement. Spitssometimes form across estuary mouths, ortwo may approach each other from twofacing headlands (a double spit). A com-mon feature is a seaward end that curvestoward the land. This may be produced bywave refraction or two different sets ofwaves may be in action, one causing accu-mulation and the other the rounding of theend.

spodosol /spoh-dŏ-sôl/ One of thetwelve soil orders of the US SOIL TAXON-OMY including ashen or podzolized soils,characterized by a spodic B horizon con-taining humus and silicate clays with alu-minum or iron that has been moved downthe profile from the bleached A horizonabove. This includes the true PODZOLS, andthe PODZOLIC SOILS and groundwater pod-zols. Spodosols develop mostly in humidcool temperate climates or on acidic parentmaterials.

spodumene /spoj-û-meen/ A lithium-bearing clinopyroxene. See pyroxene.

sponges See Porifera.

spontaneous potential log (SP log) Asubsurface logging technique that continu-ously records the spontaneous potentialpresent at different depths. This resultsfrom electrochemical reactions betweenthe drilling mud and formation fluids, andsmall electromotive forces resulting fromthe penetration of drilling fluids into theformation.

spot height A height above the verticaldatum, represented on a map by a dot lo-cator with the elevation printed beside thedot. Unlike bench marks or trigonometri-

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cal points, spot heights are not identifiableon the ground.

spreading rate The rate at which newocean floor is added to the oceanic plates atconstructive PLATE BOUNDARIES. It is there-fore directly related to the rate at whichnew ocean floor is produced.

spring A place where water flows out ofthe ground because the water table inter-sects the surface. Thus the water flows outfrom above an impervious rock, such asclay, shale, or slate.

spring equinox See equinox.

spring line The line where a water tableintersects the surface of the ground, be-tween layers of pervious and imperviousrocks, and there is a spring or springs. Seespring.

spring tide A TIDE that has a relativelylarge range, rises and falls to the greatestextent from mean tide level, and occurs ator near the times of full and new moon. Aperiod of spring tides allows waves tobreak farther up or down the beach, and atthose times when strong onshore winds co-incide with the time of high water, this maylead to flooding of the hand behind thebeach. A wider tract of beach is intermit-tently uncovered compared with a periodof NEAP TIDES. Also, tidal currents tend tobe stronger during spring tides than duringneap tides.

spur An area of high ground that pro-jects into lower ground. A spur in theupper parts of a river valley may form IN-TERLOCKING SPURS, but farther down thevalley they are likely to be eroded.

squall A sudden increase of wind speedlasting for several minutes before dyingaway quickly. To be classed as a squall, thewind should rise by at least 16 knots (8m s–1) and reach at least 22 knots (11m s–1). A squall is often, although not nec-essarily, accompanied by heavy rain or athunderstorm.

stability The state of the atmospherewhere vertical air movement is limitedowing to the slow rate of cooling of the EN-VIRONMENTAL LAPSE RATE. If the environ-mental lapse rate is less than the SATURATED

ADIABATIC LAPSE RATE then the state isknown as absolute stability, because no ris-ing thermal can cool at a slower rate and sowill quickly become colder than the envi-ronment and sink back. These conditionsare typical of summer anticyclones.

stabilized dune A coastal dune in whichthe cutting off of direct wind action by thegrowth of another dune line in front, andthe development of a virtually continuousvegetation cover, has ended sand move-ment by the wind. These dunes generallyhave a very subdued topography, unlike aMOBILE DUNE, and a species-rich vegetation.

stable zone A part of the Earth’s crustthat is not undergoing mountain building(orogeny) or deformation through crustalmovements. Such zones are generally nearthe centers of continents, away from plateboundaries.

stack A small pillarlike island just off-shore, generally at the end of a headland. Ithas been detached from the headland bythe erosive action of waves, which mayhave first formed a cave that was graduallyenlarged. See natural arch; natural bridge.

stage 1. The point to which a landscapehas evolved in a CYCLE OF EROSION, whichdetermines its appearance. Landscapesused to be labeled youthful, mature, or se-nile, but this labeling is now being droppedbecause most landscapes are polycyclic (seepolycyclic landscape), so one cannot at-tribute their appearance purely to the stageof evolution in the current cycle, and be-cause evidence is increasingly being pro-duced that landforms do not adjuststeadily with time through equally longstages: they evolve very rapidly at first andthen achieve a steady state of little furtherchange.2. A division of rock in the StandardStratomeric scheme of stratigraphic classi-fication (see chronostratigraphy). It indi-

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cates the body of rock that has formed dur-ing one AGE. A number of stages groupedtogether constitute a SERIES and the stagesthemselves are formed of several CHRONO-ZONES. Stages are often spoken of in exclu-sively biostratigraphic terms but they maybe calibrated and defined by methods notinvolving fossils. They are usually namedafter geographical localities and have theending -ian, as in the Oxfordian, a stage ofthe Jurassic System.3. (gauge height) The height of a water sur-face in a river or stream above an estab-lished fixed datum level.

stagnant ice An ice mass that no longerreceives an adequate supply of ice in the ac-cumulation zone to maintain movement.The ice melts downward from the surface,meltwater frequently forming lakes mar-ginal to the ice, which may subsequently beevidenced by deltaically-bedded terrace de-posits. Debris-covered ice will melt moreslowly than that exposed, and in this waylarge masses can be isolated, resulting inKETTLE HOLE formation at a later stage. Thelarge amounts of meltwater present are re-sponsible for the characteristic ice-stagna-tion features, namely KAMES and ESKERS.

stalactites and stalagmites Two typesof SPELEOTHEMS that occur in limestonecaverns. Stalactites are tapering pendantsprojecting downward from the rocks oflimestone caverns. They are formed fromdrips of water containing dissolved cal-cium carbonate, which on evaporation de-posit a small trace of solid material.Stalactites take many thousands of years toform and are usually associated with sta-lagmites, which develop on the cave floorin a similar way. Stalagmites tend to bemuch broader at their bases than stalac-tites, and in time they extend upward.Eventually stalagmite and stalactite mayjoin to form a complete pillar.

standard atmosphere The idealized at-mospheric structure in terms of tempera-ture and pressure for all heights. It is usedfor the calibration of altimeters, etc. Sur-face characteristics are taken as 15°C and1013.25 mb for temperature and pressure

with a lapse rate of 6.5°C/km up to 11 kmat the tropopause.

standard parallel The line of latitudeselected in the construction of a map pro-jection that is projected at its true length.

standing wave (stationary wave) A typeof water wave in which there is a surfaceoscillation that does not travel along thesea’s surface. The surface oscillates up anddown between fixed points (nodes). A crestat one moment becomes a trough at thenext, and so on. Water particles have max-imum horizontal travel at the nodes andmaximum vertical travel at the antinodes(or loops). The height of a standing wave isapproximately twice that of the initial inci-dent waves. Such a situation is best ob-served when waves approach a verticalbarrier such as a cliff or seawall and are re-flected so as to meet the incoming waves.The depth of water must be sufficient toprevent the incoming waves from break-ing. Standing waves may also be producedat sea, for instance when similar PROGRES-SIVE WAVE trains meet from opposite direc-tions. See also clapotis.

star dune A complex DUNE that has astar shape when seen from above, and hasslip faces on three or more arms that radi-ate from a central point. Star dunes growvertically and are the largest type of desertdune; they are also believed to be the old-est. They occur in areas with multidirec-tional wind regimes. Star dunes occur inthe Grand Erg Oriental in the central Sa-hara and in the Badain Jaran Desert,China.

staurolite /stor-ŏ-lÿt/ A mineral with anapproximate composition Fe2+Al4Si2-O10(OH)2. It shows orthorhombic symme-try and is brown to yellow. It has astructure of alternating layers of kyaniteand a composition Fe2+(OH)2 but withsome replacement Mg↔Fe2+. Staurolite isfound in medium-grade regionally meta-morphosed argillaceous rocks, often in as-sociation with kyanite and almandine.

steatite /stee-ă-tÿt/ See soapstone.

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steering The control of the direction andspeed of movement of pressure systems bysome atmospheric factor. Thermal steer-ing, or movement in the direction of thethermal wind, works quite well for nonin-tensifying depressions, but in developinglows and anticyclones it is less reliable.

Steinmann trinity /stÿn-măn/ The com-mon association within geosynclinal sedi-ments of SPILITES, CHERT, and SERPENTINITES

(and other ultramafic rocks). The trinity isnamed for the German geologist GustavSteinmann. See also ophiolite.

stemflow The portion of precipitationor irrigation water that, having been inter-cepted by a plant, flows down the stem toreach the ground surface. See also intercep-tion; throughfall.

steppe An area of flat open temperategrassland in an arid region that will notsupport trees, particularly that of south-eastern Europe and Asia. If the land isplowed, it provides fertile soil for growingcereal crops. In this respect, it is similar toPRAIRIE.

Stevenson screen An instrument shel-ter: the standard housing for meteorologialinstruments, named for the Scottish engi-neer Thomas Stevenson (1818–87). It con-sists of a wooden box on legs, the base ofthe box being 1 m above the ground, inwhich thermometers are placed. Ventila-tion is provided by louvered sides, which innormal conditions allow adequate airmovement but prevent solar radiationreaching the thermometer bulbs. Access isprovided by (in the N hemisphere) thenorth-facing side of the screen beinghinged; this again prevents sunlight affect-ing the temperature records when recordsare being taken.

stibnite /stib-nÿt/ A lead-gray lustrousmineral form of antimony sulfide, Sb2S3.Crystallizing in the orthorhombic system,it occurs in hydrothermal veins and re-placement deposits, often with lead, mer-cury, and silver. It is the principal source ofantimony.

stilbite /stil-bÿt/ A white, gray, or red-brown mineral form of a hydratedaluminosilicate of calcium, sodium, andpotassium, (Ca,Na2K2)(Al2Si7O18).7H2O.It crystallizes in the monoclinic system assheaflike aggregates in hydrothermal veinsin igneous rocks. It is a member of the ZE-OLITE group of minerals.

stishovite /stish-ŏ-vÿt/ A very denseproduct of QUARTZ, SiO2, produced at ex-tremely high pressures, possibly followinga meteorite impact. See also silica minerals.

stock A discordant igneous intrusion,usually of coarse-grained granitic rock, re-sembling a batholith but having a surfaceexposure of less than 50 sq km. Compareboss.

stone polygons or circles A form ofperiglacial PATTERNED GROUND found inflat areas. The polygons or circles consistof fine-grained material, which is usuallysaturated with water and frequentlydomed upward, surrounded by slightlyraised walls of coarser stones, which pro-ject into the soil for several centimeters.The diameter of these features, which mayoccur either in large groups or in total iso-lation, varies considerably: the largerforms are several meters across and arefound in areas of long-continued freezingand extensive summer thaw, whereassmaller examples reflect more frequentfreeze-thaw cycles. Their method of forma-tion is still not totally clear, but is probablyconcerned with pressures set up by re-peated freezing and thawing, which movethe coarser fragments both upward andoutward.

stone stripes A type of periglacial PAT-TERNED GROUND that forms in a similarway to STONE POLYGONS OR CIRCLES whenthe processes responsible are acting underthe influence of gravity on a slope. The re-sult is a series of alternating coarse and fineparallel downslope stripes of stone. Thecoarse lines can vary from a few centime-ters to about two meters wide, while thefiner stripes between them are often at leasttwice this width. Stone polygons developed

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on flat ground may become elongated to-ward a slope and then grade into stripes asthe slope angle increases.

stony-iron meteorite See meteorite.

stony meteorite See meteorite.

storm 1. In general, a period of strongwinds, often accompanied by heavy rain.Major storms are called CYCLONES, HURRI-CANES, or TYPHOONS, depending on wherethey occur.2. In the BEAUFORT SCALE, force 11 withwind speeds of 56–63 knots.

storm beach An accumulation of coarsebeach material found at the very highestlevels of a beach profile, far above the high-est points reached by high spring tides.These boulders and shingle blocks attainsuch a height as a result of the extremepower of very infrequent storm waves. Al-though their overall effect on a beach is de-structive, the SWASH of these waves canthrow some material to great heights andrapid percolation of water prevents its sub-sequent removal by the BACKWASH.

storm hydrograph See hydrograph.

storm surge The rapid rising of waterlevel during a storm as a result of windstresses acting on the sea’s surface, or aPROGRESSIVE WAVE resulting from certainmeteorological circumstances. If this situa-tion occurs at the time of high SPRING TIDES,the tidal levels may significantly exceedpredicted values and serious flooding anddamage may result. This occurred with dis-astrous results at Galveston, Texas, in1850. In August 2005 Hurricane Katrinaproduced a storm surge of up to 9 mwhich, in combination with the wave ac-tion and extremely high winds, caused se-vere flooding and damage in the affectedparts of the Gulf Coast of Louisiana andMississippi; the failure of levées and flood-walls left parts of New Orleans under 6 mof water.

stoss /shtohss, stoss/ The side of a hillthat faces the direction from which a glac-

ier is coming. It is exposed to the abrasiveaction of the moving glacier, and as a resultusually has gentle slopes and rounded fea-tures.

strain The deformation of a body ofrock as a result of stress. It may be a changein shape, volume, or both. See heteroge-neous strain; homogeneous strain.

strain hardening The increasingstrength of a rock as it deforms plastically.

strain rate The rate at which a materialdeforms. This is usually expressed as a per-centage by which the original length is al-tered per second.

strain-slip cleavage (crenulation cleav-age) A type of cleavage superimposedupon slaty cleavage. It is typified by tabu-lar bodies of rock between regularly spacedcleavage planes.

strait A comparatively narrow water-way that links two sea areas or other largebodies of water. Some authorities, how-ever, restrict the term to gaps in isthmianlinks or island chains.

stratification /strat-ă-fă-kay-shŏn/ Thecondition shown by sedimentary rocks ofbeing disposed in horizontal layers or BEDS,known as strata. The term is also occasion-ally applied to igneous bodies that displayanalogous parallel textures.

stratigraphic succession /strat-ă-graf-ik/The series of the Earth’s rock strataarranged in sequence from the earliest tothe latest in age. See also geologictimescale.

stratigraphic trap See trap.

stratigraphy /stră-tig-ră-fee/ The branchof geology concerned with the descrip-tion and classification of bodies of rockand their CORRELATION with one another.Various aspects of this are dealt with inLITHOSTRATIGRAPHY, BIOSTRATIGRAPHY, andstratomeric and chronomeric CHRONO-STRATIGRAPHY. This separation of the vari-

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ous concepts involved in stratigraphy isnecessary if different levels of inference areto be distinguished.

stratocumulus /stray-toh-kyoo-myŭ-lŭs/A layer cloud within which weak convec-tion is taking place. It often forms exten-sive sheets, which have dark areasseparated either by small areas of clear skyor by thinner cloud. It occurs most fre-quently in winter anticyclones or over coolocean areas where anticyclones are domi-nant.

stratomere /strat-ŏ-meer/ (in stratigra-phy) Any segment of a sequence of rock.Stratomeres are not of standard uniformsize nor need they have been formed duringequal intervals of geologic time.

stratopause /strat-ŏ-pawz/ The bound-ary between the stratosphere and mesos-phere. It is found at a height of about 50km and represents the temperature maxi-mum of about 0°C resulting from energyrelease in the ozone layer.

stratosphere /strat-ŏ-sfeer/ That layerof the atmosphere above the troposphereextending to a height of about 50 km. Thelower stratosphere is almost isothermal buttemperatures increase gradually to theirsecondary maximum of about 0°C at thestratopause. Winds in the stratosphere canbe quite strong, especially the POLAR NIGHT

JET STREAM.

stratovolcano /stray-toh-vol-kay-noh/See composite volcano.

stratum /stray-tŭm, strat-ŭm/ See bed.

stratus /stray-tŭs, strat-ŭs/ A layer cloudwith a fairly uniform base found at lowerlevels in the atmosphere or actually at theground surface in the form of hill fog. Itmay give drizzle or Scotch mist.

streak A colored mark left when a min-eral is drawn across a piece of unglazedporcelain. It is characteristic of the mineraland commonly used as an aid to identifica-tion.

stream A body of water on the Earth’ssurface flowing within a natural channel.The term is applied to all such flows, rang-ing from the smallest creek to large rivers.

stream capacity See capacity (def. 1).

stream discharge hydrograph See hy-drograph.

stream frequency See drainage density.

streamline A line that is parallel to theinstantaneous direction of the wind field atall points along it, showing overall airmovement. The term is used particularly intropical areas where isobars are of little usein indicating the wind field because of thebreakdown of the geostrophic relationship.

stream load See load.

stream order The classification of astream in an integrated drainage patternthat is broken down into stream segmentsand the segments then ranked in a hierar-chy according to their allotted order.

At the head of the basin, the first smalltributaries are first order; two or morefirst-order tributaries unite to give a sec-ond-order tributary; two or more second-order tributaries unite to produce athird-order tributary, etc., up to the major

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1 1 1 1 1

1

1

1

1

22

2

3

4

3

3

2 2

2

2

22

Stream order in a hypothetical drainagepattern

stream of the basin, whose order definesthe order of the basin. Quantitative com-parisons can then be made between basins,and studies made of the relationships be-tween orders. Studies of this kind have pro-duced apparent laws of drainage basindevelopment. See also bifurcation ratio.

stream terrace See river terrace.

stress An applied force per unit area,causing rocks to undergo STRAIN. Stress cantake the form of tension, compression, orshear.

striation /strÿ-ay-shŏn/ A line that hasbeen cut or scratched into a rock surfaceover which a glacier has passed. Such linesare usually short, but sometimes exceedone meter, and are best seen on gently slop-ing faces of fine-grained hard rock, up andover which the ice moved. Ice itself cannotcause such abrasions; the projections ofrock debris, held rigidly in the basal ice, actas the cutting tools. Each striation can in-dicate two diametrically opposed direc-tions of ice movement, and they have oftenbeen used to discover movements of for-mer ice masses.

strike The direction along a rock stra-tum at right angles to the true dip.

strike line (structure contour) A linejoining points of equal elevation above orbelow a selected stratigraphic datum.

strike–slip fault (tear fault, transcur-rent fault, wrench fault) A vertical or near-vertical fault in which the displacement ishorizontal and motion is parallel to thestrike of the fault. The movement along thefault is described as left lateral (or sinistral)if the block on the side of the fault oppositeto an observer moves to the left, and rightlateral (or dextral) if the block moves to theright. The San Andreas Fault zone is a well-known example of a particular type ofstrike–slip fault, the TRANSFORM FAULT.

stromatolite /strŏ-mat-ŏ-lÿt/ A lami-nated concentric structure formed ofcalcium carbonate and produced by blue-

green bacteria (cyanobacteria). Fossilizedstromatolites dating back to Precambrian(Archean) times have been found.

stromatoporoid /strom-ă-toh-por-oid/An extinct organism that built laminatedreef structures in the Paleozoic. They wereparticularly common during the SilurianPeriod. Their taxonomic affinities are ob-scure but they may belong to the Hydro-zoa, a class of the phylum CNIDARIA. Theyand the tabulate corals (see Anthozoa),which were also important reef-builders inthis era, seem to have preferred differentconditions, for they are rarely found in as-sociation.

Strombolian See volcano.

strontianite /stron-shee-ă-nÿt/ A color-less, white, yellow, or green mineral formof strontium carbonate, SrCO3. It crystal-lizes in the orthorhombic system and oc-curs in nodules and veins in limestonerocks. It is used as a source of strontiumand its compounds.

structural geology The branch of geol-ogy that deals with the structural featuresof rocks, their analysis, and their descrip-tion, including the forces that create thefeatures.

structural high A geologic structure ofpositive relief, such as a dome or anticline.

structural low A geologic structure ofnegative relief, such as a syncline or basin.

structural trap See trap.

structure contour See strike line.

subaerial erosion /sub-air-ee-ăl/ ERO-SION that takes place at the surface of theEarth.

subcrop /sub-krop/ The disposition of abed of rock beneath the surface and its con-tact with the undersurface of younger beds.The term is used by the petroleum industryin the reconstruction of the paleogeogra-phy of an area.

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subcrustal convection /sub-krus-tăl/Gross movement of semifluid magma inthe Earth’s mantle resulting from convec-tion, which in turn causes the movement ofthe crustal plates. See plate tectonics.

subduction zone /sub-duk-shŏn/ (destruc-tive plate boundary) An area on theEarth’s surface where two lithosphericplates are converging and one, containingoceanic lithospheric material, is beingoverriden by the other, which may beoceanic or continental. The overriddenplate is forced down to descend into the as-thenosphere in a process known as subduc-tion. Where the lithospheric platedescends, it forms a deep-sea TRENCH, along linear depression that is the deepestfeature of the Earth’s surface, and its posi-tion is marked by a series of earthquakefoci. With descent, the lithospheric ma-terial is subjected to greater temperaturesand pressures, as a result of which somestarts to melt to form magna, which thenrises upward through the overlying crust toproduce a linear chain of volcanoes paral-lel to the trench. If the oceanic lithospheresubducts below oceanic lithospheric ma-terial the volcanoes form an island volcanicarc. See also plate tectonics.

subglacial /sub-glay-shăl/ Describingthe region underneath a glacier or ice sheet.

subhedral /sub-hee-drăl/ Describingcrystals with partially developed crystalforms. Compare anhedral; euhedral.

sublimation /sub-lă-may-shŏn/ Theprocess by which ice is converted to vaporand water vapor into ice with no interven-ing liquid state. Chemically, the term isused only for the first process.

submarine bar An offshore accumula-tion of sand, more or less parallel with thecoastline, that never becomes exposed bythe tide. Submarine bars occur in areas inwhich the tidal range is extremely smalland usually form in groups of two or three.They are formed at the point at which steepstorm waves break. Seaward of this pointmaterial is moved toward the land, then

after the wave has broken movement is sea-ward. Thus sand accumulates from two di-rections, although an optimum size isreached eventually, beyond which no fur-ther increase in height is possible, owing tothe effect of the bar on waves passing it.The outermost bar, in deepest water, is in-frequently fully developed and is formedby the rare strong storm waves; it is unaf-fected by the smaller storm waves respon-sible for the bars nearer the shore. Theinnermost bar tends to move its position aswave heights vary; movement is seawardwith increasing wave height and landwardwith decreasing height.

submarine canyon A deep steep-sidedvalley or trench cut into the continentalshelf or continental or insular slope. Manyare deeply incised into the solid rock of thesea floor, with V-shaped cross profiles, andsome form deep winding gorges. Well-known submarine canyons include thoseoff the mouth of the Congo River, theHudson River, and Cape Breton in SFrance. Canyons may have tributary inci-sions and and these sometimes display adendritic pattern; others occur singly or aspart of a system of subparallel incisions.These features are thought to have origi-nated through erosion by submarine TUR-BIDITY CURRENTS. See also sea valley.

submarine contour See contour.

submarine eruption A volcanic erup-tion from a vent in the oceanic crust on thesea floor. Most of these eruptions takeplace on SEAMOUNTS or along MID-OCEAN

RIDGES.

submarine fan A fan-shaped mass ofsediment normally found at the lower endof a SUBMARINE CANYON. Much of this ma-terial may have been deposited under theaction of submarine slides and turbidityflows. For example, a very large fan existsat the lower end of the Congo Canyon,where slides or turbidity flows occur about50 times annually. The Mississippi Delta isflanked on its western side by a trough val-ley that leads, at a depth of some 1800 m,into a large submarine fan. In both cases,

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very substantial amounts of muddy sedi-ments are carried to the coast by the rivers.

submarine valley See sea valley.

submergence The covering of land bythe sea, either because the land has sunk orsea level has risen. See also cycle of erosion;fiord; ria.

subsequent stream A stream that hasdeveloped on a weak substructure, such asa clay vale, or in regional joint or fault pat-terns, at right angles to streams consequentto the dip of the slope. Subsequent streamsowe their development to the acceleratedrate of headward erosion they are able toperform in these weaker areas, rapidly ex-tending themselves and often coming to in-tegrate and dominate the drainage pattern.See also consequent stream; obsequentstream.

subsidence The widespread downwardmovement of air associated with surfaceDIVERGENCE. The rate of descent is usuallyfairly slow, of the order of a few meters perhour, but this results in adiabatic warmingand a low relative humidity. Near theground, mixing takes place so that extremevalues of relative humidity are unusual.The weather associated with large-scalesubsidence is always dry although cloudlayers may result from mixing with moisterair near the ground.

subsoil See profile.

subsolidus /sub-sol-ă-dŭs/ A mixture ofcompounds, such as minerals, below theirmelting points in which chemical reactionsmay still take place (in the solid state).

subsurface eluviation The process bywhich water falling on the soil and sinkingin eventually moves downslope as a sub-surface flow; as it does so it dissolves ortransports soil and rock material with it.This process has tended in the past to beoverlooked in studies of slope evolution.However, evidence has shown that inplaces it can be very effective.

subtropical high (subtropical anticy-clone) One of the anticyclones in the beltof semipermanent high pressure in the sub-tropics.

subtropical jet stream See jet stream.

succession The vertical sequence ofrock units in a particular place. A numberof local successions may be combined toproduce an idealized succession demon-strating the sequential order of strata overa larger region.

suevite /swee-vÿt/ A type of BRECCIA con-sisting of angular fragments of rock in aglass matrix.

sugar loaf An INSELBERG, especially inthe S coastal area of Brazil.

sulfur A native element (S) of character-istic yellow color found around volcanicvents and hot springs.

sulfur bacteria Bacteria that release ele-mental sulfur as a result of their metabo-lism. They may, for example, oxidizehydrogen sulfide (to obtain energy) or re-lease sulfur through a type of PHOTOSYN-THESIS. See also iron bacteria.

sulfurous smog See smog.

sulfur oxidation A process in soil sci-ence analogous to nitrification, in this casethe bacterium Thiobacillus converting in-organic sulfur compounds into the sulfateform. This may occur solely by chemicalmeans but most appears to be biochemicalin nature.

summer solstice See solstice.

sun crack See desiccation crack.

sunshine Direct radiation received fromthe Sun. It is one of the climatological el-ements recorded at most observing sta-tions.

sunshine recorder Sunshine is fre-quently measured by the CAMPBELL–STOKES

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RECORDER, which consists of a glass spherethat focuses the Sun’s rays onto a card,burning a trace on the card wheneverbright sunshine occurs. The length of thetrace indicates sunshine duration. Al-lowance is made for the seasonal variationin declination of the Sun. As it records onlybright sunshine many hours are notrecorded, especially in early morning orlate evening, or in conditions of severehaze. No distinction is made between thedifferent heating capacities of winter andsummer sunshine. The Campbell–Stokesrecorder is named for an employee of theBritish Board of Health, Reverend J. F.Campbell, who originally designed it. Itwas subsequently improved upon by theBritish mathematician and physicist SirGeorge Gabriel Stokes (1819–1903). In theUSA the MARVIN SUNSHINE RECORDER hasbeen used as the standard instrument of theNational Weather Service; the time scale issupplied by a chronograph. It is named forthe American meteorologist Charles Fred-erick Marvin (1858–1943). A number ofautomatic electronic sunshine recordershave been developed for use in automaticweather stations.

sunstone A translucent type of oligo-clase (see feldspar) that appears to glowwith a reddish light, caused by tiny parallelinclusions of HEMATITE. See also moon-stone.

supercooling The cooling of a liquid toa temperature below its normal freezingpoint. This is a very common phenomenonin the atmosphere because ice does notform in cloud droplets at 0°C because ofthe sparsity of suitable CONDENSATION nu-clei. Supercooling is essential to the BERG-ERON–FINDEISEN THEORY of precipitationformation.

superficial deposits Material of Holo-cene and Pleistocene age that lies on top ofthe solid geology, being extraneous ma-terial transported and deposited by variousgeomorphological processes. They includeglacial drifts, terrace gravels, alluvium ofrivers, raised beach deposits, windblownsand and loess, solifluction deposits, etc.

Often various relict weathering horizonsare included in this term, but this is notstrictly correct because these materials arerelated to the rocks beneath.

supergroup The largest division in thehierarchy of the lithostratigraphical classi-fication of bodies of rock (see lithostratig-raphy; stratigraphy). A supergroup isformed of two or more associated and ad-jacent GROUPS.

superimposition The most commonprocess by which a drainage pattern be-comes discordant. Drainage originates on aland surface and develops a conformablepattern with the structures in that surface;as dissection proceeds this surface iseroded away, and the drainage is let downonto the newly exposed rocks below.These have different structures and litholo-gies from the original surface, and so thedrainage pattern no longer conforms to thestructure over which it flows. Hence it hasbeen superimposed from a former coveronto the one it currently occupies. Once su-perimposition has occurred, the drainagemay adjust to the structures of its new sur-face, and mask its true origin. On Dart-moor, England, the E–W and N–Srectangular pattern superimposed from theformer Eocene and Cretaceous cover hasbeen supplemented by a NW–SE andNE–SW pattern following the joint patternof the granite. Current drainage is there-fore a mixture of accordant and discordantstreams.

superposition (principle of superposi-tion) The principle that if one set ofstrata occurs on top of another in a succes-sion the upper unit was formed later (ex-cept in the case of strata that have beenoverturned tectonically). This principleprovides the basis of understanding the se-quence of events in time as shown by a par-ticular succession of rocks. It was firstproposed by the Danish scientist NicolausSteno (1638–87) in 1669.

supersaturation The state of air thatcontains more water vapor than theamount required to saturate it. However,

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the abundance of condensation nuclei usu-ally prevent this situation arising in the at-mosphere.

superstructure The higher levels withinan orogenic belt, which have deformed in abrittle manner as a result of their near-sur-face position. These levels have sufferedonly minor metamorphism. Compare in-frastructure.

supervolcano An exceptionally largevolcano that begins as a boiling reservoir ofmagma risen from the mantle to within theEarth’s crust, building in pressure until itfinally erupts in a massive and devastatingexplosion. Unlike the majority of volca-noes, which are cone-shaped, supervolca-noes may be immense CALDERAS and can behard to detect. For example, the magma-filled caldera (about 70 km by 30 km) ofYellowstone National Park was only de-tected in the 1960s through infrared SATEL-LITE imagery. Yellowstone has been on aregular eruption cycle of approximately600 000 years, the last eruption beingsome 640 000 years ago. Calculations havebeen made that indicate that during the20th century parts of the caldera rose byover 70 cm, raising concerns that the over-due eruption may be imminent.

During the largest volcanic eruptionsforces are great enough to eject volcanicdust, debris, and gases, including sulfurdioxide, into the upper atmosphere. Thesulfur dioxide combines with water toform droplets of sulfuric acid, which forma reflective barrier to incoming solar radia-tion with the effect of reducing tempera-tures. The last supervolcano to erupt wasToba, in Sumatra, 74 000 years ago. Thevolcanic winter that ensued created aglobal catastrophe and is believed by somegeneticists to have pushed human life to thebrink of extinction.

supplementary contour A contour linethat may be shown (often as a dashed line)on maps of areas of detailed or irregular re-lief patterns in addition to the standard in-terval for the area. For example, in an areawhere the land is generally level but thereare several isolated small hills, a contour

interval of 100 ft (40 m) may not show upthe hills. (See diagram.)

supratenuous folding /soo-pră-ten-yoo-ŭs/ A form of folding that results fromdifferential compaction. Sediments arebent around more competent materialssuch as concretions or coral reefs.

surf Broken water resulting from turbu-lent wave activity between the outer limitof the SURF ZONE and the swash-backwashzone on the beach.

surface boundary layer See boundarylayer.

surface runoff The part of rainfall thatruns off the surface of the land and doesnot filter into the ground or evaporate intothe atmosphere.

surface water Water that stays at ornear the surface of the land, as opposed toGROUNDWATER.

surf beat A type of long wave activitythat is evident at the coast by irregular os-cillations of water level in the surf zone.These oscillations have periods of severalminutes. A possible cause is the arrival ofgroups of higher-than-average waves,which have the effect of piling up water inthe nearshore zone and causing a reflectionof long-wave energy. The presence of surfbeat has been demonstrated by precisionwave recorders set up in the offshore zone.

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200200200

275275275250250250225225225

200200200

100100100

100 ft contour interval with supplementarycontours to show a small hill

surf zone (breaker zone) The strip ofwater along the shore in which breakingwaves are actively dissipating their energy.They do this by a process that involves sur-face turbulence at a time when the wavesor swell have become unstable and break.Usually this is because of their encounter-ing very shallow water, but it may also bedue to their meeting with opposing cur-rents or wind. This complete breakdown ofwaves or swell must not be confused withbreaking seas or so-called white horses, inwhich there is only partial collapse, at sea,of wave crests (see breaker). The surf zoneis characteristically wide off gently slopingcoasts and relatively narrow off steeplyshelving coasts. The width of the surf zonefluctuates with constantly changing waveconditions; also, it is usually relativelywide in the case of sand beaches and nar-rower in the case of shingle or boulderbeaches.

surge See sea surge.

surging glacier A glacier that has a briefperiod of comparatively rapid flow. It oc-curs when accumulated ice in an ice reser-voir reaches a critical amount andsuddenly surges downward at up to a hun-dred times the normal speed.

surveying The construction of mapsand plans by accurately recording the rela-tive positions and heights of features on theEarth’s surface, and plotting them to somesuitable scale. This is achieved by measur-ing distances, directions, and heights, themajor surveying methods being: TRIANGU-LATION, TRAVERSING, CHAINING, PLANE

TABLING, TACHEOMETRY, and LEVELING. Seealso geodetic surveying.

suspended load 1. The sediment car-ried in SUSPENSION within a body of water,beneath waves or in current flow, as op-posed to the sediment that moves wholly orintermittently in contact with the bottom(bed load and saltating load). Shorewardof wave break-point, a great deal of finematerial may be carried in suspension be-neath sea waves. (This also occurs indeeper water offshore provided that the

material is sufficiently fine-grained.) Ex-periment has shown that at the break-pointof waves and in the swash zone, sandy ma-terial held in suspension is fairly evenly dis-tributed from the sea surface down to itsbed. Suspended load transport is of greatimportance in the dispersal of pollutants inthe sea.2. The fine sediment carried in suspensionby a body of flowing air.

suspension A type of transport in whichfine particles of sediment are carried inwater. The particles remain in suspension,held up by eddies caused by turbulence.The material being carried in this way iscalled the SUSPENDED LOAD. It is the sus-pended particles that make some riverslook milky or muddy.

swale /swayl/ See full.

swallow hole A SINKHOLE that is usuallythe site at which a stream disappears un-derground. See diagram.

swamp An area of soft wet land that haspoor drainage and is generally water-logged. The dominant vegetation consistsof trees, such as eucalyptus, mangroves,maples, palms, and willows, depending onthe climate. See also bog; marsh.

swash The movement of a fairly thin

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stream disappears undergroundat swallow hole, usuallyconical and occurring wherestream meets permeable rock

surface stream

stream reappears usuallyat the base of permeablelayer of rock and continuesas surface stream

Swallow hole

layer of turbulent water up a beach, fol-lowing the breaking of a wave. This watercan move material, the extent of the move-ment depending on the nature of the wavesand of the beach. Most landward move-ment is achieved by flat waves, withinwhich the oscillation of water particlestends to be elliptical. The oscillation insteeper waves is more circular, and thebreaking water strikes the beach materialfrom directly above, giving equal opportu-nity for up and down beach movements.

Some of the swash water infiltrates intothe beach material and the rest returns tothe surf zone as BACKWASH. The edge ofswash action is frequently obvious fromsuch signs as the wetted area of beach andthe line of seaweed, driftwood, mica flakes,etc., left at the upper limit of swash flow.Swash action on a falling tide may lead tothe formation of swash channels, as watertends to become channeled on its returnflow down the beach.

S wave See secondary wave.

swell 1. Waves that, having developedunder wind stress in the generating area,have sufficient energy to travel beyond thewind field or to survive after the wind hasdropped. They may travel into an area inwhich the winds are far weaker or absentor they may ultimately travel into anotherwind field. Such swell waves are usuallyfairly regular in form, with somewhatrounded crests. Their energy may enablethem to travel considerable distances. Ingeneral, swell decreases in height and in-creases in length as it travels away from awind field. Swells from several directionsmay be superimposed and swell may alsocarry relatively short and sharp-crestedwaves, which are generated by local winds.2. A dome or anticline, especially one thatrises from the sea floor without reachingthe surface.

syenite /sÿ-ĕ-nÿt/ A coarse-grained inter-mediate igneous rock in which the propor-tions of alkali feldspar or feldspathoids aredominant over sodic plagioclase; in factmany syenites contain no plagioclase. Theintermediate plutonic igneous rocks and

their volcanic equivalents are divided asshown in the table.

Syenites range from oversaturatedquartz-bearing types through saturatedtypes containing neither quartz norfeldspathoid to undersaturated strongly al-kaline varieties containing little or nofeldspar. Division may also be made ac-cording to the composition of the feldsparinto potassic (orthoclase, microcline),sodipotassic (perthites), and sodic (albite,oligoclase) types. The mafic minerals areusually hornblende and biotite but themore alkaline varieties contain sodic pyri-boles. Common accessory minerals includemagnetite, apatite, and sphene.

Nordmarkite is an oversaturatedsodipotassic syenite containing microp-erthite, quartz, aegirine, and riebeckite.With an increase in the amount of quartzto over 10%, nordmarkite passes into al-kali granite. Perthosite and pulaskite aresodipotassic leucocratic syenites. In theformer, the feldspar is perthite and in thelatter, antiperthite. Larvikite is a distinc-tive variety containing dark blue feldsparsdisplaying strong SCHILLER. The feldsparsare antiperthitic oligoclase and orthoclaseand, in addition, the rock contains aggre-gates of mafic minerals, titanaugite,olivine, and biotite. Shonkinite is a satu-rated potassic melasyenite containing or-thoclase feldspar and mafic minerals.Nepheline is the commonest feldspathoid

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INTERMEDIATE PLUTONIC ROCKSWITH VOLCANIC EQUIVALENTS

Plutonic Volcanic(sodic plagioclase) (andesine/oligoclase)

syenite trachyte, phonolite(dominant alkali feldspar/feldspathoid)

syenodiorite trachyandesite(monzonite) (latite)

(alkali feldspar = plagioclase feldspar)

diorite andesite(dominant plagioclase feldspar)

present in undersaturated syenites butother varieties contain sodalite, analcime,and leucite. Some of the many differentkinds of alkali syenite are:

Borolanite, containing orthoclase andnepheline-orthoclase intergrowths termedpseudo-leucite, thought to have beenformed from the breakdown of leucite; ma-lignite, containing potassic feldspar andnepheline; foyaite, containing a perthiticfeldspar and nepheline; litchfieldite, con-taining potassic feldspar, albite, andnepheline. Syenites containing nephelineand albite are called monmouthite or mar-iupolite; in the former nepheline predomi-nates, in the latter, albite. The feldspar-freetypes constitute the IJOLITE series.

Microsyenites are the medium-grainedequivalents and the plutonic rock namesapply with the prefix micro-. Most are por-phyritic and contain phenocrysts of ortho-clase. The volcanic equivalents of syenitesand alkali syenites are trachytes andphonolites.

Syenitic rocks are found in intrusivecomplexes often associated with continen-tal rifting, in eroded volcanic islands of al-kaline affinity, and within differentiateddikes and sills. See also diorite; syenodior-ite.

syenodiorite /sÿ-en-oh-dÿ-ŏ-rÿt/ (mon-zonite) A coarse-grained acid and inter-mediate rock that lies chemically andmineralogically between syenites and dior-ites. Syenodiorites are characterized by ap-proximately equal amounts of alkalifeldspar and plagioclase of oligoclase-an-desine composition. The mafic mineralsare usually either hornblende or biotite, orboth, but in some varieties augite occurs,often rimmed by hornblende. Syenodoritescontaining accessory quartz are calledquartz-monzonites and with an increase inthe amount of quartz pass into adamellites.With a change to plagioclase of more calciccomposition and an increase in the amountof mafic minerals, syenodiorites pass intosyenogabbros.

Syenodiorites usually occur in smallvolumes associated with granite masses.The medium-grained varieties are termedmicromonzonites or microsyenodiorites

and the volcanic equivalents are the tra-chyandesites or latites. The alkali feldsparin trachyandesites is usually sanidine, oc-curring as microlites in a pilotaxitic or tra-chytic groundmass set with plagioclasephenocrysts. Trachyandesites occur in as-sociation with andesites in calc-alkalineorogenic volcanic suites. Compare diorite;syenite.

syenogabbro /sÿ-en-oh-gab-roh/ See al-kali gabbro.

symbiosis /sim-bÿ-oh-sis, sim-bee-/ Theclose association that exists between twoorganisms of different species in whicheach partner benefits from the relationship.A common example of symbiosis is the re-lationship between flowering plants andsome insects, in which the insects feed onthe nectar of the plant, pick up grains ofpollen, and later transfer them to anotherflower (thus pollinating it). See also para-site.

symmetry See crystal symmetry.

symplectite /sim-plek-tÿt/ An inter-growth of two minerals, one mineral beingriddled with complex wormlike inclusionsof the other, as in MYRMEKITE.

syncline /sing-klÿn/ A basin-shaped foldin which the beds dip toward each other.See diagram at FOLD.

synclinorium /sing-klă-nor-ee-ŭm, -noh-ree-/ A syncline of regional extent, whichis composed of a series of minor folds.

synecology /sin-ee-kol-ŏ-jee/ The scien-tific study of interactions within ecologicalCOMMUNITIES, and between communitiesand their environment.

synform /sin-form/ A synclinal structurecomposed of sediments whose precisestratigraphic relationships are not known.See diagram at FOLD.

syngenetic /sin-jĕ-net-iki/ Describingmineral deposits that formed at the same

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time and by the same process as the rock inwhich they occur.

synkinematic /sing-kin=ĕ-mat-ik/ De-scribing minerals that are formed during aperiod of deformation.

synoptic chart /si-nop-tik/ A chart ormap that summarizes the weather condi-tions over an extensive area at a given mo-ment in time. It includes isobars to showatmospheric pressures, temperature meas-urements, wind speeds and directions,cloud cover, etc.

synoptic index A method of assessingthe sequences of pressure systems in aquantitative manner. Instead of isolatedwind readings the overall direction of sys-tem movement is taken into account, thenaggregated to give 10-day or monthly to-tals.

synoptic meteorology The section ofmeteorology concerned with the descrip-tion and analysis of synoptic weather in-formation, i.e. the weather over a wide

area at a particular moment in time, whichis shown on a SYNOPTIC CHART. This is builtup from surface weather observations ofsuch elements as wind velocity, air temper-ature, cloud amount, dew-point tempera-ture, and pressure tendency. Isobars areconstructed from pressure observationsand fronts inserted where appropriate. It ismost directly concerned with day-to-dayweather forecasting.

syntexis /sin-teks-iss/ See assimilation.

synthetic aperture radar See radar.

system A division of rock in the Stan-dard Stratomeric scale of stratigraphicclassification (see chronostratigraphy). Itindicates the body of rock that has formedduring one PERIOD. System and period gen-erally have the same name; for example,the Triassic System is the body of rockformed during the Triassic Period. A sys-tem, which does not have a standard uni-form thickness, is formed of a number ofSERIES grouped together.

synkinematic

tabular Describing a crystal or rock for-mation that is thin, wide, and long –shaped like a table.

tacheometry /tak-ee-om-ĕ-tree/ (tachyme-try) A surveying method for finding thelocation and height of points using atheodolite (set up at a point of knownheight and position) and a leveling staff(placed at the unknown points). The direc-tions of the unknown points are obtainedby measuring horizontal angles from aknown direction, while the distance fromthe theodolite position is calculated using aformula involving the difference in staffreadings for two cross-hairs on the dia-phragm of the theodolite, and the angle ofelevation or depression, derived from thevertical circle reading. Another formula,using the staff reading of the central hairand the angle of elevation, permits theheight of the point to be calculated. Theheight of the theodolite must always betaken into account whenever this methodis being used. See also traversing.

tachylite /tak-ă-lÿt/ A dark basic glassformed by the rapid chilling of BASALT andfound particularly at the margins of dikes.

taconite /tak-ŏ-nÿt/ A fine-grained iron-rich laminar sedimentary rock, commonlyhaving layers of chert. The iron may be inthe form of carbonate, oxide, silicate, orsulfide. It is used as a low-grade ore of iron.

taenite /tee-nÿt/ An alloy of iron andnickel (with 27–65% nickel) that occurs iniron METEORITES.

taiga /tÿ-gă/ (Boreal forest) A large re-gion of coniferous forests that occupies Nlatitudes from Canada, across Scandinavia,

to Siberia. It has cool summers and verycold winters, with frozen subsoil for muchof the year. Fallen needles from the treescreate PODZOL soil.

talc /tal’k/ A white or pale green mineralwith a mica-like layered structure andcomposition Mg3(Si4O10)(OH)2. Talc isgreasy to the touch and very soft, having ahardness of only 1 on the Mohs’ scale. It isformed during the hydrothermal alterationof ultrabasic and basic rocks and duringlow-grade thermal metamorphism ofsiliceous dolomites. Rocks known as soap-stone or steatite consist almost wholly oftalc.

talus /tal-ŭs/ See colluvium.

tangential folding A type of foldingproduced in homogeneous rocks, analo-gous to the bending of a block of rubberwith circles drawn on it; the circles becomedistorted but there is no sliding movementof any kind present.

tantalite /tan-tă-lÿt/ A black mineralform of mixed tantalates and niobates ofiron and manganese, (Fe,Mn)(Ta,Nb)2O6.It crystallizes in the orthorhombic system,and occurs in alluvial deposits, pegmatites,and granites. It is the chief source of tanta-lum.

taphonomy /ta-fon-ŏ-mee/ The study ofthe processes affecting an organism fromits death to its possible fossilization.Taphonomy elucidates the many differ-ences that exist between a fossil assem-blage and the community (or communities)of living plants or animals from which itcame (see thanatocoenosis). It is a neces-

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sary preliminary to investigations in PALEO-ECOLOGY. See also actuopaleontology.

taphrogeosyncline /taf-roh-jee-oh-sing-klÿn/ See rift valley.

tarn A small lake, usually occupying aCIRQUE.

tar pit A deposit of natural BITUMEN in adepression at the surface of the land. Ani-mals sometimes become trapped in suchpits, which are a good source of recent fos-sils.

tar sand A type of sedimentary rock thatcontains commercially useful amounts ofASPHALT.

taxonomy /taks-on-ŏ-mee/ 1. The studyand practice of naming organisms and clas-sifying them into a hierarchy of groupsbased on the similarities between them. It isimplied that this also represents their evo-lutionary relationships. Thus the membersof a lower group show more similaritiesand are therefore more closely related thanthose of a higher group. The group at thebase of the hierarchy is the SPECIES. Thegroups above the level of species, in as-cending order, are GENUS, FAMILY, ORDER,CLASS, PHYLUM, and KINGDOM. Any of thesegroups may be subdivided into smallerones; for example a class may contain sev-eral subclasses.2. See Soil Taxonomy.

tear fault See strike–slip fault.

tectogene /tek-tŏ-jeen/ A buckling of theEarth’s crust that results in the down-buck-ling of granitic rocks deep into the Earth’scrust to form the roots of mountains. Italso results in the upward-buckling of theshallower levels in the crust to form oro-genic mountain belts.

tectonic breccia /tek-tonn-ik/ A type ofBRECCIA that has been formed throughmovements of the Earth’s crustal plates(see plate tectonics).

tectonic plate See lithospheric plate.

tectonics The study of the Earth’s defor-mational movements and their effect onsedimentation and geomorphology.

tektite /tek-tÿt/ See meteorite.

Teleostei /tel-ee-oss-tee-ÿ, tee-lee-/ Thegroup of bony fish dominant since the Cre-taceous, occurring in both freshwater andmarine environments. The body covering isusually reduced to thin scales of bone.Teleosts show great morphological varia-tion and many fossil species are known.

telluric current /tĕ-loor-ik/ A naturallyoccurring electric current at or near theEarth’s surface.

tellurometer /tel-yŭ-rom-ĕ-ter/ A sur-veying instrument for precise distancemeasurement. Like the GEODIMETER it is in-direct in its measurements. The time re-quired for radio microwaves to travel fromone point to another is determined and cor-rections applied for the meteorologicalconditions to convert this time to that in avacuum. The distance is then calculatedfrom the known velocity of the waves, andfurther corrections applied for slope andaltitude. The best results are obtainablealong a line with few obstacles, in moder-ate sunshine, with a light breeze and lowrelative humidity.

temperate Describing a moderate cli-mate, such as that of the mid-latitudesnorth and south of the Equator, character-ized by warm summers and cool winters. Itextends from the Tropics of Cancer andCapricorn to the Arctic and Antarctic Cir-cles, and thus lies between the tropics andthe frigid zone.

temperate glacier A glacier containingconsiderable amounts of water above,within, and beneath the glacier ice. Waterat the ice-rock boundary promotes easysliding and consequently such glaciers flowfaster than other types. This movementproduces erosion of the abrasive type,while the presence of water allows freezingand thawing at the ice margins, an impor-tant factor in GLACIAL PLUCKING.

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temperate grassland A type of GRASS-LAND that occurs in the world’s TEMPERATE

regions. It includes the PRAIRIES of NorthAmerica and the STEPPES of Russia. Theyhave warm wet summers and cold drywinters, making them ideal pasture and,when plowed, good land for growing cerealcrops.

temperature An index of the heat con-tent of a substance, which determines theflow of heat between one substance andanother. Because the specific heat capacityof different substances varies appreciably,temperature cannot be equated directlywith heat. A variety of scales are in use. Inmost parts of the world the Celsius (centi-grade) scale is used. For scientific work, theABSOLUTE TEMPERATURE was formerly usedbut temperature is now measured inKELVINS. See also maximum temperature;minimum temperature; soil temperature.

temperature inversion See inversionlayer.

temporary hardness See hard water.

tension crack (gash; fissure; or fracture)A crack that develops in competent beds asthey are deformed. The rock is put undertension and fractures. This usually happensat several sites and sets of tension cracksdevelop en echelon. These fractures areusually infilled by secondary minerals suchas calcite.

tension joint A joint produced as a re-sult of tension, usually developed parallelto the fold axis of an anticline.

tephigram A type of THERMODYNAMIC

DIAGRAM used in meteorology that showsthe changing properties of the atmospherewith height. The properties displayed on thediagram are: isotherms, i.e. lines of con-stant temperature; dry adiabats; isobars,i.e. lines of constant pressure; saturatedadiabats; and saturation mixing ratio lines.

tephra /teff-ră/ (ejectamenta) Fragmen-tal materials thrown out by a volcano, in-

cluding bombs, ash, cinders, pumice, andlapilli. See pyroclastic rock.

tephrite /teff-rÿt/ An olivine-free type ofALKALI BASALT.

terminal moraine An accumulation ofTILL material that develops against thefront of a glacier. Such moraines are bestformed when the front is slowly advancingor is stationary, because in these circum-stances the front will be high and fairlysteep. Such forms will finally be depositedon the retreat of the glacier, thereby mark-ing its farthest limit from the source. Seealso moraine.

termite Ants and termites are abundantin tropical soils, their activity being con-spicuous in the form of termite mounds,which generally are about one meter high.Their main benefit appears to be in im-proving the structure of the soil.

terra fusca /te-ra foo-ska/ A soil that,like TERRA ROSSA, develops on limestone ina warm subcontinental climate. Terrafusca are brown clay loam soils, neutral orslightly alkaline in reaction, occurringunder cork oak or maquis vegetation. Be-neath the brown A horizon is a relict red Bhorizon, suggesting that they may be de-graded terra rossas.

terrain correction See topography cor-rection.

terra rossa /ross-a/ Red soil that devel-ops on hard limestone in warm subconti-nental climates. These soils typify the karstareas adjacent to the Mediterranean, andalso occur in S Australia, Texas, centralSpain, and Israel. They typically have aclay-loam texture, with some free sesqui-oxides, a variable depth, and low base sta-tus; they pass directly and abruptly to theparent material below, with per-haps ahumus surface horizon. Their origin is un-certain: they were formerly thought to rep-resent a weathering residue. They maydevelop from TERRA FUSCA on deforesta-tion, because they occur only under gar-rigue vegetation. They are related in some

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way to RENDZINAS, which seem to occurmore typically on soft limestone (e.g.chalk); it may also be that the terra rossasoils are more mature, because their lowerbase status and content of free sesqui-oxides shows more intense weathering.

terra roxa /roks-a/ A Brazilian red earth(see ferrallitic soil) with a high percentageof titanium and manganese oxides and alow aluminum oxide:iron oxide ratio, de-veloped on a parent material of basic ig-neous rocks. Compare terra rossa.

terrigenous deposit /tĕ-rij-ĕ-nŭs/ A sed-iment from land washed into the sea asmud, where it slumps down the continentalshelf under gravity or the affect of currents.The deposits are named after their colors,and include blue mud, gray mud, greenmud, and red mud. The color, in turn, de-pends on the composition.

Tertiary /ter-shee-air-ee, -shă-ree/ A sub-division of the CENOZOIC Era that followedthe CRETACEOUS nearly 65 million yearsago, and lasted some 63 million years untilthe beginning of the QUATERNARY. The Ter-tiary was formerly regarded as an era, butbecause of its short length was subse-quently classified as a period. The divisionsof the Tertiary, which previously formedperiods, are now regarded as epochs. Theyare the PALEOCENE, EOCENE, OLIGOCENE,MIOCENE, and PLIOCENE. Although this def-inition of the Tertiary as a period has beenwidely used, it has not been give formalstatus. The International Commission onStratigraphy now recognizes the Cenozoicas comprising the PALEOGENE and NEOGENE

Periods. Some authorities have suggestedthat the Tertiary should be regarded as asubera.

During the Tertiary the spatial distribu-tion of the continents was similar to that oftoday. There was intense volcanic activityin NW Scotland, N Ireland, the Faeroe Is-lands, and Greenland associated with therifting and separation of Eurasia andNorth America between Scandinavia andGreenland. Massive eruptions of basalticlava occurred on the Deccan trap. Australiaand South America separated from Antarc-

tica. Episodes of orogenic movements ex-tended through the period with the forma-tion of major mountain belts, including theformation of the Alps and Himalayas (at-tributed to the movement north of Indiaand its collision with Asia), the Rockies,and the Andes of South America. Extensivevolcanic activity took place along the westcoast of North and South America.

The Tertiary saw the evolution andgradual increase in abundance of moderninvertebrates and mammals, with a corre-sponding reduction of primitive groups.The modern angiosperms became the dom-inant plants.

teschenite /tesh-ĕ-nÿt/ A nepheline-freetype of ALKALI GABBRO.

Tethys Sea /teth-is/ (Tethyan Ocean) Alarge ocean that existed before the north-ward movement of Africa relative to Eu-rope, of which the Mediterranean Sea is thesurviving remnant. Within this ocean,which existed for a long period of time, thesediments of the Alps and Himalayanmountains were deposited. As the oceanclosed as a result of SEA-FLOOR SPREADING

this wedge of sediments was compressedinto the present-day mountain chains andassociated orogenic features.

tetragonal /te-trag-ŏ-năl/ See crystalsystem.

tetrahedrite /tet-ră-hee-drÿt/ A gray sul-fide mineral that contains copper and anti-mony, sometimes also with iron andarsenic, typical composition (Cu,Fe)12-Sb4S13. It crystallizes in the terahedral sys-tem, and occurs in veins associated withcopper, silver, and zinc. It is used as asource of copper.

texture The small-scale structures rec-ognized in hand specimens and THIN SEC-TIONS of rocks, originating from thegeometrical relationships of the mineralsthat constitute the rocks. Some of the tex-tural terms in common use are: 1. Terms describing grain size: coarse-,.

medium-, and fine-grained, APHANITIC,HYALINE.

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2. Terms describing the degree of crys-tallinity: HOLOCRYSTALLINE, HYALOCRYS-TALLINE, HYALINE.

3. Terms describing grain shape: EUHE-DRAL, SUBHEDRAL, ANHEDRAL, IDIOMOR-PHIC, ALLOTRIOMORPHIC, HYPIDIO-MORPHIC.

4. Terms describing the relationships be-tween grains: EQUIGRANULAR, INTER-GRANULAR, OPHITIC, POIKILITIC, PORPHY-RITIC, APHYRIC, poikiloblastic, PILO-TAXITIC, SPHERULITIC, interstitial.

thanatocoenosis /th’an-ă-toh-see-noh-sis/(death assemblage) An assemblage offossils composed of the remains of animalsthat have been accumulated by variousagencies after their death and thus may nothave lived together in life. They may havebecome affected by scavengers and by me-chanical breakage in the process. Most fos-sil occurrences are thanatocoenoses.Compare biocoenosis.

theodolite /th’ee-od-ŏ-lÿt/ An accuratesurveying instrument used for the measure-ment of horizontal and vertical angles,consisting of a telescope that can be rotatedin both the horizontal and vertical planes.

theralite /th’e-ră-lÿt/ An analcime-freetype of ALKALI GABBRO.

thermal /th’er-măl/ A volume of air thatpossesses buoyancy relative to its coolersurroundings. Thermals tend to arise as aresult of strong insolational heating at theground or a steep environmental lapse rate.Some will reach the condensation levelproducing cumulus clouds, others may losetheir buoyancy earlier. The thermal risesby slowly mixing with its cooler surround-ings at its upward margin and by entrain-ment in its wake. The mixing withenvironmental air means that strictly thethermal is not cooling adiabatically, but inpractical terms the difference is rarely sig-nificant.

thermal depression See thermal low.

thermal efficiency A concept devisedby Thornthwaite (see Thornthwaite classi-

fication) for use in his climate classificationscheme. It is a measure of the amount ofheat given to a specific area expressed interms of the potential evapotranspirationthat would result.

thermal equator The line of highestmean surface air temperature. Becauseland areas tend to absorb heat more thanoceanic areas, the thermal equator has amean position in the N hemisphere. In July,it reaches about 20°N over the continentsand in December is located at about the ac-tual Equator except in E South Americawhere it does extend farther south overAmazonia.

thermal low (thermal depression, heatlow) An area of low pressure producedby intense solar heating of the ground sur-face. The heating reduces the density of theair, which often results in rising air and afall in surface pressure. The best examplesof thermal lows are the monsoon lows,such as those over the Thar Desert. Thesedo not give rise to rain because they arecapped by strong anticyclones, which pre-vent all thermals reaching condensationlevel.

thermal metamorphism See contactmetamorphism.

thermal spring (hot spring) A springthat produces hot water from under-ground, heated by geothermal energy. If itejects steam as well as boiling water, it istermed a GEYSER. See also smoker.

thermal wind A theoretical wind usedto indicate the horizontal temperature field

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vector of upper wind28 knots 280°

21 knots 255°

vector of wind at base of layer 12 knots 315°

thermal wind

Thermal wind

in a layer of air. Actual winds change in thevertical as a result of variations in the ther-mal field at higher levels. The thermal windis a measure of the difference between thewind at the top and at the base of the layer.It is obtained by subtracting the geo-strophic wind vector at the base of thelayer from that at the top. See diagram.

thermocline /th’er-nŏ-klÿn/ A subsur-face layer of water within the ocean inwhich there is a marked decrease in watertemperature as the depth increases. Thewater layer above the thermocline is essen-tially homogeneous or isothermal: the dif-ferences in temperature and salinity in thevertical column are minimal. This upper-most layer seldom exceeds 100 m in thick-ness. Hence, the thermocline is located atthe bottom of the surface water masses.The maximum vertical temperature gradi-ent is generally found at depths between100 and 200 m. Some distance beneath thethermocline, in depths greater than 200 to300 m, the water masses are surprisinglyconstant in their characteristics, the tem-perature gradient rapidly decreasing to-ward the ocean floor.

thermodynamic diagram /th’er-moh-dÿ-nam-ik/ A diagram on which the proper-ties of a parcel of air may be indicated interms of its pressure, temperature, and hu-midity. Adiabatic lapse rates are also fre-quently included on the diagram, fromwhich it is possible to make assessments ofatmospheric stability and cloud base.

thermodynamics The branch ofphysics concerned with the study of themovements or relations of heat.

thermograph /th’er-mŏ-graf, -grahf/ Aself-recording thermometer, which consistsof a temperature-sensing mechanism and asystem of time portrayal. A continuousrecord of temperature can be obtainedfrom it.

thermohaline circulation /th’er-moh-hal-ÿn, -hay-lÿn/ The circulation of seawater within the oceans that arises fromdifferences of salinity and temperature,

which cause differences in density. Oceanwater circulation tends to be controlled bytwo principal factors: the wind stressesthat affect the sea’s surface and the distrib-ution of density within the water masses.These two factors are closely related andinfluence one another. Thermohaline con-vection, largely due to the cooling of sur-face waters at high latitudes, gives rise topronounced currents operating in theoceans.

thermometer An instrument for meas-uring temperature by recording a change ina heat-dependent property of a substance.The property chosen will depend on suchfactors as the accuracy required and therange of temperatures to be measured. Themost common method relies on noting thelength of a column of mercury (or alcoholwhere low temperatures are involved) en-closed in a sealed glass capillary rising froma small bulb. The length of the columnvaries as the mercury expands and con-tracts with changing temperature. Otherproperties used include electrical resistance(resistance thermometer), the variation inthe pressure of a gas kept at constant vol-ume (gas thermometer), and the magnitudeof the EMF produced by a bimetallic junc-tion (thermocouple).

A dry- and wet-bulb thermometer con-sists of two thermometers side by side, oneof which has its bulb enclosed in a wetmuslin bag. The difference in readings be-tween the two thermometers enables therelative humidity of the atmosphere to becalculated from standard tables (the wet-bulb thermometer will have a lower tem-perature due to the cooling effect as theliquid evaporates).

thermophile /th’er-mŏ-fÿl/ A plant thatcan survive in very hot climates.

thermoremanent magnetization /th’er-moh-rem-ă-nĕnt/ (TRM) The permanentmagnetism possessed by igneous rocksafter they have solidified from the moltenstate. It is produced in the rocks by theEarth’s magnetic field, and the direction ofthe magnetism in ancient rocks provides

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evidence of the history of the Earth’s geol-ogy.

thermosphere /th’er-mŏ-sfeer/ The layerof the Earth’s atmosphere above themesosphere, in which the temperature in-creases with height. The pressure rangesfrom about 0.01 mb to 10–8 mb. The ther-mosphere falls within the IONOSPHERE.

thickness (in meteorology) The verticalthickness of air between any two specifiedpressure levels. The most common layer isthe 1000 mb–500 mb, but others could beused. The thickness of the layer is directlyproportional to its mean temperature,warm temperatures being associated with athick atmosphere.

thin section A thin slice of rock, cut andground to a uniform standard thickness(usually 0.33 mm) and mounted on a glassslide for petrographical study under amicroscope.

tholeiite /th’oh-lee-ÿt/ One of the threemain types of BASALT.

thomsonite /tom-sŏn-ÿt/ A white min-eral form of hydrated sodium calcium alu-minum silicate, NaCa2(Al5Si5O20).6H2O.It crystallizes in the orthorhombic system,and occurs in crevices in basic igneousrocks and in amygdales within lava. It is amember of the ZEOLITE group of minerals.

thorn forest An area of thorny SCRUB

that occurs in regions that have prolongeddry weather. The plants generally havelong roots (to reach underground water),thick bark (to prevent water loss), andsharp thorns (to discourage browsing ani-mals).

Thornthwaite classification A systemof classification based on climatic effi-ciency, or the capacity of a climate to sup-port the growth of plant communities. It isbased on available moisture, the annualvariation of temperature, and the degree ofassociation between the temperature andprecipitation regimes. The climate of anarea is described by a four-digit label from

which it can be identified and its character-istics in terms of rainfall, temperature,moisture index, and concentration of ther-mal efficiency understood. Because of itscomplexity and difficulties, it has not be-come widely used since its final versionappeared. It is named for the Americanclimatologist and geographer C. WarrenThornthwaite (1899–1963).

threshold The lip of a CIRQUE, whichmay act as a dam, holding back water afterthe ice has melted and creating a tarn.

threshold wind speed The lowest windspeed that permits air to pick up grains ofrock dust, sand, or soil. See aeolian trans-port.

throughfall The portion of precipita-tion or irrigation water that falls through acovering of vegetation, without being in-tercepted, to reach the ground surface. Seealso interception; stemflow.

throughflow The rainwater that flowsdown a hillside through the soil. It occurswhen more rain falls on the surface of theground than can be absorbed quicklydownward by the soil.

throw The vertical change in level of apreviously continuous bed of rock as a re-sult of faulting.

thrust A low-angle reverse fault (see dia-gram at FAULT) that extends over a largedistance. In thrusting large rock bodies canbe displaced over each other, with the over-riding block moving upward as it passesover the lower block. It results from a prin-cipal stress configuration in which themaximum and intermediate stresses arehorizontal, while the minimum stress isvertical. The Moine Thrust of NW Scot-land, which is 150 km long, carries Pre-cambrian rocks far over Cambrian andother Precambrian rocks. These large dis-placements are often facilitated by gravityand the presence of fluids in the pores ofthe rocks undergoing deformation.

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thulite /thoo-lÿt/ A pink manganese-con-taining type of zoisite. See epidote.

thunder The noise heard accompanyinga LIGHTNING FLASH resulting from the sud-den heating and expansion of air as theflash passes through the ionized atmos-phere. Because the speed of sound is far lessthan that of light, thunder is always heardafter the flash. An approximate measure ofdistance from the storm is 1 mile for every5 seconds between flash and thunder.

thunderstorm A storm that is producedfrom CUMULONIMBUS cloud and is accom-panied by thunder and LIGHTNING, andusually strong winds and rain. It involveslarge convection currents, which can resultin very heavy rain, often causing flooding.If the convection currents carry drops ofwater high enough, they freeze and as a re-sult hail may accompany the rain.

tidal current 1. The periodic horizontalflow that occurs in response to the rise andfall of the tide. Such a current, like the ver-tical oscillation of the tide, arises from thegravitational attraction between the Earth,the Sun, and the Moon. Near the coast,tidal currents tend to be rectilinear and re-versing. Farther offshore, they tend to flowin a more rotary manner. Tidal currents areresponsible for the transport across shelfareas of large quantities of sediment, partlyalong the sea floor, partly in suspensionabove it. Because they are reversing cur-rents near the coast, the residual flow inone or other direction often determines thenet direction of sediment transport. Tidalcurrents, which in limited situations attainspeeds even in excess of 9 or 10 knots, tendto flow most rapidly at the sea surface, butrather more slowly near the seabed. Bot-tom friction accounts for this difference.Tidal currents create several distinctivemorphological features, including tidalbanks and channels, and depressionseroded out of soft rocks.2. The movement of water between twoplaces differentially affected by the rise andfall of the tides. Two tidal regions may op-erate in close proximity, such that thewater level at one point is, at any particu-

lar time, higher than at a neighbouringpoint. A tidal current will be producedcausing water to move from the high to thelow area. Although moving with some ve-locity, tidal currents are not noted as ero-sive agents, but may be important in thetransport of material thrown into suspen-sion by waves. The most notable tidal cur-rents operate within straits where high tideat each end occurs at different times.

tidal flat A wide flat area of barren ormarshy land that is covered and uncoveredat high and low tides. It is made up of un-consolidated sediments. See marsh; mud-flat.

tidal hypothesis A theory of the originof the Solar System, which suggested that astar approached close to the Sun, and thatthe tidal attraction coupled with the Sun’sunstable nature caused some of the Sun’smass to be torn off, later to cool as theplanets. The theory has fallen into disfa-vor. See nebular hypothesis.

tidal limit The highest point in an estu-ary or river inlet that is reached by seawater at high tide.

tidal prism The quantity of sea waterthat floods into or ebbs out of a harbor, es-tuary, or other sea inlet as a result of tidalflow (not taking into account the freshwa-ter discharge from streams, rivers, orcanals). It is usually measured in cubic me-ters. The stability of tidal inlets is largelydetermined by the relationships that existbetween the tidal prism and littoral drift:the ebb flow, reinforced by freshwater dis-charge, tends to evacuate sediments frominlets whereas the flood flow, reinforced bythe littoral drift, tends to infill inlets withsediments. Dredging is often necessary tomaintain the desired balance because mostinlets tend toward progressive siltation.

tidal range The difference in height be-tween the water level at high and low tide.This range is never a fixed value becausewherever tides occur there is always varia-tion between great ranges, associated withthe especially high and particularly low

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tides of spring type, and small ranges asso-ciated with the neap tides. Whenever a sin-gle value for tidal range is quoted for aplace, it is a mean value. A major geomor-phological importance of the tidal range isthat it controls how much of a beach pro-file will come under the influence of waveaction, and therefore the nature of thecoastal features.

tidal stream A movement of water intoand out of bays, estuaries, and other re-stricted coastal openings, associated withthe rise and fall of the tides. These streamsmay move comparatively rapidly and cancause considerable erosion both in floodand ebb. In the fine sediments of estuariesdifferent streams are utilized by the risingand falling tide, and the results of their ero-sion are complex patterns of interdigitatingchannels, termed flood and ebb channelsaccording to their mode of formation. Seealso tidal current.

tidal wave See tsunami.

tide The regular rising and falling ofwater level resulting from the gravitationalattraction that exists between the Earth,the Sun, and the Moon. Depending on theirrelative positions, either SPRING TIDES orNEAP TIDES result. Tidal predictions can bemade, although other influences such aslocal meteorological conditions make veryaccurate predictions difficult. With the riseand fall of the tide, the horizontal move-ment of water gives rise to TIDAL CURRENTS.The response of the oceans to tide-produc-ing forces is based on a series of amphi-dromic points (see amphidromic system),around which the tide oscillates in themanner of a PROGRESSIVE WAVE. Diurnaltides are those with one low water and onehigh water during the tidal day; semidiur-nal tides have two low waters and two highwaters during the tidal day.

tie See chaining.

tierra caliente /tye-rah kal-yen-tay/ Thehottest regions of Central and South Amer-ica. They occur on the coastal plains, onthe lower slopes of the Andes near the

Equator, and on the mountainsides at alti-tudes of up to about 1000 m. The temper-ature is constant and rainfall heavythroughout the year – conditions that favorthe growth of tropical rainforest.

tierra fria /free-ah/ The cold regions ofCentral and South America. They occurhigher in the Andes, at up to 3000 m nearthe Equator. Rainfall is spread throughoutthe year, and the natural vegetation isconiferous forest on the lower slopes withscrub at higher altitudes.

tierra templada /tem-plah-dah/ TheTEMPERATE regions of Central and SouthAmerica. They occur at altitudes of be-tween 1000 and 2000 m in the equatorialAndes. Natural forest covers much of theland, although some areas have beencleared (and the mountain slopes terraced)for agriculture.

tiger’s-eye A yellow-brown type ofQUARTZ, colored by iron oxide impurities.Fibrous inclusions of crocidolite give themineral its shiny banded appearance, mak-ing it popular as a semiprecious gemstone.

tight fold A type of fold with parallel orslightly diverging limbs, with an interlimbangle of less than 30°. See diagram at FOLD.

till (boulder clay) The unstratified ma-terial deposited by glaciers and ice sheets.Characteristically unsorted, it contains an-gular material ranging from clay-sized par-ticles to huge boulders. These may exhibitSTRIATIONS and can be derived from a widevariety of rock types. See also ablation till;lodgment till.

till fabric The general pattern of orien-tation of elongate stones within TILL ma-terial. Analysis of the positioning ofpebbles has shown that in many cases alarge proportion are located with theirlong axes parallel with the direction of for-mer ice flow, and with a low angle of dipdownstream. Frequently another minorconcentration exists, consisting of stoneswhose long axes are perpendicular to thedirection of movement. It is believed that

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the former were moved by sliding and thelatter by rotation about their long axes.Tills of different ages have been distin-guished on the evidence of their differentstone orientations.

timberline (treeline) The highest alti-tude on a mountainside or the highest lati-tude at which trees will grow. Trees do notgrow beyond this line because it is too cold,there is insufficient soil, or there is notenough rainfall. See also snow line.

time-distance curve (T.D. curve) Agraph that shows the relation between thearrival times of various seismic waves andtheir distances to the epicenter of an earth-quake. Such curves provide seismologistswith various kinds of data, such as infor-mation about discontinuities in the rocksthrough which the waves pass.

time series A set of data arranged inorder of occurrence with equal time inter-vals between each value. Such series arecommon in climatology because standardrecords are taken at equal intervals and ex-tend over many years. A time series can befor individual values or for averaged valuessuch as annual mean temperature. Time se-ries can be analyzed by a variety of statisti-cal techniques such as spectrum orharmonic analysis.

tinguaite /ting-gwă-lÿt/ See phonolite.

titanaugite /tÿ-tăn-aw-jÿt/ A titanium-rich variety of AUGITE. See pyroxene.

titanite /tÿ-tăn-ÿt/ See sphene.

tombolo /tom-bŏ-loh/ A form of coastalSPIT, composed of sand or shingle, thatlinks an offshore island to the mainland.Two tombolos (double tombolos) may linkone island to different parts of the coast,enclosing a lagoon between them. Theusual existence of comparatively shallowwater in the straits between island andcoast assists in the reduction of wave en-ergy in this area, thereby promoting depo-sition of material. Tideless areas seemespecially suited to tombolo formation, the

Mediterranean containing many examples.Where the coastal outline is suitable, nor-mal longshore movements of beach ma-terial may produce a spit, which eventuallyextends to reach an island.

tonalite /toh-nă-lÿt/ A quartz-diorite.See diorite.

topaz /toh-paz/ A variably colored or-thorhombic mineral of compositionAl2(SiO4)(OH,F)2 found in acid igneousrocks, such as granites and pegmatites.Some types are used as semiprecious gem-stones.

topoclimatology /top-ŏ-klÿ-mă-tol-ŏ-jee/ The study of the interaction betweentopography and climate at the local scale.Stress is placed on the varying slope condi-tions (as they affect radiation receipt), thelocal water balance, raindrop trajectories,the geostrophic wind flow, and gravita-tional winds. These modifications of theheat and water balances produce a distinc-tive topoclimate, which may be widely dif-ferent from the macroclimate as recordedin an instrument shelter on a horizontalsurface.

topography /tŏ-pog-ră-fee/ The relief,drainage, roads, vegetation, and culturalfeatures of the Earth’s surface.

topography correction (terrain correc-tion) The correction of gravity measure-ments for the effects of local topography,because hills and valleys cause variations inthe strength of the Earth’s gravitationalfield.

toposequence See catena.

topset bed See delta deposit.

topsoil The dark fertile soil at the sur-face, the A HORIZON.

tor A small hill projecting abruptly froma gently undulating land surface. Tors usu-ally consist of stacks of joint blocks, whichremain in situ and attached to bedrock atdepth, together with many collapsed

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blocks. The most favored explanation oftor formation postulates two cycles: thefirst involves initial differential DEEP

WEATHERING, the least jointed rocks beingthe least weathered; the subsequent periodof EXHUMATION exposes the former undu-lating WEATHERING FRONT and leaves theunweathered blocks as tors. Many tors dis-play UNLOADING sheets, which must havedeveloped as exhumation proceeded.

torbanite /tor-bă-nÿt/ A type of darkbrown OIL SHALE that contains 70–80% ofcarbonaceous material.

tornado (twister) A violently rotatingcolumn of air characterized by a funnel-shaped cloud, which may reach to theground surface, accompanied by a roaringnoise. On a local scale the tornado re-presents the greatest wind intensity of allsurface weather conditions. Most ane-mometers and even barographs are dam-aged if a tornado passes overhead, but fallsof up to 200 mb and wind speeds of up to500 km per hour are believed to be experi-enced. The tornado moves across countrywith the wind flow and direction of thewind at a higher level, leaving a swath ofdestruction wherever the funnel cloud hasreached the ground. The precise origins arenot fully understood, but in their severestform the following environmental condi-tions are required: warm moist air at lowlevels, an INVERSION at about 2000 m, atongue of dry air between 850 and 700 mb,and a triggering mechanism to initiate theindividual storm. These factors occur mostfrequently in the mid-West of the USA,where tornadoes are a great problem.However, tornadoes of lower intensityoccur in many countries. Tornadoes andother severe weather are tracked in theUSA by a system of radar stations that useDoppler radar.

torrid zone The tropical areas between23.5°N and 23.5°S.

tourmaline /toor-mă-lin, -leen/ The tour-maline minerals all have the general for-mula NaR3

2+Al6B3Si6O27(OH,F)4 where R= Fe2+, Mg, or (Al + Li). They have trigonal

symmetry and are variably colored al-though black varieties are common. Tour-maline is found in veins and pegmatitesassociated with granites.

The pneumatolytic alterations ofgranitic minerals caused by the introduc-tion of boron is called tourmalinization.The rock luxullianite is a tourmalinizedgranite in which the biotite and much ofthe alkali feldspar has been replaced by ra-diating aggregates of acicular tourmaline.Quartz is the only mineral to survive tour-malinization and the end product is a tour-maline-quartz assemblage known as shorlrock. See pneumatolysis.

tourmalinization /toor-mă-lin-ă-zay-shŏn/See pneumatolysis; tourmaline.

trace element (in geology) An elementthat occurs in a rock or mineral in verysmall quantities (much less than 1%), andis not regarded as an essential part of itscomposition.

trace fossil A fossilized remnant of theeffects of an organism in the past, ratherthan the remains of the organism itself.Trace fossils may be the fossilized feces ofan organism or structures produced by itthat have been preserved in the sediment,such as tracks and burrows. ACTUOPALEON-TOLOGY is used in relating these fossils tothe species that produced them. Trace fos-sils are classified in a taxonomic systemanalogous to that applied to the organismsthemselves, characteristic trace fossilsbeing described as ichnospecies. They areoften used in elucidating the paleoecologyof a species.

trachyandesite /tray-kee-an-dĕ-zÿt, trak-ee-/ A type of SYENODIORITE.

trachybasalt /tray-kee-bă-sawlt, -bass-awlt, trak-ee-/ The fine-grained equiva-lent of syenogabbro (see alkali gabbro),displaying mineralogical and chemical fea-tures intermediate between TRACHYTES andALKALI BASALTS. Hawaiite and mugeariteare members of the alkali basalt volcanicsuite containing andesine and oligoclasefeldspar, respectively, together with

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basaltic mafic minerals. These lavas aredistinguished from andesites largely onchemical grounds and field associations.Benmoreite is chemically intermediate be-tween mugearite and soda trachyte.

trachyte /tray-kÿt, trak-ÿt/ The volcanicequivalent of syenite, ranging from over-saturated to undersaturated in composi-tion. Some varieties contain sanidine andoligoclase feldspars, others contain a singleanorthoclase feldspar. Feldspars occurboth as phenocrysts and as close-packedmicrolites in the groundmass. The micro-lites usually have a subparallel flow orien-tation and swirl around the phenocrysts,imparting a characteristic trachytic tex-ture. Mafic minerals include biotite,clinopyroxene, and hornblende. In thesodic varieties, soda pyriboles and fayaliteare common. Trachytes may contain up to10% accessory quartz. Oversaturatedsodic varieties are termed pantelleritic tra-chytes which, with an increase in theamount of quartz, pass into pantellerites(alkali rhyolites). Varieties containing ac-cessory nepheline are termed phonolitictrachytes. When the nepheline contentreaches 10%, phonolitic trachytes passinto phonolites. Kenytes are a variety ofsodic phonolitic trachyte containing dis-tinctive rhombic phenocrysts of anortho-clase.

Trachytes are the intermediate mem-bers of the alkaline basalt volcanic suiteand are found mainly in ocean islands andareas of continental rifting and vulcanism.

traction load See bed load.

trade winds The predominantly east-erly winds that blow steadily over theocean areas and less steadily over conti-nental interiors in the tropics, convergingtoward the Equator. They play a very im-portant role in the GENERAL CIRCULATION

OF THE ATMOSPHERE by evaporating mois-ture in large quantities from the tropicalseas, which helps maintain the global heatbalance by removing surplus heat fromthese areas. On the eastern side of the trop-ical oceans, the trade winds are very steadyin direction and speed and have a very

strong low-level inversion, preventing thevertical development of cloud. These areas,such as the Canary Islands or the Galapa-gos Islands, tend to have low rainfall.Crossing the oceans increases the depth ofthe moist layer near the surface, the inver-sion becomes less intense, and disturbancesmore frequent, so that on the western sidesof the oceans the trade winds provide anadequate rainfall in most areas.

trajectory /tră-jek-tŏ-ree/ (in meteorol-ogy) A line drawn to indicate the actualmovement of a particle of air over a certaintime interval. Ideally the path should bethree-dimensional, but as little is normallyknown in detail about the vertical compo-nent, the horizontal projection of the pathis portrayed. The term is also used for thedownward motion of a precipitation parti-cle as far as it can be assessed from upperair information.

transcurrent fault See strike–slip fault.

transform fault A fault present withinthe Earth’s oceanic crust (see diagram atFAULT). Such faults have strike-slip dis-placement and are orientated at right an-gles to the MID-OCEAN RIDGES (constructiveplate boundaries), which they offset forseveral tens or even hundreds of kilome-ters. These faults lie parallel to small cir-cles, the axis of which is the axis ofrotation for the relative motion of theplates on each side. Transform faults formconservative plate boundaries, one of thethree types of PLATE BOUNDARY betweenlithospheric plates (the other two beingconstructive and destructive plate bound-aries). Along conservative plate boundariesthe lithospheric plates are sliding past eachother.

transgression Flooding of land area bya positive movement of BASE LEVEL, result-ing in onshore migration of the high-watermark. A transgression results in the short-ening of rivers by the drowning of formervalleys, leading to the creation of fiords,rias, and estuaries: these features are rela-tively shortlived because rapid aggradationsoon fills them up with silt. Past transgres-

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sions at a particular point can be identifiedby studying the deposits left. Fossil remainsmay also show changes from freshwater tomarine types. In the postglacial periodthere have been two major transgressions,the first one in the Boreal period(7600–5500 BC) being the most important,which have elevated sea level to its currentextent after the low stand of the last glacialperiod. Marine transgression can be localor worldwide.

transpiration The process of water lossfrom the surface of a plant. Moisture isdrawn up from the soil by the plant rootsand passes up through the stem to theleaves where it evaporates as water vaporfrom stomata (pores) on the surface ofleaves. The movement of water up throughthe plant is known as the transpirationstream. The process cools leaf surfaces,which in turn cools surrounding air.

transport (in geology) The link in thegeomorphological system between sites oferosion and sites of deposition. The mediaof running water, wind, and currents trans-port material in varying proportions bybed creep, saltation, and suspension. Inrivers, bed creep is dominant; in air, salta-tion is most important. Solution can alsobe important in rivers. Waves transport bybeach drift of pebbles and sand in the surfzone, and the setting up of LONGSHORE

CURRENTS. Ice transports as a solidmedium, by shunting material in front, in-corporating material in its base, and carry-ing slope-eroded material on its top.Gravity is inherent in most transport; onslopes it is directly responsible for down-slope movements and it provides energyfor flowing water.

transverse Mercator projection AMAP PROJECTION constructed in the sameway as the MERCATOR PROJECTION but hav-ing the cylinder tangential to a meridianrather than to the Equator. It is usedmainly for small areas with a north–southorientation, and for all British OrdnanceSurvey maps. Rhumb lines on this projec-tion are curved. The projection is named

for the Dutch cartographer and seismolo-gist Gerardus Mercator (1512–94).

transverse valley A valley that cutsacross the prevailing geologic structure ofthe land, or at right angles to the generalalignment of the underlying rock strata.

trap (oil trap) A geologic structure inwhich hydrocarbons (oil and/or naturalgas) become trapped and accumulate. Hy-drocarbons migrate away from theirsource rock under pressure and accumulatewhere an effective barrier to their furthermigration exists. Traps can occur in a vari-ety of ways. Since hydrocarbons, both oiland gas, are less dense than water, theytend to accumulate in the highest points ofthe trap, displacing most of the water pre-viously present in the rock pores. The bar-riers to further migration can result fromtectonic activity, as in the case of fault, un-conformity, and anticlinal traps, and fromlateral changes in the properties of the rockbody, as in the case of facies change trapsand reef traps. When more than one type oftrap exists in the same place it is termed acomposite trap. A distinction is made be-tween a structural trap, formed by tectonicactivity (such as folding and faulting), anda stratigraphic trap, where, for example, alayer of fine sediments may act as an im-permeable cap above coarser sedimentarylayers. See also salt dome. See illustrationoverleaf.

traversing /tră-ver-sing/ A surveyingmethod for finding position by measure-ment of direction and distance, using eithera prismatic compass and tape (compasstraverse), or a theodolite and leveling staff(tacheometric traverse). If the starting andfinishing points are of known position,whether a single or two distinct stations,then the traverse is closed. If only one sta-tion, at one end of a traverse line, is ofknown position the traverse is open or un-closed. The former type is better because itallows easier correction if errors are made.Unlike a compass traverse, a tacheometrictraverse will not only fix position but alsoheight. To obtain heights for points fixedby compass and tape, a leveling traverse

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must be run, starting and finishing at apoint of known height. See tacheometry.

travertine /trav-er-tin, -teen/ Deposits ofcalcium carbonate formed by precipitationfrom hot springs.

treeline See timberline.

tree-ring analysis See dendrochronol-ogy.

trellis drainage A drainage pattern thatdevelops where two sets of structural con-trols of a different type occur at right an-gles. If a steep regional slope is crossed byvarying hard and soft lithologies, shortstream segments will follow the slopes,while the soft lithologies will rapidly de-velop long subsequent streams that willcome to dominate the pattern. One exam-ple is in the Jura Mountains of France; an-other is the Ridge and Valley province inthe Appalachian mountains of the easternUSA.

A similar pattern can also arise if glacia-tion of an area of former consequentstreams proceeds at right angles to the ini-tial drainage. Subsequents will develop fol-lowing the grain of the glacial advance, asexpressed in fluted ground moraine ordrumlins, and come to dominate the sys-tem.

tremolite /trem-ŏ-lÿt/ A monoclinicmineral of the AMPHIBOLE group.

trench (deep-sea trench, foredeep, ocean

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Fig. 1: Unconformity trap

Fig. 2: Facies change

Fig. 3: Fault trap

Fig. 4: Anticlinal trap

Fig. 5: Salt dome trap

Types of trap

strike of lith

ologies

directionof slope

Trellis drainage

trench) A long narrow deep furrow inthe Earth’s crust, generally developed atthe margin of a continent where an oceanicplate is being subducted (see subductionzone), and often bordered by an ISLAND

ARC. Trenches can also develop where oneoceanic plate overrides another. Theymark the site of a destructive PLATE BOUND-ARY and the surface expression of a BENIOFF

ZONE. Most trenches have steep V-shapedwalls and are often terraced, with a flatfloor that is formed by a sediment infill.They are the deepest parts of the oceans:the greatest known depth (10 911 m) wasrecorded in the Mariana Trench, and simi-lar depths occur in the Tonga and KurilTrenches.

trend The general alignment, direction,or bearing of a geological feature such as afold. See diagram at FOLD.

triangulation /trÿ-ang-gyŭ-lay-shŏn/ Theaccurate location of a number of points bydividing the area containing them into a se-ries of triangles for which the values of in-ternal angles and the lengths of sides areascertained. Only one side of one triangleneed be measured physically, because onceall the angles are known, the side lengthscan be computed using trigonometry. Aftera BASELINE has been measured very accu-rately, the angles are measured using aTHEODOLITE, usually a one-second instru-ment, which is set up in turn at all the sta-tions to be fixed. Triangulation schemes

are the basis of all surveying because theyprovide the locations of the few initial con-trol points, from which subsequent detailmapping may take place using other meth-ods.

Triassic /trÿ-ass-ik/ (Trias) The first pe-riod of the MESOZOIC Era, beginning about246 million years ago, at the end of the PER-MIAN, and lasting for some 40 million yearsuntil the beginning of the JURASSIC. The Tri-assic System has three main divisions. TheLower Triassic consists of the Gries-bachian, Nammalian, and Spathian Stages;the Middle Triassic comprises the Anisianand Ladinian Stages; and the Karnian, No-rian, and Rhaetian Stages form the UpperTriassic. The name reflects the threefold di-vision of these rocks in Germany: Bunter, alower unit of continental red-colored sedi-ments; Muschelkalk, a middle unit of ma-rine limestone, sandstone, and shale; andKeuper, an upper unit of continental rocks.A wide variety of sedimentary rocks is evi-dent from the Triassic, with some igneous.Toward the end of the Triassic the super-continent of Pangaea, which extendedfrom pole to pole, started to show signs ofbreaking up with rifting along the Tethysseaway.

Triassic faunas, following the Late Pa-leozoic extinctions, show a typically Meso-zoic character. Modern corals appeared,ammonites developed, and bivalves re-placed the declining brachiopods. The evo-lution of the Reptilia produced such

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XX

X

orogenic belt withearthquake activityisland arc

trench

lithospherelithospherelithosphere

asthenosphere earthquakes(Benioff zone)

Trench

diverse types as the dinosaurs and the ma-rine ichthyosaurs and plesiosaurs.

tributary /trib-yŭ-tair-ee/ A secondarystream or river that flows into a largerriver. There may be many tributaries, all ofwhich collect rainwater and groundwaterfrom the main river’s catchment area.Rivers that flow through semiarid or aridareas tend to have few tributaries. See alsodistributary.

triclinic /trÿ-klin-ik/ See crystal system.

tridymite /trid-ă-mÿt/ A white crys-talline high-temperature derivative ofQUARTZ, SiO2. It crystallizes in the or-thorhombic system, and occurs in cavitiesin acid volcanic rocks. See silica minerals.

trigonal /trig-ŏ-năl/ See crystal system.

Trilobita /trÿ-lŏ-bÿ-tă/ An extinct classof the phylum ARTHROPODA whose mem-bers are common and widely distributed asfossils. Trilobites had an oval flattened seg-mented body divided longitudinally intothree lobes and transversely into three re-gions: an anterior cephalon, a thorax, anda posterior pygidium. They possessed alarge number of appendages, including onepair of antennae. Most trilobites wereabout 50 mm in length although a fewreached over 500 mm. The first fossils arefound in lower Cambrian rocks; the groupdeclined from the Ordovician Period on-ward and by the end of the Paleozoic it wasextinct. The trilobites formed a wide-spread, diverse, and rapidly evolving groupand their fossils are of great value in thecalibration and correlation of Lower Pal-eozoic rocks.

triple junction A point at which threelithospheric plate boundaries meet.

tripoli /trip-ŏ-lee/ A pale-colored poroussedimentary rock containing silica. It ispowdered and used as an abrasive for pol-ishing.

TRM See thermoremanent magnetiza-tion.

trochoidal wave /troh-koi-dăl/ See cy-cloidal wave.

troctolite /trok-tŏ-lÿt/ A GABBRO consist-ing of olivine and plagioclase feldspar oflabradorite composition.

trona /troh-nă/ A white, gray, or yellow-ish mineral mixture of hydrated sodiumcarbonate and sodium hydrogen carbon-ate, Na2CO3.NaHCO3.2H2O. It crystal-lizes in the monoclinic system, and occursas tabular crystals or evaporite deposits. Itis used as a source of sodium compounds.

trondhjemite /tron-yem-ÿt/ A coarse-grained igneous rock consisting mainly ofquartz and plagioclase, with some biotite;it lacks feldspar. See granite.

trophic level /troh-fik/ The position anorganism occupies in a FOOD CHAIN. Themain levels are PRIMARY PRODUCERS (suchas plants, at the lowest level), PRIMARY

CONSUMERS (such as herbivores, at the nextlevel), and secondary consumers (such ascarnivores, usually at the highest level).

tropical black soil A black heavy-tex-tured soil characteristic of tropical lowlandareas, either plainlands or the downslopeends of CATENAS, which are subject to sea-sonal drying and cracking due to theshrinking and swelling of the dominantmontmorillonitic clay during dry and wetperiods respectively, caused by the abilityof the clay to take up water in its lattice.During the wet swelling periods, soil ispushed up to form swells (gilgai) and in thedry phases cracks develop, which infillthemselves from the surface, thereby main-taining a kind of cycle, which makes thesoils self-mulching or inverting. There arepossibly 40 or so different names for thesesoils, including vertisol, mbuga, tir, blackcotton soil, grumusol, morgatitic, blackearth, black turf, and regur, each of whichdenotes a regional variant differing in somesmall way. As a group, they are similar toCHERNOZEMS, but have less organic matterand differences in the surface horizons, duemainly to the higher temperature andgreater evaporation of their environment.

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Tropical black soils have indistinct hori-zons due to their self-mulching nature,often with calcareous or gypseous concre-tions; silt and clay content is high (possibly85%+), pH is neutral or alkaline, and par-ent materials typically basic, e.g. marine al-luvium, limestone, or basalt.

tropical cyclone 1. A low-pressure sys-tem of tropical latitudes. It is a generalterm used to describe a cyclonic storm ofany intensity, distinctions of wind strengthbeing made by other terms such as tropicaldepression, tropical storm, and hurricane(or typhoon). The diagram shows the dis-tribution of tropical cyclones, which areclearly restricted to certain areas. They de-velop only in oceanic areas where the seatemperature is above 27°C and at least 5°of latitude away from the Equator. It ap-pears that the warm sea surface must pro-vide energy through evaporation, 27°Crepresenting its lower limit, and as theCORIOLIS EFFECT is zero at the Equator, thestorm must be at least 5° away before rota-tional components of airflow can develop.The precise origins of tropical cyclones aredebatable, but some form of initial distur-bance in the airflow seems to be a prereq-uisite.2. See hurricane.

tropical easterlies See trade winds.

tropical grassland A type of GRASSLAND

that occurs in the world’s tropical regions.These areas have seasonal rainfall followedby prolonged droughts preventing thegrowth of trees. See also savanna.

tropical podzol (groundwater podzol;geant podzol) A soil developed in low-lying tropical areas, often adjacent to river-ine, marine, or deltaic areas, subject to afluctuating water table. They are charac-terized by their striking colors and overde-veloped A2 horizon, which can be twometers deep. They develop by the leachingof sesquioxides and organic material to theB horizon, with precipitation taking placeat the level of the water table.

tropical rainforest (equatorial rainfor-est) Lush evergreen forest that grows inthe tropics, generally in regions between 5°north and south of the Equator. There areconstant high temperatures and it rainsthroughout the year. Such forests support avery large number of different species ofanimals and plants. The soil, however, isshallow and poor, and easily eroded awayif trees are felled. Even if the soil is used foragriculture after the trees have gone, the

tropical rainforest

areas with sea-surface temperatureof the warmest month above 27°C

Distribution of tropical cyclones

soil nutrients are consumed after only afew seasons and the ground becomes nolonger suitable for crops.

Tropic of Cancer The line of latitude23°30′N. At the summer SOLSTICE in the Nhemisphere, the Sun is directly overhead atthe Tropic of Cancer, the most northerlylatitude at which this occurs.

Tropic of Capricorn The line of lati-tude 23°30′S. At the summer SOLSTICE inthe S hemisphere, the Sun is directly over-head at the Tropic of Capricorn, the mostsoutherly latitude at which this occurs.

tropopause /trop-ŏ-pawz/ The bound-ary between the troposphere and thestratosphere. It represents the point atwhich temperatures stop falling before theisothermal conditions of the lower stratos-phere. On occasion, there may be morethan one tropopause. The height of thetropopause varies, being higher abovewarm air than cold. Consequently its meanheight is greatest over tropical latitudes atabove 16 km but only about 8 km overpolar latitudes. It also changes because ofdifferent weather conditions. The tropo-pause is not a continuous surface betweenthe tropics and poles, but is broken at thelatitude of the subtropical jet stream and issuddenly lower on the poleward side.These breaks enable mixing to take placebetween tropospheric and stratosphericair, which would otherwise not occur be-cause the isothermal layer of the stratos-phere acts as a stable inversion.

tropophyte /trop-ŏ-fÿt/ A plant that cansurvive in both dry and wet climates, suchas deciduous trees.

troposphere /trop-ŏ-sfeer/ The lowestlayer of the atmosphere, where almost allweather phenomena develop. It takes itsname from the Greek word tropos mean-ing a turn: it is the atmospheric layer whereturning and convective mixing is domi-nant.

trough /troff/ A pressure system on aweather chart distinguished by an isobaric

pattern of low pressure having muchgreater length than width and a concavecurvature of the isobars toward the maindepression center. All fronts occupytroughs of low pressure, but not all troughsare frontal. They are normally associatedwith precipitation and cloud.

trowal A trough of warm air aloft. Thisindicates the presence of a layer of warmerair in the upper atmosphere, which is not incontact with the ground surface. This typeof situation occurs in an occlusion.

truncated spur A SPUR that formerlyprotruded into a valley but has been partlyor completely separated from the highground by the action of a moving glacier.

tsunami /tsoo-nah-mee/ A seismic seawave generated in the ocean by submarineearthquakes, explosive volcanic eruptions,or mass slides underwater. Although fairlyrare, such waves can have catastrophiceffects when they do occur. Perhaps of lim-ited height (about a meter) in the openocean, they may travel hundreds or eventhousands of kilometers, and they attainconsiderable heights (up to tens of meters)in shallow coastal water and at breakpoint.Their length at sea may be 150 km ormore, and they usually travel at highspeeds, probably between 500 and 1000km per hour. They have caused immensehavoc and destruction and heavy loss oflife, especially along low-lying coasts. Thecatastrophic Asian tsunami of December26, 2004, which resulted in over 283,000deaths and considerable damage, wascaused by a massive earthquake of magni-tude 9 that had its epicenter 30 km belowthe seafloor, 250 km south-southeast ofBanda Aceh, Indonesia. It occurred as a1200-km stretch of the Indian lithospheric(tectonic) plate was thrust some 20 munder the Burma plate, the thrust-faultingreleasing elastic strains that had accumu-lated in the SUBDUCTION ZONE, and raisingthe seafloor by several meters. The conse-quent tsunami waves caused damage alongthe coastlines of 13 countries including In-donesia (with waves up to 10 m high on thecoastline of Sumatra), Sri Lanka, India,

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Thailand, Myanmar, Malaysia, The Mal-dives, and the East African countries of So-malia and Kenya.

A landslide-generated tsunami, some-times known as a landslide surge is a largeand often destructive wave caused by thefalling of a mass of rock, or perhaps partsof a glacier, from cliffs or hill slopes bor-dering a body of water. Such events havebeen witnessed on the open coast and inlakes. Because of the generally unpre-dictable nature of rockfalls and rockslides,detailed studies of landslide surges haverarely been possible. One rockfall in LituyaBay, Alaska, which resulted from a 1958earthquake, caused the fiord water toswamp some 500 m up the mountainsideon the opposite shore of the fiord. A 15-mhigh wave was generated along the fiord,traveling at an estimated 160 km per hour.Somewhat similar occurrences have beenrecorded in Japan, Norway, and elsewhere,but they are comparatively rare.

Some scientists have predicted thatthere is potential for a landslide to be gen-erated on the island of La Palma in the Ca-nary Islands through the collapse of part ofa volcano during a future eruption. Thismay generate a tsunami capable of crossingthe Atlantic Ocean to bring damage to theCaribbean and eastern seaboard of the USA.

Tsunamis are sometimes incorrectlycalled tidal waves.

tufa /tew-fă/ Deposits of calcium carbon-ate formed by precipitation from waterand including stalagmites, stalactites, flow-stone, and travertine.

tuff A type of PYROCLASTIC ROCK madeup principally of small fragments of con-solidated volcanic ash.

tundra /tun-dră/ A flat or undulating re-gion located north of the TAIGA in the sub-arctic lowlands of North America, Europe,and Asia. Summers are cool although theymay be sunny; for more than six months ofthe year in winter temperatures never riseabove freezing and there is a thick layer ofPERMAFROST. No trees grow on the tundraand the main vegetation is grasses, lichens,and mosses.

turbidite /ter-bă-dÿt/ A sediment de-posited in water by TURBIDITY CURRENTS.Because the speed of a turbidity current isproportional to the square root of effectivedensity, its lower part will be more heavilycharged with sediment and will movefaster than the less heavily charged upperpart. For this reason the particle size of thesediment transported decreases from thebase of the flow upward, a consequencebeing that a form of sediment grading, orgraded bedding, occurs in the deposit ulti-mately laid down. A typical sequence insuch a deposit is for coarse material at thebase to give way to finer material above,and possibly be covered in turn by lutite. Ifa series of turbidity flows occur in the samearea, a rhythmic pattern of sedimentationarises, with superimposed layer upon layerof graded turbidite material. Many sandyturbidites contain a significant amount ofsilt and clay.

turbidity /ter-bid-ă-tee/ 1. (in meteorol-ogy) The property of the clear atmospherethat leads to a reduction of solar radiation,usually due to particles of dust and smoke,which lead to attenuation rather than mo-lecular scattering. It is therefore a measureof atmospheric pollution either natural orresulting from human activities.2. (in oceanography) The stirring up of sed-iment by water.

turbidity current A flow of dense sedi-ment and water. Such currents developwith the stirring of sediment, resulting inthe formation of a layer that is denser thanthe surrounding water. Such a situation oc-curs if strong wave action, a submarineslide, or an earthquake disturbance affectsthe sea floor in an area in which sediments,especially the finer-grained ones, are rea-sonably abundant. In the case of a flatseabed and in the absence of currents, thesuspended material would tend to settleout again, but where the seabed slopes (e.g.on CONTINENTAL SLOPES) the suspendedmaterial usually begins to flow, mostrapidly when the slope is steep, and therebyinitiates a turbidity current. Turbidity cur-rents can be very dense and may possiblyattain speeds of 80 km per hour or more.

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They have been powerful enough to breaksubmarine cables and to erode channels inthe deep-sea floor. They lead to sedimentdeposits in characteristic morphologicalforms.

turbulence Irregular movements of afluid, such as air in the lower atmosphereor water in a rapidly flowing river. See tur-bulent flow.

turbulent flow The usual type of move-ment characteristic of air and water in na-ture, in which the net forward movementof the fluid has superimposed upon it achaotic pattern of secondary eddies, carry-ing molecules from one layer in the flow toeither higher or lower layers. This constantexchange of molecules speeds up slowerlayers and slows down faster ones; the ed-dies are principally responsible for creatinghydrodynamic lift, so important in sedi-ment transport. LAMINAR FLOW becomesturbulent when the viscosity of the waterfalls as temperature increases, releasing theliquid from the confines of flowing purelyas laminar layers. Alternatively, increasedvelocity, increased bed roughness, or a de-crease in water depth can all promote tur-bulent flow. In waterfalls and rapids thegreatly increased velocity and lowering ofthe water surface promotes a special typeof turbulence, ‘shooting’ flow, which accel-erates erosion and leads to rapid loweringof the falls or regrading of the rapids.

The scale of turbulence in the atmos-phere is wide, ranging from small eddiesrising above a strongly heated ground sur-face to mid-latitude depressions, whichrepresent large-scale turbulence in thewesterlies. It is important in the atmos-phere as an effective mechanism of disper-sal.

turquoise A complex mineral form ofhydrated, copper aluminum phosphate,CuAl6(PO4)4(OH)8.5H2O, valued for itslight blue or green color. It crystallizes inthe triclinic system, and occurs in nodules,small masses, or thin veins in various rocks.It is used as a semiprecious gemstone.

twilight The period of weak daylight

after the Sun has set below the horizon inthe evening or before it rises in the morn-ing. It can last for up to an hour in Arcticand Antarctic regions, but is over in a fewminutes near the Equator.

twinning The growth of two or morecrystals so that they join together, either in-tergrown or in contact, with different ori-entations of the crystal axes. It can occurwith several minerals, such as calcite, fluo-rite, gypsum, feldspar, staurolite, and ru-tile.

twister See tornado.

type locality 1. (in paleontology) Theplace from which a fossil that is the TYPE

SPECIMEN of a species comes.2. (in stratigraphy) The site of the outcropof rock used to define a particular strati-graphical division. See formation; lithos-tratigraphy; stratigraphy; type section.

type section The particular outcrop ofrock that has been selected as the standardsection for the definition of the limits of alithostratigraphical unit because it clearlydemonstrates the characteristic lithologicalfeatures of the unit. The place in which itoccurs is known as the TYPE LOCALITY. Seealso formation; lithostratigraphy; stratig-raphy.

type species The species regarded astypifying the GENUS to which it belongs.The type species is often the species whosecharacteristics were used in defining thegenus.

type specimen A specimen of an organ-ism that was orginally selected and pre-served to define the morphologicalcharacteristics of a SPECIES. It may not nec-essarily be typical of the species as a whole.

typhoon A TROPICAL CYCLONE or HURRI-CANE with winds above 64 knots (117 kmper hour or Force 12 on the BEAUFORT

SCALE). The term is used for such storms inthe W and N Pacific Ocean and comesfrom a Chinese word meaning great wind.

turbulence

Udden–Wentworth scale (Wentworthscale) A scale of particle or grain size insediments or sedimentary rocks. The grain-size classification is: boulder, over 256mm; cobble, 64–256 mm; pebble, 4–64mm; granule, 2–4 mm; sand (with subdivi-sions very fine, fine, medium, coarse, andvery coarse), 0.06–2.0 mm; silt (with sub-divisions very fine, fine, medium, andcoarse), 4–62.5 µm; clay, 0.06–4.0 µm. It isnamed for the American geologists JohanAugust Udden (1859–1932) who proposedthe original scale in 1898, and C. K. Went-worth, who produced a modified and ex-tended version of the scale in 1922. Seealso particle size; phi scale.

ugrandite /yoo-gran-dÿt/ A chemical se-ries of GARNET minerals.

ulexite /yoo-leks-ÿt/ A white mineralform of a hydrated sodium calcium borate,NaCaB5O9.8H2O. It crystallizes in the tri-clinic system as rounded masses of silkyhairlike fibers, and occurs as evaporite de-posits in arid regions and lake basins. It isused as a source of boron.

ultisol /ul-tă-sôl/ One of the twelve soilorders from the US SOIL TAXONOMY, denot-ing highly weathered soil formed in sub-tropical to tropical climates, which has asurface horizon containing residual ironoxides and an illuvial horizon beneath richin clay. Base status and fertility is low. It isso named because it contains soils at the ul-timate stage of weathering and includes thered-yellow podzolics, red-brown lateritics,and associated hydromorphic variants.

ultrabasic rock /ul-tră-bay-sik/ An ig-neous rock containing less than 45% silica(by weight). Most ultramafic rocks (con-

sisting largely of ferromagnesian minerals)are ultrabasic but some pyroxenites mayhave more than 45% silica and are basicrather than ultrabasic in composition. Cur-rent usage has tended to equate the two,but strictly, ultrabasic and ultramafic arechemical and mineralogical terms respec-tively. See acid rock; ultramafic rock.

ultramafic rock /ul-tră-maf-sik/ An ig-neous rock consisting largely of FERROMAG-NESIAN MINERALS. Such rocks may bedivided into three groups: 1. Peridotites, inwhich olivine is dominant and feldspar ab-sent (dunite, harzburgite, lherzolite,wehrlite). 2. Pyroxenites, consisting whollyof orthopyroxenes and clinopyroxenes(bronzitite, websterite). 3. Picrites, whichcontain accessory feldspar (oceanite, an-karamite).

With an increase in the amount of pla-gioclase, picrites grade into gabbros. Mostultramafic rocks are plutonic and occur inlayered intrusions, ophiolite complexes,zoned ultrabasic bodies, and as nodules inbasaltic lavas.

ulvöspinel /ûl-vŏ-spi-nel/ See spinel.

umber (raw umber) A greenish-brownearthy pigment that contains oxides of ironand manganese, together with silica andlime. It can be roasted to give dark brownburnt umber. See also sienna.

unaka /yû-nay-kă/ An area of highground that rises above a PENEPLAIN (whichhas been eroded away around it), largerthan a MONADNOCK.

uncompahgrite /un-kŏm-pah-grÿt/ Seeijolite.

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unconformable /un-kŏn-for-mă-băl/ Seediscordant (def. 2).

unconformity / un-kŏn-for-mă-tee/ Asurface representing a period of nondeposi-tion or erosion separating rocks of differ-ent ages. Some unconformites show amarked angularity, the beds above andbelow the unconformity surface having dif-ferent dips and strikes, whereas other un-conformities can be detected only bypaleontological means. Compare discon-formity.

underground drainage In limestoneareas solution can take place to such an ex-tent that joints, bedding planes, and otherfissures become progressively enlarged,forming hollows (SINKHOLES), UVALAS, orPOLJES) on the surface, and cavern systemsunderground. Linked cave systems canlead the water of a stream from a sinkholeat one point to a spring some considerabledistance away, thereby eliminating surfacedrainage for that stretch.

underground stream A stream thatflows underground for all or part of itscourse, most commonly in limestone re-gions. The erosive action of such streamscan carve various formations under-ground. See underground drainage.

undersaturated rock A type of igneousrock that is deficient in silica and thereforecontains FELDSPATHOIDS. Undersaturatedrocks include ijolites, nephelinites, and al-kali basalts. See silica saturation.

underthrust A low-angled fault result-ing in the movement of the foot wall rela-tive to the hanging wall.

undertow A seaward flow of water be-neath, or in the vicinity of, breaking waves.Some authorities claim that the backwashof waves constitutes a periodic sheet-flowseaward, describing this as an undertow.The term has also been used (erroneously)to mean the RIP CURRENT flows that occurwithin fairly confined zones.

uniformitarianism /yoo-nă-for-mă-tair-

ee-ă-niz-ăm/ (actualism) The theory,now generally accepted, that all geologicchanges have occurred by the gradual ef-fect of processes that have been operatingover a long period of time and are stillgoing on today. Compare catastrophism.

units For general scientific purposes, SI

UNITS are now widely used throughout theworld, However, in some disciplines oldersystems of units still persist. For example,in meteorology and climatology many el-ements are still measured in Imperial unitsand knots continue to be used for wind ve-locity and ocean currents in most coun-tries. Although the SI unit of pressure is thepascal, it has been agreed that the millibarwill continue to be used in meteorology.

unloading (sheeting; pseudo-bedding;pressure release) A weathering processresulting in the division of rock masses intosheets separated by fractures. These sheetstend to increase in frequency and decreasein thickness toward the top of an exposure;they are concentric, and essentially followthe topographic outline of the ground sur-face. The usual explanation is that the frac-tures are due to the expansion of the rockon reduction of the confining pressure,achieved by the removal of overlying ma-terials by erosion. These features occurmost frequently in granites.

unmixing See exsolution.

upper-air observation See sounding(def. 2).

upslope fog A form of ADVECTION FOG

resulting from the cooling of moist air tosaturation point as it is forced to rise up thewindward slope of uplands. It has the ap-pearance of stratus cloud from below.

upthrown Describing the side of aFAULT that appears to have moved upwardrelative to the other side (by an amountcalled the upthrow). See also heave.

upwarp The uplifting or uparching of alarge area of the Earth’s crust, usually in re-sponse to isostacy. The area of Scandinavia

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has gradually been rising after having beendepressed by the weight of the ice duringthe last ice age.

upwelling An ascending water current,not necessarily from the bottom of the sea,which transports colder water up into thesurface layers of the sea. Such currentsoccur particularly in tropical and subtrop-ical seas, and in the water flanking theAntarctic, especially in those regions wheresurface currents diverge (DIVERGENCE

zones). Upwellings also occur in some in-land or marginal sea areas, including theCaspian Sea and Black Sea. They may stemfrom strong offshore winds that tend todrag the surface water layer seaward so tobe replaced by colder water from beneath.Upwellings occur off the coasts of Califor-nia, Peru, and Ghana. They tend to operateseasonally and to form cold zones withinotherwise warm sea areas. They are of pri-mary importance in bringing to the surfacewaters that are rich in nutrients; these inturn may support thriving fisheries. Shiftsin the zones of upwellings have proved dis-astrous to the fishing industry.

uralitization /yû-ral-ă-tă-zay-shŏn/ Thelate-stage or metasomatic alterations (seemetasomatism) of primary igneous pyrox-enes to fibrous pale green amphiboles.

uraninite /yû-ran-ă-nÿt/ A black radio-active uranium oxide mineral, UO2, foundas an accessory in acid igneous rocks and inhydrothermal veins.

urban climate The distinctive modifica-

tion of the macroclimate produced by ex-tensive urban areas. The marked differencein the nature of the surface in rural andurban areas alters the local heat balance tosuch an extent that, even in an area of uni-form topography, the city evolves its ownclimate. This has become the subject ofmuch research in recent years and to dis-tinguish it from the macroclimate deter-mined from Stevenson screens at standardexposures, it is called the urban climate.See also heat island.

urban heat island See heat island.

Urstromtäler /oor-strohm-tah-ler/ In NEurope, wide trenches or valleys eroded bymeltwater from the ice sheet as it retreatednorthward at the end of the last ICE AGE.

urtite /oor-tÿt/ See ijolite.

U-shaped valley See glacial trough.

uvala /yoo-vah-lă/ A large depressionfound in limestone areas, resulting fromsolution (see limestone solution). Theymay be several kilometers in diameter, andfrequently result from the coalescence of anumber of adjacent DOLINES. In such a casethe uvala may have a scalloped margin. Seealso polje.

uvarovite /oo-vah-rŏ-fÿt/ A rare brightgreen member of the GARNET group of min-erals, Ca3Cr2Si3O12. It occurs in chromite-rich serpentinites. It is used as asemiprecious gemstone.

uvarovite

vadose water /vay-dohs/ Groundwaterabove the water table in permeable rock,e.g. limestone. Underground streams inthis zone flow with free air surfaces. Com-pare phreatic water.

vagrant benthos See benthos.

valley A long depression, lower than thesurrounding terrain, generally formed bythe erosive action of a glacier or river. Itswidth and depth may be as little as a fewmeters or many kilometers. The cross-sec-tional shape depends on the prevailingrock type and its origin (see glacial trough;V-shaped valley). Young valleys, in moun-tainous areas, tend to be steep and narrow,whereas in lower regions the valleys aregenerally broader and less steep. See alsoriver.

valley breeze The daytime equivalent ofthe MOUNTAIN WIND. It is formed by greatersolar heating on the inclined mountainslopes relative to the air at the same levelover the valley. Because wind speeds tendto be higher during the day as a result ofgeneral turbulence and convection, the val-ley breeze is more easily masked than themountain wind.

valley glacier An accumulation of icemoving down a preexisting valley and re-stricted in width by the valley walls. Suchglaciers may develop as a result of themerging of a number of CIRQUE GLACIERS invalley-head locations at times of deterio-rating climate, in which case they areknown as the Alpine type, or they may bedeveloped at the very edge of an ICE CAP orICE SHEET, when they are known as the out-let type. If the supply of ice to the sourceareas is sufficient, valley glaciers can ex-

tend down-valley to comparatively low al-titudes.

vanadinite /vă-nay-dă-nÿt/ A brilliantorange, red, or brown mineral, a chlorideand vanadate of lead, Pb5(VO4)3Cl. Itforms hexagonal crystals or fibrous massesin deposits of lead ores, and is used as asource of vanadium.

vapor pressure The pressure exerted bythe molecules of a liquid or solid that es-cape from the surface. In meteorology it isthat part of the total atmospheric pressureexerted by any water vapor that is present.Vapor pressure is measured indirectly fromdry- and wet-bulb temperatures using ta-bles or a humidity slide rule to obtain theprecise value.

variolitic /va-ree-ŏ-lit-ik/ Denoting atexture occurring in basaltic glasses similarto SPHERULITIC texture in acid glassy rocks.Variolites usually consist of radial orsheaflike aggregates of plagioclase crystals.

Variscan /va-ră-skan/ The period ofmountain-building in late Paleozoic timethat affected Europe and includes both theArmorican and Hercynian phases.

variscite /va-ră-sÿt/ A greenish mineralform of hydrated aluminum phosphate,AlPO4.2H2O, which occurs as nodularmasses.

varve /varv/ A thin bed of sediment inglacial lakes that represents a seasonal in-crement. Spring and summer glacial melt-ing produces a sudden influx of coarsesediment upon which finer materials settleduring the remainder of the year. These an-

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nual rhythms can be used in dating eventswithin the PLEISTOCENE.

vauclusian /vaw-kloo-zhee-ăn/ A springthat flows from underground in a lime-stone region.

veering In meteorology, a clockwisechange in wind direction, such as fromsouthwesterly to westerly. See also back-ing.

vegas In Spain, the local name for irri-gated lowland areas used for agriculture.

vein A thin sheetlike deposit of mineralsin a crack, fault, or joint in rock. Mostminerals in veins were deposited after vol-canic activity by hot fluids (gases or liq-uids) that solidify when they cool.

veld /velt, felt/ (veldt) A large area ofgrassland that occurs in southern Africa,where it is too dry for trees to grow. Somehas been plowed to grow cereal crops, suchas corn and wheat.

vent (conduit) The subterranean passagefrom the underlying magma chamberthrough which volcanic products, i.e. lava,ashes, and vapor, are discharged at theEarth’s surface. Where a volcano has onlyone such hole, debris accumulates aroundit as a roughly symmetrical cone: this istermed a CENTRAL-VENT VOLCANO. Wherethere is more than one eruptive site, usuallyalong a large fracture in the crust, it istermed a FISSURE ERUPTION or volcano.

ventifact /ven-tă-fakt/ A pebble or stoneworn and polished by the action of windand sand, and found in the gravel-strewnparts of the deserts. If a ventifact becomesworn in such a way that a roughly triangu-lar cross section evolves, it is known as aDREIKANTER (German = three sides).

vermiculite /ver-mik-yŭ-lÿt/ See clayminerals.

Vertebrata The subphylum of the Chor-data to which FISH, AMPHIBIA, REPTILIA, AVES

(birds), and MAMMALIA belong. They pos-

sess an internal skeleton of cartilage orbone, including a jointed backbone formedof vertebrae. Vertebrates are active animalsand consequently are usually bilaterallysymmetrical. The sense organs and feedingapparatus are grouped anteriorly, wherethey and the brain are protected by a skull.Vertebrates have a dorsal hollow nervecord and, in most higher groups, two pairsof limbs. Fossil evidence indicates that ver-tebrates appeared in the Ordovician (seeOstracodermi), since when they haveevolved to occupy a great variety of envi-ronmental niches.

vertical corrasion /kŏ-ray-zhŏn/ Abra-sive erosion (corrasion) on a rocky riverbed that acts downward and forms POT-HOLES, producing a gradual lowering of thebed.

vertisol /ver-tă-sôl/ One of the twelvesoil orders of the US SOIL TAXONOMY, de-noting mixed and inverting soils of theTROPICAL BLACK SOIL group. Vertisols aresplit into uderts, which crack open for lessthan three months in a year, and usterts,which open and close more than once ayear.

vesicle /vess-ă-kăl/ A small rounded cav-ity within lavas formed by bubbles of gascoming out of solution during the solidifi-cation of the lava. The concentration ofvesicles at the top surfaces of lava flowsserve as a way-up criterion. Elongate hol-low tubes formed by the escape of steamthrough lavas flowing over wet ground arecalled pipe vesicles. Highly vesicular lavafragments are termed pumice. See alsoamygdale; pyroclastic rock.

vesiculation /vĕ-sik-yŭ-ler/ The forma-tion of small cavities in molten igneousrock as it cools. The gases, previously in so-lution, form bubbles as the pressure de-creases when the magma rises nearer thesurface. If a great deal of gas is released, anexplosive volcanic eruption may occur.

Vesuvian See volcano.

vesuvianite /vĕ-soo-vee-ă-nÿt/ See idocrase.

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viscous flow Flowage that occurred inonce molten rocks, for example lavas, orimperfect fluids, such as asphalt. This re-sults in highly complex fold patterns, eachfold being distinct from and related to itsneighbors.

viscous lava Slow-flowing acidic LAVA,which tends to build up steep volcaniccones. See also lava flow.

visibility (in meteorology) The greatestdistance that the eye can see. If there is amarked variation with direction then thelowest visibility is recorded, because visi-bility is mostly used for aviation purposesand therefore the minimum value is impor-tant. Reports at night are based on dis-tances to unfocused lights of moderate orknown intensity.

vitrophyre /vit-rŏ-fÿr/ A PORPHYRITIC

lava in which the groundmass is whollyglassy.

vogesite /voh-jĕ-sÿt/ A type of LAMPRO-PHYRE that contains biotite and horn-blende, and in which ORTHOCLASE

predominates.

volatile (fugitive) A substance, normallygaseous, that is dissolved under pressure ina magma. The main volatile constituents ofmagmas are water and carbon dioxide, to-gether with smaller quantities of chlorine,hydrochloric acid, fluorine, hydrofluoricacid, hydrogen sulfide, sulfur, and boroncompounds.

The solubility of volatile substances in-creases with pressure so that during the as-cent of magma and the attendant loweringof pressure, the magma is unable to retainthese constituents in solution. Volatilesconcentrate at the top of the magma cham-ber in a residual fluid from which peg-matites, hydrothermal veins, and mineraldeposits may originate. During escape,volatiles effect mineralogical changes onthe early-formed crystals (PNEUMATOLYSIS).

Volcanic eruptions are accompanied bythe release of large quantities of steam andcarbon dioxide. The explosive release ofthese gases produces pyroclastic rocks.

Water and gases are also released at minorvents, geysers, hot springs, fumaroles, andsolfataras.

volcanic ash Fragments of PYROCLASTIC

ROCK less than 2 mm cross.

volcanic bomb See pyroclastic rock.

volcanic breccia A type of BRECCIA con-sisting of angular fragments of volcanicrock more than 65 mm across.

volcanic dome See lava dome.

volcanicity /vol-kă-niss-ă-tee/ (vulcani-city) The various processes involved withthe formation of magma and how it movesbeneath and through the Earth’s crust.

volcanic neck (volcanic plug) A cylin-drical column of solidified magma that oc-cupied the core of an inactive volcano. It isexposed by erosion of the less resistant sur-rounding volcanic rock formations. Seeneck.

volcanic rock Any igneous rock that isformed on the surface of the Earth as a re-sult of volcanic action. The chief basic vol-canic rock is BASALT; its acidic equivalent isRHYOLITE. Because of their rapid cooling,such rocks consist of very small micro-scopic crystals; even more rapid cooling (aswith a submarine volcano) results in theformation of glassy OBSIDIAN.

volcanism /vol-kă-niz-ăm/ (vulcanism)The various processes involved in the ejec-tion of molten rock (magma, which be-comes lava) from a volcano or hot waterand steam from a FUMAROLE or GEYSER.

volcano A fissure or vent on the Earth’ssurface connected by a conduit to theEarth’s interior, from which lava, gas, andpyroclastic material are erupted. Each vol-canic eruption has its own characteristics,but they can be classified generally as: 1.Hawaiian Fairly quiet eruptions in whichfluid lava is erupted from fissures or pits,with gas being liberated freely. When theeruption is accompanied by spurting gases,

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incandescent spray is thrown into the air.2. Strombolian More violent than theHawaiian type, the eruption taking placemore spasmodically as trapped gas escapesfrom a more viscous lava confined in acrater. Eruptions may be every few min-utes. During violent activity bombs areejected, which may be accompanied bylava flows. 3. Vulcanian Characterized byviscous lava whose surface rapidly solidi-fies. Beneath this crust gas accumulatesand builds up pressure until the crust shat-ters. This results in large quantities of py-roclastic deposits ranging from largebombs to fine ash. 4. Vesuvian Similar to 2and 3 but with gas-charged lava being shotviolently up into the air, emptying the lavacolumn to a considerable depth. 5. PlinianA culmination of type 4 in which a violentblast of gas rises to a height of several kilo-meters. The gas and vapor on reaching thisheight spreads into a large cloud. The fall-out of ash is low, being confined to ma-terial removed from the conduit. 6. PeleanVery violent eruptions accompanied bynuées ardentes and hot avalanches of inco-herent constantly expanding self-explosivelava, lubricated by hot gases and vapors.Such eruptions are characteristic of highlyviscous lava, whose gas content cannotreadily escape.See also supervolcano.

volcanology /vol-kă-nol-ŏ-jee/ (vulcanol-ogy) The scientific study of volcanoes,including their origins, action, structure,and classification. Volcanologists are alsoconducting research into the prediction ofvolcanic eruptions.

vorticity /vor-tiss-ă-tee/ The three-di-mensional rotation of a fluid about an axis,measured as a vector of twice the local rateof rotation of an individual fluid element.As rotation occurs in most aspects of at-mospheric motion, vorticity is very impor-tant. Even if air is still relative to the Earth,it will still be rotating in space because ofthe Earth’s rotation. To distinguish thesetwo types, relative vorticity is taken as ro-tation relative to the Earth’s surface; it ispositive if cyclonic, i.e. rotating in the same

direction as the Earth, and negative if anti-cyclonic. Absolute vorticity is the relativevorticity plus the component of the Earth’srotation about its axis; this is zero at theEquator and a maximum at the poles.

V-shaped valley Traditionally, fluvialvalleys of youthful and mature rivers aresaid to have a V cross profile, with steepwalls and narrow bottoms, while by senil-ity, the broadening floodplain producesmore of a U-shape. Irrespective of age,most valleys display a V-shape near theirsource, with some trend toward a broaden-ing bottom near their mouth. Streams withsandy and gravelly banks tend to have a V-shape as the material slumps on erosion ofthe foot slope to its angle of rest, and rain-fall landing on the valley sinks in and doesnot run off and sculpture the gentle sidewalls; conversely, valleys in cohesive siltyor claylike material tend to maintain theirwalls as the river cuts sideways, leaving aflat bottom, with steep side walls andhence a U-shape. Climate can also be im-portant: shady (north- and west-facing inthe N hemisphere) valley sides have a dif-ferent microclimate from sunny (south-and east-facing in the N hemisphere) wallsand this can influence the rate and type ofprocesses acting, hence lowering the twoside walls at different rates to produce anirregular valley shape. If precipitation ishigh, the valley walls become clothed indense vegetation, which slows down ero-sion, maintaining a V-shape, whereas inarid zones where there is no vegetation U-shapes often result from increased erosionof valley walls.

vugh /vug, vûg/ (vug) A cavity within avolcanic rock lined with deuteric or SEC-ONDARY MINERALS. Compare amygdale;miarolitic cavity.

Vulcanian See volcano.

vulcanicity See volcanicity.

vulcanism See volcanism.

vulcanology See volcanology.

vulcanology

wacke /wak-ĕ/ A type of young SAND-STONE with poorly sorted grains in a matrixof clay and fine silt.

wackestone /wak-ĕ-stohn/ A type oflimestone consisting of grains of calciumcarbonate in a matrix of fine-grained limemud.

wadi /wod-ee, wah-dee/ A normally dryvalley in a desert or semidesert environ-ment. In such locations rain falls very in-frequently but when it does so it is often inthe form of violent downpours of limitedduration. As normal fluvial activity doesnot exist, large amounts of weathering de-bris accumulate in situ. Channel flow fol-lowing a desert cloudburst can have aconsiderable erosional effect, cutting thesenarrow valleys with nearly vertical wallsinto the weathered rocks. Wadis may be re-licts from times of wetter climate.

wake The trail of eddies or vorticitiesthat develop on the leeward side of an ob-struction to wind flow. Large vertical ve-locities may be induced in essentiallyhorizontal flow by this method, as on thelee side of hills, buildings, or wind breaks.

waning slope The gentle concave foots-lope lying beneath the FREE FACE. The wan-ing slope is equivalent to the pediment, bywhich name it is more commonly referredto. It comprises the gentle slope of the val-ley floor, terminating in a drainage line orfloodplain. Waning slopes are said to be-come more widespread at the latter part ofthe cycle of erosion, as the dominantprocess becomes retreat of hillsides (back-cutting), rather than downcutting of val-leys. See also waxing slope.

warm front A strong thermal gradientin the atmosphere in which, from its direc-tion of movement, warm air is replacingcold air. The slope of a warm frontal sur-face is about 1 in 150, and so weather phe-nomena associated with it precede thesurface front by many kilometers. The firstindication is the spreading of cirrus cloudsacross the sky, followed by cirrostratus, al-tostratus, then nimbostratus as the rainstarts falling. Temperatures rise, relativehumidity increases, and the wind veers asthe front passes. Warm fronts are a majorcontributor to annual rainfall totals incoastal locations of temperate latitudes.Compare cold front.

warm occlusion See occlusion.

warm rain Rain falling from clouds thatare entirely warmer than freezing andwhich therefore must have developed bythe COALESCENCE process alone.

warm sector The region of warmer airexisting in a mid-latitude DEPRESSION be-tween warm and cold fronts. It eventuallydisappears from the surface during the evo-lution of the depression as the cold frontcatches up the warm front (see occlusion).Weather in the warm sector is usually no-ticeably milder than the preceding and fol-lowing air streams. Precipitation is veryvariable, varying from none if there is astrong ridge of high pressure to heavy, es-pecially over mountain areas, if the warmsector is potentially or conditionally unsta-ble.

wash See overland flow.

washout A filled-in river channel thatcuts into preexisting sediments. It is usually

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developed on deltas and other areas ofslow-flowing rivers. The channels are usu-ally infilled with sands, which often con-tain fragments of the eroded bed. They arevery common in the Coal Measures anddeltaic Jurassic beds. The sand body infill-ing the channel is sometimes known as ahorse.

wash slope (alluvial toe-slope) Thatpart of a hill slope lying at the foot of theCONSTANT SLOPE, formed by the washingout of fines from the scree in the constantslope. This fine material, often lying in theriver valley, has a lesser angle of rest thanthe coarser constant slope debris, and sothere is a gentle break of slope between thetwo components.

waste mantle See regolith.

water balance (in meteorology) Themovement of water between and withinthe atmosphere and ground surface on aglobal scale. It involves the balance be-tween precipitation, evaporation, advec-tion of moisture in the air, ocean-currentcirculations, and river runoff on land.There is a long-term balance, so that noarea of the Earth is continuously losing orgaining moisture. The values of the com-ponents of the water balance are oftenknown only very approximately, especiallyover the ocean areas.

water cycle See hydrologic cycle.

waterfall A vertical fall of water at asteep break in the long profile of a stream.This may be the product of base-level fallproducing a large KNICKPOINT, local dis-placement due to an earthquake, or irregu-larities in the long profile produced by hardbands of rock through which the river hasbeen unable to grade a smooth profileowing to lack of erosive power or insuffi-cient time. In areas of horizontal strata,breaks through individual rock beds willoften result in a steep fall. Once created thefalls immediately begin to retreat upstreamand become less pronounced owing to low-ering of their height through the concen-tration of erosion on the lip. Deposition

below the falls also obscures the break andtends to restore a smooth long profile.

water gap A pass through a ridge ofmountains through which a stream or riverflows.

watershed 1. See divide.2. See drainage basin.

waterspout The oceanic equivalent of aTORNADO. It occurs over seas or large lakeswhen a funnel-shaped cloud descends froma cumulonimbus cloud to produce violentagitation of the sea surface and a very localintense rotation of wind. Waterspouts arenever as severe or long-lasting as the vari-ety of tornado found in the mid-West ofthe USA.

water table See groundwater.

water vapor The distribution of watervapor in the atmosphere varies greatly,with largest amounts in the humid tropicsand least in polar regions and to a lesser ex-tent the desert areas. As well as being nec-essary for precipitation, water vapor hasanother important meteorological role inabsorbing long-wave or terrestrial radia-tion. With carbon dioxide, it acts as a veryeffective mechanism for preventing the lossof radiation that would otherwise occur.The mean temperature of the Earth istherefore partly a response to the presenceof water vapor; without it, it would be appreciably lower. Water vapor alsorepresents the highest energy state of water and so it can act as an energy store.When condensation to liquid or sublima-tion to solid takes place, energy is released,which may give rise to an increase of tem-perature.

water wave See wave def. 2.

wave 1. A periodic disturbance or varia-tion in a medium (solid, liquid, or gas) orin space that transfers energy from oneplace to another.2. (water wave) An undulation or defor-mation of a water surface. The period ofwater waves ranges from a second or so to

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several hours. The size of waves rangesfrom tiny capillary waves (only a few cen-timeters long) to the large storm waves,tidal waves, and long waves over 150 kmin length. Many types of waves are wind-induced, directly or indirectly; others arisebecause of submarine earthquake distur-bances or submarine sediment slides. Seealso constructive wave; destructive wave;ocean wave; sea wave; tsunami.

wave base The lowest limit of orbitalmotion beneath waves. Below the sea sur-face in deep water, the orbits of water par-ticles remain, as with surface particles,almost circular, but the diameter of the or-bits decreases with depth. For every 1/9thof the wavelength in depth, the size of thewater particle orbits are roughly halved. Ata depth that is equivalent to the wave-length, the orbit size is 1/535th of the orbitsize at the surface. Where orbital motionbecomes almost negligible, wave base isreached. The concept of wave base as-sumes importance in problems of sedimenttransport beneath waves, because bedmovements due to waves are influenced bythe wave-induced oscillatory currents ex-perienced close to the seabed.

wave-cut platform (abrasion platform;shore platfrom) An irregular gently slop-ing bare rock platform extending out to seaand usually backed by cliffs. The mecha-nism of platform erosion is still not totallyunderstood: it is not clear whether thewaves are themselves responsible for therock breakdown or merely for the removalof debris weathered by subaerial processes.Whichever processes are active, however, itis the wearing back of the cliffs that causesthe enlargement of the platform. Becausethese forms will not develop where there isa covering of beach material, the most fa-vorable sites are headlands.

wave cyclone A low-pressure systemdeveloping as a wave along a FRONT. Mostmid-latitude DEPRESSIONS form in this way.

wave diffraction (in oceanography)The transfer of the energy of wave crests asthey negotiate a structure, such as a break-

water or sharp promontory. Diffractionusually leads to a marked reduction inwave height. The various diffraction pat-terns that result from, for example, thepassage of sea waves through a restrictedharbor entrance, can be calculated mathe-matically provided the characteristics ofthe waves and the receiving basin areknown. Diffraction may allow some of theenergy of incident waves to affect the waterarea in the lee of a breakwater.

wave forecasting The prediction of fu-ture wave conditions using empirical ob-servations of wave conditions that havealready occurred or theoretical calcula-tions. Such calculations take into accountpredicted meteorological conditions andsuch factors as FETCH distance. Of the var-ious methods of wave forecasting that havebeen devised, two main types haveemerged: those that take account of the sig-nificant height and period of the predictedwaves (see significant wave), and thosethat take account of the total WAVE SPEC-TRUM. Wave forecasting is of great impor-tance to navigation and to studies ofexpected coastal changes.

wave hindcasting /hÿnd-kast-ing, -kahst-/A consideration of wave conditions thatmay have applied at an earlier period andin a particular area. The method employedusually involves synoptic wind charts andother wind data for the period in question,and (if available) hydrographic chartscovering that same period. The method isuseful if coastal changes have been care-fully documented and have to be ac-counted for.

wave ray See orthogonal.

wave refraction (in oceanography) Theturning of wave crests so that they becomemore parallel to the coastline as they ap-proach it. The process is due to the effectsof the shallowing of water: as the depth ofwater decreases so the wave velocity de-creases; consequently, a wave crest ap-proaching the land obliquely will beslowed initially at the end nearest theshore, while the other end will continue

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moving faster until the crest has becomealmost parallel to the coastline. Wave en-ergy can be represented diagrammaticallyby lines drawn perpendicular to the crests.As refraction takes place on an indentedcoast more energy becomes concentratedon the headlands than the bays, thereby fa-voring coastal straightening.

wave spectrum 1. The complete arrayof waves present on a water surface in aparticular place at a particular time. Care-ful measurements at sea or in a scale modelenable graphic wave records to be pro-duced, which show such an array of waves.A typical wave spectrum in the open seawill nearly always consist of waves ofmany different heights and periods super-imposed one upon the other. The measuredwave records enable their size and fre-quency distributions to be determined.2. A graphic classification of waves, vary-ing from capillary waves with a period ofless than 0.1 second to the transtidal waveswith periods of 24 hours or more. Most ofthe energy of such a spectrum would lie intwo of the period ranges: the ordinary tidesand the ordinary gravity waves.

waxing slope The convex slope at theupper part of a hill-slope profile. Accord-ing to the South African geologist LesterKing, the waxing slopes are most promi-nent at the early stages of the cycle of ero-sion, when the main processes are theincision, rather than widening, of valleys.These slopes are said to be dominated byCREEP. See also slope convexity; waningslope.

way-up structures Sedimentary struc-tures that can be used to determine the wayup of a series of unfossiliferous beds. Typ-ical way-up structures are cross bedding,mud cracks, ripple marks, graded bedding,included fragments of older beds, convo-lute bedding, burrows, roots, and geopetalcavities.

weather The state of atmospheric condi-tions at any one place and time. The totalsynthesis of these individual weather statesis CLIMATE. The state of the weather hasbeen classified into a code form for use inSYNOPTIC METEOROLOGY. In this way, cur-rent and past weather can quickly be iden-tified, although greatest stress is placed onprecipitation or other weather factorslikely to be considered important in fore-casting or in their impact on humans.

Efforts have been made to modify theweather, but without much success. Themost publicized work has been on CLOUD

SEEDING, but hurricane steering and fogclearance have been attempted. Because ofthe vast amounts of energy involved in theweather and the large area affected, theprospects for large-scale modifications arenot hopeful. Unintentional modificationsare far more numerous, such as the effectsof towns on weather (see urban climate), orthe effect of irrigating arid lands.

weather forecast See forecast.

weathering The process of breakdownand alteration of rock on the Earth’s sur-face in response to the changes in environ-mental conditions since the time of theirformation. During formation many rockswere subjected to great pressures or hightemperatures, away from the effects of at-mospheric air and water. Under present-day conditions these rocks are atcomparatively low temperatures and pres-sures and will naturally be influenced bythe presence of air and water. The resultingproducts of weathering are materials morenearly in equilibrium with their environ-ment than those from which they were de-rived. See also chemical weathering; deepweathering; differential weathering; me-chanical weathering; organic weathering.

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waxing slope

free face

waning slope

Waxing slope

weathering front The junction betweensound and weathered rock within a weath-ering profile. Certain rocks, especiallydense types with well-developed jointing,such as granite and basalt, display ex-tremely sharp transitions, whereas inporous or fissile rocks the junction is fre-quently so unclear that there is no realweathering front.

weather station An installationequipped with instruments for making me-teorological observations. See also auto-matic weather station.

websterite /web-ster-ÿt/ An ultramaficrock consisting of hypersthene and diop-side.

wedge (in meteorology) A region of highpressure that extends from an ANTICY-CLONE. It is narrower than a RIDGE OF HIGH

PRESSURE.

wehrlite /weer-lÿt/ An ultramafic rockconsisting largely of olivine with accessoryaugite.

welded tuff See pyroclastic rock.

Wentworth scale See Udden–Wentworthscale.

westerlies The main winds blowing be-tween 40° and 70° latitude. It is in thewesterly circulation that depressions formand maintain the vital meridional heat ex-change, and in this zone the strongestundisturbed wind flows are found.

West Wind Drift A circumpolar cur-rent that constitutes one of the Earth’slarge and significant permanent ocean cur-rents. In the S part of the Pacific Ocean, itoccupies a wide tract of water and gener-ally flows in an easterly direction to thesouth of the subtropical convergence zone.At about latitude 55°S, the Antarctic Con-vergence cuts across the West Wind Drift.In the S Atlantic Ocean, the West WindDrift occupies the wide tract of water thatlies approximately between latitudes 35°Sand 63°S. It moves with surprising con-

stancy, and is often characterized by verylarge sea waves and swell.

wet-bulb depression The temperaturedifference between the dry- and wet-bulbTHERMOMETERS in a Stevenson screen. It isa measure of the amount of cooling result-ing from evaporation of water on the wet-bulb thermometer. Relative humidity ofthe air can be calculated from this valueusing tables.

wet day In Britain, a period of 24 hourscommencing 9 a.m. GMT in which 1.0 mmor more of rainfall is recorded. The concepthas its limitations because 1 mm could fallfrom a single heavy shower or be the resultof prolonged drizzle throughout the day.See also rain day.

wetland See marsh; swamp.

wetting-and-drying weathering Themechanical breakdown of fine-grainedrocks through alternate wetting and dry-ing, causing surface FLAKING or major split-ting of blocks into two or more largepieces. A possible explanation involves thefact that water is a polar liquid, with posi-tive charge on the hydrogen atoms andnegative charge on the oxygen atom. Linesof mutually attracted water molecules maydevelop within the rocks on wetting anddrying and these may subsequently exertthe expansive forces that cause fracture.

whirlpool A small rotating area ofwater in a lake, river, or the sea; largerwhirlpools may become funnel-shaped. Itmay be caused by the shape of rocks on thebed or result when two opposed currentsmeet and circulate around each other.

whirlwind A small revolving verticaleddy of air, which whirls around a locallow-pressure area. Stronger whirlwindsmay be heard, but they are more frequentlynoticed because they pick up small piecesof litter or debris. They are formed duringperiods of local heating and instability, ap-pearing as small-scale TORNADOES.

white alkali soil See solonchak.

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white ice Ice that contains trapped air,located at or near the surface of theground, often on a glacier. As it descendsunder further layers of ice, the pressureforces out the air and the ice appears blue.This change is often accompanied by melt-ing (see regelation).

white-out A meteorological conditionthat occurs in polar regions when the sky isuniformly overcast, or there is driftingsnow or blizzard conditions, in combina-tion with complete snow cover: the hori-zon disappears and everything thenbecomes white with no shadows or identi-fiable landmarks.

white smoker See hydrothermal vent.

Wilson cycle A theory for the cyclicaldevelopment of ocean basins that relatesSEA-FLOOR SPREADING to continental geol-ogy. It is named for the Canadian geo-physicist John Tuzo Wilson (1908–93)who first formulated it in the1960s. The se-quence begins with the opening of theocean, then progresses with its develop-ment, through to its destruction, a processthat may be repeated throughout history.In the opening stage a continental plate be-gins to split or rupture and form rift valleys(e.g. the Great Rift Valley of East Africa).In the following stage there is subsidenceaccompanied by sea-floor spreading withthe formation of a narrow sea (e.g. the RedSea separating the Arabian Peninsula fromAfrica). The next stage sees the develop-ment of a wide ocean basin (e.g. the At-lantic) with continued expansion as moltenrock erupts to create new oceanic lithos-phere along the mid-ocean ridges; sedimen-tary wedges form on the continentalshelves. In the following stage the oceanbasin begins to close: subduction bound-aries form and the oceanic lithosphere issubsumed down into the asthenosphereforming a trench and associated island arc.The closing continues with subductionzones close to the continents and the for-mation of volcanic arcs; the compressionand uplift of accretionary wedges formsnew mountain ranges. The stage is reachedin which the ocean basin finally closes and

the continents converge in a collision zoneto form a continental suture. See also platetectonics.

wilting point The point at which aplant has extracted all the available waterfrom a soil and can no longer exert enoughforce to remove the remaining water. Oncethis point is reached the cells of the plantlose their turgor and it wilts. The wiltingpoint varies between different soils accord-ing to particle size; fine particles hold watermore tightly so it is less easily removed byplants.

wind The horizontal movement of airrelative to the Earth’s surface. Air move-ment results from thermal differences,which produce pressure variations, or bydynamic factors such as divergence of theair flow itself. Wind is one of the basic el-ements of weather. It is measured in knotsand its direction is that from which itblows. Wind speeds at the ground surfacecan be very variable from absolute calmduring anticyclones up to 200 knots in atornado.

windbreak Any barrier having the de-liberate effect of reducing surface windspeeds. It can be artificial or natural, al-though the latter is more frequently calleda shelter-belt. The degree of shelter that abarrier offers depends upon its height, itsdegree of permeability, and distance. For adense barrier, wind speeds fall dramati-cally immediately to the leeward, but soonrecover to the prevailing wind speed. Witha barrier that offers about 50% permeabil-ity, the effect reaches a maximum in termsof overall reduction of wind speed and dis-tance affected downwind. Even at a dis-tance of forty times the height of thebarrier there is still some reduction.

wind-chill index An index of the effectsupon living creatures of cold winds. Thestronger a cold wind is blowing the morerapid will be the rate of heat removal froma mammal. Consequently, cold but calmconditions are more physiologically ac-ceptable than warmer temperatures in astrong wind. A number of wind-chill in-

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dices have been used worldwide. That usedin North America was based on theSiple–Passel equation, which was devisedfrom experiments conducted in the 1940sin Antarctica by the American geographersPaul Siple and Charles Passel. In 2001 anew revised Wind-chill Temperature Indexwas implemented in the USA and Canada.This index: calculates wind speed at an av-erage height of 1.5 m (typical height of theadult human face) based on readings fromthe standard anemometer height of 10 m;uses modern heat-transfer theory; is basedon a human face model; has the thresholdfor calm wind at 4.8 km per hour; and as-sumes clear night sky conditions.

wind drift current A current that re-sults from wind stresses on the sea’s sur-face, part of the wind energy producingsurface waves and part generating thesecurrents. Indirectly, the wind is responsiblefor most of the currents in the surface andnear-surface waters. Because of the Earth’srotation and the resulting Coriolis effect,the wind-induced currents in the openocean have a mean direction approxi-mately 45° to the wind direction (some-what less than this near the surface, andgreater than this at depth). The currentsflow to the right of the wind in the N hemi-sphere and to the left of the wind in the Shemisphere. A wind blowing fairly hardfor a long period may produce a thick ho-mogeneous layer of isothermal water, forexample, under the influence of the steadytrade winds. In the shallower water nearthe coast, onshore winds tend to drag thesurface water shoreward, while offshorewinds cause the reverse to happen; in bothcases, the related currents near to theseabed may be flowing in the reverse direc-tion to the surface wind-induced current.

wind erosion See eolian erosion.

wind gap A gap (col) or notch in a ridgeor hilltop, through which the wind canblow and may whistle. It may have beenformed by RIVER CAPTURE or glacial action.

window (fenster) An outcrop of rocklying beneath a recumbent fold or thrust.

Such rocks are exposed as an enclosed out-crop, as a result of erosional processes cut-ting down through the overlying rock massto the younger rock beneath.

wind rose A diagram for illustrating thefrequency of wind directions over a speci-fied time period. It is normally split intoeight or sixteen points of the compass andcan also give an indication of the frequencyof wind speeds within specified ranges bydirection, as shown in the diagram over-leaf.

wind set-down The lowering of thestill-water level on the down-wind flank ofa body of water because of wind stressesacting on the water surface, or the differ-ence in levels between the up-wind anddown-wind flanks resulting from windstress. Like WIND SET-UP the term is usuallyapplied to the phenomenon as it affectsreservoirs, lakes, estuaries, and other bod-ies of water that are relatively limited insize. One example is the Plate River estuaryof Argentina: here winds blowing up (ordown) the estuary cause set-up (or set-down) and also change the current pat-terns. Statistical analyses of wind datashow that for some 220 days per year, tidallevels differ by up to a half a meter or morefrom the predicted values.

wind set-up The raising of the still-water level on the down-wind flank of abody of water because of wind stresses act-ing on the water surface, or the differencein levels between the up-wind and down-wind flanks resulting from wind stress.

wind shear The local rate of change ofwind velocity, usually at right angles to thehorizontal air flow. Vertical shear may alsobe considered.

windward Describing the side of a hillor other feature that faces into the prevail-ing wind.

winter solstice See solstice.

witherite /with-er-ÿt/ A white, yellow-ish, or gray mineral form of barium car-

376

wind drift current

377

bonate, BaCO3. It crystallizes in the or-thorhombic system, and generally occursin veins or in association with the lead oreGALENA. It is used as a source of barium.See also carbonate minerals.

wold In E or NE England, a chalk hill.

wolframite /wucirc;l-frăm-ÿt/ A red-brown to black mineral form of iron man-ganese tungstate, (Fe,Mn)WO4. Itcrystallizes in the monoclinic system, andoccurs mainly in pegmatites and hy-drothermal veins in quartz. It is the chiefsource of tungsten. See also scheelite.

wollastonite /wucirc;l-ă-stŏn-ÿt/ A whitetriclinic calcium silicate mineral of compo-sition CaSiO3. It is found in metamor-phosed limestones and in some alkalineigneous rocks, such as nepheline SYENITES.

woodland An area where the dominantvegetation is trees that form a canopy, butnot as dense as a forest. In temperate cli-mates, the mild moist climate favors wood-

land of mixed deciduous trees (replaced byevergreens in the Mediterranean area),which grow between the tropical forestsfarther south and the coniferous forestsfarther north.

World Weather Watch (WWW) Aprogram planned by the World Meteoro-logical Organization to provide a world-wide coverage of surface and upperatmospheric data. This is then used to pro-vide an initial detailed data set for numeri-cal models of the atmosphere.

worm cast See earthworm.

wrench fault See strike–fault.

wulfenite /wucirc;l-fĕn-ÿt/ A yellow ororange mineral form of lead molybdate,PbMoO4. It crystallizes in the tetragonalsystem, and occurs as earthy or granularaggregates in lead deposits. It is used as asource of molybdenum.

WWW See World Weather Watch.

WWW

variableand calm

9.0%

0–3 knots

4–10 knots

11–21 knots22–23 knots

% of total wind flow

0 5 10 15

N

S

Wind rose

378

xanthophyllite /zan-th’ŏ-fil-ÿt/ A brit-tle yellowish form of hydrated calciummagnesium aluminosilicate. It crystallizesin the monoclinic system, and is a memberof the MICA group of minerals.

xenoblastic /zen-ŏ-blas-tik/ Describingcrystals in metamorphic rocks that are an-hedral, exhibiting no crystal faces.

xenocryst /zen-ŏ-krist/ A crystal superfi-cially resembling a PHENOCRYST, which isnot in equilibrium with the other mineralsin an igneous rock. It is commonly an-hedral, having suffered resorption due toreaction with the magma.

xenolith /zen-ŏ-lith/ An inclusion withinan igneous rock body. The inclusion maybe a block of country rock that has beencaught up in the intrusion, but has not beencompletely assimilated. It may alterna-tively be a block of the igneous body itselfthat solidified at an earlier period andtherefore has a slightly different composi-tion.

xenomorphic /zen-ŏ-mor-fik/ Describ-ing an igneous or metamorphic rock whosemineral crystals do not have characteristiccrystal faces, caused by disturbance duringcrystallization.

xerophyte /zeer-ŏ-fÿt/ A plant that cantolerate drought or extremely dry climaticconditions. Their adaptations include longroots (to reach underground water), smallor thick leaves (to reduce water loss duringtranspiration), water storage cells in fleshystems, and dense hairs (to trap moist air).Cacti are typical xerophytic plants.yardang /yar-dang/ A narrow steep-sided ridge that occurs in arid regions.

Yardangs are caused by erosion by wind-borne sand (corrasion), and often lie pa-rallel to the direction of the prevailingwind.

yazoo stream /yă-zoo, ya-zoo/ A tribu-tary stream that flows for a consider-able distance parallel to the main stream,from which it is separated by a naturallevée, before joining the main stream.Yazoo streams are named for the YazooRiver, which flows alongside the Mis-sissippi River for 320 km before joiningit near Vicksburg, Mississippi.

yellow ocher See ocher.

yield point The point at which a rockceases to deform elastically to an appliedforce, and beyond which further force willcause it to fracture.

Young’s modulus A stretch modulus(E), equal to the ratio of the stress on across-sectional area of a rod of material tothe longitudinal strain. It is named for theBritish physicist, physician, and Egyptolo-gist Thomas Young (1773–1829).

zenith The point in the sky that is verti-cally above an observer, It is frequentlyused in radiation geometry for calculatingsolar radiation input.

zenithal projection See azimuthal pro-jection.

zeolites /zee-ŏ-lÿts/ A large group ofminerals with a general formula(Na2,K2,Ca,Ba,Sr) ((Al,Si)O2)n.xH2O. Ze-olites have a structure similar to feldsparsand feldspathoids and consist of a very

XYZ

open framework of linked (Si,Al)O4 tetra-hedra, with the metal cations filling thelarge cavities in the structure, which alsocontains loosely held water molecules. Thecompositions of some of the commonly oc-curring zeolites are as follows:

natrolite Na2(Al2Si3O10).2H2Omesolite Na2Ca2(Al2Si3O10)3.8H2Oscolecite Ca(Al2Si3O10).3H2Othomsonite NaCa2(Al5Si5O20).6H2Oheulandite (CaNa2)(Al2Si7O18).6H2Ophillipsite(½Ca,Na,K)3(Al3Si5O16).6H2O

harmatome Ba(Al2Si6O16).6H2Ostilbite (Ca,Na2,K2)(Al2Si7O18).7H2Ochabazite Ca(Al2Si4O12).6H2Olaumontite Ca(Al2Si4O12).4H2OMost zeolites are colorless or white and

are relatively soft, varying in hardnessfrom 3.5 to 5 on Mohs’ scale. Natrolite,mesolite, scolecite, and thomsonite have fi-brous habits. Zeolites are commonly foundas late-stage minerals in AMYGDALES inbasic lavas. They also occur as alterationproducts after feldspars and feldspathoids.The occurrence of zeolites on a regionalscale in volcanic rocks and sediments isconsidered to indicate a very low-grademetamorphism termed the zeolite facies.

zeugen /tsoi-gĕn/ A type of PEDESTAL

ROCK whose column comprises tabularslabs. The variation in profile is the resultof differential erosion of the rock by wind-borne particles.

zinc blende See sphalerite.

zincite /zink-ÿt/ An orange to deep-redmineral oxide of zinc and manganese,(Zn,Mn)O. It crystallizes in the hexagonalsystem, and occurs as masses in metamor-phosed limestone. It is used as a source ofzinc.

zinnwaldite /zin-wawld-ÿt/ See mica.

zircon /zer-kon/ A zirconium silicatemineral of composition ZrSiO4, found asan accessory mineral in intermediate andacid igneous rocks. Being very hard and re-sistant to weathering, it is a common detri-tal mineral in sediments.

zoisite /zoh-ă-sÿt/ A member of the EPI-DOTE group of minerals that crystallizes inthe orthorhombic system.

zonal circulation Wind circulationwith a dominant west–east directionalcomponent. The main zonal circulationsare the WESTERLIES of the mid-latitudes andthe EASTERLIES of the tropical oceans. East-erly winds are normally defined as beingnegative. Compare meridional circulation.

zonal index A measure of the strengthof the zonal circulation in a specified areaand time period. The index most com-monly used is that for the N AtlanticOcean between latitudes 35°N and 55°Nbased on the differences in mean pressurealong these lines across the ocean.

zonal soil A soil occurring over a widearea because of the dominance of the bio-climatic factor in soil formation, which de-termines soil-forming processes. Where thebioclimatic influence is locally replaced byrelief and drainage, INTRAZONAL SOILS

occur. Over a century ago the Russian N.M. Sibertsev (1860–1900) conceived soilas a climatic succession running from thetundra soils to laterite. This simplificationis increasingly regarded as outmoded be-cause of the location of many soils outsidetheir zone through the local influence ofsome other factor (e.g. podzols on sandyparent materials in the brown earth zone)and the climatic change of the recent past,which has varied zonal boundaries.

zone 1. (in metamorphism) A spatial di-vision in an area that has undergone META-MORPHISM, based on the first appearance ofsuccessive index minerals within the meta-morphosed rocks in the progression to-ward rocks of the highest metamorphicGRADE. The progressive nature of meta-morphism was developed by the British ge-ologist George Barrow (1853–1932)working in the Scottish Highlands, wherehe identified a number of zones in peliticrocks. The index minerals occurring inPELITES in order of increasing grade are:chlorite, biotite, almandine garnet, stauro-lite, kyanite, and sillimanite. A line drawn

379

zone

on a map representing the first appearanceof a particular index mineral is termed anisograd. These are lines of equal grade andlink rocks originating under similar physi-cal conditions of metamorphism. The al-mandine zone, for instance, is the zoneoccurring between the almandine and stau-rolite isograds. However, in rocks of othercompositions, an index mineral may ap-pear at a higher or lower grade than thatcorresponding to its first appearance inpelitic rocks. Different mineral zones havebeen correlated with different rock typesand this correlation is embodied in the con-cept of metamorphic FACIES.

Sequences of metamorphic zones otherthan those proposed by Barrow have beenidentified in other areas. In NE Scotland,the Buchan zones (named for this ScottishHighland region) are defined by the indexminerals staurolite, cordierite, andalusite,and sillimanite. The Buchan zones repre-sent a different metamorphic gradient in-volving relatively lower pressures thanthose represented by the Barrow zones.2. (in stratigraphy) The fundamental divi-sion used in biostratigraphical methods ofcalibrating and correlating rock succes-sions. A zone is demarcated by the fossils itcontains and is usually named after a ZONE

FOSSIL; when applied to a zone the name ofthe fossil should be italicized. Examples ofvarious types of zone are as follows. An as-semblage zone is characterized by the oc-currence of a particular assemblage offossils; an acme zone is characterized bythe particular abundance of a single speciesor fossil group; a total-range zone com-prises the entire thickness of rocks in whicha fossil occurs; a concurrent-range zone

consists of the strata in which the ranges ofparticular fossil species or groups overlap.In some situations a zone in biostratigra-phy may correspond to a CHRONOZONE inthe chronostratigraphic scale.3. See seismic zone.4. See crystal zoning.5. See morphogenetic zone.

zone fossil (index fossil) A fossil speciesthat is used in BIOSTRATIGRAPHY to delimita ZONE. Ideally a zone fossil should have alimited vertical range in the succession andshould in life have been rapidly and widelydistributed to permit correlation over alarge area. Several species, forming a wholezonal assemblage, may be used in the defi-nition of a biostratigraphical division.

zone of aeration The zone below theground surface and above the water tablein which the pore spaces and openingswithin the soil, sediments, and rock con-tain mainly air.

zone of saturation (phreatic zone) Azone below the ground surface where allcracks and pore spaces in sediments orrock are filled with water. The zone’supper boundary forms the water table (seegroundwater); phreatic water.

zooplankton /zoh-ŏ-plank-tŏn/ See plank-ton.

zweikanter /tsvÿ-kan-ter/ A pebble withtwo facets, formed by the erosive action ofwindblown sand in desert regions. See alsodreikanter.

zone fossil

380

APPENDIXES

actinium Ac 89 227*

aluminum Al 13 26.982

americium Am 95 243*

antimony Sb 51 112.76

argon Ar 18 39.948

arsenic As 33 74.92

astatine At 85 210

barium Ba 56 137.327

berkelium Bk 97 247*

beryllium Be 4 9.012

bismuth Bi 83 208.98

bohrium Bh 107 262*

boron B 5 10.811

bromine Br 35 79.904

cadmium Cd 48 112.411

calcium Ca 20 40.078

californium Cf 98 251*

carbon C 6 12.011

cerium Ce 58 140.115

cesium Cs 55 132.905

chlorine Cl 17 35.453

chromium Cr 24 51.996

cobalt Co 27 58.933

copper Cu 29 63.546

curium Cm 96 247*

darmstadtium Ds 110 269*

dubnium Db 105 262*

dysprosium Dy 66 162.50

einsteinium Es 99 252*

erbium Er 68 167.26

europium Eu 63 151.965

fermium Fm 100 257*

fluorine F 9 18.9984

francium Fr 87 223*

gadolinium Gd 64 157.25

gallium Ga 31 69.723

germanium Ge 32 72.61

gold Au 79 196.967

hafnium Hf 72 178.49

hassium Hs 108 265*

helium He 2 4.0026

holmium Ho 67 164.93

hydrogen H 1 1.008

indium In 49 114.82

iodine I 53 126.904

iridium Ir 77 192.217

iron Fe 26 55.845

krypton Kr 36 83.80

lanthanum La 57 138.91

lawrencium Lr 103 262*

lead Pb 82 207.19

lithium Li 3 6.941

lutetium Lu 71 174.967

magnesium Mg 12 24.305

manganese Mn 25 54.938

meitnerium Mt 109 266*

mendelevium Md 101 258*

mercury Hg 80 200.59

molybdenum Mo 42 95.94

neodymium Nd 60 144.24

neon Ne 10 20.179

neptunium Np 93 237.048

nickel Ni 28 58.69

niobium Nb 41 92.91

383

The Chemical Elements(* indicates the nucleon number of the most stable isotope)

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m

Appendixes

nitrogen N 7 14.0067

nobelium No 102 259*

osmium Os 76 190.23

oxygen O 8 15.9994

palladium Pd 46 106.42

phosphorus P 15 30.9738

platinum Pt 78 195.08

plutonium Pu 94 244*

polonium Po 84 209*

potassium K 19 39.098

praseodymium Pr 59 140.91

promethium Pm 61 145*

protactinium Pa 91 231.036

radium Ra 88 226.025

radon Rn 86 222*

rhenium Re 75 186.21

rhodium Rh 45 102.91

roentgenium Rg 111 272*

rubidium Rb 37 85.47

ruthenium Ru 44 101.07

rutherfordium Rf 104 261*

samarium Sm 62 150.36

scandium Sc 21 44.956

seaborgium Sg 106 263*

selenium Se 34 78.96

silicon Si 14 28.086

silver Ag 47 107.868

sodium Na 11 22.9898

strontium Sr 38 87.62

sulfur S 16 32.066

tantalum Ta 73 180.948

technetium Tc 43 99*

tellurium Te 52 127.60

terbium Tb 65 158.925

thallium Tl 81 204.38

thorium Th 90 232.038

thulium Tm 69 168.934

tin Sn 50 118.71

titanium Ti 22 47.867

tungsten W 74 183.84

ununbium Uub 112 285*

ununtrium Uut 113 284*

ununquadium Uuq 114 289*

ununpentium Uup 115 288*

ununhexium Uuh 116 292*

uranium U 92 238.03

vanadium V 23 50.94

xenon Xe 54 131.29

ytterbium Yb 70 173.04

yttrium Y 39 88.906

zinc Zn 30 65.39

zirconium Zr 40 91.22

Appendixes

384

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m

385

Appendixes

Web Sites

An important general site for information about the Earth Sciences is that of the USGeological Survey. It has sections covering all branches of the subject.

US Geological Survey www.usgs.gov

Other useful sites are given below.

GeologyAmerican Geophysical Union www.agu.org

American Geological Institute (AGI) www.agiweb.org

Earth Sciences on the Web http://geology.com

The Geological Society of America www.geosociety.org

International Union of Geological Sciences www.iugs.org

MineralsAthena Mineralogy http://un2sg4.unige.ch/athena/mineral/

mineral.html

Mindat www.mindat.org

Web Mineral http://webmineral.com

OceanographyNOAA Ocean Explorer http://oceanexplorer.noaa.gov

Ocean Biogeographic Information System http://www.iobis.org

United Nations Atlas of the Oceans www.oceansatlas.com

Meteorology, Climate, and the Atmosphere

American Meteorological Society, Boston www.ametsoc.org/AMS

Climate Prediction Center www.cpc.ncep.noaa

Hadley Centre for climate prediction and www.met-office.gov.uk/research/research, UK hadleycentre/index

Mount Washington Observatory www.mountwashington.org

NASA's Earth Observatory www.earthobservatory.nasa.gov

National Oceanic and Atmospheric www.noaa.govAdministration (NOAA)

University Corporation for Atmospheric www.edu/ucar/indexResearch

US Environmental Protection Agency www.epa.gov

World Meteorological Organization www.wmo.ch

WW2010 (weather world 2010 project) www.2010.atmos.uiuc.edu/(Gh)/homeUniversity of Illinois

Appendixes

386

El Niño

El Niño site www.pmel.noaa.gov/toga-tao/el-nino-story

Satellites

European Space Agency (ESA) www.esa.int

EUMETSAT www.eumetsat.de

NASA www.nasa.gov

NESDIS www.nesdis.noaa.gov

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