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igneous rock

ContentsArticles

Andesite 1Basalt 3Diorite 10Gabbro 12Granite 15Obsidian 23Pegmatite 27Pumice 30Rhyolite 32Scoria 34

ReferencesArticle Sources and Contributors 36Image Sources, Licenses and Contributors 38

Article LicensesLicense 40

Andesite 1

AndesiteFor the extinct cephalopod genus, see Andesites.

Andesite Igneous Rock

Photomicrograph of andesite in thin section (between crossed polars)

Composition

IntemediateMajor minerals: plagioclase (often andesine) and pyroxene and/orhornblendeAccessory minerals: magnetites, biotite, sphene, quartz

A sample of andesite (dark groundmass) withamygdaloidal vesicules filled with zeolite.

Diameter of view is 8 cm.

Andesite Mount arnov (Vtnik)

Andesite ( /ndsat/) is an extrusive igneous, volcanic rock, ofintermediate composition, with aphanitic to porphyritic texture. In ageneral sense, it is the intermediate type between basalt and dacite. Themineral assemblage is typically dominated by plagioclase pluspyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite,biotite, and garnet are common accessory minerals.[1] Alkali feldsparmay be present in minor amounts. The quartz-feldspar abundances inandesite and other volcanic rocks are illustrated in QAPF diagrams.Relative alkali and silica contents are illustrated in TAS diagrams.

Classification of andesites may be refined according to the mostabundant phenocryst. Example: hornblende-phyric andesite, ifhornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonicdiorite. Andesites are characteristic of subduction zones, such as thewestern margin of South America. The name andesite is derived fromthe Andes mountain range.

Genesis of andesite

Andesite is typically formed at convergent plate margins but mayoccur in other tectonic settings. Intermediate volcanic rocks are createdvia several processes:

1. Hydration melting of peridotite and fractional crystallization2. Melting of a subducted slab containing sediments1. Magma mixing between felsic rhyolitic and mafic basaltic magmas

in an intermediate reservoir prior to emplacement or eruption.

Andesite 2

Via fractional crystallisationAndesitic magma in island arc regions (i.e., active oceanic margins) comes from the interplay of the subducting plateand the mantle wedge, the wedge-shaped region above the subducting plate.Water in the subducted oceanic lithosphere 'boils off' from the slab by dehydration of hydrous minerals such asamphibole, zeolites, chlorite etc., which are formed in the oceanic lithosphere during hydrothermal circulation at themid-ocean-ridge. As these minerals are subjected to greenschist or blueschist metamorphism during subduction, theychange to more stable, anhydrous forms, releasing water and soluble elements into the overlying wedge of mantle.The slab itself, or the overlying mantle wedge, may melt. If the slab melts, it may include subducted sediment aswell. The water and initial slab melts rise into the mantle wedge, prompting melting of the peridotite to producebasaltic magma with a distinctive enrichment of soluble elements (e.g., K, Ba, and Pb) compared to insolubleelements (e.g., Nb and Ti).On its way to the surface, the melt stalls and cools, enabling the fractional crystallization of silica poor minerals, thusraising the silica content of the remaining melt and resulting in andesitic magma.

Via magma mixingBasaltic magma may also mix with rhyolitic magma. This usually occurs in continental arc areas such as the Andes,where the high geothermal gradient above the subducted plate, and hydrothermal flows within the mantle wedge maycreate an underplate of softened, partially molten continental crust of intermediate or felsic composition. Basalticmagmas intruded into this anomalously hot zone will prompt partial melting of the crust, and may mix with thesemelts to produce intermediate compositions, typically andesite to trachyte in composition.Alternatively, the basaltic melt may heat up the overlying arc, prompting partial melting, and may even assimilatesediments, previous volcanic rocks, etcetera, whilst undergoing fractional crystallisation. These rocks aresubordinate due to the difficulty in assimilating sufficient cold material by magmas without cooling to a degree thatthey become immobile.Ultimately, the resultant composition of andesite and intermediate magmas is the result of fractional crystallisation,assimilation, partial melting and contamination by the subducted slab. These may take considerable effort to resolvethe individual components.In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129)that were discovered in the Graves Nunatak Icefield during the US Antarctic Search for Meteorites 2006/2007 fieldseason. This possibly points to a new mechanism to generate andesite crust.[2]

References[1] Blatt, Harvey and Robert J. Tracy, 1996, Petrology, Freeman, ISBN 0-7167-2438-3[2] Scientists Find Evidence of Asteroids with Earth-Like Crust (http:/ / newswise. com/ articles/ view/ 547877/ ) Newswise, Retrieved on

January 19, 2008.

Origins of the Continental Crust (http:/ / nsw. royalsoc. org. au/ journal_archive/ 132_34. html#Arculus) Island arc magmatism (http:/ / www. nsm. buffalo. edu/ courses/ gly206/ SubductionMagmas. pdf) Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge (http:/ / www.

margins. wustl. edu/ Eugene_PDF/ SubFac_abstract_Gaetani. pdf)

Andesite 3

External links Igneous Rock Textures (http:/ / geology. csupomona. edu/ alert/ igneous/ texture. htm)

Basalt

Basalt Igneous Rock

Composition

Mafic: amphibole and pyroxene, sometimes plagioclase, feldspathoids, and/or olivine.

Basalt ( /bslt/, /bslt/, or /beslt/)[1] [2] is a common extrusive volcanic rock. It is usually grey to black andfine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals ina fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey. According to the official definition,basalt is defined as an aphanitic igneous rock that contains, by volume, less than 20% quartz and less than 10%feldspathoid and where at least 65% of the feldspar is in the form of plagioclase.On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed onEarth's Moon, Mars, Venus, and even on the asteroid Vesta. Source rocks for the partial melts probably include bothperidotite and pyroxenite (e.g., Sobolev et al., 2007). The crustal portions of oceanic tectonic plates are composedpredominantly of basalt, produced from upwelling mantle below ocean ridges.The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of thiscomposition with a phaneritic (coarse) groundmass are generally referred to as diabase (also called dolerite) orgabbro.

Columnar basalt at Sheepeater Cliff in Yellowstone

Etymology

The word "Basalt" is derived from Late Latin basaltes,misspelling of L. basanites "very hard stone," whichwas imported from Ancient Greek basani'ty*s(basanites), from ba'sano*s (basanos, "touchstone") andoriginated in Egyptian bauhun "slate."[3]

Uses

Basalt is used in construction (e.g. as building blocks orin the groundwork), making cobblestones (fromcolumnar basalt) and in making statues. Heating andextruding basalt yields stone wool, an excellent thermalinsulator.

Basalt 4

Vesicular basalt at Sunset Crater, Arizona. US quarter for scale.

Types

Large masses must cool slowly to form a polygonaljoint pattern.

Tholeiitic basalt is relatively poor in silica and poor in sodium.Included in this category are most basalts of the ocean floor,most large oceanic islands, and continental flood basalts such asthe Columbia River Plateau.

MORB (Mid Ocean Ridge Basalt), is characteristically low inincompatible elements. MORB is commonly erupted only atocean ridges. MORB itself has been subdivided into varietiessuch as NMORB and EMORB (slightly more enriched inincompatible elements).[4] [5]

High alumina basalt may be silica-undersaturated or-oversaturated (see normative mineralogy). It has greater than17% alumina (Al2O3) and is intermediate in compositionbetween tholeiite and alkali basalt; the relatively alumina-rich composition is based on rocks without phenocrystsof plagioclase.

Alkali basalt is relatively poor in silica and rich in sodium. It is silica-undersaturated and may containfeldspathoids, alkali feldspar and phlogopite.

Boninite is a high-magnesium form of basalt that is erupted generally in back-arc basins, distinguished by its lowtitanium content and trace element composition.

Basalt 5

Petrology

Photomicrograph of a volcanic (basaltic) sandgrain; upper picture is plane-polarized light,

bottom picture is cross-polarized light, scale boxat left-center is 0.25 millimeter. Note white

plagioclase 'microlites' in cross-polarized lightpicture, surrounded by very fine grained volcanic

glass.

The mineralogy of basalt is characterized by a preponderance of calcicplagioclase feldspar and pyroxene. Olivine can also be a significantconstituent. Accessory minerals present in relatively minor amountsinclude iron oxides and iron-titanium oxides, such as magnetite,ulvospinel, and ilmenite. Because of the presence of such oxideminerals, basalt can acquire strong magnetic signatures as it cools, andpaleomagnetic studies have made extensive use of basalt.

In tholeiitic basalt, pyroxene (augite and orthopyroxene or pigeonite)and calcium-rich plagioclase are common phenocryst minerals. Olivinemay also be a phenocryst, and when present, may have rims ofpigeonite. The groundmass contains interstitial quartz or tridymite orcristobalite. Olivine tholeiite has augite and orthopyroxene or pigeonitewith abundant olivine, but olivine may have rims of pyroxene and isunlikely to be present in the groundmass.

Alkali basalts typically have mineral assemblages that lackorthopyroxene but contain olivine. Feldspar phenocrysts typically arelabradorite to andesine in composition. Augite is rich in titaniumcompared to augite in tholeiitic basalt. Minerals such as alkali feldspar,leucite, nepheline, sodalite, phlogopite mica, and apatite may bepresent in the groundmass.

Basalt has high liquidus and solidus temperatures--values at the Earth'ssurface are near or above 1200 C (liquidus) and near or below 1000 C (solidus); these values are higher than thoseof other common igneous rocks.

The majority of tholeiites are formed at approximately 50-100km depth within the mantle. Many alkali basalts maybe formed at greater depths, perhaps as deep as 150-200km. The origin of high-alumina basalt continues to becontroversial, with interpretations that it is a primary melt and that instead it is derived from other basalt types (e.g.,Ozerov, 2000).

Geochemistry

Columnar basalt flows in Yellowstone NationalPark, USA.

Relative to most common igneous rocks, basalt compositions are richin MgO and CaO and low in SiO2 and the alkali oxides, i.e., Na2O +K2O, consistent with the TAS classification.

Basalt generally has a composition of 45-55 wt% SiO2, 2-6 wt% totalalkalis, 0.5-2.0 wt% TiO2, 5-14 wt% FeO and 14 wt% or more Al2O3.Contents of CaO are commonly near 10 wt%, those of MgO commonlyin the range 5 to 12 wt%.

High alumina basalts have aluminium contents of 17-19 wt% Al2O3;boninites have magnesium contents of up to 15% MgO. Rarefeldspathoid-rich mafic rocks, akin to alkali basalts, may have Na2O +K2O contents of 12% or more.

MORB basalts and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges. They are tholeiites particularly low in total alkalis and in incompatible trace elements, and they have

Basalt 6

relatively flat REE patterns normalized to mantle or chondrite values. In contrast, alkali basalts have normalizedpatterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatibleelements. Because MORB basalt is considered a key to understanding plate tectonics, its compositions have beenmuch studied. Although MORB compositions are distinctive relative to average compositions of basalts erupted inother environments, they are not uniform. For instance, compositions change with position along the Mid-Atlanticridge, and the compositions also define different ranges in different ocean basins (Hofmann, 2003).Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have beenmuch-studied, so as to learn about evolution of the Earth's mantle. Isotopic ratios of noble gases, such as 3He/4He,are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiite (normalized toatmospheric values), but to 15-24+ for ocean island basalts thought to be derived from mantle plumes.

Morphology and textures

An active basalt lava flow

The shape, structure and texture of a basalt is diagnostic of how andwhere it erupted -- whether into the sea, in an explosive cinder eruptionor as creeping pahoehoe lava flows, the classic image of Hawaiianbasalt eruptions.

Subaerial eruptions

Basalt which erupts under open air (that is, subaerially) forms threedistinct types of lava or volcanic deposits: scoria; ash or cinder(breccia); and lava flows.

Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight"frothy" texture to the rock. Basaltic cinders are often red, coloured by oxidized iron from weathered iron-richminerals such as pyroxene.`A`a types of blocky, cinder and breccia flows of thick, viscous basaltic lava are common in Hawaii. Pahoehoe is ahighly fluid, hot form of basalt which tends to form thin aprons of molten lava which fill up hollows and sometimesforms lava lakes. Lava tubes are common features of pahoehoe eruptions.Basaltic tuff or pyroclastic rocks are rare but not unknown. Usually basalt is too hot and fluid to build up sufficientpressure to form explosive lava eruptions but occasionally this will happen by trapping of the lava within thevolcanic throat and build up of volcanic gases. Hawaii's Mauna Loa volcano erupted in this way in the 19th century,as did Mount Tarawera, New Zealand in its violent 1886 eruption. Maar volcanoes are typical of small basalt tuffs,formed by explosive eruption of basalt through the crust, forming an apron of mixed basalt and wall rock breccia anda fan of basalt tuff further out from the volcano.Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites, quartz or calciteare frequently found.

Basalt 7

Columnar basalt

Columnar jointed basalt in Turkey

During the cooling of a thick lava flow, contractional joints or fracturesform. If a flow cools relatively rapidly, significant contraction forcesbuild up. While a flow can shrink in the vertical dimension withoutfracturing, it cannot easily accommodate shrinking in the horizontaldirection unless cracks form; the extensive fracture network thatdevelops results in the formation of columns. The topology of thelateral shapes of these columns can broadly be classed as a randomcellular network. These structures are often erroneously described asbeing predominantly hexagonal. In reality, the mean number of sidesof all the columns in such a structure is indeed six (by geometricaldefinition), but polygons with three to twelve or more sides can beobserved.[6] Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may resultin very small (

Basalt 8

Life on basaltic rocksThe common corrosion features of underwater volcanic basalt suggest that microbial activity may play a significantrole in the chemical exchange between basaltic rocks and seawater. The significant amounts of reduced iron, Fe(II),and manganese, Mn(II), present in basaltic rocks provide potential energy sources for bacteria. Recent research hasshown that some Fe(II)-oxidizing bacteria cultured from iron-sulfide surfaces are also able to grow with basaltic rockas a source of Fe(II).[7] In recent work at Loihi Seamount, Fe- and Mn- oxidizing bacteria have been cultured fromweathered basalts.[8] The impact of bacteria on altering the chemical composition of basaltic glass (and thus, theoceanic crust) and seawater suggest that these interactions may lead to an application of hydrothermal vents to theorigin of life.

Distribution

Paran Traps, Brazil

Basalt is one of the most common rock types in the world. Basalt is therock most typical of large igneous provinces. The largest occurrencesof basalt are in the ocean floor that is almost completely made up bybasalt. Above sea level basalt is common in hotspot islands and aroundvolcanic arcs, specially those on thin crust. However, the largestvolumes of basalt on land correspond to continental flood basalts.Continental flood basalts are known to exist in the Deccan Traps inIndia, the Chilcotin Group in British Columbia, Canada, the ParanTraps in Brazil, the Siberian Traps in Russia, the Karoo flood basaltprovince in South Africa, the Columbia River Plateau of Washingtonand Oregon.

Many archipelagoes and island nations have an overwhelming majority of its exposed bedrock made up by basaltdue to being above hotspots, for example, Iceland and Hawaii

Ancient Precambrian basalts are usually only found in fold and thrust belts, and are often heavily metamorphosed.These are known as greenstone belts, because low-grade metamorphism of basalt produces chlorite, actinolite,epidote and other green minerals.

Lunar and Martian basaltThe dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows. These rocks weresampled by the manned American Apollo program, the robotic Russian Luna program, and are represented amongthe lunar meteorites.Lunar basalts differ from their terrestrial counterparts principally in their high iron contents, which typically rangefrom about 17 to 22 wt% FeO. They also possess a stunning range of titanium concentrations (present in the mineralilmenite), ranging from less than 1 wt% TiO2, to about 13 wt.%. Traditionally, lunar basalts have been classifiedaccording to their titanium content, with classes being named high-Ti, low-Ti, and very-low-Ti. Nevertheless, globalgeochemical maps of titanium obtained from the Clementine mission demonstrate that the lunar maria possesses acontinuum of titanium concentrations, and that the highest concentrations are the least abundant.Lunar basalts show exotic textures and mineralogy, particularly shock metamorphism, lack of the oxidation typicalof terrestrial basalts, and a complete lack of hydration. While most of the Moon's basalts erupted between about 3and 3.5 billion years ago, the oldest samples are 4.2 billion years old, and the youngest flows, based on the agedating method of "crater counting," are estimated to have erupted only 1.2 billion years ago.Basalt is also a common rock on the surface of Mars, as determined by data sent back from the planet's surface andby Martian meteorites.

Basalt 9

Alteration of basalt

MetamorphismBasalts are important rocks within metamorphic belts, as they can provide vital information on the conditions ofmetamorphism within the belt. Various metamorphic facies are named after the mineral assemblages and rock typesformed by subjecting basalts to the temperatures and pressures of the metamorphic event. These are: Blueschist facies Eclogite facies Granulite facies Greenschist facies Zeolite faciesMetamorphosed basalts are important hosts for a variety of hydrothermal ore deposits, including gold deposits,copper deposits, volcanogenic massive sulfide ore deposits and others.

WeatheringCompared to other rocks found on Earth's surface, basalts weather relatively fast. Chemical weathering of basaltminerals release cations such as calcium, sodium and magnesium, which give basaltic areas a strong buffer capacityagainst acidification. Calcium released by basalts binds up CO2 from the atmosphere forming CaCO3 acting thus as aCO2 trap. To this it must be added that the eruption of basalt itself is often associated with the release of largequantities of CO2 into the atmosphere from volcanic gases.Carbon sequestration in basalt has been studied as a means of removing carbon dioxide, produced by humanindustrialization, from the atmosphere. Underwater basalt deposits, scattered in seas around the globe, have theadded benefit of the water serving as a barrier to the re-release of CO2 into the atmosphere.

[9]

References[1] basalt (http:/ / encarta. msn. com/ encnet/ features/ dictionary/ DictionaryResults. aspx?refid=1861589073) definition - Dictionary - MSN

Encarta. Archived (http:/ / www. webcitation. org/ 5kwPtNIhx) 2009-10-31.[2] Yourdictionary.com (http:/ / www. yourdictionary. com/ basalt)[3] Etymonline.com (http:/ / www. etymonline. com/ index. php?term=basalt)[4] See the PETDB database (http:/ / www. petdb. org/ ).Hyndman, Donald W. (1985). Petrology of igneous and metamorphic rocks (2nd ed.

ed.). McGraw-Hill. ISBN0-07-031658-9.[5] Blatt, Harvey and Robert Tracy (1996). Petrology (2nd ed. ed.). Freeman. ISBN0-7167-2438-3.[6] D. Weaire and N. Rivier. Contemporary Physics 25 1 (1984), pp. 55-99[7] Katrina J. Edwards, Wolfgang Bach and Daniel R. Rogers, Geomicrobiology of the Ocean Crust: A Role for Chemoautotrophic Fe-Bacteria,

Biol. Bull. 204: 180-185. (April 2003) Biolbull.org (http:/ / www. biolbull. org/ cgi/ content/ full/ 204/ 2/ 180)[8] Templeton, A.S., Staudigel, H., Tebo, B.M. (2005). Diverse Mn(II)-oxidizing bacteria isolated from submarine basalts at Loihi Seamount,

Geomicrobiology Journal, v. 22, 129-137. OGI.edu (http:/ / www. ebs. ogi. edu/ tebob/ pdfs/ Templeton GeomicroJ. pdf)[9] Mongabay.com (http:/ / news. mongabay. com/ 2010/ 0104-hance_ccs. html)

A. Y. Ozerov, The evolution of high-alumina basalts of the Klyuchevskoy volcano, Kamchatka, Russia, based onmicroprobe analyses of mineral inclusions. Journal of Volcanology and Geothermal Research, v. 95, pp.65-79(2000).

A. W. Hofmann, Sampling mantle heterogeneity through oceanic basalts: isotopes and trace elements. Treatiseon Geochemistry Volume 2, pages 61-101 Elsevier Ltd. (2003). ISBN 0-08-044337-0 In March, 2007, the articlewas available on the web at MPG.de. (http:/ / www1. mpch-mainz. mpg. de/ ~geo/ hofmann/ Hofmann.mantle_heterogen1. pdf)

A. V. Sobolev and others, The amount of recycled crust in sources of mantle-derived melts. Science, v. 316,pp.412-417 (2007). Sciencemag.org (http:/ / www. sciencemag. org/ cgi/ content/ abstract/ 316/ 5823/ 412)

Basalt 10

External links Basalt Columns (http:/ / giantcrystals. strahlen. org/ europe/ basalt. htm) Basalt in Northern Ireland (http:/ / geographyinaction. co. uk/ / Geology files/ Basalt. html) Lava - water interface (http:/ / www. geology. sdsu. edu/ how_volcanoes_work/ lava_water. html) Petrology of Lunar Rocks and Mare Basalts (http:/ / web. archive. org/ web/ 20080607121825/ http:/ / www.

union. edu/ PUBLIC/ GEODEPT/ COURSES/ petrology/ moon_rocks/ ) Pillow lava USGS (http:/ / volcanoes. usgs. gov/ Products/ Pglossary/ PillowLava. html)

Diorite

Diorite

Diorite classification on QAPF diagram

Diorite ( /darat/) is a grey to dark grey intermediate intrusiveigneous rock composed principally of plagioclase feldspar (typicallyandesine), biotite, hornblende, and/or pyroxene. It may contain smallamounts of quartz, microcline and olivine. Zircon, apatite, sphene,magnetite, ilmenite and sulfides occur as accessory minerals.[1] It canalso be black or bluish-grey, and frequently has a greenish cast.Varieties deficient in hornblende and other dark minerals are calledleucodiorite. When olivine and more iron-rich augite are present, therock grades into ferrodiorite, which is transitional to gabbro. Thepresence of significant quartz makes the rock type quartz-diorite (>5%quartz) or tonalite (>20% quartz), and if orthoclase (potassiumfeldspar) is present at greater than ten percent the rock type grades intomonzodiorite or granodiorite. Diorite has a medium grain size texture,occasionally with porphyry.

Diorites may be associated with either granite or gabbro intrusions,into which they may subtly merge. Diorite results from partial meltingof a mafic rock above a subduction zone. It is commonly produced involcanic arcs, and in cordilleran mountain building such as in theAndes Mountains as large batholiths. The extrusive volcanic equivalentrock type is andesite.

Diorite 11

Occurrence

Diorite

Diorite is a relatively rare rock; source localities include Leicestershire;UK[2] (one name for microdiorite - Markfieldite - exists due to the rockbeing found in the village of Markfield), Sondrio, Italy; Thuringia andSaxony in Germany; Finland; Romania; Northeastern Turkey; centralSweden; Scotland; the Darrans range of New Zealand; the AndesMountains; the Isle of Guernsey; Basin and Range province andMinnesota in the USA; Idahet in Egypt

An orbicular variety found in Corsica is called corsite.

Historic use

Diorite Porphyry vase from predynastic AncientEgypt, ca. 3600 BC; approx 30 cm.

Diorite is an extremely hard rock, making it difficult to carve and workwith. It is so hard that ancient civilizations (such as Ancient Egypt)used diorite balls to work granite. Its hardness, however, also allows itto be worked finely and take a high polish, and to provide a durablefinished work.

One comparatively frequent use of diorite was for inscription, as it iseasier to carve in relief than in three-dimensional statuary. Perhaps themost famous diorite work extant is the Code of Hammurabi, inscribedupon a 2.23m (7ft 4in) pillar of black diorite. The original can be seentoday in Paris' Muse de Louvre.[3] The use of diorite in art was mostimportant among very early Middle Eastern civilizations such asAncient Egypt, Babylonia, Assyria and Sumer. It was so valued inearly times that the first great Mesopotamian empire -- the Empire ofSargon of Akkad -- listed the taking of diorite as a purpose of militaryexpeditions.

Although one can find diorite art from later periods, it became morepopular as a structural stone and was frequently used as pavement dueto its durability. Diorite was used by both the Inca and Mayancivilizations, but mostly for fortress walls, weaponry, etc. It was especially popular with medieval Islamic builders.In later times, diorite was commonly used as cobblestone; today many diorite cobblestone streets can be found inEngland, Guernsey and Scotland, and scattered throughout the world in such places as Ecuador and China. Althoughdiorite is rough-textured in nature, its ability to take a polish can be seen in the diorite steps of St. Paul's Cathedral,London, where centuries of foot traffic have polished the steps to a sheen.

Diorite 12

References[1] Blatt, Harvey and Robert J. Tracy (1996) Petrology, W. H. Freeman, 2nd edition, p. 53 ISBN 0-7167-2438-3[2] Midland Quarry Products (http:/ / www. mqp. co. uk/ cliffe-hill-quarry. htm)[3] The Louvre: Law Code of Hammurabi (http:/ / www. louvre. fr/ llv/ oeuvres/ detail_notice. jsp?CONTENTcnt_id=10134198673226487&

CURRENT_LLV_NOTICEcnt_id=10134198673226487& FOLDERfolder_id=9852723696500800& bmUID=1156475018923&bmLocale=en)

Gabbro

Gabbro specimen; Rock Creek Canyon, easternSierra Nevada, California.

Close-up of gabbro specimen; Rock CreekCanyon, eastern Sierra Nevada, California.

Photomicrograph of a thin section of gabbro.

Gabbro ( /bro/) refers to a large group of dark, coarse-grained,intrusive mafic igneous rocks chemically equivalent to basalt. Therocks are plutonic, formed when molten magma is trapped beneath theEarth's surface and cools into a crystalline mass.

The vast majority of the Earth's surface is underlain by gabbro withinthe oceanic crust, produced by basalt magmatism at mid-ocean ridges.

Gabbro 13

Petrology

A gabbro landscape on the main ridge of theCuillin, Isle of Skye, Scotland.

Gabbro as a xenolith in a granite, eastern SierraNevada, Rock Creek Canyon, California.

Gabbro is dense, greenish or dark-colored and contains pyroxene,plagioclase, amphibole, and olivine (olivine gabbro when olivine ispresent in a large amount).

The pyroxene is mostly clinopyroxene; small amounts oforthopyroxene may be present. If the amount of orthopyroxene issubstantially greater than the amount of clinopyroxene, the rock is thena norite. Quartz gabbros are also known to occur and are probablyderived from magma that was over-saturated with silica. Essexitesrepresent gabbros whose parent magma was under-saturated withsilica, resulting in the formation of the feldspathoid mineral nepheline.(Silica saturation of a rock can be evaluated by normative mineralogy).Gabbros contain minor amounts, typically a few percent, ofiron-titanium oxides such as magnetite, ilmenite, and ulvospinel.

Gabbro is generally coarse grained, with crystals in the size range of1mm or greater. Finer grained equivalents of gabbro are calleddiabase, although the vernacular term microgabbro is often used whenextra descriptiveness is desired. Gabbro may be extremely coarsegrained to pegmatitic, and some pyroxene-plagioclase cumulates areessentially coarse grained gabbro, although these may exhibit acicularcrystal habits.

Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially when plagioclaseoikocrysts have grown earlier than the groundmass minerals.

Distribution

Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and plagioclase, or aspart of a layered intrusion as a cumulate formed by settling of pyroxene and plagioclase. Cumulate gabbros are moreproperly termed pyroxene-plagioclase orthocumulate.

Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes as parts of zones IIIand IV (sheeted dyke zone to massive gabbro zone). Long belts of gabbroic intrusions are typically formed atproto-rift zones and around ancient rift zone margins, intruding into the rift flanks. Mantle plume hypotheses mayrely on identifying mafic and ultramafic intrusions and coeval basalt volcanism.

UsesGabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides.Ocellar varieties of gabbro can be used as ornamental facing stones, paving stones and it is also known by the tradename of 'black granite', which is a popular type of graveyard headstone used in funerary rites. It is also used inkitchens and their countertops, also under the misnomer of 'black granite'.

EtymologyGabbro was named by the German geologist Christian Leopold von Buch after a town in the Italian Tuscany region.Essexite is named after the type locality in Essex County, Massachusetts, US.

Gabbro 14

External links Ocean drilling program gabbro petrology [1]

Scientists find the elusive gabbro [2]

References[1] http:/ / www-odp. tamu. edu/ publications/ prelim/ 176_PREL/ 176IGNE. HTML[2] http:/ / www. livescience. com/ forcesofnature/ 060420_earth_drill. html

Granite 15

Granite

Granite Igneous Rock

Granite containing potassium feldspar, plagioclase feldspar, quartz, and biotite and/or amphibole

Composition

Potassium feldspar, plagioclase feldspar, and quartz; differing amounts of muscovite, biotite, and hornblende-type amphiboles.

Granite ( /rnt/) is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usuallyhave a medium- to coarse-grained texture. Occasionally some individual crystals (phenocrysts) are larger than thegroundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture issometimes known as a porphyry. Granites can be pink to gray in color, depending on their chemistry andmineralogy. By definition, granite is an igneous rock with at least 20% quartz by volume. Granite differs fromgranodiorite in that at least 35% of the feldspar in granite is alkali feldspar as opposed to plagioclase; it is the alkalifeldspar that gives many granites a distinctive pink color. Outcrops of granite tend to form tors and rounded massifs.Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureoleor hornfels. Granite is usually found in the continental plates of the Earth's crust.Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespreaduse as a construction stone. The average density of granite is between 2.65[1] and 2.75 g/cm3, its compressivestrength usually lies above 200 MPa, and its viscosity at standard temperature and pressure is 3-6 1019 Pas.[2]

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such acrystalline rock.Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous rocks. Petrographicexamination is required for identification of specific types of granitoids.[3]

Granite 16

Mineralogy

Orbicular granite near the town of Caldera, northern Chile

Granite is classified according to the QAPF diagram forcoarse grained plutonic rocks and is named accordingto the percentage of quartz, alkali feldspar (orthoclase,sanidine, or microcline) and plagioclase feldspar on theA-Q-P half of the diagram. True granite according tomodern petrologic convention contains bothplagioclase and alkali feldspars. When a granitoid isdevoid or nearly devoid of plagioclase, the rock isreferred to as alkali granite. When a granitoid containsless than 10% orthoclase, it is called tonalite; pyroxeneand amphibole are common in tonalite. A granitecontaining both muscovite and biotite micas is called abinary or two-mica granite. Two-mica granites aretypically high in potassium and low in plagioclase, andare usually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite. Granite haspoor primary permeability but strong secondary permeability.

Chemical composition

A worldwide average of the chemical composition of granite, by weight percent:[4]

The Stawamus Chief is a granite monolith inBritish Columbia

SiO2 72.04% (silica) Al2O3 14.42% (alumina) K2O 4.12% Na2O 3.69% CaO 1.82% FeO 1.68% Fe2O3 1.22% MgO 0.71% TiO2 0.30% P2O5 0.12% MnO 0.05%Based on 2485 analyses

OccurrenceGranite is currently known only on Earth, where it forms a major part of continental crust. Granite often occurs asrelatively small, less than 100km stock masses (stocks) and in batholiths that are often associated with orogenicmountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of graniticintrusions. In some locations, very coarse-grained pegmatite masses occur with granite.Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is ofPrecambrian age. Granitic rock is widely distributed throughout the continental crust and is the most abundantbasement rock that underlies the relatively thin sedimentary veneer of the continents.

Granite 17

Origin

Close-up of granite exposed in Chennai, India.

Granite is an igneous rock and is formed from magma. Granitic magmahas many potential origins but it must intrude other rocks. Most graniteintrusions are emplaced at depth within the crust, usually greater than1.5kilometres and up to 50km depth within thick continental crust.The origin of granite is contentious and has led to varied schemes ofclassification. Classification schemes are regional and include French,British, and American systems.

Geochemical origins

Various granites (cut and polished surfaces)

Granitoids are a ubiquitous component of the crust. They havecrystallized from magmas that have compositions at or near a eutecticpoint (or a temperature minimum on a cotectic curve). Magmas willevolve to the eutectic because of igneous differentiation, or becausethey represent low degrees of partial melting. Fractional crystallisationserves to reduce a melt in iron, magnesium, titanium, calcium andsodium, and enrich the melt in potassium and silicon - alkali feldspar (rich in potassium) and quartz (SiO2), are twoof the defining constituents of granite.

Close-up of granite from Yosemite National Park,valley of the Merced River

This process operates regardless of the origin of the parental magma tothe granite, and regardless of its chemistry. However, the compositionand origin of the magma which differentiates into granite, leavescertain geochemical and mineral evidence as to what the granite'sparental rock was. The final mineralogy, texture and chemicalcomposition of a granite is often distinctive as to its origin. Forinstance, a granite which is formed from melted sediments may havemore alkali feldspar, whereas a granite derived from melted basalt maybe richer in plagioclase feldspar. It is on this basis that the modern"alphabet" classification schemes are based.

Chappell & White classification system

The letter-based Chappell & White classification system was proposed initially to divide granites into I-type granite(or igneous protolith) granite and S-type or sedimentary protolith granite.[5] Both of these types of granite are formedby melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment,respectively.

M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced fromcrystallized mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basaltinto granite via fractional crystallisation.A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy andgeochemistry. These granites are formed by melting of the lower crust under conditions that are usually extremelydry. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite.[6] [7]

Granite 18

GranitizationAn old, and largely discounted theory, granitization states that granite is formed in place by extreme metasomatismby fluids bringing in elements e.g. potassium and removing others e.g. calcium to transform the metamorphic rockinto a granite. This was supposed to occur across a migrating front. The production of granite by metamorphic heat isdifficult, but is observed to occur in certain amphibolite and granulite terrains. In-situ granitisation or melting bymetamorphism is difficult to recognise except where leucosome and melanosome textures are present in gneisses.Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as atransitional between the two, but are not technically granite as they do not actually intrude into other rocks. In allcases, melting of solid rock requires high temperature, and also water or other volatiles which act as a catalyst bylowering the solidus temperature of the rock.

Ascent and emplacement

Roche Rock, Cornwall

The Cheesewring, a granite tor on the southern edge ofBodmin Moor, Cornwall

The ascent and emplacement of large volumes of granite withinthe upper continental crust is a source of much debate amongstgeologists. There is a lack of field evidence for any proposedmechanisms, so hypotheses are predominantly based uponexperimental data. There are two major hypotheses for the ascentof magma through the crust: Stokes Diapir Fracture PropagationOf these two mechanisms, Stokes diapir was favoured for manyyears in the absence of a reasonable alternative. The basic idea isthat magma will rise through the crust as a single mass throughbuoyancy. As it rises it heats the wall rocks, causing them tobehave as a power-law fluid and thus flow around the plutonallowing it to pass rapidly and without major heat loss.[8] This isentirely feasible in the warm, ductile lower crust where rocks areeasily deformed, but runs into problems in the upper crust which isfar colder and more brittle. Rocks there do not deform so easily:for magma to rise as a pluton it would expend far too much energyin heating wall rocks, thus cooling and solidifying before reachinghigher levels within the crust.

Nowadays fracture propagation is the mechanism preferred bymany geologists as it largely eliminates the major problems ofmoving a huge mass of magma through cold brittle crust. Magmarises instead in small channels along self-propagating dykes whichform along new or pre-existing fault systems and networks ofactive shear zones (Clemens, 1998).[9] As these narrow conduitsopen, the first magma to enter solidifies and provides a form of insulation for later magma.

Granitic magma must make room for itself or be intruded into other rocks in order to form an intrusion, and severalmechanisms have been proposed to explain how large batholiths have been emplaced:

Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust Assimilation, where the granite melts its way up into the crust and removes overlying material in this way Inflation, where the granite body inflates under pressure and is injected into position

Granite 19

Most geologists today accept that a combination of these phenomena can be used to explain granite intrusions, andthat not all granites can be explained entirely by one or another mechanism.

Natural radiationGranite is a natural source of radiation, like most natural stones. However, some granites have been reported to havehigher radioactivity thereby raising some concerns about their safety.Some granites contain around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite,gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts.Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where theuranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites.Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite maybe susceptible to higher doses of radiation than other communities.[10] Cellars and basements sunk into soils overgranite can become a trap for radon gas, which is formed by the decay of uranium.[11] Radon can also be introducedinto houses by wells drilled into granite.[12] Radon gas poses significant health concerns, and is the number twocause of lung cancer in the US behind smoking.[12]

There is some concern that materials sold as granite countertops or as building material may be hazardous to health.Dan Steck of St. Johns University, has stated[13] that approximately 5% of all granites will be of concern, with thecaveat that only a tiny percentage of the tens of thousands of granite slabs have been tested. Various resources fromnational geological survey organizations are accessible online to assist in assessing the risk factors in granite countryand design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements anddwellings.A study of granite countertops was done (initiated and paid for by the Marble Institute of America) in November2008 by National Health and Engineering Inc of USA, and found that all of the 39 full size granite slabs that weremeasured for the study showed radiation levels well below the European Union safety standards (section 4.1.1.1 ofthe National Health and Engineering study) and radon emission levels well below the average outdoor radonconcentrations in the US.[14]

Uses

Antiquity

Life-size elephant and other creatures carved ingranite; Mahabalipuram, India.

The Red Pyramid of Egypt (c.26th century BC), named for the lightcrimson hue of its exposed granite surfaces, is the third largest ofEgyptian pyramids. Menkaure's Pyramid, likely dating to the same era,was constructed of limestone and granite blocks. The Great Pyramid ofGiza (c.2580 BC) contains a huge granite sarcophagus fashioned of"Red Aswan Granite." The mostly ruined Black Pyramid dating fromthe reign of Amenemhat III once had a polished granite pyramidion orcapstone, now on display in the main hall of the Egyptian Museum inCairo (see Dahshur). Other uses in Ancient Egypt include columns,door lintels, sills, jambs, and wall and floor veneer.[15] How theEgyptians worked the solid granite is still a matter of debate. Dr.Patrick Hunt[16] has postulated that the Egyptians used emery shown tohave higher hardness on the Mohs scale.

Many large Hindu temples in southern India, particularly those built by the 11th century king Rajaraja Chola I, were made of granite. There is a large amount of granite in these structures. They are comparable to the Great Pyramid of

Granite 20

Giza.[17]

Modern

Building

Quarrying granite for the Mormon Temple, UtahTerritory, in Little Cottonwood Canyon

Polished red granite tombstone

Granite has been extensively used as a dimension stone and as flooringtiles in public and commercial buildings and monuments. Aberdeen inScotland, which is constructed principally from local granite, is knownas "The Granite City". Because of its abundance, granite wascommonly used to build foundations for homes in New England. TheGranite Railway, America's first railroad, was built to haul granitefrom the quarries in Quincy, Massachusetts, to the Neponset River inthe 1820s. With increasing amounts of acid rain in parts of the world,granite has begun to supplant marble as a monument material, since itis much more durable. Polished granite is also a popular choice forkitchen countertops due to its high durability and aesthetic qualities. Inbuilding and for countertops, the term "granite" is often applied to alligneous rocks with large crystals, and not specifically to those with agranitic composition.

Other uses

Curling stones are traditionally fashioned of Ailsa Craig granite. Thefirst stones were made in the 1750s, the original source being AilsaCraig in Scotland. Because of the particular rarity of the granite, thebest stones can cost as much as US$1,500. Between 6070 percent ofthe stones used today are made from Ailsa Craig granite, although theisland is now a wildlife reserve and is no longer used for quarrying.[18]

In some areas granite is used for gravestones and memorials. Granite isa hard stone and requires skill to carve by hand. Modern methods ofcarving include using computer-controlled rotary bits and sandblastingover a rubber stencil. Leaving the letters, numbers and emblemsexposed on the stone, the blaster can create virtually any kind ofartwork or epitaph.

Engineering

Engineers have traditionally used polished granite surfaces to establisha plane of reference, since they are relatively impervious andinflexible. Sandblasted concrete with a heavy aggregate content has anappearance similar to rough granite, and is often used as a substitute when use of real granite is impractical. A mostunusual use of granite was in the construction of the rails for the Haytor Granite Tramway, Devon, England, in 1820.Granite block is usually processed into slabs and after can be cut and shaped by a cutting center.

Granite 21

Granite was used for cobblestones on the St.Louis riverfront and for the piers of the Eads

Bridge (background).

Rock climbing

The granite peaks of the Torres del Paine in theChilean Patagonia

Granite is one of the rocks most prized by climbers, for its steepness,soundness, crack systems, and friction. Well-known venues for graniteclimbing include Yosemite, the Bugaboos, the Mont Blanc massif (andpeaks such as the Aiguille du Dru, the Mountains of Mourne, theAiguille du Midi and the Grandes Jorasses), the Bregaglia, Corsica,parts of the Karakoram (especially the Trango Towers), the FitzroyMassif, Patagonia, Baffin Island, Ogawayama, the Cornish coast andthe Cairngorms.

Granite rock climbing is so popular that many of the artificial rockclimbing walls found in gyms and theme parks are made to look andfeel like granite.

Half Dome, Yosemite, a classic granite dome andpopular rock climbing

References[1] "Basic Rock Mechanics" (http:/ / webpages. sdsmt. edu/ ~lstetler/ merlot/

rock_mechanics. htm). Webpages.sdsmt.edu. . Retrieved 2010-05-09.[2] Kumagai, Naoichi; Sadao Sasajima, Hidebumi Ito (15 February 1978). "Long-term

Creep of Rocks: Results with Large Specimens Obtained in about 20 Years andThose with Small Specimens in about 3 Years" (http:/ / translate. google. com/translate?hl=en& sl=ja& u=http:/ / ci. nii. ac. jp/ naid/ 110002299397/ & sa=X&oi=translate& resnum=4& ct=result& prev=/ search?q=Ito+ Hidebumi& hl=en).Journal of the Society of Materials Science (Japan) (Japan Energy Society) 27 (293):157161. . Retrieved 2008-06-16.

[3] "Granitoids - Granite and the Related Rocks Granodiorite, Diorite and Tonalite"(http:/ / geology. about. com/ od/ more_igrocks/ a/ granitoids. htm).Geology.about.com. 2010-02-06. . Retrieved 2010-05-09.

[4] Harvey Blatt and Robert J. Tracy (1997). Petrology (2nd ed.). New York: Freeman. p.66. ISBN0-7167-2438-3.[5] Chappell, B.W. and White, A.J.R., 2001. Two contrasting granite types: 25 years later. Australian Journal of Earth Sciences v.48, p.489-499.[6] Boroughs, S., Wolff, J., Bonnichsen, B., Godchaux, M., and Larson, P., 2005, Large-volume, low-18O rhyolites of the central Snake River

Plain, Idaho, USA: Geology 33: 821824.[7] C.D. Frost, M. McCurry, R. Christiansen, K. Putirka and M. Kuntz, Extrusive A-type magmatism of the Yellowstone hot spot track 15th

Goldschmidt Conference Field Trip AC-4. Field Trip Guide, University of Wyoming (2005) 76 pp., plus an appended map.[8] Weinberg, R. F., and Podladchikov, Y., Diapiric ascent of magmas through power-law crust and mantle, 1994, J. Geophys. Res., 99,

9543-9559[9] Clemens, John (1998). "Observations on the origins and ascent mechanisms of granitic magmas". Journal of the Geological Society of

London 155 (Part 5): 84351. doi:10.1144/gsjgs.155.5.0843.

Granite 22

[10] "Radiation and Life" (http:/ / world-nuclear. org/ education/ ral. htm). World Nuclear Association. July 2002. . Retrieved 2010-02-04.[11] "Decay series of Uranium" (http:/ / www. world-nuclear. org/ images/ info/ decayseries. gif). . Retrieved 2008-10-19.[12] "Radon and Cancer: Questions and Answers" (http:/ / www. cancer. gov/ cancerTopics/ factsheet/ Risk/ radon). National Cancer Institute. .

Retrieved 2008-10-19.[13] Steck, Daniel J. (2009). "Pre- and Post-Market Measurements of Gamma Radiation and Radon Emanation from a Large Sample of

Decorative Granites" (http:/ / www. aarst. org/ proceedings/ 2009/PRE-AND_POST-MARKET_MEASUREMENTS_OF_GAMMA_RADIATION_AND_RADON_EMANATION_FROM_A_LARGE_SAMPLE_OF_DECORATIVE_GRANITES.pdf). .

[14] Natural Stone Countertops and Radon (http:/ / www. marble-institute. com/ industryresources/ countertops_radon_wp. pdf) - EnvironmentalHealth and Engineering - Assessing Exposure to Radon and Radiation from Granite Countertops.

[15] James A. Harrell. "Decorative Stones in the Pre-Ottoman Islamic Buildings of Cairo, Egypt" (http:/ / www. eeescience. utoledo. edu/Faculty/ Harrell/ Egypt/ Mosques/ CAIRO_Rocks_1. htm). . Retrieved 2008-01-06.

[16] "Egyptian Genius: Stoneworking for Eternity" (http:/ / hebsed. home. comcast. net/ hunt. htm). . Retrieved 2008-01-06.[17] "The Lost Temples of India" (http:/ / video. google. com/ videoplay?docid=8931191297840928556& q=Lost+ temples+ India) (video). .

Retrieved 2008-01-06.[18] "National Geographic News Puffins Return to Scottish Island Famous for Curling Stones" (http:/ / news. nationalgeographic. com/ news/

2004/ 10/ 1027_041027_curling_stones. html). News.nationalgeographic.com. . Retrieved 2009-07-30.

External links The Emplacement and Origin of Granite (http:/ / www. geologynet. com/ granite1. htm)

Obsidian 23

Obsidian

Obsidian

Obsidian from Lake County, OregonGeneral

Category Volcanic glass

Chemical formula 7075% SiO2,plus MgO, Fe3O4

Identification

Color Black

Fracture Conchoidal

Mohs scale hardness ~ 5 to 5.5

Luster Vitreous

Specific gravity ~ 2.5

Optical properties Translucent

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. Obsidian iscommonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemicalcomposition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition ofatomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Because ofthis lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use asprojectile points and blades, and its modern use as surgical scalpel blades.[1] [2]

Origin and propertiesPliny's Natural History features volcanic glass called "Obsidianus", so named from its resemblance to a stone foundin Ethiopia by one Obsius.[3]

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition istoo complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually darkin color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly ofSiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite andrhyolite. Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineralcrystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated bythe presence of water. Obsidian has low water content when fresh, typically less than 1% water by weight,[4] butbecomes progressively hydrated when exposed to groundwater, forming perlite. Tektites were once thought by manyto be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.

Obsidian 24

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Ironand magnesium typically give the obsidian a dark green to brown to black color. Very few samples are nearlycolorless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glassproduce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining fromthe lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles canproduce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbowobsidian).

OccurrenceObsidian can be found in locations which have experienced rhyolitic eruptions. It can be found in Argentina,Armenia, Azerbaijan, Canada, Chile, Greece, El Salvador, Guatemala, Iceland, Italy, Japan, Kenya, Mexico, NewZealand, Peru, Scotland and the United States. Obsidian flows which may be hiked on are found within the calderasof Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, and at InyoCraters east of the Sierra Nevada in California. Yellowstone National Park has a mountainside containing obsidianlocated between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many otherwestern U.S. states including Arizona, Colorado, New Mexico, Texas, Utah, Washington,[5] Oregon[6] and Idaho.Obsidian can also be found in the eastern U.S. state of Virginia.

Obsidian arrowhead

Historical use

Obsidian was valued in Stone Age cultures because, like flint, it couldbe fractured to produce sharp blades or arrowheads. Like all glass andsome other types of naturally occurring rocks, obsidian breaks with acharacteristic conchoidal fracture. It was also polished to create earlymirrors.

Modern archaeologists have developed a relative dating system,obsidian hydration dating, to calculate the age of obsidian artifacts.

Middle East

In Ubaid in the 5th millennium BC, blades were manufactured fromobsidian mined in today's Turkey.[7]

Ancient Egyptians used obsidian imported from the easternMediterranean and southern Red Sea regions. Obsidian was also usedin ritual circumcisions because of its deftness and sharpness.[8]

Obsidian 25

Obsidian talus at Obsidian Dome, California

Americas

Lithic analysis can be instrumental in understanding prehispanicgroups in Mesoamerica. A careful analysis of obsidian in a culture orplace can be of considerable use to reconstruct commerce, production,distribution and thereby understand economic, social and politicalaspects of a civilization. This is the case in Yaxchiln, a Maya citywhere even warfare implications have been studied linked withobsidian use and its debris.[9] Another example is the archeologicalrecovery at coastal Chumash sites in California indicating considerabletrade with the distant site of Casa Diablo, California in the Sierra

Nevada Mountains.[10]

Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated; including carved and workedobsidian for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted ina wooden body. Called a macuahuitl, the weapon was capable of inflicting terrible injuries, combining the sharpcutting edge of an obsidian blade with the ragged cut of a serrated weapon.

Counterclockwise from top: obsidian, pumice andrhyolite (light color)

Native American people traded obsidian throughout the Americas.Each volcano and in some cases each volcanic eruption produces adistinguishable type of obsidian, making it possible for archaeologiststo trace the origins of a particular artifact. Similar tracing techniqueshave allowed obsidian to be identified in Greece also as coming fromMelos, Nisyros or Yiali, islands in the Aegean Sea. Obsidian cores andblades were traded great distances inland from the coast.

Glass Mountain, a large obsidian flow atMedicine Lake Volcano

In Chile obsidian tools from Chaitn Volcano have been found as faraway as in Chan-Chan 400km north of the volcano and also in sites400km south of it.[11] [12]

Easter Island

Obsidian was also used on Rapa Nui (Easter Island) for edged toolssuch as Mataia and the pupils of the eyes of their Moai (statues).

Current useObsidian has been used for blades in surgery, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, the cutting edge of the blade being only about 3 nanometers thick.[13] Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope; when examined even under an electron microscope an obsidian blade is still smooth and even. One study found that obsidian incisions produced narrower scars, fewer inflammatory cells, and less granulation tissue in a group of rats.[14] Don Crabtree produced obsidian blades for surgery and other purposes[13] , and has written articles on the

Obsidian 26

subject.

Pig carved in snowflake obsidian, 10 centimeters(4 in) long. The markings are spherulites.

Obsidian is also used for ornamental purposes and as a gemstone. Itpossesses the property of presenting a different appearance accordingto the manner in which it is cut: when cut in one direction it is jetblack; in another it is glistening gray. "Apache tears" are small roundedobsidian nuggets embedded within a grayish-white perlite matrix.

Plinths for audio turntables have been made of obsidian since the1970s; e.g. the greyish-black SH-10B3 plinth by Technics.

ReferencesNotes[1] Primitive Technology: A Book of Earth Skills (http:/ / books. google. com. au/ books?id=HlwUo0IccoMC& pg=PT90& lpg=PT90&

dq=obsidian+ surgical+ scalpel& source=web& ots=tO8nfz3GFt& sig=7rQSy1-tT3-mm_Pp7nhyxOXjyTc& hl=en#PPT90,M1) DavidWescott

[2] Supplier of modern obsidian surgical scalpels with information on use (http:/ / www. finescience. com/ commerce/ ccc1065-obsidian-scalpels.htm)

[3] Encyclopedia.com (http:/ / www. encyclopedia. com/ doc/ 1O27-obsidian. html)[4] "Perlite - Mineral Deposit Profiles, B.C. Geological Survey" (http:/ / www. em. gov. bc. ca/ mining/ GeolSurv/ MetallicMinerals/

MineralDepositProfiles/ profiles/ r12. htm). . Retrieved 2007-11-20.[5] Washington Obsidian Source Map (http:/ / www. obsidianlab. com/ image_maps/ image_maps. html#wa)[6] Oregon Obsidian Sources (http:/ / www. sourcecatalog. com/ or/ s_or. html)[7] "In Syria, a Prologue for Cities" (http:/ / www. nytimes. com/ 2010/ 04/ 06/ science/ 06archeo. html). The New York Times. 5 April 2010.[8] Harrell, James A. (2000). "Stone in Ancient Egypt" (http:/ / www. eeescience. utoledo. edu/ faculty/ harrell/ egypt/ Stone Use/

Harrell_Stones_text. htm). University of Toledo. .[9] Brokmann, Carlos, Tipologa y anlisis de la obsidiana de Yaxchiln, Chiapas, Coleccin Cientfica, no.422, INAH, 2000, 284p.[10] C.Michael Hogan (2008) Morro Creek, ed. by A. Burnham (http:/ / www. megalithic. co. uk/ article. php?sid=18502)[11] (Spanish) Pino, Mario and Navarro, Rayen. Geoarqueologa del sitio arcaico Chan-Chan 18 (http:/ / www. scielo. cl/ scielo.

php?pid=S0716-02082005000100004& script=sci_arttext). Revista Geolgica de Chile, 2005.[12] Naranjo, Jos A; Stern, Charles R (December 2004). Holocene tephrochronology of the southernmost part (4230'-45S) of the Andean

Southern Volcanic Zone (http:/ / www. scielo. cl/ scielo. php?pid=S0716-02082004000200003& script=sci_arttext). 31. Revista geolgica deChile. pp. 225240. ISSN0716-0208. OCLC61022562. . Retrieved 2008-05-02.

[13] Buck, BA (March 1982). "Ancient technology in contemporary surgery". The Western journal of medicine 136 (3): 265269.ISSN0093-0415. OCLC115633208. PMC1273673. PMID7046256.

[14] Disa, JJ; Vossoughi, J; Goldberg, NH (October 1993). "A comparison of obsidian and surgical steel scalpel wound healing in rats" (http:/ /www. ncbi. nlm. nih. gov/ pubmed/ 8415970?dopt=AbstractPlus). Plastic and reconstructive surgery 92 (5): 884887.doi:10.1097/00006534-199392050-00015. ISSN0032-1052. OCLC121212765. PMID8415970. . Retrieved 2007-11-20.

External links Mindat with location data (http:/ / www. mindat. org/ min-8519. html) Obsidian: Mineral galleries (http:/ / mineral. galleries. com/ minerals/ mineralo/ obsidian/ obsidian. htm) USGS Obsidian photo glossary (http:/ / volcanoes. usgs. gov/ images/ pglossary/ obsidian. php)

Pegmatite 27

Pegmatite

Pegmatite with blue corundum crystals

Pegmatite containing lepidolite, tourmaline, andquartz from the White Elephant Mine in the

Black Hills, South Dakota

A pegmatite is a very coarse-grained, intrusive igneous rockcomposed of interlocking grains usually larger than 2.5cm in size;[1]

such rocks are referred to as pegmatitic.

Most pegmatites are composed of quartz, feldspar and mica; in essencea granite. Rarer intermediate composition and mafic pegmatitescontaining amphibole, Ca-plagioclase feldspar, pyroxene and otherminerals are known, found in recrystallised zones and apophysesassociated with large layered intrusions.

Crystal size is the most striking feature of pegmatites, with crystalsusually over 5cm in size. Individual crystals over 10 meters acrosshave been found, and the world's largest crystal was found within apegmatite.Similarly, crystal texture and form within pegmatitic rock may betaken to extreme size and perfection. Feldspar within a pegmatite maydisplay exaggerated and perfect twinning, exsolution lamellae, andwhen affected by hydrous crystallization, macroscale graphic texture isknown, with feldspar and quartz intergrown. Perthite feldspar within apegmatite often shows gigantic perthitic texture visible to the nakedeye.

Petrology

Crystal growth rates in pegmatite must be incredibly fast to allow gigantic crystals to grow within the confines andpressures of the Earth's crust. For this reason, the consensus on pegmatitic growth mechanisms involves acombination of the following processes; Low rates of nucleation of crystals coupled with high diffusivity to force growth of a few large crystals instead of

many smaller crystals High vapor and water pressure, to assist in the enhancement of conditions of diffusivity High concentrations of fluxing elements such as boron and lithium which lower the temperature of solidification

within the magma or vapor Low thermal gradients coupled with a high wall rock temperature, explaining the preponderance for pegmatite to

occur only within greenschist metamorphic terranesDespite this consensus on likely chemical, thermal and compositional conditions required to promote pegmatitegrowth there are three main theories behind pegmatite formation;

Pegmatite 28

Theoryname

Theory

Metamorphic pegmatite fluids are created by devolatilisation (dewatering) of metamorphic rocks, particularly felsic gneiss, to liberate the rightconstituents and water, at the right temperature

Magmatic pegmatites tend to occur in the aureoles of granites in most cases, and are usually granitic in character, often closely matching thecompositions of nearby granites. Pegmatites thus represent exsolved granitic material which crystallises in the country rocks

Metasomatic pegmatite, in a few cases, could be explained by the action of hot alteration fluids upon a rock mass, with bulk chemical and texturalchange.

Metasomatism is currently not well favored as a mechanism for pegmatite formation and it is likely thatmetamorphism and magmatism are both contributors toward the conditions necessary for pegmatite genesis.

Mineralogy

Pegmatitic granite, Rock Creek Canyon, easternSierra Nevada, California. Note pink potassium

feldspars and cumulate-filled chamber.

The mineralogy of a pegmatite is in all cases dominated by some formof feldspar, often with mica and usually with quartz, being altogether"granitic" in character. Beyond that, pegmatite may include mostminerals associated with granite and granite-associated hydrothermalsystems, granite-associated mineralisation styles, for example greisens,and somewhat with skarn associated mineralisation.

It is however impossible to quantify the mineralogy of pegmatite insimple terms because of their varied mineralogy and difficulty inestimating the modal abundance of mineral species which are of only atrace amount. This is because of the difficulty in counting andsampling mineral grains in a rock which may have crystals fromcentimeters to meters across.Garnet, commonly almandine or spessartine, is a common mineral within pegmatites intruding mafic andcarbonate-bearing sequences. Pegmatites associated with granitic domes within the Archaean Yilgarn Cratonintruding ultramafic and mafic rocks contain red, orange and brown almandine garnet.

Tantalum and niobium minerals (columbite, tantalite, niobite) are found in association with spodumene, lepidolite,tourmaline, cassiterite in the massive Greenbushes Pegmatite in the Yilgarn Craton of Western Australia, considereda typical metamorphic pegmatite unassociated with granite.

GeochemistryPegmatite is difficult to sample representatively due to the large size of the constituent mineral crystals. Often, bulksamples of some 5060kg of rock must be crushed to obtain a meaningful and repeatable result. Hence, pegmatite isoften characterised by sampling the individual minerals which comprise the pegmatite, and comparisons are madeaccording to mineral chemistry.Geochemically, pegmatites typically have major element compositions approximating "granite", however, whenfound in association with granitic plutons it is likely that a pegmatite dike will have a different trace elementcomposition with greater enrichment in large-ion lithophile (incompatible) elements, boron, beryllium, aluminium,potassium and lithium, uranium, thorium, cesium, et cetera.Occasionally, enrichment in the unusual trace elements will result in crystallisation of equally unusual and rare minerals such as beryl, tourmaline, columbite, tantalite, zinnwaldite and so forth. In most cases, there is no particular genetic significance to the presence of rare mineralogy within a pegmatite, however it is possible to see some causative and genetic links between, say, tourmaline-bearing granite dikes and tourmaline-bearing pegmatites within

Pegmatite 29

the area of influence of a composite granite intrusion (Mount Isa Inlier, Queensland, Australia).

Economic importancePegmatites are important because they often contain rare earth minerals and gemstones, such as aquamarine,tourmaline, topaz, fluorite, apatite and corundum, often along with tin and tungsten minerals, among others. Forexample, beautiful crystals of aquamarines and topaz can be found in pegmatites in the mountains of Colorado andIdaho.Pegmatites are the primary source of lithium either as spodumene, lithiophyllite or usually from lepidolite (Li-mica).The only used source for caesium is also a mineral from a zoned pegmatite, pollucite. The majority of the world'sberyllium is sourced from non-gem quality beryl within pegmatite. Tantalum, niobium, rare-earth elements aresourced from a few pegmatites worldwide, notably the Greenbushes Pegmatite. Bismuth, molybdenum and tin havebeen won from pegmatite, but this is not yet an important source of these metals.

NomenclaturePegmatites can be classified according to the elements or mineral of interest, for instance "lithian pegmatite" todescribe a Li-bearing or Li-mineral bearing pegmatite, or "boron pegmatite" for those containing tourmaline.There is often no meaningful way to distinguish pegmatites according to chemistry due to the difficulty of obtaininga representative sample, but often groups of pegmatites can be distinguished on contact textures, orientation,accessory minerals and timing. These may be named formally or informally as a class of intrusive rock or within alarger igneous association.While difficult to be certain of derivation of pegmatite in the strictest sense, often pegmatites are referred to as"metamorphic", "granitic" or "metasomatic", based on the interpretations of the investigating geologist.

OccurrenceWorldwide, notable pegmatite occurrences are within the major cratons, and within greenschist-facies metamorphicbelts. However, pegmatite localities are only well recorded when economic mineralisation is found.Within the metamorphic belts, pegmatite tends to concentrate around granitic bodies within zones of low mean strainand within zones of extension, for example within the strain shadow of a large rigid granite body. Similarly,pegmatite is often found within the contact zone of granite, transitional with some greisens, as a late-stagemagmatic-hydrothermal effect of syn-metamorphic granitic magmatism. Some skarns associated with granites alsotend to host pegmatites.Aplite and porphyry dikes and veins may intrude pegmatites and wall rocks adjacent to intrusions, creating aconfused sequence of felsic intrusive apophyses (thin branches or offshoots of igneous bodies) within the aureole ofsome granites.

References

Footnotes[1] USGS pegmatite definition (http:/ / vulcan. wr. usgs. gov/ LivingWith/ VolcanicPast/ Notes/ pegmatite. html), , retrieved 2009-08-28

Notations Tan, Li-ping, 1966, Major Pegmatite Deposits of New York State, New York State Museum Bulletin No. 408

Pumice 30

Pumice

Specimen of highly porous pumice from Teidevolcano on Tenerife, Canary Islands. Density of

specimen approximately 0.25 g/cm3; scale incentimeters.

Pumice ( /p.ms/) is a textural term for a volcanic rock that is asolidified frothy lava typically created when super-heated, highlypressurized rock is violently ejected from a volcano. It can be formedwhen lava and water are mixed. This unusual formation is due to thesimultaneous actions of rapid cooling and rapid depressurization. Thedepressurization creates bubbles by lowering the solubility of gases(including water and CO2) dissolved in the lava, causing the gases torapidly exsolve (like the bubbles of CO2 that appear when a carbonateddrink is opened). The simultaneous cooling and depressurizationfreezes the bubbles in the matrix.

Properties

Illustrates the porous nature in detail.

Rocks from the Bishop tuff, uncompressed withpumice on left; compressed with fiamme on right.

Pumice is composed of highly microvesicular glass pyroclastic withvery thin, translucent bubble walls of extrusive igneous rock. It iscommonly, but not exclusively of silicic or felsic to intermediate incomposition (e.g., rhyolitic, dacitic, andesite, pantellerite, phonolite,trachyte), but basaltic and other compositions are known. Pumice iscommonly pale in color, ranging from white, cream, blue or grey, togreen-brown or black. It forms when volcanic gases exsolving fromviscous magma nucleate bubbles which cannot readily decouple fromthe viscous magma prior to chilling to glass. Pumice is a commonproduct of explosive eruptions (plinian and ignimbrite-forming) andcommonly forms zones in upper parts of silicic lavas. Pumice has anaverage porosity of 90%, and initially floats on water.

Scoria differs from pumice in being denser. With larger vesicles andthicker vesicle walls, it sinks rapidly. The difference is the result of thelower viscosity of the magma that forms scoria. When larger amountsof gas are present, the result is a finer-grained variety of pumice knownas pumicite. Pumice is considered a glass because it has no crystalstructure. Pumice varies in density according to the thickness of thesolid material between the bubbles; many samples float in water. Afterthe explosion of Krakatoa, rafts of pumice drifted through the PacificOcean for up to 20 years, with tree trunks floating among them.[1] Infact, pumice rafts disperse and support several marine species.[2] In1979, 1984 and 2006, underwater volcanic eruptions near Tonga created large pumice rafts, some as large as 30kilometres that floated hundreds of kilometres to Fiji.[3]

Pumice 31

A 15 centimeter (6 inch) piece of pumicesupported by a rolled-up U.S. 20-dollar bill

demonstrates its very low density.

There are two main forms of vesicles. Most pumice contains tubularmicrovesicles that can impart a silky or fibrous fabric. The elongationof the microvesicles occurs due to ductile elongation in the volcanicconduit or, in the case of pumiceous lavas, during flow. The other formof vesicles are subspherical to spherical and result from high vaporpressure during eruption.

Uses

Pumice is widely used to make lightweight concrete or insulativelow-density breeze blocks. When used as an additive for cement, afine-grained version of pumice called pozzolan is mixed with lime toform a light-weight, smooth, plaster-like concrete. This form ofconcrete was used as far back as Roman times. Roman engineers used it to build the huge dome of the Pantheon andas construction material for many aqueducts.

It is also used as an abrasive, especially in polishes, pencil erasers, cosmetic exfoliants, and the production ofstone-washed jeans. "Pumice stones" are often used in beauty salons during the pedicure process to remove dry andexcess skin from the bottom of the foot as well as calluses. It was also used in ancient Greek and Roman times toremove excess hair.[4] Finely ground pumice is added to some toothpastes and heavy-duty hand cleaners (such asLava soap) as a mild abrasive. Pumice is also used as a growing substrate for growing horticultural crops.

References[1] De Vantier, L.M. (September 1992). "Rafting of tropical marine organisms on buoyant coralla" (http:/ / www. int-res. com/ articles/ meps/ 86/

m086p301. pdf). Marine Ecology Progress Series 86: 301302. doi:10.3354/meps086301. . Retrieved 2007-07-14. "Trunks and pumice thatwashed ashore at Keeling Atoll in the early 1900's had been drifting for some 20 years, since the eruption of Krakatoa 1000 km to thenortheast in 1883 (Wood-Jones 1912)".

[2] Bryan, S.E.; A. Cook, J.P. Evans, P.W. Colls, M.G. Wells, M.G. Lawrence, J.S. Jell, A. Greig, R. Leslie (2004). "Pumice rafting and faunaldispersion during 20012002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga" (http:/ / earth. geology.yale. edu/ ~je84/ mystuff/ research/ bryanetal2004. pdf). Earth and Planetary Science Letters 227: 135154. Bibcode2004E&PSL.227..135B.doi:10.1016/j.epsl.2004.08.009. . Retrieved 2007-07-14. "The abundance and variety of fouling taxa, coupled with the long dispersal trajectory(>3500km) and period of pumice floatation (1year), confirm the importance of sea-rafted pumice as a long-distance dispersal mechanismfor marine organisms".

[3] "New Island and pumice raft in the Tongas" (http:/ / earthobservatory. nasa. gov/ Newsroom/ NewImages/ images. php3?img_id=17461).NASA Earth Observatory. National Aeronautics and Space Administration. 16 November 2006. .

[4] Sherrow, V. 2001. For appearance' sake: the historical encyclopedia of good looks. Greenwood, 312pp. ISBN 1-57356-204-1

External links University of Oxford image of pumice. (http:/ / www. earth. ox. ac. uk/ ~oesis/ rocks/ ign8. html) Retrieved

2010-09-27. Analytical identification of a single source pumice from Greek shores and ancient sites in the Levant. (http:/ / uoa.

academia. edu/ CharalamposVasilatos/ Papers/ 265386/Analytical_identification_of_a_single_source_pumice_from_Greek_shores_and_ancient_sites_in_the_Levant)

On the occurrence of a pumice-rich layer in Holocene deposits of western Peloponnesus, Ionian Sea, Greece. Ageomorphological and geochemical approach. (http:/ / uoa. academia. edu/ CharalamposVasilatos/ Papers/ 88430/On_the_occurrence_of_a_pumice-rich_layer_in_Holocene_deposits_of_western_Peloponnesus_Ionian_Sea_Greece._A_geomorphological_and_geochemical_approach)

Rhyolite 32

Rhyolite

Rhyolite Igneous Rock

Composition

Felsic: igneous quartz and alkali feldspar (orthoclase, sanidine and sodic plagioclase), biotite and hornblende.

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, seeRhyolite/Aquacade.

Rhyolite is an igneous, volcanic (extrusive) rock, of felsic (silica-rich) composition (typically > 69% SiO2 see theTAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage isusually quartz, alkali feldspar and plagioclase (in a ratio > 1:2 see the QAPF diagram). Biotite and hornblende arecommon accessory minerals.

Rocks from the Bishop tuff, uncompressed withpumice on left; compressed with fiamme on right.

Geology

Rhyolite can be considered as the extrusive equivalent to the plutonicgranite rock, and consequently, outcrops of rhyolite may bear aresemblance to granite. Due to their high content of silica and low ironand magnesium contents, rhyolite melts are highly polymerized andform highly viscous lavas. They can also occur as breccias or involcanic plugs and dikes. Rhyolites that cool too quickly to growcrystals form a natural glass or vitrophyre, also called obsidian. Slowercooling forms microscopic crystals in the lava and results in texturessuch as flow foliations, spherulitic, nodular, and lithophysal structures.Some rhyolite is highly vesicular pumice. Many eruptions of rhyolite

are highly explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites.

Rhyolite 33

HistoryIn North American pre-historic times, rhyolite was quarried extensively in eastern Pennsylvania in the United States.Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County, where as many as fiftysmall quarry pits are known.[1]

Name

Top stone is obsidian (vitrophyre), below that ispumice and in lower right corner is rhyolite (light

color)

A sample of Rhyolite from the Conical Hill domeat the head Lyttelton Harbour, Banks Peninsula,

New Zealand

The name rhyolite was introduced into science by the German travelerand geologist Ferdinand von Richthofen after his explorations in theRocky Mountains in the 1860s.

References[1] Beckerman, Ira. National Register of Historic Places Inventory/Nomination:

Carbaugh Run Rhyolite Quarry Site (36AD30). National Park Service, 1981, 2.

External links

University of North Dakota description of rhyolite (http:/ / volcano.und. edu/ vwdocs/ vwlessons/ lessons/ Slideshow/ Igrocks/ Igrock9.html)

Information from rocks-rock.com (http:/ / www. rocks-rock. com/rhyolite. html)

Scoria 34

Scoria

Scoria

Scoria is a volcanic rock containing many holes or vesicles. It is mostgenerally dark in color (generally dark brown, black or red), andbasaltic or andesitic in composition. Scoria is relatively low in mass asa result of its numerous macroscopic ellipsoidal vesicles, but incontrast to pumice, all scoria has a specific gravity greater than 1, andsinks in water. The holes or vesicules form when gases that weredissolved in the magma come out of solution as it erupts, creatingbubbles in the molten rock, some of which are frozen in place as therock chills and solidifies. Scoria may form as part of a lava flow,typically near its surface, or as fragmental ejecta (lapilli, blocks andbombs), for instance in Strombolian eruptions that form steep-sided scoria cones. Most scoria is composed of glassyfragments, and may contain phenocrysts.

The word scoria comes from the Greek , skria, rust. An old name for scoria is cinder.

ComparisonsScoria differs from pumice, another vesicular volcanic rock, in having larger vesicles and thicker vesicle walls, andhence is denser. The difference is probably the result of lower magma viscosity, allowing rapid volatile diffusion,bubble growth, coalescence, and bursting.

FormationAs rising magma encounters lower pressures, dissolved gases are able to exsolve and form vesicles. Some of thevesicles are trapped when the magma chills and solidifies. Vesicles are usually small, spheroidal and do not impingeupon one another; instead they open into one another with little distortion.Volcanic cones of scoria can be left behind after eruptions, usually forming mountains with a crater at the summit.An example is Mount Wellington, Auckland in New Zealand, which like the Three Kings in the south of the samecity has been extensively quarried. Quincan, a unique form of Scoria, is quarried at Mount Quincan in Far NorthQueensland, Australia.

Scoria 35

Uses

Tuff moai with red scoria pukao onits head

Scoria has several useful characteristics that influence how it is used. It issomewhat porous, has high surface area and strength for its weight, and often hasstriking colours. Scoria is often used in landscaping and drainage works.[1] It isalso commonly used in barbecue grills.[2]

Scoria can be used for high-temperature insulation.The quarry of Puna Pau on Rapa Nui/Easter Island was the source of a redcoloured scoria which the Rapanui people used to carve the pukao (or topknots)for their distinctive moai statues, and to carve some moai from.

Images

Fresh scoria sometimes has ablue sheen to its surface.

Scoria of varioushues exists on

Mount Tarawera,from its 1886

eruption.

Holocene scoria-producingvolcano near Veyo, Utah

Photomicrographof a volcanic lithic

fragment (sandgrain) derived

from scoria; upperpicture is

plane-polarizedlight, bottom

picture iscross-polarized

light, scale box atleft-center is 0.25

millimeter.

References[1] Three Kings Quarry (http:/ / www. winstoneaggregates. co. nz/ Auckland. php?location=Three Kings), Winstone Aggregates.[2] Rock Types and Rocks Found in Michigan (http:/ / www. michigan. gov/ documents/ deq/ GIMDL-GGRF_307777_7. pdf)

Article Sources and Contributors 36

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