Pearly white, translucent crystals of selenite gypsum (CaSO4 2

Pearly white, translucent crystals of selenite gypsum(CaSO4 � 2H2O). Lechuguilla Cave, New Mexico.(Photograph by D. Bunnell.)

2

Some readers of this book will have taken a prior course inphysical geology. These students will have been intro-duced to rocks and minerals and may use this chapter as areview. For other readers, a basic knowledge of earth ma-terials sets the stage for an understanding of the processesthat have shaped our planet. Minerals and rocks (and thefossils within rocks) are the indispensible documents fromwhich the history of the Earth is deciphered.

�MINERALS

Minerals are important to all of us. They are the rawmaterials needed to manufacture the products that sus-tain our modern technologic society. For geologists,the search for new mineral deposits has always been akey objective. Minerals, however, are also of interest toearth scientists because of the evidence they provideabout past events. Many minerals form only within anarrow range of physical conditions and therefore canbe used to diagnose the pressures and temperatures in-volved in the formation of mountain ranges and volca-noes. Some minerals develop exclusively in ocean waterand provide evidence of the incursion of seas across for-mer areas of dry land. Others form under conditions ofexcessive aridity and are used to locate the arid tropicalbelts of long ago. The magnetic properties of certainminerals provide clues to drifting continents, thewidening of oceans, and changes in the planet’s mag-netic poles. As a further aid to deciphering Earth his-tory, some minerals contain radioactive elements thatpermit us to determine the ages of rocks and structuresformed at particular times in the geologic past.

Common Rock-Forming Minerals

Minerals are naturally occurring solid, inorganic sub-stances that have a definite chemical composition orrange of compositions as well as distinctive propertiesthat reflect their composition and regular internalatomic structure. Minerals can usually be identified bysuch physical properties as color, hardness, density,crystal form, and cleavage. Cleavage refers to the ten-dency of some minerals to break smoothly along cer-tain planes of weakness.

35

What stuff ’tis made of, whereof it is born, I am tolearn.Shakespeare, The Merchant of Venice

O U T L I N E

� MINERALSCommon Rock-Forming Minerals

� ROCKSRock ConversionsIgneous RocksSedimentary RocksMetamorphic Rocks

C H A P T E R 2

Earth Materials: A PhysicalGeology Refresher

More than 3000 minerals have already been discov-ered and described, but most of these are rarely en-countered. For our present purposes, it is important toconsider only those minerals that compose the bulk ofcommon rocks. Among these, the most important arethe silicate minerals.

SILICATE MINERALS About 75 percent by weight ofthe Earth’s crust is composed of the two elements oxy-gen and silicon (Table 2-1). These elements usuallyoccur in combination with such abundant metals asaluminum, iron, calcium, sodium, potassium, and mag-nesium to form a group of minerals called the silicates.A single family of silicates, the feldspars, comprisesabout one-half of the material of the crust, and a singlemineral species called quartz represents a sizable por-tion of the remainder.

Quartz The principal silicate minerals and some oftheir properties are listed in Table 2-2. The mineralquartz (SiO2) is one of the most familiar and impor-tant of all the silicate minerals. It is common in manydifferent families of rocks. In general appearance,quartz is a glassy, colorless, gray or white mineral.Quartz is relatively hard (see Mohs’ scale in the foot-note of Table 2-2) and will easily scratch glass. Whenquartz grows in an open cavity, it may form the hexag-onal crystals (Fig. 2-1) that are prized by mineral col-lectors. More frequently, the crystal faces cannot bediscerned because the orderly addition of atoms dur-ing crystal growth was interrupted by contact withother growing crystals and results in an aggregate thatexhibits crystalline texture. Such common minerals aschert, flint, jasper, and agate are sedimentary varietiesof quartz. Chert (Fig. 2-2) is a dense, hard, white min-eral composed of sub-microcrystalline quartz that

forms as a result of the precipitation of silicon dioxideby either biologic or chemical means. Flint is the pop-ular name for a dark gray or black variety of chert inwhich the dark color results from inclusions of organicmatter. Red or red-brown chert is called jasper. Agateis a submicroscopic variety of quartz characterized bybands of differing colors.

The origin of chert is a complex problem madeeven more difficult by the fact that different varietiesare formed by somewhat different processes. Somecherts are replacements of earlier carbonate rocks.Others appear to have formed as a result of the solu-tion and reprecipitation of silica from the siliceousskeletal remains of organisms. Small amounts of chertmay also be precipitated directly from concentratedaqueous solutions.

The Feldspars The feldspars are the most abundantconstituents of rocks, composing about 60 percent ofthe total weight of the Earth’s crust. There are twomajor families of feldspars: the orthoclase, or potas-sium, group (Fig. 2-3), which comprises the potassiumaluminosilicates; and the plagioclase group, whichcomprises the aluminosilicates of sodium and calcium(Fig. 2-4). Members of the plagioclase group exhibit awide range in composition—from a calcium-rich endmember called anorthite (CaAl2Si2O8) to a sodium-rich end member called albite (NaAlSi3O8). Betweenthese two extremes, plagioclase minerals containingboth sodium and calcium occur. The substitution ofsodium for calcium, however, is not random but is gov-erned by the temperature and composition of the par-ent material. Thus, by examining the feldspar contentof a once-molten rock, it is possible to infer the physi-cal and chemical conditions under which it originated.Feldspars are nearly as hard as quartz and range in

36 � Chapter 2. Earth Materials: A Physical Geology Refresher

TABLE 2-1 Abundances of Chemical Elements in the Earth’s Crust*

PercentageElement Percentage by Number Percentageand Symbol by Weight of Atoms by Volume

Oxygen (0) 46.6 62.6 93.8Silicon (Si) 27.7 21.2 0.9Aluminum (Al) 8.1 6.5 0.5Iron (Fe) 5.0 1.9 0.4Calcium (Ca) 3.6 1.9 1.0Sodium (Na) 2.8 2.6 1.3Potassium (K) 2.6 1.4 1.8Magnesium (Mg) 2.1 1.9 0.3All other elements 1.5 — —

100.0 100.0† 100.0†

*Based on Mason, B. 1966. Principles of Geochemistry. New York: John Wiley & Sons. Note thehigh percentage of oxygen in the Earth’s crust.†Includes only the first eight elements.

color from white or pink to bluish gray. They have goodcleavage in two directions, and the resulting flat, oftenrectangular surfaces are useful in identification. Thoseplagioclase feldspars with abundant sodium tend, alongwith potassium feldspars, to occur in silica-rich rockssuch as granite. Calcium-rich plagioclases occur in suchrocks as the Hawaiian lavas (called basalts).

Micas Mica is a family of silicate minerals easily rec-ognized by its perfect and conspicuous cleavage (Fig.2-5) along one directional plane. The two chief vari-eties are the colorless or pale-colored muscovite mica,which is a hydrous potassium aluminum silicate, andthe dark-colored biotite mica, which also containsmagnesium and iron. Both muscovite and biotite arecommon rock-forming silicates.

Hornblende Hornblende (Fig. 2-6) is a vitreous, blackor very dark green mineral. It is the most commonmember of a larger family of minerals called amphi-

boles, which have generally similar properties. Becauseof its content of iron and magnesium, hornblende(along with biotite, augite, and olivine) is designated aferromagnesian or mafic mineral. Crystals of horn-blende tend to be long and narrow.

Augite Just as hornblende is only one member of afamily of minerals called amphiboles, augite is an im-portant member of the pyroxene family, in which manyother mineral species also occur. Like hornblende, it isa ferromagnesian mineral and thus dark-colored. Anaugite crystal is typically rather stumpy in shape, withgood cleavages developed along two planes that arenearly at right angles.

Olivine As you might guess from its name, this glassy-looking iron and magnesium silicate often has an olivegreen color. Olivine (Fig. 2-7) is present in dark rockssuch as basalt. Along with pyroxene and minor calciumplagioclase, it is also an important component of the

Minerals � 37

TABLE 2-2 Common Rock-Forming Silicate Minerals

Silicate Mineral Composition Physical Properties

Quartz Silicon dioxide Hardness of 7 (on scale of 1 to 10)*; will not cleave (fractures (silica, SiO2) unevenly); specific gravity: 2.65

Orthoclase Aluminosilicates of Hardness of 6.0–6.5; cleaves well in two directions; pink or feldspar group potassium white; specific gravity: 2.5–2.6

Plagioclase Aluminosilicates of Hardness of 6.0–6.5; cleaves well in two directions; white or feldspar group sodium and calcium gray; may show striations on cleavage planes; specific

gravity: 2.6–2.7

Muscovite mica Aluminosilicates of Hardness of 2–3; cleaves perfectly in one direction, yielding potassium with water flexible, thin plates; colorless; transparent in thin sheets;

specific gravity: 2.8–3.0

Biotite mica Aluminosilicates of Hardness of 2.5–3.0; cleaves perfectly in one direction, magnesium, iron, yielding flexible, thin plates; black to dark brown; potassium, with water specific gravity: 2.7–3.2

Pyroxene group Silicates of aluminum, Hardness of 5–6; cleaves in two directions at 87� and 93�; calcium, magnesium, black to dark green; specific gravity: 3.1–3.5and iron

Amphibole group Silicates of aluminum, Hardness of 5–6; cleaves in two directions at 56� and 124�; calcium, magnesium, black to dark green; specific gravity: 3.0–3.3and iron

Olivine Silicates of magnesium Hardness of 6.5–7.0; light green; transparent to translucent; and iron specific gravity: 3.2–3.6

Garnet group Aluminosilicates of iron, Hardness of 6.5–7.5; uneven fracture; red, brown, or yellow; calcium, magnesium, specific gravity: 3.5–4.3

and manganese

*The scale of hardness used by geologists was formulated in 1822 by Frederich Mohs. Beginning with diamond as the hardest mineral, hearranged the following table:

10 Diamond 8 Topaz 6 Feldspar 4 Fluorite 2 Gypsum9 Corundum 7 Quartz 5 Apatite 3 Calcite 1 Talc

Ferr

omag

nesi

an m

iner

als

ultramafic rock called peridotite. Peridotite is a promi-nent rock type in the Earth’s mantle, the rocky layerthat lies beneath the crust.

Clay Minerals The word clay is so familiar to us that wemay not appreciate its importance. Nearly seventy-fivepercent of the surface of continents is covered by clayminerals. They are the most abundant materials de-posited in modern and ancient oceans. Shale, the mostabundant of sedimentary rocks, is about 50 percentclay. Clay minerals are an essential component of agri-cultural soils, serving to bind soil particles together,and hold moisture and nutrients.

Clay minerals are silicates of hydrogen and alu-minum with additions of magnesium, iron, and potas-sium. Their basic structure is similar to that of mica,but because individual flakes are extremely small (some


FIGURE 2-1 A large crystal of quartz. This specimen is14 cm (about 5.5 inches) tall.

FIGURE 2-2 Chert. This variety (called novaculite) has agrainy texture that makes it useful as a grinding stone.

FIGURE 2-3 Orthoclase. (A) Crystals of orthoclase(potassium feldspar). (B) A cleavage fragment of orthoclase.

(A)

(B)

are comparable in size to some viruses). Their mica-likeform can be seen only with the magnification providedby an electron microscope (Fig. 2-8). Unlike many ofthe silicate minerals already described, clay mineralsform as a result of weathering of other aluminum sili-cate minerals such as feldspars. As a group, clays areknown by their amorphous form, softness, low density,and ability to become plastic when wet.

Geologists recognize that there are two differentuses for the term clay. As described above, a clay min-eral is a hydrous aluminum silicate with mica-likeform. However, the term clay can also be used to de-note particles of sediment with a diameter of less than1/256 mm.

NONSILICATE MINERALS Approximately 8 percentof the Earth’s crust is composed of nonsilicate miner-als. These include a host of carbonates, sulfides, sul-fates, chlorides, and oxides. Among these groups, thecarbonates, such as calcite and dolomite, are themost important. Calcite (CaCO3), which is the mainconstituent of limestone and marble, forms in manyways. It is secreted as skeletal material by certain in-vertebrate animals, precipitated directly from seawater, or formed as dripstone in caverns. Calcite is

Minerals � 39

FIGURE 2-4 The variety of plagioclase feldspar knownas labradorite. The mineral often displays beautiful blueand gold reflections as well as fine striations on cleavageplanes. (Courtesy of Wards Natural Science Establishment, Inc.,Rochester, NY.)

FIGURE 2-5 Muscovite mica, exhibiting itscharacteristic perfect cleavage in one plane. The blade ofthe screwdriver is shown here separating pieces of themineral along its cleavage plane.

FIGURE 2-6 Hornblende. The black scaly flakes on thesurface are biotite mica. Specimen is 6 cm long. On the basisof its dark color, what metallic elements are likely to be present inthis mineral?

FIGURE 2-7 Olivine.

easily recognized by its rhombohedron-shapedcleaved fragments (Fig. 2-9) and by the fact that an ap-plication of dilute hydrochloric acid on its surface willcause effervescence (the bubbles are liberated carbondioxide).

The term dolomite is used both for the carbonate min-eral that has the chemical formula CaMg(CO3)2 andfor the rock composed largely of that mineral. (The al-ternate term dolostone is also used occasionally to des-ignate dolomite rock.) In the mineral dolomite, cal-cium and magnesium occur in approximately equalproportions.

Aragonite (CaCO3) is another carbonate mineralthat occurs in a different crystal form and more rarely

than either calcite or dolomite. Most of us have seen itas the inner “mother-of-pearl” layer of clam shells. Be-cause both calcite and aragonite are two differentforms of the same compound, they are spoken of aspolymorphs of calcium carbonate.

Other common nonsilicate minerals include com-mon rock salt, or halite (NaCl), and gypsum. Halite iseasily recognized by its salty taste and the fact that itcrystallizes and cleaves to form cubes (Fig. 2-10). Gyp-sum is a soft, hydrous calcium sulfate (CaSO4 � 2H2O).The variety of gypsum called satinspar has a fine, fi-brous structure, whereas the variety known as selenite(Fig. 2-11) will split into thin plates. The finely crys-talline, massive variety known as alabaster is usedwidely in carvings and sculpture because of its uniformtexture and softness. Halite and the various gypsumminerals are sometimes referred to as evaporites be-


FIGURE 2-8 Electron micrograph of the clay mineralkaolinite. The flaky, stack-of-cards character of the claycrystals is a manifestation of their silicate sheet structure.(Magnified 32,000 times; courtesy of Kevex Corporation.)

FIGURE 2-9 Calcite, showing the characteristicrhombohedral shape of cleavage pieces. (Courtesy ofWards Natural Science Establishment, Inc., Rochester, NY.)

FIGURE 2-10 Halite. (Courtesy of Wards Natural ScienceEstablishment, Inc., Rochester, NY.) �? ? How do these cleavedpieces of halite differ from the cleavage pieces of calcite in Figure 2-9?

FIGURE 2-11 A cluster of selenite gypsum crystals.(Courtesy of Wards Natural Science Establishment, Inc.,Rochester, NY.)

cause they are often precipitated from bodies of waterthat have been subjected to intense evaporation.

If you were to make a list of the elements that com-pose the minerals described thus far, the list would be asurprisingly short one. Only eight elements make up thebulk of these minerals, and only the same eight are abun-dant in the Earth’s crust as well. As shown in Table 2-1,these abundant elements are oxygen, silicon, aluminum,iron, calcium, sodium, potassium, and magnesium.

�ROCKS

Rock Conversions

We have noted that minerals form under specific condi-tions of temperature and pressure. It is therefore possi-ble to know the origin of the rocks containing thoseminerals. Geologists have agreed on a fundamental divi-sion of rocks into three great families according to dif-ference of origin. Igneous rocks are those that havecooled from a molten state. Sedimentary rocks consist ofmaterials formed from the weathered products of pre-existing rocks that have been transported, deposited, andlithified. Metamorphic rocks are any that have beenchanged from previously existing rocks by the action of

heat, pressure, and associated chemical activity. Thechanges may include a recrystallization of the previousminerals or growth of entirely new minerals.

It is important to remember that the rocks of any ofthe three major rock families are not immutable. TheEarth’s crust is dynamic and everchanging. Any sedi-mentary or metamorphic rock may be partially orcompletely melted to produce igneous rocks, and anypreviously existing rock of any category can be com-pressed and altered during mountain building to pro-duce metamorphic rocks. The weathered and erodedresidue of any family of rock can be observed todaybeing transported to the sea for deposition and con-version into sedimentary rocks. These changes can besummarized in a schematic diagram that is designatedthe rock cycle (Fig. 2-12).

Although rocks are classified into groups that havehad similar origin, the identification of rocks is notbased on origin but on characteristics. For identifica-tion, it is necessary to know general appearance as wellas mineral composition and physical properties. Tex-ture (size, shape, and arrangement of constituent min-erals) and mineral composition are essential for identi-fication. Inferences regarding the origin of the rock arebased on geologic observations and experimentation.

Rocks � 41

FIGURE 2-12 Geologic processes actcontinuously on Earth to change onetype of rock into another.

Igneous Rocks

Igneous rocks constitute over 90 percent by volume ofthe Earth’s crust, although their great abundance maygo unnoticed because they are extensively covered bysedimentary rocks. Much of our mountain scenery issculpted in igneous rocks. Recent volcanic activity (Fig.2-13) provides an often spectacular reminder of thefiery processes that produce igneous rocks (the wordigneous comes from the Latin ignis, meaning “fire”). Itis an appropriate name for rocks that develop fromcooling masses of molten material derived from excep-tionally hot parts of the Earth’s interior. Magma is theterm used to describe this mixture of molten silicatesand gases while it is still beneath the surface. If it shouldpenetrate to the surface, it becomes lava (Fig. 2-14).

COOLING HISTORY OF IGNEOUS ROCKS Igneous rocksthat formed from magma that had penetrated intoother rocks and solidified before reaching the surfaceare termed intrusive or plutonic igneous rocks. Verylarge masses of such rocks are sometimes called plu-tons. Their exposure at the Earth’s surface results fromcrustal uplift and erosional removal of overlying rocks.In contrast, extrusive or volcanic igneous rocks formfrom melts that have reached the Earth’s surface. Thisgroup includes rocks formed from lava erupted fromvolcanoes or lava that has welled out of fissures.

The grain size of igneous rocks is an index to theirhistory of cooling (Fig. 2-15). Magmas deep within theEarth lose heat slowly and retain water. Water tends to

inhibit formation of myriads of crystal nuclei. Thus,there is time and space for the growth of larger crystalsaround fewer nuclei. In typical intrusive rocks such asgranite, diorite, and gabbro, the intergrown crystalsare large enough to be seen readily without magnifica-tion. In contrast, extrusive igneous rocks have a finertexture in which crystals are too small to be seen withthe unaided eye. The structure of such rocks reflectssudden chilling of molten silicates as they were ejectedat the surface of the Earth. When a lava is extruded,there is not enough time for the growth of large crys-tals. Furthermore, water from the melt is quickly lostto the atmosphere, and without water, many tiny crys-tals form rather than fewer crystals that can grow to alarger size.

One such extrusive rock is basalt, composed of fer-romagnesian minerals and tiny rectangular grains ofplagioclase feldspars. Obsidian is an extrusive rockthat cooled so rapidly that there was insufficient timefor crystallization; the melt therefore froze into a glass(Fig. 2-16). Obsidian forms from lavas that have lostmost of their dissolved gases. If the lava still containsdissolved gases, bubbles can be released and form cavi-ties called vesicles. Pumice is a vesicular glass. In asense, it is actually a solidified silicate froth. Tuff is avolcanic rock composed of consolidated ash.

If coarsely crystalline igneous rocks indicate slowcooling and finely crystalline ones indicate rapid cool-ing, then what would be the cooling history of a rockwith large crystals immersed in a very fine-grained ma-trix? Such rocks are said to have porphyritic texture(Fig. 2-15C). The large crystals (phenocrysts) wereformed slowly, deep within the Earth, and were thenswept upward and incorporated into the lava as it hard-ened at the surface.


FIGURE 2-13 A stream of molten lava and a firefountain pour from Pu’u’O’o vent, Hawaii, 1986. Thelava solidifies to form an extrusive igneous rock. (J. D.Griggs, U.S. Geological Survey.)

FIGURE 2-14 Front of a lava flow advancing over anolder flow, Kilauea Volcano, Hawaii, February 1990.The advancing flow is breaking into a jagged, rough kind oflava termed aa. The underlying older flow exhibits the ropytexture of pahoehoe lava. (Courtesy of J. Plaut.)

COMPOSITION OF IGNEOUS ROCKS Both texture andcomposition are used in naming and classifying ig-neous rocks (Fig. 2-17). Granite, for example, refers toa coarsely textured rock composed largely of feldsparand quartz. Rhyolite has the same composition as gran-ite but has a very fine-grained texture (Fig. 2-18).

The minerals in an igneous rock reflect the propor-tions of such elements as silicon, oxygen, aluminum,calcium, iron, magnesium, sodium, and potassium pres-ent in the magma from which the rock was formed.These eight elements combine in various ways to formfeldspars, ferromagnesian minerals, micas, and quartz,which are the constituents of igneous rocks. If themagma is particularly rich in silica, for example, it islikely that quartz will be present in the rock formed bysolidification (crystallization) of the melt. In general,intrusive igneous rocks are richer in silica than are ex-trusive rocks. This observation is related to the fact thatgreater amounts of silica in a magma increase its viscos-ity (“thickness,” or resistance to flow). The thick liquidcannot readily work its way to the Earth’s surface.

As noted earlier, granite (Fig. 2-18A) is a silica-rich,relatively light-colored intrusive rock composed pri-marily of potassium feldspar, quartz, sodium plagio-clase, hornblende, and mica. It is derived from magmasso rich in silica that after all chemical linkages withmetallic atoms are satisfied, enough silicon and oxygenstill remain to form quartz grains. These form an inter-locking network with feldspars and micas to form thecrystalline texture of granite. Granodiorite is anotherquartz-bearing igneous rock in which plagioclase is thedominant feldspar mineral. Quartz-bearing rocks suchas granite and granodiorite are loosely termed graniticrocks.

Basalt (Fig. 2-19) is a fine-grained extrusive rockderived from a low-silica melt rich in iron and magne-sium. Because the silica percentage is low, quartzgrains are rarely seen in basalt. Basaltic lavas have lowviscosity and are able to flow for considerable dis-tances across the Earth’s surface before solidifying.Because of the presence of ferromagnesian mineralsand gray calcium plagioclase, low-silica rocks such asbasalt tend to be black, dark gray, or green.

CRYSTALLIZATION OF MAGMA Not all minerals formsimultaneously during the crystallization of a magma.For example, if the magma originally had a basaltic

Rocks � 43

FIGURE 2-15 Differences in the texture of igneousrocks. (A) A coarse-grained intrusive rock called gabbro.(B) A porphyry with large phenocrysts of potassiumfeldspar. It is used here in the ornamental posts outside St.Paul’s Cathedral in London. (C) A fine-grained igneousrock known as andesite. �? What is the origin of this type ofporphyritic texture? On the basis of color alone, what feldsparmineral is represented by the large phenocrysts?

(A)

(B)

(C)

composition, olivine, pyroxene, and calcium feldsparwill be among the earliest minerals to crystallize. Oftenthese first-formed crystals are larger and more perfectthan those that formed later because there was amplespace for growth and an abundance of elements to in-corporate in their crystal structure. The minerals thatcrystallize later must grow within the remaining spacesand thus tend to be smaller and less perfect. Also, min-erals enclosed in other minerals must have formed be-fore the enclosing mineral. When rocks are viewed mi-croscopically, these observations permit one todetermine the order in which specific minerals crystal-lized in a magma.

In the early 1900s an eminent geologist named N. L. Bowen studied the order in which minerals crys-tallize as a magma is slowly cooled. Bowen made artifi-cial magmas by melting powdered rock samples in a


FIGURE 2-16 The volcanic glass obsidian. �? How doesobsidian form?

FIGURE 2-17 Mineral composition, texture, and other properties of common igneousrocks. The abundance of a particular mineral in an igneous rock can be estimated from thethickness of its colored area beneath the rock name. The scale on the left side can be used toestimate the percentage of each mineral. For example, granite at about the midpoint on thefigure would be composed of about 26 percent quartz, 25 percent potassium feldspar, 31percent sodium-rich plagioclase, 12 percent amphibole, 5 percent biotite, and 1 percentmuscovite. �? What would be the composition of a midpoint basalt? What minerals might you expectto find in the porphyry shown in Figure 2-15B?

steel cylinder called a bomb. The bomb was heateduntil the powder melted and was then slowly cooled tothe temperature and pressure selected for the experi-ment. That temperature and pressure was maintainedfor a period of a few months so that some mineralscould crystallize within the melt. Thus, there would bea mixture of crystals and uncrystallized melt within thebomb. The bomb was then plunged into cold water oroil, causing the mixture to solidify. The melt solidified

as glass, and within the glass the crystals were pre-served. Bowen repeated the experiment many times,forming crystals at different temperatures beforequick-cooling them. By identifying the crystals thatformed at each temperature, he was able to determinethe order of crystallization of minerals in a coolingmagma. For a melt of basaltic composition, Bowenfound that olivine would crystallize first and at thehighest temperature. Pyroxene and calcium plagioclase

Rocks � 45

FIGURE 2-18 Granite and rhyolite. (A) Granite, a coarse-grained intrusive rock that hasthe same composition as its intrusive, fine-grained equivalent, rhyolite (B). The lighter-colored grains in the granite are potassium feldspar and quartz, whereas the black minerals arehornblende and biotite. These minerals are also present in the rhyolite but cannot be seenwith the unaided eye.

(A) (B)

FIGURE 2-19 Basalt. (A) A hand specimen of basalt. (B) A thin section of basalt viewed withpolarized light. Tabular crystals are plagioclase and brightly colored crystals are iron-magnesian silicates. Width of field of view is 2 mm.

(A) (B)

would form next, followed by hornblende, biotite, andmore sodium-rich feldspars. The order in which theminerals crystallized came to be called Bowen’s Reac-tion Series (Fig. 2-20). It is called a reaction series be-cause early formed crystals react with the melt to yield anew mineral further down in the series. For example,silica in the melt would react with olivine crystals by si-multaneously dissolving and precipitating the dissolvedcomponents to form pyroxene, and pyroxene crystalswould begin to grow in spaces between earlier formedcrystals. At still lower temperatures, the melt wouldsimilarly react with pyroxene to form hornblende, andsubsequently hornblende would react to form biotite.For the complete sequence to occur, however, the orig-inal magma must have sufficient magnesium, iron, andcalcium. If the melt is rich in silica but deficient in theseelements, the first-formed crystals might be biotite andsodic feldspars, followed by orthoclase, muscovite, andquartz. Thus, reaction is prevented if silica in the melt isexhausted in forming the early minerals. It is also pre-vented if minerals are removed from contact with themagma after they have crystallized.

On the right branch of Bowen’s chart, one can tracethe changes that occur as calcium-rich plagioclase re-acts with the magma to produce varieties of plagioclasethat are successively richer in sodium. Because the pla-gioclase minerals maintain the same basic crystal struc-ture but change continuously in their content of cal-cium and sodium, the right side of the diagram is calledthe continuous series. The left side of the diagram depictsreactions that result in minerals of distinctly differentstructure. It is therefore called the discontinuous series.The three minerals at the base of the chart do not reactwith the melt. By the time they crystallize, little liquidremains, and the residues of silicon, aluminum, oxy-gen, and potassium join to form orthoclase, muscovite,and quartz.

One can see evidence of the reactions describedabove when igneous rocks are examined microscopi-cally. One observes plagioclase crystals in which theinnermost layers or zones are more calcium-rich andthe outer zones more sodium-rich. Similarly, reactionrims of pyroxene can be observed around olivine, andrims of hornblende around pyroxene.


FIGURE 2-20 A depiction of Bowen’s Reaction Series. Note that the earliest minerals tocrystallize are olivine and calcium-rich plagioclase. As crystallization proceeds, each mineralreacts with the melt to form the mineral beneath it. �? What minerals are likely to form whenpyroxene crystals react with the remaining liquid magma? Are plagioclase crystals in a granite likely tobe of the sodium-rich or calcium-rich variety?

VOLCANIC ACTIVITY Although deciphering the his-tory of a granitic mass is certainly intellectually stimu-lating, it is unlikely to evoke the feelings of awe and ex-citement one experiences when viewing volcanicactivity. Volcanoes are basically vents in the Earth’ssurface through which hot gases and molten rock flowfrom the Earth’s interior. The extrusions may be quietor explosive. Quiet eruptions are exemplified by theHawaiian volcanoes and are frequently characterizedby truly enormous outpourings of low-viscosity lava(Fig. 2-21). The lava spreads widely to form the gentleslopes of a shield volcano. Explosive eruptions arecaused by the sudden release of molten rock, driven up-ward by large pockets of compressed steam and gases.Explosive eruptions can literally blow the volcaniccone to bits. The explosive eruption of Krakatoa in1883 was heard 5000 km away and was responsible forthe death of 36,000 people.

Water is an important cause of explosive volcanism.It accounts for the violence of many Pacific rim volca-noes, such as Washington’s Mount St. Helens. Thewater released by these volcanoes as steam and vapor isactually recycled sea water. At midocean ridges, seawater reacts with extruding basalts to form hydrous

minerals. These water-bearing minerals are then con-veyed by sea-floor spreading to subduction zones. Asthe crust descends into the subduction zone, water isreleased from the hydrous minerals. Pressures build inareas containing the trapped water until they becomesufficient to cause explosive release.

There are, of course, all degrees of volcanic activitybetween quiet and violently explosive. Perhaps in re-sponse to changes in the composition of the melt, somevolcanoes have even been known to shift from one typeof activity to another. Volcanic activity also includessuccessive outpourings of lava from great fissures toform lava plateaus that extend over thousands of squarekilometers. Such fiery floods produced the Columbiaand Snake River Plateau as well as the Deccan Plateauof India.

By far, the most abundant kind of volcanic rock isbasalt. It underlies the ocean basins, has been built intomidoceanic ridges, and has accumulated sufficiently insuch places as Hawaii and Iceland to have producedsubstantial land area.

How and at what depth did this great volume ofbasalt originate? To answer this question, it is neces-sary to refer briefly to a model of the Earth’s interiorthat has been formulated by the study of earthquakewaves. The model depicts the Earth’s basaltic crust as athin zone averaging about 6 kilometers thick and over-lying the mantle of denser olivine- and pyroxene-richrocks. The boundary between the crust and the mantleis recognized by an abrupt change in the velocity ofearthquake waves as they travel downward into theEarth. For many years geologists believed that basalticlavas originated from the lower part of the basalticcrust. However, several recent lines of evidence sug-gest that the basaltic lavas may have come from moltenpockets of upper mantle material. For example, pre-sent-day volcanic activity is closely associated withdeep earthquakes that occur within the mantle far be-neath the crust. It is likely that fractures produced bythese earthquakes could serve as passages for the escapeof molten material to the surface. A detailed study ofearthquake shocks from particular volcanic eruptionsin Hawaii indicates that the erupting lavas were derivedfrom pockets of molten material within the upper man-tle at depths of about 100 kilometers. A weak plasticzone (called the “low-velocity zone”) in the mantle ap-pears to represent the level at which the lavas origi-nated. The mechanism by which they developed iscalled partial melting. Partial melting (Fig. 2-22) isthat general process by which a rock subjected to hightemperature and pressure is partly melted and the liq-uid component is moved to another location. At thenew location, the separated liquid may solidify intorocks that have a different composition from the parentmass. The word “partial” in the expression partialmelting refers to the fact that some minerals melt at

Rocks � 47

FIGURE 2-21 A river of lava formed during the 1984dual eruption of the Mauna Loa and Kilauea volcanoes,Hawaii. (Courtesy of I. Duncan.)

lower temperatures than do others, and so for a timethe material being melted resembles a hot slush com-posed of liquid and still-solid crystals. The molten frac-tion is usually less dense than the solids from which itwas derived and thus tends to separate from the parentmass and work its way toward the surface. In this way,melts of basaltic composition separated from denserrocks of the upper mantle and eventually made theirway to the surface to form volcanoes.

Many complex and interrelated factors controlwhere in the mantle partial melting may occur andeven whether it will occur at all. Generally, heat in ex-cess of 1500�C is required, but the precise temperaturefor melting is also influenced by pressure and the watercontent of the rock. As pressure increases, the tempera-ture at which particular minerals melt also rises. Thus,a rock that would melt at 1000�C near the surface willnot melt in deeper zones of higher pressure until itreaches far greater temperatures. Water has an effectopposite that of pressure, for its presence will allow arock to start melting at lower temperatures and shal-

lower depths than it would have otherwise. Laboratoryexperiments indicate that the melting of “dry” mantlerock can occur at depths of about 350 kilometers butthat the presence of only a little water can cause partialmelting and yield basaltic liquid from depths as shallowas 100 kilometers.

Not all lavas found at the Earth’s surface arebasaltic. Volcanoes of the more explosive type that arelocated at the edge of continents around the Pacific andin the Mediterranean extrude a lava called andesite.Andesite contains more silica than basalt does, and itslava is more viscous. This greater resistance to flowcontributes to the gas containment that precedes ex-plosive volcanic activity. Andesites are intermediate insilica content between the rocks of the continentalcrust and those of the oceanic crust of the Earth.

Andesitic rocks may originate in more than one way.Some emplacements result from originally basalticmagmas in which minerals such as olivine and pyrox-ene form early and settle out, thus leaving the remain-ing melt relatively richer in silica. This process is calledfractional crystallization (Fig. 2-23). Other andesitesmay result from the melting of basaltic ocean crust andsiliceous marine sediments as they descend into hotzones of the mantle. The water-rich and silica-enriched melts of andesitic composition are able to risebuoyantly and erupt along volcanic island arcs. We willexamine this interesting idea more fully in Chapter 5,which deals with plate tectonics.

Sedimentary Rocks

Sedimentary rocks are rocks composed of consolidatedsediment—particles and chemical compounds that arethe product of weathering and erosion of any previously


FIGURE 2-22 A conceptual illustration of the processof partial melting of peridotite to form basalt. Withrelief of pressure at divergent plate boundaries, rocks of theupper mantle begin to melt and release liquid that onsolidification would form basalt. Basaltic lava extrudedalong divergent zones of tectonic plates is then movedlaterally in the course of sea-floor spreading.

FIGURE 2-23 An andesitic melt resulting fromfractional crystallization of a basaltic magma.

existing rock or soil. Components of sediments mayrange from large boulders to the molecules dissolved inwater. Sediment is deposited through such agents aswind, water, ice, or mineral-secreting organisms. Theloose sediment is converted to coherent solid rock byany of several processes: precipitation of a cementingmaterial around individual grains, compaction, or crys-tallization. These processes constitute lithification.

The most obvious feature of sedimentary rocks istheir occurrence in beds or layers called strata. Stratifica-tion is commonly the result of changes in the conditionsof deposition that cause materials of a somewhat differ-ent nature to be deposited for a period of time. For exam-ple, the velocity of a stream might decrease, causing par-ticles to settle out that might otherwise have stayed insuspension. In another situation, the kind of materialsbrought into a given depositional site by streams mightchange, and there would then be a corresponding changein composition of the accumulating layers.

Shale, sandstone, and carbonate rocks (such as lime-stone) constitute the most abundant sedimentaryrocks. Sandstones are composed of grains of quartz,feldspar, and other particles that are cemented or oth-erwise consolidated. Shale consists largely of very fineparticles of quartz and abundant clay. Shale is the mostabundant of sedimentary rocks. The carbonates arerocks formed when carbon dioxide in water combineswith oxides of calcium and magnesium.

DERIVATION OF SEDIMENTARY MATERIALS Sedimen-tary rocks must have originally come from the disin-tegration and decomposition of older rocks. Com-monly, the older rocks are igneous; indeed, thesewere once the only rocks on Earth. It is therefore in-structive to review the manner in which the commoncomponents of sedimentary rocks might be derivedfrom an abundant kind of igneous rock, such as gran-odiorite (Fig. 2-24).

Rocks � 49

FIGURE 2-24 A conceptual diagram showing how the weathering of granitic rockyields quartz grains for quartz sandstone, clay for shale, and calcium for limestone. Ifweathering is not too severe, detrital grains of feldspar will also be included in sands andsandstones. For simplicity, minor mineral components are not included. �? What is the mostabundant, relatively insoluble product of the weathering of orthoclase and plagioclase? Which mineral ismost stable (least likely to experience dissolution during chemical weathering)?

Consider first the quartz in the granodiorite. Quartzwill persist almost unchanged during weathering. It isone of the most chemically stable of all the commonsilicate materials. As the parent rock is gradually de-composed, quartz grains tend to be washed out andcarried away to be deposited as sand that will one daybecome sandstone (Fig. 2-25).

The feldspars decay more readily than quartz. Theyare primarily aluminum silicates of potassium, sodium,and calcium. In the weathering process, the last threeelements are largely dissolved and carried away by so-lutions (although some may remain in soils within clayminerals). Ultimately, they reach the sea, where theymay stay in solution, or they may be precipitated as lay-ers of sediment. Under suitable conditions, loss of CO2from ocean water can result in the precipitation of cal-cite. If large quantities of lake or sea water are evapo-rated, evaporites such as halite or gypsum may beformed. Of course, not all the feldspars and micas inthe granodiorite necessarily decay. Some may persist asdetrital grains that become incorporated into sand-stones and other sediments.

The most voluminous product of the weathering of feldspars is clay. As described earlier, potassium,

sodium, and calcium are largely dissolved duringweathering. Aluminum and silicon are left behind toform the chief ingredients of clay, which then finds itsway into the making of shales and claystones.

Biotite is another mineral in the granodiorite sourcerock. Decomposition of biotite, which is a potassium,magnesium, and iron aluminosilicate, yields solublepotassium and magnesium carbonates, small amountsof soluble silica, iron oxides, and clay. The iron oxidesserve to color many sedimentary rocks in shades ofbrown and red.

VARIETY AMONG SEDIMENTARY ROCKS Sedimentaryrocks are classified according to their composition andtexture. The term texture refers to the size and shape ofthe individual grains and to their arrangement in therock. A rock that has a clastic texture is composed ofgrains and broken fragments (clasts) of pre-existingminerals, rocks, and fossils. Other sedimentary rocksare composed of a network of intergrown crystals andtherefore have a crystalline texture.

Clastic Rocks. The fragments of pre-existing materi-als that compose clastic sedimentary rocks range insize from huge boulders to microscopic particles. Par-ticle size is particularly useful in classifying theserocks, which include conglomerates, sandstones, silt-stones, and shales. Conglomerate (Fig. 2-26) is com-posed of water-worn, rounded particles larger than 2 mm in diameter. The term breccia is reserved forclastic rocks composed of fragments that are angular(Fig. 2-27) but similar in size to those of conglomer-ates. In sandstones, grains range between 1/16 mmand 2 mm in diameter (Table 3-1). The varieties ofsandstone are then subdivided mainly according tocomposition. Siltstones are finer than sandstones(1/16 mm to 1/256 mm). Shales may contain abun-dant clay minerals, which are flaky minerals that align


FIGURE 2-25 Exposure of weathered granodiorite,showing an accumulation of quartz grains and partiallydecomposed feldspar resulting from the disintegrationof the parent rock. �? In addition to the quartz and feldspar,what other weathering product is present in the debris beneaththe granodiorite?

FIGURE 2-26 Conglomerate. The pebbles in thisparticular sample are chert in a carbonate matrix. Thelargest pebbles are about 2.5 cm in diameter. �? How doconglomerates differ from breccias?

parallel to bedding planes. As a result, shales charac-teristically split into thin slabs parallel to beddingplanes. This property is termed fissility. Rocks lack-ing fissility but composed of clay-sized particles arecalled claystones or mudstones.

Most clastic sedimentary rocks are composed ofparticles of minerals like quartz, feldspar, and mica. Be-cause these are all silicate minerals, such rocks aretermed siliciclastics. Sandstones, shales, and mostconglomerates are siliciclastics. Approximately 75 per-cent of all sedimentary rocks are siliciclastics.

Carbonate Rocks. The minerals that compose carbon-ate rocks are primarily calcite, aragonite, anddolomite. Calcite (CaCO3) and aragonite (CaCO3)have the same chemical composition but differ in crys-tal form. (Aragonite is relatively unstable and is usuallyconverted to calcite.) Calcite is the predominant min-eral in limestone. Dolomite—CaMg(CO3)2—predom-inates in the rock dolomite. (Here both mineral androck bear the same name.) Carbonate rocks usually alsocontain variable amounts of impurities, including ironoxide, clay, and particles of sand and silt swept into thedepositional environment by currents.

Limestones The most abundant limestones are ofmarine origin and have formed as a result of precipita-tion of calcite or aragonite by organisms and the incor-poration of skeletons of those organisms into sedimen-tary deposits. Inorganic precipitation of carbonateminerals may also form deposits of limestone. The im-portance of this process is questionable, however, be-cause the precipitation is nearly always closely associ-ated with photosynthetic and respiratory activities oforganisms or with the release of tiny particles of arago-nite upon the decay of green algae. Strictly speaking, itappears that very few marine limestones are the resultof direct chemical precipitation.

After the calcium carbonate has accumulated, it be-comes recrystallized or otherwise consolidated into in-durated rock that may be variously colored-from white,through tints of brown, to gray. Limestones tend to bewell stratified (Fig. 2-28), frequently contain nodulesand inclusions of chert, and are often highly fossiliferous(containing fossils). The rock may range in texture fromcoarsely granular to very fine-grained and aphanitic.

Rocks � 51

FIGURE 2-27 A breccia (bed across center) composedof angular fragments or clasts of Precambrianmetamorphic rock (quartzite). Mosaic Canyon, DeathValley, California. (Photograph by R.F. Dymek.)

FIGURE 2-28 Roadcuts exposinglimestone strata are familiar totravelers on interstate highwaysthroughout the Mississippi Valleyregion. In this roadcut near HouseSprings, Missouri, beds dip about35�. The beds are the limb of amonocline (a fold with only onelimb).

In general, limestones consist of one of a combina-tion of textural components such as micrite, carbon-ate clasts, oölites, or carbonate spar. Micrite is excep-tionally fine-grained carbonate mud. Carbonateclasts are sand- or gravel-sized pieces of carbonate.The most common clasts are either bioclasts (skeletalfragments of marine invertebrates) or oöids (Fig. 2-29), which are spherical grains formed by the pre-

cipitation of carbonate around a nucleus. Sparry car-bonate is a clear, crystalline carbonate that is nor-mally deposited between the clasts as a cement or hasdeveloped by replacement of calcite. These texturalcategories of limestone as seen through the micro-scope are shown in Figure 2-30. They permit classifi-cation of particular samples as micritic limestone,clastic limestone, oölitic limestone, or sparry (crys-talline) limestone.

There are many varieties of limestone. Chalk is asoft, porous variety that is composed largely of calcare-ous, microscopic skeletal remains of marine planktonic(floating) animals and plants. Lithographic limestoneis a dense, micritic limestone once widely used as anetching surface in printing illustrations. Some lime-stones consist almost entirely of the skeletal remains ofreef corals and other frequently skeletonized marineinvertebrates.

Dolomite As we have already noted, dolomite is arock composed of the calcium-magnesium mineral ofthe same name. Like limestone, it occurs in extensivestratified sequences, and it is not easily distinguish-able from limestone. The usual field test for distin-guishing dolomite from limestone is to apply cold, di-lute hydrochloric acid. Unlike limestone, which


E N R I C H M E N T

Glass from Sand

Glass, a product formed by melting and cooling quartz sandand minor amounts of certain other common materials, isso familiar to us that we sometimes forget its importance.In fact, it would be difficult to imagine a world withoutglass. There would be no goblets, bottles, mirrors, or win-dows; no glass laboratory retorts, tubes, or beakers; no elec-tric light bulbs or neon signs; no glass lenses for cameras,microscopes, telescopes, or motion picture projectors; andno fiber optics for use in viewing the inside of the humanbody. We see glass used everywhere in our buildings, elec-tronic equipment, and medical devices. We not only see it,we see through it.

Glass has a long and colorful history. Natural glass, suchas obsidian, was used by prehistoric humans to make ar-rowheads and knife blades. Human-made glass was manu-factured by Egyptians as long ago as 3000 B.C. At Ur inMesopotamia, glass beads nearly 4500 years old have beenfound in archaeologic excavations. The Greeks and Romanslearned the art of making glass ornaments, bottles, vases,and trinkets that were prized throughout the ancient world.During the Middle Ages, Europeans produced beautifulstained-glass windows for cathedrals. Blood-red stainedglass was made by adding copper compounds to the moltensilica. The Europeans knew that cobalt gave the glass arich, deep-blue color; manganese turned it purple; and an-timony provided golden yellows. Iron oxides were added tocolor glass green or brown.

Today, the sands used in making glass have a silica con-tent as high as 95.0 to 99.8 percent. Sources of such ex-ceptionally pure sands are the Ordovician St. Peter Sand-stone of Missouri and Illinois and the sandstone membersof the Devonian Oriskany Formation of West Virginia andPennsylvania. These clean, pure sandstones were de-posited in shallow marine environments where wave actioncould remove clay impurities and concentrate grains ofsand-sized quartz. Both sandstones are composed of grainsthat have been eroded from older sandstones, and this re-working has contributed to their extraordinary purity.

In the manufacture of window glass, the mixture pre-pared for melting consists of about 72 percent silica fromquartz grains. Certain metallic oxides, such as soda (Na2O)and lime (CaO), serve to lower the temperature required formelting. That temperature for soda-lime-silica glass is 600to 700�C. Once molten, the batch (as it is called) is cooled,poured, rolled, or blown into the shapes desired. Com-pounds of boron and aluminum can be added to provideheat-resistant glass, as in Pyrex� cookware. Fine cut glassand so-called crystal are a silica-lead-soda glass known notonly for its brilliance but for its musical tone as well. Byvarying the kinds and amounts of metallic oxides, glass canbe produced for a great variety of special uses and prod-ucts. There is little doubt that it will always be an essentialpart of life in this age of technology.

FIGURE 2-29 Oöids in an oölitic limestone. (CopyrightWilliam E. Ferguson.)

bubbles readily, dolomite effervesces only slightly, ifat all.

The origin of dolomites is somewhat problematic.The mineral dolomite is not secreted by organismsduring shell building. Direct precipitation from seawater does not normally occur today, except in a fewenvironments where the sediment is steeped in ab-normally saline water. Such an origin is not consid-ered adequate to explain the thick sequences ofdolomitic rock commonly found in the geologicrecord. The most widely believed theory for the ori-gin of dolomites is that they result from partial re-placement of calcium by magnesium in the originalcalcareous sediment. However, it is not known forhow long or at what time in the history of the rock thisdolomitization occurs.

Other Sedimentary RocksChert We have previously mentioned a form of

microcrystalline quartz called chert (SiO2), noting itsoccurrence as nodules in limestones (Fig. 2-31). Theorigin of these nodules is still being debated amongpetrologists, although the majority believe that theyform as replacements of carbonate sediment by silicadissolved in sea water trapped in the sediment. Somecherts occur in areally extensive layers and thus qualifyas monomineralic rocks. These so-called beddedcherts are thought to have formed from the accumula-tion of the siliceous remains of diatoms and radiolariaand from subsequent reorganization of the silica into amicrocrystalline quartz. Silica from the dissolution ofvolcanic ash is believed to enhance the process; indeed,many bedded cherts are found in association with ashbeds and submarine lava flows.

Evaporites As indicated by their name, evaporitesare chemically precipitated rocks that are formed as aresult of evaporation of saline water bodies. Onlyabout three percent of all sedimentary rocks consist ofevaporites. Evaporite sequences of strata are com-posed chiefly of such minerals as gypsum, anhydrite,

halite, and associated calcite and dolomite. Gypsum isa hydrous calcium sulfate, whereas anhydrite lackswater. Halite has the same composition as ordinarytable salt. The conditions required for precipitation ofthick sequences of marine evaporites include warm,relatively arid conditions and periodic addition of seawater to the evaporating marine basin. Evaporites,

Rocks � 53

FIGURE 2-30 Textures of limestones as seen in thin section under a microscope. (A)Aphanitic limestone or micrite. (B) Bioclastic limestone with fine-grained sparry calcite ascement. (C) Oölitic limestone. (D) Sparry or crystalline limestone.

FIGURE 2-31 White nodular chert in tan limestone.Fern Glen Formation, west of St. Louis, Missouri.

however, are also precipitated from inland lakes in aridregions such as the Basin and Range Province of theUnited States.

Coal Coal is a black or dark-brown combustiblerock formed during the biochemical and physical alter-ation of plant material that has accumulated in a swampor marshlike environment. For coal to form, the plantmatter must accumulate under water or be quicklyburied so that it does not have access to air. If this werenot the case, the carbon would be lost as oxygen com-bined with carbon from the plant matter to form car-bon dioxide (and water).

Metamorphic Rocks

Sir Charles Lyell recognized that igneous or sedimen-tary rocks, if subjected to high temperature, pressure,and the chemical action of solutions and gases, can bealtered to quite different kinds of rocks. Lyell em-ployed the term metamorphism (from the Latin meta-morphosis, meaning “change of form”) to describe thisprocess. It is still used today to describe alterations inrocks brought about by physical or chemical changes inthe environment that are intermediate between thosethat result in igneous rocks and those that produce sed-imentary rocks. Any previously existing rock may beconverted to a metamorphic rock, and the changes pri-marily involve recrystallization of minerals in the rockwhile it remains in the solid state. In the process of re-crystallization, the textural characteristics of the parentrock may be changed, while at the same time new min-erals develop that are stable under the new conditionsof pressure and temperature. New elements need notbe introduced; instead, those that are already presentare incorporated into different and often denser miner-als. Variations in heat and pressure may result in differ-ent kinds of metamorphic rocks, even from the sameparent material.

METAMORPHISM Alterations of rock immediately ad-jacent to igneous intrusions constitute contact meta-morphism (Fig. 2-32). The changes that occur in theintruded rock are largely the result of high temperaturesand the emanation of chemically active fluids that ac-company igneous intrusions. Such factors as the size ofthe magmatic body, its composition and fluidity, and thenature of the intruded rock also influence the kind anddegree of contact metamorphism. Important ore de-posits are commonly situated in metamorphosed rocksurrounding intrusives. Examples of such deposits in-clude magnetite and copper ores in metamorphic zonesaround granite intrusives in the Urals, central Asia, theAppalachian Mountains, Utah, and New Mexico.

Regional or dynamothermal metamorphism isa type of rock alteration that is areally extensive andoccurs under the conditions of great confining pres-sures and heat accompanying deep burial and moun-

tain building. In a subsequent chapter, we will discusshow rocks deposited in crustal troughs adjacent tocontinents may be compressed into mountain sys-tems and thus be regionally metamorphosed. Meta-morphic index minerals known to form under spe-cific temperature and pressure conditions are used todecipher the history of growth of these ancientmountainous regions, even when only the roots ofthe ranges remain. Figure 2-33 shows the tempera-tures at which certain metamorphic index mineralsform in rock that is being subjected to heat and pres-sure. Note that the mineral chlorite forms at temper-atures of 50�C to 300�C. The metamorphic indexmineral garnet forms at higher temperatures andpressures, and sillimanite indicates the highest levelof temperature and pressure.

KINDS OF METAMORPHIC ROCKS Because any rockcan be metamorphosed in a number of different ways,


FIGURE 2-32 The Granite Peak aureole, located 12miles east of Lincoln, Montana. An aureole is the zone ofcontact metamorphism surrounding an igneous intrusion.This metamorphic aureole is developed around a graniticstock that has intruded into dolomite country rock. Theintensity of metamorphism diminished outward from thegranite margin, and particular assemblages of metamorphicminerals occur within each contact metamorphic zone.(Simplified from Melson, W. G. 1966. Am. Mineralogist51:404, Fig. 1.).

there are hundreds of different kinds of metamorphicrocks. However, we need only consider the most abun-dant kinds. It is convenient to divide metamorphicrocks into two groups based on the presence or absenceof foliation. Foliation is laminated structure in a meta-morphic rock resulting from a parallel alignment ofplaty or elongate mineral grains.

Foliated Metamorphic RocksSlate In slate, the foliation is microscopic and

caused by the parallel alignment of minute flakes ofsilicates such as mica. The planes of foliation are quitesmooth, and the rock may be split along these planesof “slaty cleavage.” The planes of foliation may lie atany angle to the bedding in the parent rock, and thischaracteristic helps to differentiate slate from denseshale. Slate is derived from the regional metamor-

phism of shale. As heat and pressure are applied to theclay minerals in shale, they are converted to chloriteand mica, two platy minerals that contribute to therock’s fissility.

Phyllite The texture in phyllite is also very fine, al-though some grains of mica, chlorite, garnet, or quartzmay be visible. Phyllite surfaces often develop a wrin-kled aspect and are more lustrous than slate. Phylliterepresents an intermediate degree of metamorphismbetween slate and schist. The parent rocks are com-monly shale or slate.

Schist The platy or needlelike minerals in schisthave grown sufficiently large to be visible to the un-aided eye; the minerals tend to be segregated into dis-tinct layers. Schists are named according to the mostconspicuous mineral present. Thus, there are micaschists (Fig. 2-34), amphibole schists, chlorite schists,

Rocks � 55

FIGURE 2-33 Changes in minerals that will develop during the progressivemetamorphism of shale. As the parent rock is subjected to low-grade metamorphism,chlorite and muscovite develop. The shale is metamorphosed to slate and then phyllite. Withincreasing temperature and pressure, muscovite will be joined by biotite, garnet, andstaurolite. As temperatures approach the level of high-grade metamorphism, kyanite andsillimanite appear. Beyond 800A7C, the rock may be melted.

and many others. Shales are the usual parent rocks forschists, although some schists are derived from fine-grained volcanic rocks.

Gneiss This is a coarse-grained (Fig. 2-35), evenlygranular rock. Foliation results from segregation ofminerals into bands rich in quartz, feldspar, biotite, oramphibole (Fig. 2-36). Foliation is coarse and appearsless distinct than in schist. High-silica igneous rocksand sandstones are the usual parent rocks for gneisses.

Nonfoliated Metamorphic RocksMarble A fine to coarsely crystalline rock,

marble (Fig. 2-37) is composed of calcite or dolomiteand therefore is relatively soft. (It can be scratched withsteel.) Marble is derived from limestone or dolomite.

Quartzite A fine-grained, often sugary-texturedrock, quartzite (Fig. 2-38) is composed of intergrownquartz and therefore is very hard. The rock will breakthrough, rather than around, constituent grains, andmay be any color. Quartzite is derived from quartzsandstone.

Greenstone A dark-green rock, greenstone has atexture so fine that mineral components, except forscattered larger crystals, cannot be seen without mag-nification. It is derived by the low-grade metamor-phism of low-silica volcanic rocks such as basalt.

Hornfels This very hard, fine-grained rock is oftenstudded with small crystals of mica and garnet that haveno preferred orientation. Hornfels may form from shaleor other fine-grained rocks that are intensely heated attheir contact with intrusive igneous bodies.

THE HISTORICAL SIGNIFICANCE OF METAMORPHICROCKS We have noted that the conditions for meta-morphism are developed in regions that have been sub-jected to intense compressional deformation. Such re-gions of the Earth’s crust either now have, or once had,great mountain ranges. Thus, where large tracts oflow-lying metamorphic terrane are exposed at theEarth’s surface, geologists conclude that crustal upliftand long periods of erosion leveled the mountains.


FIGURE 2-34 Steeply inclined mica schists splitting apart along foliation planes. GreatSmoky Mountain National Park, North Carolina-Tennessee.

FIGURE 2-35 Coarse (gneissic) foliation developed ina quartz-feldspar-biotite gneiss.

Metamorphic rock exposures at many localities acrossthe eastern half of Canada represent the truncatedfoundations of ancient mountain systems.

From studies of the mineralogic composition ofmetamorphic rocks, it is often possible for geologists toreconstruct the conditions under which the rocks werealtered and then to make inferences about the direc-tions of compressional forces, pressures, temperatures,and the nature of parent rocks. Investigators are aidedin these studies by the knowledge that specific meta-morphic minerals form and are stable within finite lim-its of temperature and pressure. Maps of metamorphicfacies, or zones of rocks that formed under specificconditions, can be constructed. Commonly, such mapsdelineate broad bands of metamorphic rocks, each ofwhich formed under sequentially more intense condi-tions of pressure and temperature.

Imagine a terrane that was once underlain by a thicksequence of calcareous shales, was subjected to compres-sion to produce mountains, and then experienced loss of

those mountains by erosion. One might then begin a tra-verse across the eroded surface on unmetamorphosedshales that were not involved in the mountain building.These shales would contain only unaltered sedimentaryminerals. Progressing farther toward the area of most in-tense metamorphism, one might see that the shales hadgiven way to slates bearing the green metamorphic min-eral chlorite. Still farther along the traverse, schists con-taining intermediate-grade metamorphic mineralswould appear. Finally, one might come upon coarsely fo-liated schists containing minerals that form only underhigh temperature and pressure. Although many charac-teristics of metamorphic rocks reveal the conditionsunder which they formed, they may also contain clues asto conditions on the Earth prior to metamorphism. Forexample, the discovery of a marble formed a billion yearsago by metamorphism of a limestone indicates that thecomposition of the Earth’s atmosphere and ocean 1 bil-lion years ago was similar to that in which carbonates canbe formed today.

Rocks � 57

FIGURE 2-36 An exposure of the Monson Gneiss of NewHampshire exbibiting plasticdeformation resulting fromintense pressure and heat. The dark bands are composed ofhornblende, biotite, and plagioclase,whereas the lighter bands are rich inquartz and white feldspar. (Courtesyof Robert Tucker.)

FIGURE 2-37 Marble. On close examination, one candiscern the lustrous cleavage surfaces of the calcite crystals.Width of specimen is about 9.0 cm

FIGURE 2-38 Hand specimen of an attractive greenquartzite. �? What simple physical test would convince you thatthis specimen was quartzite and not a green marble?


Rocks are the materials of which the Earth is composed.Rocks are themselves composed of minerals, and minerals, inturn, are constructed of chemical elements. The most com-mon elements in rocks are oxygen, silicon, aluminum, iron,calcium, sodium, potassium, and magnesium. Rocks andminerals are the materials from which geologists make inter-pretations of ancient environments and past geologic hap-penings. Silicate minerals are the most important of the rock-forming minerals. Quartz, mica, feldspar, hornblende,augite, and olivine are silicates that were initially crystallizedfrom molten rock. Clay, calcite, dolomite, gypsum, halite,and certain varieties of quartz (such as chert and flint) areformed by processes of weathering and precipitation or de-position at temperatures that prevail at the Earth’s surface.

Rocks are aggregates of minerals. Igneous rocks are thosethat have cooled and solidified from a molten silicate body. Ifsolidified beneath the Earth’s surface, the igneous rocks aretermed intrusive. Granite is an example. Igneous rocks, suchas those formed from the lavas flowing from volcanoes, aretermed extrusive. Basalt is an example. The texture of ig-neous rocks provides an index to their cooling history, in thatcoarse-grained varieties cooled slowly (and are mainly intru-sive), whereas fine-grained varieties cooled rapidly (and aremainly extrusive).

Melting and crystallization of rock materials in the labo-ratory have provided an understanding of the causes of ig-neous rock diversity. Among the mechanisms that provide

for a variety of rock types are fractional crystallization, asseen in Bowen’s Reaction Series, and partial melting, aprocess by which rock is incompletely melted and moltenfractions are removed.

Calcite is the main constituent of the carbonate rock lime-stone, and the mineral dolomite forms a carbonate rock thatis also called dolomite. In water bodies in which intense evap-oration occurs, evaporite minerals such as gypsum and halitemay be precipitated. Clastic sedimentary rocks are composedof fragments of weathered rock that have been transportedsome distance from their place of origin. Transported peb-bles and cobbles, for example, may be lithified to form therock called conglomerate, sand grains may form sandstone,and particles of clay may accumulate to form shale.

Metamorphism refers to all the processes by which rocks ofany kind undergo mineral and textural changes in the solidstate in response to changing physical and chemical condi-tions. The agents of metamorphism include heat, pressure,and chemically active solutions. The basic kinds of metamor-phism are contact and regional. Contact metamorphism oc-curs around the margins of bodies of molten rock. Unlike con-tact metamorphism, which may be relatively local, regionalmetamorphism occurs on a large (regional) scale and is usuallyassociated with mountain building. The rocks in the regionalbelts are well foliated and divisible into distinct metamorphiczones or facies, characterized by minerals that formed in re-sponse to particular conditions of pressure and temperature.

S U M M A R Y

1. What is a mineral? What characteristics of a true mineralsuch as quartz or feldspar would not be present in a piece ofglass?2. What are the eight most abundant elements found inrocks and minerals?3. Why are silicate minerals important in geology? Whichsilicates might one expect to find in granite? Which silicatesoccur in sedimentary rocks?4. Which igneous rock best approximates the compositionof the continental crust? The oceanic crust?5. With regard to the origin of igneous rocks, describe whatis meant by partial melting and fractional crystallization.6. What inferences can be drawn from the color of igneousrocks? From the grain size of igneous rocks?7. List the clastic sedimentary rocks in order of increasinglyfiner grain size.8. What mineral groups discussed in this chapter are par-ticularly common in sedimentary rocks?9. List the foliated metamorphic rocks in order of increas-ingly coarser foliation.10. If you were a Stone-Age (Paleolithic) human and had tochoose between limestone and chert as the material for aspearhead, which would you select? Why?11. Imagine that you are a geologist making a study of anarea in which there are tabular layers of basalt lying be-tween sedimentary strata. Some of the basalt layers repre-

sent lava that once flowed out upon the surface of theground. Others were formed when basaltic magma was injected along sedimentary bedding surfaces. The formerare called lava flows and the latter sills.

a. Which of the accompanying figures represents alava flow?b. Cite two lines of evidence that support your interpre-tation.c. In which of the figures is the age of the bed above thebasalt older than the basalt itself?

Q U E S T I O N S F O R R E V I E W A N D D I S C U S S I O N

Web Sites � 59

Arem, J. 1991. Rocks and Minerals. Phoenix: GeosciencePress.

Blatt, H. and Tracy, R. J. 1996. Petrology: Igneous, Sedimen-tary, and Metamorphic, 2d ed., New York: W. H. Freeman.

Decker, R. W. and Decker, B. B. 1991. Mountains of Fire: TheNature of Volcanoes. New York: Cambridge UniversityPress.

Fisher, R. V., Heiken, G., and Hulen, J. B. 1997. Volcanoes:Crucibles of Change. Princeton: Princeton UniversityPress.

Pough, F. H. 1960. A Field Guide to Rocks and Minerals. Cam-bridge, MA: Houghton Mifflin.

Raymond, L. A. 1995. Petrology. Dubuque, Iowa: Wm. C.Brown.

Robinson, G. W. 1994. Minerals: An Illustrated Exploration ofthe Dynamic World of Minerals and Their Properties. NewYork: Simon & Schuster.

Sigurdson, H. (ed) 2000. The Encyclopedia of Volcanoes. NewYork: Academic Press.

R E A D I N G S

The Earth Through Time Student Companion Web Site(www.wiley.com/college/levin) has online resources to helpyou expand your understanding of the topics in this chapter.Visit the Web Site to access the following:1. Illustrated course notes covering key concepts in each

chapter;2. Online quizzes that provide immediate feedback;

3. Links to chapter-specific topics on the web;4. Science news updates relating to recent developments in

Historical Geology;5. Web inquiry activities for further exploration;6. A glossary of terms;7. A Student Union with links to topics such as study skills,

writing and grammar, and citing electronic information.

W E B S I T E S

Pearly white, translucent crystals of selenite gypsum (CaSO4 2

Documents

Transcript of Pearly white, translucent crystals of selenite gypsum (CaSO4 2