isbn , custom edition, by frederick k. lutgens and edward ... · processes driven by heat from...

Foundations of Earth Science, Custom Edition, by Frederick K. Lutgens and Edward J. Tarbuck. Published by Pearson Custom Publishing. Copyright © 2008 by Pearson Education, Inc.

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Climbers scaling the vertical face of ElCapitan in Yosemite National Park,California. (Photo by Ron Niebruggel/Mira)

F O C U S O N L E A R N I N G

To assist you in learning the important concepts in thischapter, you will find it helpful to focus on the follow-ing questions:

1. What are the three groups of rocks and the geologic processesinvolved in the formation of each?

2. What two criteria are used to classify igneous rocks?

3. What are the two major types of weathering and the processesassociated with each?

4. What are the names and environments of formation for somecommon detrital and chemical sedimentary rocks?

5. What are the names, textures, and environments of formationfor some common metamorphic rocks?

Rocks: Materials of the SolidEarth

2C H A P T E R

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38 Chapter 2 Rocks: Materials of the Solid Earth

Why study rocks? You have already learned that rocksand minerals have great economic value. Further-more, all Earth processes in some way depend on

the properties of these basic materials. Events such as vol-canic eruptions, mountain building, weathering, erosion, andeven earthquakes involve rocks and minerals. Consequently,a basic knowledge of Earth materials is essential to under-standing Earth phenomena.

Every rock contains clues about the environment inwhich it formed. For example, some rocks are composed en-tirely of small shell fragments. This tells Earth scientists thatthe particles making up the rock originated in a shallow ma-rine environment. Other rocks contain clues that indicate theyformed from a volcanic eruption or deep in the Earth duringmountain building (Figure 2.1). Thus, rocks contain a wealthof information about events that have occurred over Earth’slong history.

We divide rocks into three groups, based on their modeof origin. The groups are igneous, sedimentary, and meta-morphic. Before examining each group, we will view the rockcycle, which depicts the interrelationships among these rockgroups.

Earth as a System: The Rock Cycle

Earth Materials� The Rock Cycle

Earth is a system. This means that our planet consists of manyinteracting parts that form a complex whole. Nowhere is thisidea better illustrated than when we examine the rock cycle(Figure 2.2). The rock cycle allows us to view many of the in-terrelationships among different parts of the Earth system. Ithelps us understand the origin of igneous, sedimentary, andmetamorphic rocks and to see that each type is linked to theothers by the processes that act upon and within the planet.Learn the rock cycle well; you will be examining its interre-lationships in greater detail throughout this chapter andmany other chapters as well.

The Basic Cycle

We begin at the top of Figure 2.2. Magma is molten materialthat forms inside Earth. Eventually, magma cools and solidifies.This process, called crystallization, may occur either beneath

Figure 2.1 Rocks contain information about the processes that produce them. This large exposure of igneous rocks located in the Sierra Nevada,California, was once a molten mass found deep within Earth. (Photo by Brian Bailey/Getty Images)

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When magmaor lava coolsand solidifies,igneous rock

forms.

Magma forms whenrock melts deepbeneath Earth’s

surface.

Uplift,weathering,

transportation,and

deposition

Sediment iscompacted and

cemented to formsedimentary rock.

When sedimentaryrock is buried deep

in the crust, heatand pressure (stress)cause it to become

metamorphicrock.

Magma

IgneousRock

SedimentSedimentary

Rock

MetamorphicRock

Heat andpressure

Weathering breaksdown rock that is transported and

deposited assediment.

Uplift,weathering,

transportation,and

deposition

Lava

Melting

Heat

Weathering/transport

Mel

ting

Crystallization

Met

amor

phis

m

Lithification

Figure 2.2 Viewed over long spans, rocks are constantly forming, changing, and reforming.The rock cycle helps us understand the origin of the three basic rock groups. Arrows representprocesses that link each group to the others.

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the surface or, following a volcanic eruption, at the surface. Ineither situation, the resulting rocks are called igneous rocks.

If igneous rocks are exposed at the surface, they will un-dergo weathering, in which the day-in and day-out influencesof the atmosphere slowly disintegrate and decompose rocks.The materials that result are often moved downslope by grav-ity before being picked up and transported by any of a numberof erosional agents, such as running water, glaciers, wind, orwaves. Eventually, these particles and dissolved substances,called sediment, are deposited. Although most sediment ulti-mately comes to rest in the ocean, other sites of deposition in-clude river floodplains, desert basins, swamps, and sand dunes.

Next, the sediments undergo lithification, a term mean-ing “conversion into rock.” Sediment is usually lithified intosedimentary rock when compacted by the weight of overly-ing layers or when cemented as percolating groundwater fillsthe pores with mineral matter.

If the resulting sedimentary rock is buried deep withinEarth and involved in the dynamics of mountain building orintruded by a mass of magma, it will be subjected to greatpressures and/or intense heat. The sedimentary rock willreact to the changing environment and turn into the thirdrock type, metamorphic rock. If metamorphic rock is sub-jected to still higher temperatures, it will melt, creatingmagma, which will eventually crystallize into igneous rock,starting the cycle all over again.

Although rocks may seem to be unchanging masses, therock cycle shows that they are not. The changes, however, taketime—great amounts of time. In addition, the rock cycle is op-erating all over the world, but in different stages. Today, newmagma is forming under the island of Hawaii, while the Col-orado Rockies are slowly being worn down by weatheringand erosion. Some of this weathered debris will eventually becarried to the Gulf of Mexico, where it will add to the alreadysubstantial mass of sediment that has accumulated there.

Alternative Paths

The paths shown in the basic cycle are not the only ones thatare possible. To the contrary, other paths are just as likely tobe followed as those described in the preceding section. Thesealternatives are indicated by the blue arrows in Figure 2.2.

Igneous rocks, rather than being exposed to weather-ing and erosion at Earth’s surface, may remain deeply buried.Eventually, these masses may be subjected to the strong com-pressional forces and high temperatures associated withmountain building. When this occurs, they are transformeddirectly into metamorphic rocks.

Metamorphic and sedimentary rocks, as well as sedi-ment, do not always remain buried. Rather, overlying layersmay be eroded away, exposing the once buried rock. Whenthis happens, the material is attacked by weathering process-es and turned into new raw materials for sedimentary rocks.

Where does the energy that drives Earth’s rock cyclecome from? Processes driven by heat from Earth’s interiorare responsible for forming igneous and metamorphic rocks.Weathering and the movement of weathered material are ex-ternal processes powered by energy from the Sun. Externalprocesses produce sedimentary rocks.

Igneous Rocks: “Formed by Fire”

Earth Materials� Igneous Rocks

In our discussion of the rock cycle, we pointed out that ig-neous rocks form as magma cools and crystallizes. But what ismagma and what is its source? Magma is molten rock gener-ated by partial melting of rocks in Earth’s mantle and in muchsmaller amounts, in the lower crust. This molten material con-sists mainly of the elements found in the silicate minerals. Sil-icon and oxygen are the main constituents in magma, withlesser amounts of aluminum, iron, calcium, sodium, potassi-um, magnesium, and others. Magma also contains some gases,particularly water vapor, which are confined within themagma body by the weight of the overlying rocks.

Once formed, a magma body buoyantly rises towardthe surface because it is less dense than the surroundingrocks. Occasionally molten rock reaches the surface, where itis called lava. Sometimes, lava is emitted as fountains thatare produced when escaping gases propel molten rock sky-ward. On other occasions, magma is explosively ejected froma vent, producing a spectacular eruption such as the 1980eruption of Mount St. Helens. However, most eruptions arenot violent; rather, volcanoes more often emit quiet outpour-ings of lava (Figure 2.3).

Igneous rocks that form when molten rock solidifies atthe surface are classified as extrusive or volcanic (after the firegod Vulcan). Extrusive igneous rocks are abundant in west-ern portions of the Americas, including the volcanic cones ofthe Cascade Range and the extensive lava flows of the Co-lumbia Plateau. In addition, many oceanic islands, typified bythe Hawaiian Islands, are composed almost entirely of vol-canic igneous rocks.

Most magma, however, loses its mobility before reach-ing the surface and eventually crystallizes at depth. Igneousrocks that form at depth are termed intrusive or plutonic(after Pluto, the god of the lower world in classical mythol-ogy). Intrusive igneous rocks would never be exposed atthe surface if portions of the crust were not uplifted and theoverlying rocks stripped away by erosion. Exposures of in-trusive igneous rocks occur in many places, includingMount Washington, New Hampshire; Stone Mountain,Georgia; the Black Hills of South Dakota; and Yosemite Na-tional Park, California.

Did You Know?During the catastrophic eruption of Vesuvius in A.D. 79, the en-

tire city of Pompeii (near Naples, Italy) was completely buried

by several meters of pumice and volcanic ash. Centuries passed,

and new towns sprang up around Vesuvius. It was not until

1595, during a construction project, that the remains of Pompeii

came to light. Today, thousands of tourists stroll amongst the ex-

cavated remains of Pompeii’s shops, taverns, and villas.

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MOLOKAI

MAUI

HAWAII

Haleakala

HualalaiMauna Loa Kilauea

Loihi

155°156°157°

19°

20°

21°

Mauna Kea

154°

Figure 2.3 Fluid basaltic lava moves down the slopes of Hawaii’s Kilauea Volcano. (Photo by G. Brad Lewis/Getty Images—Liaison)

Magma Crystallizes to Form Igneous Rocks

Magma is basically a very hot, thick fluid, but it also containssolids and gases. The solids are mineral crystals. The liquidportion of a magma body is composed of ions that move aboutfreely. However, as magma cools, the random movements ofthe ions slow, and the ions begin to arrange themselves into or-derly patterns. This process is called crystallization. Usually,the molten material does not all solidify at the same time.Rather, as it cools, numerous small crystals develop. In a sys-tematic fashion, ions are added to these centers of crystalgrowth. When the crystals grow large enough for their edgesto meet, their growth ceases for lack of space, and crystalliza-tion continues elsewhere. Eventually, all of the liquid is trans-formed into a solid mass of interlocking crystals.

The rate of cooling strongly influences crystal size. If amagma cools very slowly, relatively few centers of crystalgrowth develop. Slow cooling also allows ions to migrate overrelatively great distances. Consequently, slow cooling results inthe formation of large crystals. On the other hand, if cooling oc-curs quite rapidly, the ions lose their motion and quickly com-bine. This results in a large number of tiny crystals that allcompete for the available ions. Therefore, rapid cooling resultsin the formation of a solid mass of small intergrown crystals.

Thus, if a geologist encounters igneous rock containingcrystals large enough to be seen with the unaided eye, itmeans the molten rock from which it formed cooled quite

slowly. But if the crystals can be seen only with a microscope,the geologist knows that the magma cooled very quickly.

If the molten material is quenched almost instantly, thereis not sufficient time for the ions to arrange themselves into acrystalline network at all. Therefore, solids produced in thismanner consist of randomly distributed ions. Such rocks arecalled glass and are quite similar to ordinary manufacturedglass. “Instant” quenching occurs during violent volcaniceruptions that produce tiny shards of glass called volcanic ash.

In addition to the rate of cooling, the composition of amagma and the amount of dissolved gases influence crystal-lization. Because magmas differ in each of these aspects, thephysical appearance and mineral composition of igneous

Did You Know?During the Stone Age, volcanic glass (obsidian) was used for

making cutting tools. Today, scalpels made from obsidian are

being employed for delicate plastic surgery because they leave

less scarring. “The steel scalpel has a rough edge, where the

obsidian scalpel is smoother and sharper,” explains Lee Green,

MD, an associate professor at the University of Michigan Med-

ical School.41


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rocks vary widely. Nevertheless, it is possible to classify ig-neous rocks based on their texture and mineral composition.We will now look at both features.

Igneous Textures

Texture describes the overall appearance of an igneous rock,based on the size and arrangement of its interlocking crystals.Texture is a very important characteristic, because it revealsa great deal about the environment in which the rock formed.You learned that rapid cooling produces small crystals,whereas very slow cooling produces much larger crystals. Asyou might expect, the rate of cooling is slow in magma cham-bers lying deep within the crust, whereas a thin layer of lavaextruded upon Earth’s surface may chill to form solid rockin a matter of hours. Small molten blobs ejected into the airduring a violent eruption can solidify almost instantly.

Igneous rocks that form rapidly at the surface or assmall masses within the upper crust have a fine-grained tex-ture, with the individual crystals too small to be seen withthe unaided eye (Figure 2.4A). Common in many fine-grainedigneous rocks are voids, called vesicles, left by gas bubblesthat formed as the lava solidified (Figure 2.5).

When large masses of magma solidify far below the sur-face, they form igneous rocks that exhibit a coarse-grained tex-ture. These coarse-grained rocks have the appearance of a massof intergrown crystals, which are roughly equal in size andlarge enough that the individual minerals can be identifiedwith the unaided eye. Granite is a classic example (Figure 2.4B).

A large mass of magma located at depth may requiretens of thousands, even millions, of years to solidify. Be-cause all materials within a magma do not crystallize at thesame rate or at the same time during cooling, it is possiblefor some crystals to become quite large before others evenstart to form. If magma that already contains some largecrystals suddenly erupts at the surface, the remainingmolten portion of the lava would cool quickly. The resultingrock, which has large crystals embedded in a matrix ofsmaller crystals, is said to have a porphyritic texture (Figure2.4D).

During some volcanic eruptions, molten rock is ejectedinto the atmosphere, where it is quenched very quickly. Rapidcooling of this type may generate rock with a glassy texture(Figure 2.4C). Glass results when the ions do not have suffi-cient time to unite into an orderly crystalline structure. In ad-dition, melts that contain large amounts of silica aremore likely than melts with a low silica content to form rocksthat exhibit a glassy texture.

Obsidian, a common type of natural glass, is similar inappearance to a dark chunk of manufactured glass (Figure2.6). Another volcanic rock that often exhibits a glassy textureis pumice. Usually found with obsidian, pumice forms whenlarge amounts of gas escape from a melt to generate a gray,frothy mass (Figure 2.7). In some samples, the vesicles arequite noticeable, whereas in others, the pumice resemblesfine shards of intertwined glass. Because of the large volumeof air-filled voids, many samples of pumice will float inwater.

(SiO2)

A. Fine-grained C. Glassy (pumice)

B. Coarse-grained D. Porphyritic

Intrusiveigneousrocks

Extrusiveigneousrocks

Figure 2.4 Igneous rock textures. A. Igneous rocks that form at or near Earth’s surface cool quickly and often exhibit a fine-grained texture. B. Coarse-grained igneous rocks form when magma slowly crystallizes at depth. C. During a volcanic eruption in which silica-rich lava is ejectedinto the atmosphere, a frothy glass called pumice may form. D. A porphyritic texture results when magma that already contains some largecrystals migrates to a new location where the rate of cooling increases. The resulting rock consists of large crystals embedded within a matrix ofsmaller crystals. (Photos courtesy of E. J. Tarbuck)


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Igneous Rocks: “Formed by Fire” 43

5 cm

Figure 2.5 Scoria is a volcanic rock that is vesicular. Vesicles form asgas bubbles escape near the top of a lava flow. (Photo fromGeoScience Resources/American Geological Institute)

Igneous Compositions

Igneous rocks are mainly composed of silicate minerals. Fur-thermore, the mineral makeup of a particular igneous rock is ul-timately determined by the chemical composition of the magmafrom which it crystallizes. Recall that magma is composed large-ly of the eight elements that are the major constituents of the sil-icate minerals. Chemical analysis shows that silicon and oxygen(usually expressed as the silica content of a magma) areby far the most abundant constituents of igneous rocks. Thesetwo elements, plus ions of aluminum (Al), calcium (Ca), sodi-um (Na), potassium (K), magnesium (Mg), and iron (Fe), makeup roughly 98 percent by weight of most magmas.

[SiO2]

As magma cools and solidifies, these elements combineto form two major groups of silicate minerals. The dark silicatesare rich in iron and/or magnesium and are relatively low insilica. Olivine, pyroxene, amphibole, and biotite mica are the com-mon dark silicate minerals of Earth’s crust. By contrast, thelight silicates contain greater amounts of potassium, sodium,and calcium rather than iron and magnesium. As a group,these minerals are richer in silica than the dark silicates. Thelight silicates include quartz, muscovite mica, and the mostabundant mineral group, the feldspars. The feldspars makeup at least 40 percent of most igneous rocks. Thus, in additionto feldspar, igneous rocks contain some combination of theother light and/or dark silicates listed earlier.

Classifying Igneous Rocks

Igneous rocks are classified by their texture and mineral com-position. Various igneous textures result from different cool-ing histories, while the mineral compositions are aconsequence of the chemical makeup of the parent magmaand the environment of crystallization.

Figure 2.6 Obsidian, a natural glass, was used by Native Americansfor making arrowheads and cutting tools. (Photo by E. J. Tarbuck;inset photo by Jeffrey Scovil)

2 cm

Figure 2.7 Pumice, a glassy rock, is very lightweight because itcontains numerous vesicles. (Inset photo by Chip Clark)

Did You Know?Quartz watches actually contain a quartz crystal to keep time.

Before quartz watches, timepieces used some sort of oscillating

mass or tuning fork. Cogs and wheels converted this mechanical

movement to the movement of the hand. It turns out that if volt-

age is applied to a quartz crystal, it will oscillate with a consis-

tency that is hundreds of times better for timing than a tuning

fork. Because of this property, and modern integrated-circuit

technology, quartz watches are now built so cheaply they are

sometimes given away in cereal boxes. Modern watches that

employ mechanical movements are very expensive indeed.


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Despite their great compositional diversity, igneousrocks can be divided into broad groups according to theirproportions of light and dark minerals. A general classifica-tion scheme based on texture and mineral composition is pro-vided in Figure 2.8.

Granitic (Felsic) Rocks Near one end of the continuum arerocks composed almost entirely of light-colored silicates—quartz and potassium feldspar. Igneous rocks in which theseare the dominant minerals have a granitic composition. Ge-ologists also refer to granitic rocks as being felsic, a term de-rived from feldspar and silica (quartz). In addition to quartzand feldspar, most granitic rocks contain about 10 percentdark silicate minerals, usually biotite mica and amphibole.Granitic rocks are rich in silica (about 70 percent) and aremajor constituents of the continental crust.

Granite is a coarse-grained igneous rock that formswhere large masses of magma slowly solidify at depth. Dur-ing episodes of mountain building, granite and related crys-talline rocks may be uplifted, whereupon the processes ofweathering and erosion strip away the overlying crust. PikesPeak in the Rockies, Mount Rushmore in the Black Hills,Stone Mountain in Georgia, and Yosemite National Park inthe Sierra Nevada are all areas where large quantities of gran-ite are exposed at the surface.

Granite is perhaps the best-known igneous rock (Figure2.9). This is partly because of its natural beauty, which is en-hanced when polished, and partly because of its abundance.Slabs of polished granite are commonly used for tombstonesand monuments and as building stones.

Rhyolite is the extrusive equivalent of granite and, likegranite, is composed essentially of the light-colored silicates

(Figure 2.9). This fact accounts for its color, which is usuallybuff to pink or light gray. Rhyolite is fine-grained and frequent-ly contains glass fragments and voids, indicating rapid coolingin a surface environment. In contrast to granite, which is wide-ly distributed as large plutonic masses, rhyolite deposits areless common and generally less voluminous. Yellowstone Parkis one well-known exception. Here rhyolite lava flows andthick ash deposits of similar composition are extensive.

Basaltic (Mafic) Rocks Rocks that contain substantialamounts of dark-colored silicate minerals (mainly pyroxene),and calcium-rich plagioclase feldspar are said to have abasaltic composition (Figure 2.9). Because basaltic rocks con-tain a high percentage of dark silicate minerals, geologistsalso refer to them as mafic (from magnesium and ferrum, theLatin name for iron). Because of their iron content, basalticrocks are typically darker and denser than granitic rocks.

Basalt is a very dark green to black fine-grained volcanicrock composed primarily of pyroxene, olivine, and plagio-clase feldspar. Basalt is the most common extrusive igneousrock. Many volcanic islands, such as the Hawaiian Islandsand Iceland, are composed mainly of basalt. Further, theupper layers of the oceanic crust consist of basalt. In the Unit-ed States, large portions of central Oregon and Washingtonwere the sites of extensive basaltic outpourings.

The coarse-grained, intrusive equivalent of basalt iscalled gabbro (Figure 2.9). Although gabbro is not commonlyexposed on the surface, it makes up a significant percentageof the oceanic crust.

Andesitic (Intermediate) Rocks As you can see in Figure 2.9,rocks with a composition between granitic and basaltic rocks are

0% to 25% 25% to 45% 45% to 85%Rock Color

(based on % of dark minerals)

Coarse-grained

Fine-grained

Porphyritic

Glassy

TEXTURE

“Porphyritic” precedes any of the above names whenever there areappreciable phenocrysts

Obsidian (compact glass)Pumice (frothy glass)

Peridotite

Uncommon

Diorite

Andesite

Granite

Rhyolite

QuartzPotassium feldspar

Sodium-richplagioclase feldspar

Gabbro

Basalt

ChemicalComposition

DominantMinerals

85% to 100%

Granitic(Felsic)

Andesitic(Intermediate) Ultramafic Basaltic

(Mafic)

AmphiboleSodium- andcalcium-rich

plagioclase feldspar

PyroxeneCalcium-rich

plagioclase feldspar

OlivinePyroxene

Komatiite(rare)

Figure 2.8 Classification of the major groups of igneous rocks based on their mineral composition and texture. Coarse-grainedrocks are plutonic, solidifying deep underground. Fine-grained rocks are volcanic, or solidify as shallow, thin plutons. Ultramafic rocksare dark, dense rocks, composed almost entirely of minerals containing iron and magnesium. Although relatively rare on Earth’ssurface, these rocks are believed to be major constituents of the upper mantle.


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Igneous Rocks: “Formed by Fire” 45

said to have an andesitic or intermediate composition afterthe common volcanic rock andesite. Andesitic rocks contain amixiture of both light- and dark-colored minerals, mainly am-phibole and plagioclase feldspar. This important category ofigneous rocks is associated with volcanic activity that is typi-cally confined to the margins of continents. When magma ofintermediate composition crystallizes at depth, it forms thecoarse-grained rock called diorite (Figure 2.9).

Ultramafic Rocks Another important igneous rock, peridotite,contains mostly the dark-colored minerals olivine and pyrox-ene and thus falls on the opposite side of the compositionalspectrum from granitic rocks (see Figure 2.8). Because peri-dotite is composed almost entirely of dark silicate minerals, itschemical composition is referred to as ultramafic. Althoughultramafic rocks are rare at Earth’s surface, peridotite is be-lieved to be the main constituent of the upper mantle.

How Different Igneous Rocks Form

Because a large variety of igneous rocks exist, it is logical to as-sume that an equally large variety of magmas must also exist.However, geologists have observed that a single volcano mayextrude lavas exhibiting quite different compositions. Data ofthis type led them to examine the possibility that magmamight change (evolve) and thus become the parent to a vari-ety of igneous rocks. To explore this idea, a pioneering inves-tigation into the crystallization of magma was carried out byN. L. Bowen in the first quarter of the twentieth century.

Bowen’s Reaction Series In a laboratory setting, Bowendemonstrated that unlike a pure compound, such as water,which solidifies at a specific temperature, magma with its di-verse chemistry crystallizes over a temperature range of atleast 200 degrees. Thus, as magma cools, certain mineralscrystallize first, at relatively high temperatures (top of Figure

2.10). At successively lower temperatures, other mineralscrystallize. This arrangement of minerals, shown in Figure2.10 became known as Bowen’s reaction series.

Bowen discovered that the first mineral to crystallizefrom a mass of magma is olivine. Further cooling results inthe formation of pyroxene, as well as plagioclase feldspar. Atintermediate temperatures the minerals amphibole and bi-otite begin to crystallize.

During the last stage of crystallization, after most of themagma has solidified, the minerals muscovite and potassi-um feldspar may form (Figure 2.10). Finally, quartz crystal-lizes from any remaining liquid. As a result, olivine is notusually found with quartz in the same igneous rock, becausequartz crystallizes at much lower temperatures than olivine.

Evidence that this highly idealized crystallization modelapproximates what can happen in nature comes from theanalysis of igneous rocks. In particular, we find that mineralsthat form in the same general temperature range on Bowen’sreaction series are found together in the same igneous rocks.For example, notice in Figure 2.10 that the minerals quartz,potassium feldspar, and muscovite, which are located in thesame region of Bowen’s diagram, are typically found togeth-er as major constituents of the igneous rock granite.

Magmatic Differentiation Bowen demonstrated that differentminerals crystallize at different temperatures. But how doBowen’s findings account for the great diversity of igneousrocks? During the crystallization process, the composition of themelt (the liquid portion of magma excluding the solid crystals)continually changes because it gradually becomes depleted inthose elements used to make the earlier formed minerals. Thisprocess, coupled with the fact that at one or more stages duringcrystallization, a separation of the solid and liquid componentsof magma can occur creates different mineral assemblages. Oneway this happens is called crystal settling. This process occurs

Intrusive(course-grained)

Diorite

Andesite

Granite

Rhyolite

Gabbro

Basalt

Granitic(Felsic)

Andesitic(Intermediate)

Basaltic(Mafic)

Extrusive(fine-grained)

Figure 2.9 Common igneous rocks. (Photos by E. J. Tarbuck)


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when the earlier formed minerals are denser (heavier) than theliquid portion and sink toward the bottom of the magma cham-ber, as shown in Figure 2.11. When the remaining melt solidi-fies—either in place or in another location if it migrates intofractures in the surrounding rocks—it will form a rock with achemical composition much different from the parent magma(Figure 2.11). The formation of one or more secondary magmasfrom a single parent magma is called magmatic differentiation.

At any stage in the evolution of a magma, the solid andliquid components can separate into two chemically distinctunits. Further, magmatic differentiation within the second-ary melt can generate additional chemically distinct fractions.Consequently, magmatic differentiation and separation of thesolid and liquid components at various stages of crystalliza-tion can produce several chemically diverse magmas and ul-timately a variety of igneous rocks.

Weathering of Rocks to Form Sediment

Earth Materials� Sedimentary Rocks

All materials are susceptible to weathering. Consider, for ex-ample, the synthetic rock we call concrete. A newly pouredconcrete sidewalk is smooth, but many years later, the samesidewalk will appear chipped, cracked, and rough, with peb-bles exposed at the surface. If a tree is nearby, its roots maygrow under the sidewalk, heaving and buckling the concrete.The same natural processes that eventually break apart a con-crete sidewalk also act to disintegrate natural rocks, regard-less of their type or strength.

Why does rock weather? Simply, weathering is the nat-ural response of Earth materials to a new environment. For in-stance, after millions of years of erosion, the rocks overlyinga large body of intrusive igneous rock may be removed. Thisexposes the igneous rock to a whole new environment at thesurface. This mass of crystalline rock, which formed deepbelow ground, where temperatures and pressures are high, isnow subjected to very different and comparatively hostilesurface conditions. In response, this rock mass will gradual-ly change until it is once again in equilibrium, or balance,with its new environment. Such transformation of rock iswhat we call weathering.

In the following sections, we will discuss the two kindsof weathering—mechanical and chemical. Mechanical weath-ering is the physical breaking up of rocks. Chemical weath-ering actually alters what a rock is, changing it into a differentsubstance. Although we will consider these two processesseparately, keep in mind that they usually work simultane-ously in nature. Furthermore, the activities of erosionalagents—wind, water, and glaciers—that transport weatheredrock particles are important. As these mobile agents moverock debris, they relentlessly disintegrate it further.

Mechanical Weathering of Rocks

When a rock undergoes mechanical weathering, it is brokeninto smaller and smaller pieces. Each piece retains the char-acteristics of the original material. The end result is manysmall pieces from a single large one. Figure 2.12 shows thatbreaking a rock into smaller pieces increases the surface areaavailable for chemical attack. An example is adding sugar towater. A chunk of rock candy will dissolve much more slow-

TemperatureRegimes

IgneousRock Types

High temperature(~1200°C)

Low temperature(~ 750°C)

Olivine

Pyroxene

Amphibole

Biotite mica

Discontinuous Series

of Crystallization

UltramaficCalcium-

rich

Plag

iocl

ase

feld

spar

Con

tinuo

us S

erie

s

of C

ryst

alliz

atio

n

Potassium feldspar

Muscovite mica

Quartz

Sodium-rich

+

+

Basaltic(Mafic)

Andesitic(Intermediate)

Granitic(Felsic)

Coo

ling

mag

ma

Bowen's Reaction Series

Figure 2.10 Bowen’s reaction series shows the sequence in which minerals crystallize from a magma. Compare this figure to themineral composition of the rock groups in Figure 2.8. Note that each rock group consists of minerals that crystallize at the same time.

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Weathering of Rocks to Form Sediment 47

Hostrock

Magmabody

A.

B.

C.

Crystallizationand settling

Crystallizationand settling

Time

Igneous activityproduces rocks havinga composition of the

initial magma

Crystallization andsettling changes thecomposition of the

remaining melt

Further magmaticdifferentiation results

in a more highlyevolved melt

Figure 2.11 Illustration of how a magma evolves as the earlierformed minerals (those richer in iron, magnesium, and calcium)crystallize and settle to the bottom of the magma chamber, leavingthe remaining melt richer in sodium, potassium, and silica A.Emplacement of a magma body and associated igneous activitygenerates rocks having a composition similar to that of the initialmagma. B. After a period of time, crystallization and settling changethe composition of the melt, while generating rocks having acomposition quite different from the original magma. C. Furthermagmatic differentiation results in another more highly evolved meltwith its associated rock types.

(SiO2).

ly than will an equal volume of sugar granules because of thevast difference in surface area. Hence, by breaking rocks intosmaller pieces, mechanical weathering increases the amountof surface area available for chemical weathering.

In nature, three important physical processes breakrocks into smaller fragments: frost wedging, expansion re-sulting from unloading, and biological activity.

Frost Wedging Alternate freezing and thawing of water isone of the most important processes of mechanical weather-ing. Water has the unique property of expanding about 9 per-cent when it freezes. This increase in volume occurs because,as ice forms, the water molecules arrange themselves into avery open crystalline structure. As a result, when waterfreezes, it expands and exerts a tremendous outward force.Here is everyday proof: Water in a car’s cooling system willfreeze in winter, expanding and cracking the engine block.This is why antifreeze is added; it lowers the temperature atwhich the solution freezes.

In nature, water works its way into every crack or voidin rock and, upon freezing, expands and enlarges the opening.After many freeze-thaw cycles, the rock is broken into pieces.This process is appropriately called frost wedging (Figure 2.13).Frost wedging is most pronounced in mountainous regionsin the middle latitudes where a daily freeze-thaw cycle oftenexists. Here, sections of rock are wedged loose and may tum-ble into large piles called talus or talus slopes that often form atthe base of steep rock outcrops (Figure 2.13).

Unloading When large masses of igneous rock are exposedby erosion, entire slabs begin to break loose, like the layers ofan onion. This sheeting is thought to occur because of the greatreduction in pressure when the overlying rock is erodedaway. Accompanying the unloading, the outer layers expandmore than the rock below and thus separate from the rockbody. Granite is particularly prone to sheeting.

Continued weathering eventually causes the slabs to sep-arate and spall, causing exfoliation domes. Excellent examples ofexfoliation domes include Stone Mountain, Georgia, and Lib-erty Cap Half Dome in Yosemite National Park (Figure 2.14).

Biological Activity Weathering is also accomplished by theactivities of organisms, including plants, burrowing animals,and humans. Plant roots in search of water grow into frac-tures, and as the roots grow, they wedge the rock apart(Figure 2.15). Burrowing animals further break down the rockby moving fresh material to the surface, where physical andchemical processes can more effectively attack it.

Chemical Weathering of Rocks

Chemical weathering alters the internal structure of miner-als by removing and/or adding elements. During this trans-formation, the original rock is altered into substances that arestable in the surface environment.

Water is the most important agent of chemical weather-ing. Oxygen dissolved in water will oxidize some materials.For example, when an iron nail is found in the soil, it willhave a coating of rust (iron oxide), and if the time of exposure


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4 square units �6 sides �1 cube �24 square units

1 square unit �6 sides �8 cubes �48 square units

.25 square unit �6 sides �64 cubes �96 square units

4 square units

1 squareunit

2

2

1.5

1.5

Figure 2.12 Chemical weathering can occur onlyto those portions of a rock that are exposed to theelements. Mechanical weathering breaks rock intosmaller and smaller pieces, thereby increasing thesurface area available for chemical attack.

has been long, the nail will be so weak that it can be brokenas easily as a toothpick. When rocks containing iron-rich min-erals (such as hornblende) oxidize, a yellow to reddish-brownrust will appear on the surface.

Carbon dioxide dissolved in water formscarbonic acid This is the same weak acid producedwhen soft drinks are carbonated. Rain dissolves some carbondioxide as it falls through the atmosphere, so normal rain-water is mildly acidic. Water in the soil also dissolves carbondioxide released by decaying organic matter. The result isthat acidic water is everywhere on Earth’s surface.

(H2CO3).(H2O)(CO2)

How does rock decompose when attacked by carbonicacid? Consider the weathering of the common igneous rock,granite. Recall that granite is composed mainly of quartz andpotassium feldspar. As the weak acid slowly reacts with crys-tals of potassium feldspar, potassium ions are displaced. Thisdestroys the mineral’s crystalline structure.

The most abundant products of the chemical break-down of feldspar are clay minerals. Because clay minerals arethe end product of chemical weathering, they are very stableunder surface conditions. Consequently, clay minerals makeup a high percentage of the inorganic material in soils.

Frost wedging

Talusslope

TalusslopeTalusslope

Figure 2.13 Frost wedging. Aswater freezes, it expands, exertinga force great enough to breakrock. When frost wedging occursin a setting such as this, the brokenrock fragments fall to the base ofthe cliff and create a cone-shapedaccumulation known as talus.(Photo by Tom & Susan Bean, Inc.)


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Weathering of Rocks to Form Sediment 49

In addition to the formation of clay minerals, some sil-ica is dissolved from the feldspar structure and is car-ried away by groundwater. The dissolved silica willeventually precipitate to produce a hard, dense sedimentaryrock (chert), fill pore spaces between mineral grains, or becarried to the ocean, where microscopic animals will buildsilica shells from it.

Quartz, the other main component of granite, is very re-sistant to chemical weathering. Because it is durable, quartzremains substantially unaltered when attacked by weak acid.As granite weathers, the feldspar crystals become dull andslowly turn to clay, releasing the once interlocked quartz grains,which still retain their fresh, glassy appearance. Although somequartz remains in the soil, much is transported to the sea andother sites, where it becomes sandy beaches and sand dunes.

To summarize, the chemical weathering of granite producesclay minerals along with potassium ions and silica, which entersinto solution. In addition, durable quartz grains are freed.

Table 2.1 lists the weathered products of some of the mostcommon silicate minerals. Remember that silicate mineralsmake up most of Earth’s crust and are composed primarily ofjust eight elements (see Figure 1.14, p. 26). When chemicallyweathered, the silicate minerals yield sodium, calcium, potas-sium, and magnesium ions. These may be used by plants or re-moved by groundwater. The element iron combines withoxygen to produce iron-oxide compounds that give soil a red-

(SiO2)

Deeppluton

Uplift anderosion

Expansionand

sheeting

A.

B.

C.

Confining pressure

Figure 2.14 Sheeting is caused by the expansion of crystalline rock as erosion removes the overlying material. When the deeply buried pluton (A)is exposed at the surface following uplift and erosion (B), the igneous mass fractures into thin slabs. The photo (C) is of the summit of Half Dome inYosemite National Park, California. It is an exfoliation dome and illustrates the onionlike layers created by sheeting. (Photo by Breck P. Kent)

Figure 2.15 Root wedging widens fractures in rocks and aids theprocess of mechanical weathering. (Photo by Tom Bean/DRK Photo)


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dish-brown or yellowish color. The three remaining elements—aluminum, silicon, and oxygen—join with water to produceclay minerals that become an important part of the soil. Ulti-mately, the products of weathering form the raw materials forbuilding sedimentary rocks, which we consider next.

Sedimentary Rocks: Compacted and Cemented Sediment

Earth Materials� Sedimentary Rocks

Recall the rock cycle, which shows the origin of sedimentaryrocks. Weathering begins the process. Next, gravity and ero-sional agents (running water, wind, waves, and glacial ice)remove the products of weathering and carry them to a newlocation where they are deposited. Usually, the particles arebroken down further during this transport phase. Followingdeposition, this sediment may become lithified, or “turned torock.” Commonly, compaction and cementation transform thesediment into solid sedimentary rock.

The word sedimentary indicates the nature of these rocks,for it is derived from the Latin sedimentum, which means “set-tling,” a reference to a solid material settling out of a fluid.Most sediment is deposited in this fashion. Weathered debrisis constantly being swept from bedrock and carried away bywater, ice, or wind. Eventually, the material is deposited inlakes, river valleys, seas, and countless other places. The par-ticles in a desert sand dune, the mud on the floor of a swamp,the gravels in a streambed, and even household dust are ex-amples of sediment produced by this never-ending process.

The weathering of bedrock and the transport and dep-osition of the weathering products are continuous. Therefore,sediment is found almost everywhere. As piles of sedimentaccumulate, the materials near the bottom are compacted bythe weight of the overlying layers. Over long periods, thesesediments are cemented together by mineral matter deposit-ed from water in the spaces between particles. This formssolid sedimentary rock.

Geologists estimate that sedimentary rocks account foronly about 5 percent (by volume) of Earth’s outer 16 kilome-ters (10 miles). However, the importance of this group of rocksis far greater than this percentage implies. If you sampled therocks exposed at Earth’s surface, you would find that thegreat majority are sedimentary (Figure 2.16). Indeed, about 75percent of all rock outcrops on the continents are sedimenta-ry. Therefore, we can think of sedimentary rocks as compris-ing a relatively thin and somewhat discontinuous layer in theuppermost portion of the crust. This makes sense becausesediment accumulates at the surface.

It is from sedimentary rocks that geologists reconstructmany details of Earth’s history. Because sediments are de-posited in a variety of different settings at the surface, therock layers that they eventually form hold many clues to pastsurface environments. They may also exhibit characteristicsthat allow geologists to decipher information about themethod and distance of sediment transport. Furthermore, itis sedimentary rocks that contain fossils, which are vital ev-idence in the study of the geologic past.

Finally, many sedimentary rocks are important econom-ically. Coal, which is burned to provide a significant portionof U.S. electrical energy, is classified as a sedimentary rock.Other major energy resources (petroleum and natural gas)occur in pores within sedimentary rocks. Other sedimentaryrocks are major sources of iron, aluminum, manganese, andfertilizer, plus numerous materials essential to the construc-tion industry.

Classifying Sedimentary Rocks

Materials accumulating as sediment have two principalsources. First, sediments may originate as solid particles fromweathered rocks, such as the igneous rocks earlier described.These particles are called detritus, and the sedimentary rocksthat they form are called detrital sedimentary rocks (Fig-ure 2.17).

The second major source of sediment is soluble materi-al produced largely by chemical weathering. When these dis-solved substances are precipitated back as solids, they arecalled chemical sediment, and they form chemical sedimenta-ry rocks. We will now look at detrital and chemical sedimen-tary rocks. (Figure 2.17)

Detrital Sedimentary Rocks Though a wide variety of min-erals and rock fragments may be found in detrital rocks, clayminerals and quartz dominate. As you learned earlier, clayminerals are the most abundant product of the chemicalweathering of silicate minerals, especially the feldspars.Quartz, on the other hand, is abundant because it is extremelydurable and very resistant to chemical weathering. Thus,when igneous rocks such as granite are weathered, individ-ual quartz grains are set free.

Geologists use particle size to distinguish among detri-tal sedimentary rocks. Figure 2.17 presents the four size cate-gories for particles making up detrital rocks. When gravel-sizeparticles predominate, the rock is called conglomerate if thesediment is rounded (Figure 2.18A) and breccia if the pieces

Table 2.1 Products of weathering

Original Weathers Released Mineral to Produce into Solution

Quartz Quartz grains Silica Feldspar Clay minerals Silica

Ions of potassium, sodium, and calcium

Hornblende Clay minerals Silica Iron minerals Ions of calcium and (limonite and magnesiumhematite)

Olivine Iron minerals Silica (limonite and Ions of magnesiumhematite)

(SiO2)

(SiO2)

(SiO2)(SiO2)

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are angular (Figure 2.18B). Angular fragments indicate thatthe particles were not transported very far from their sourceprior to deposition and so have not had corners and roughedges abraded. Sandstone is the name given rocks when sand-size grains prevail (Figure 2.18C). Shale, the most commonsedimentary rock, is made of very fine-grained sediment(Figure 2.18D). Siltstone, another rather fine-grained rock, issometimes difficult to differentiate from rocks such as shale,which are composed of even smaller clay-size sediment.

Particle size is not only a convenient method of divid-ing detrital rocks; the sizes of the component grains also pro-vide useful information about the environment in which thesediment was deposited. Currents of water or air sort the par-ticles by size. The stronger the current, the larger the particlesize carried. Gravels, for example, are moved by swiftly flow-ing rivers, rockslides, and glaciers. Less energy is required totransport sand; thus, it is common in windblown dunes, riverdeposits, and beaches. Because silts and clays settle veryslowly, accumulations of these materials are generally asso-ciated with the quiet waters of a lake, lagoon, swamp, or ma-rine environment.

Although detrital sedimentary rocks are classified byparticle size, in certain cases the mineral composition is alsopart of naming a rock. For example, most sandstones are pre-dominantly quartz-rich, and they are often referred to asquartz sandstone. In addition, rocks consisting of detrital sed-

iments are rarely composed of grains of just one size. Conse-quently, a rock containing quantities of both sand and silt canbe correctly classified as sandy siltstone or silty sandstone,depending on which particle size dominates.

Chemical Sedimentary Rocks In contrast to detrital rocks,which form from the solid products of weathering, chemicalsediments are derived from material that is carried in solutionto lakes and seas. This material does not remain dissolved inthe water indefinitely. When conditions are right, it precipi-tates to form chemical sediments. This precipitation mayoccur directly as the result of physical processes, or indirectlythrough life processes of water-dwelling organisms. Sedi-ment formed in this second way has a biochemical origin.

An example of a deposit resulting from physicalprocesses is the salt left behind as a body of saltwater evap-orates. In contrast, many water-dwelling animals and plantsextract dissolved mineral matter to form shells and other hard

Figure 2.16 Sedimentary rocks exposed in Canyonlands National Park, Utah. Sedimentary rocks occur in layers called strata. About 75 percent ofall rock outcrops on the continents are sedimentary rocks. (Photo by Jeff Gnass)

51

Did You Know?The most important and common material used for making

glass is silica, which is usually obtained from the quartz in

“clean,” well-sorted sandstones.


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parts. After the organisms die, their skeletons may accumu-late on the floor of a lake or ocean.

Limestone is the most abundant chemical sedimentaryrock. It is composed chiefly of the mineral calcite Ninety percent of limestone is biochemical sediment. The restprecipitates directly from seawater.

One easily identified biochemical limestone is coquina,a coarse rock composed of loosely cemented shells and shellfragments (Figure 2.19). Another less obvious but familiar ex-ample is chalk, a soft, porous rock made up almost entirely ofthe hard parts of microscopic organisms that are no largerthan the head of a pin (Figure 2.20).

Inorganic limestones form when chemical changes orhigh water temperatures increase the concentration of calci-um carbonate to the point that it precipitates. Travertine, thetype of limestone that decorates caverns, is one example.Groundwater is the source of travertine that is deposited incaves. As water drops reach the air in a cavern, some of thecarbon dioxide dissolved in the water escapes, causing calci-um carbonate to precipitate.

Dissolved silica precipitates to form varieties ofmicrocrystalline quartz (Figure 2.21). Sedimentary rocks com-posed of microcrystalline quartz include chert (light color), flint

(SiO2)

(CaCO3).

(dark), jasper (red), and agate (banded). These chemical sedi-mentary rocks may have either an inorganic or biochemical ori-gin, but the mode of origin is usually difficult to determine.

Very often, evaporation causes minerals to precipitatefrom water. Such minerals include halite, the chief compo-nent of rock salt, and gypsum, the main ingredient of rock gyp-sum. Both materials have significant commercial importance.Halite is familiar to everyone as the common salt used incooking and seasoning foods. Of course, it has many otheruses and has been considered important enough that peoplehave sought, traded, and fought over it for much of humanhistory. Gypsum is the basic ingredient of plaster of Paris.This material is used most extensively in the construction in-dustry for “drywall” and plaster.


ClasticTextureParticle Size Sediment Name Rock Name

Coarse(over 2 mm)

Gravel(Rounded particles)

Gravel(Angular particles)

Medium(1/16 to 2 mm)

Sand

(If abundant feldsparis present the rockis called Arkose)

Conglomerate

Breccia

Sandstone

Fine(1/16 to

1/256 mm)

Very fine(less than1/256 mm)

Mud

Mud

Siltstone

Shale

Coquina

Chalk

Chert (light colored)Flint (dark colored)

Rock Gypsum

Rock Salt

Bituminous Coal

Calcite, CaCO3

Quartz, SiO2

GypsumCaSO4•2H2O

Halite, NaCl

Altered plantfragments

Nonclastic:Fine to coarse

crystalline

CrystallineLimestone

Clastic: Visibleshells and shell

fragments looselycemented

Clastic: Various sizeshells and shell

fragments cementedwith calcite cement

Clastic: Microscopicshells and clay

Nonclastic: Very fine crystalline

Nonclastic: Fine tocoarse crystalline

Nonclastic: Fine tocoarse crystalline

Nonclastic:Fine-grained

organic matter

FossiliferousLimestone

Biochemical

Li

mestone

TextureComposition Rock Name

Detrital Sedimentary Rocks Chemical Sedimentary Rocks

Travertine

Figure 2.17 Identification of sedimentary rocks. Sedimentary rocks are divided into two major groups, detrital and chemical, basedon their source of sediment. The main criterion for naming detrital rocks is particle size, whereas the primary basis for distinguishingamong chemical rocks is their mineral composition.

Did You Know?Each year, about 30 percent of the world’s supply of salt is ex-

tracted from seawater. The seawater is pumped into ponds and

allowed to evaporate, leaving behind “artificial evaporites,”

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Sedimentary Rocks: Compacted and Cemented Sediment 53

In the geologic past, many areas that are now dry landwere covered by shallow arms of the sea that had only narrowconnections to the open ocean. Under these conditions, watercontinually moved into the bay to replace water lost by evap-oration. Eventually, the waters of the bay became saturatedand salt deposition began. Today, these arms of the sea aregone, and the remaining deposits are called evaporite deposits.

On a smaller scale, evaporite deposits can be seen insuch places as Death Valley, California. Here, following rainsor periods of snowmelt in the mountains, streams flow fromsurrounding mountains into an enclosed basin. As the waterevaporates, salt flats form from dissolved materials left be-hind as a white crust on the ground (Figure 2.22).

Coal is quite different from other chemical sedimentaryrocks. Unlike other rocks in this category, which are calcite-or silica-rich, coal is made mostly of organic matter. Close ex-amination of a piece of coal under a microscope or magnify-ing glass often reveals plant structures such as leaves, bark,and wood that have been chemically altered but are still iden-tifiable. This supports the conclusion that coal is the end prod-uct of the burial of large amounts of plant material overextended periods (Figure 2.23).

The initial stage in coal formation is the accumulation oflarge quantities of plant remains. However, special conditions

A. B.

C. D.

5 cm 5 cm

5 cm 5 cm

Figure 2.18 Common detrital sedimentary rocks. A. Conglomerate (rounded particles). B. Breccia (angular particles). C. Sandstone.D. Shale with plant fossil. (Photos by E. J. Tarbuck)

5 cm

Close up

Figure 2.19 This rock, called coquina, consists of shell fragments;therefore, it has a biochemical origin. (Photos by E. J. Tarbuck)


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are required for such accumulations, because dead plants nor-mally decompose when exposed to the atmosphere. An idealenvironment that allows for the buildup of plant material isa swamp. Because stagnant swamp water is oxygen-deficient,complete decay (oxidation) of the plant material is not possi-ble. At various times during Earth history, such environmentshave been common. Coal undergoes successive stages of for-mation. With each successive stage, higher temperatures andpressures drive off impurities and volatiles, as shown inFigure 2.23.

Lignite and bituminous coals are sedimentary rocks,but anthracite is a metamorphic rock. Anthracite forms whensedimentary layers are subjected to the folding and deforma-tion associated with mountain building.

In summary, we divide sedimentary rocks into twomajor groups: detrital and chemical. The main criterion forclassifying detrital rocks is particle size, whereas chemicalrocks are distinguished by their mineral composition. Thecategories presented here are more rigid than is the actualstate of nature. Many detrital sedimentary rocks are a mixtureof more than one particle size. Furthermore, many sedimen-tary rocks classified as chemical also contain at least smallquantities of detrital sediment, and practically all detritalrocks are cemented with material that was originally dis-solved in water.

Lithification of Sediment

Lithification refers to the processes by which sediments aretransformed into solid sedimentary rocks. One of the mostcommon processes is compaction. As sediments accumulatethrough time, the weight of overlying material compressesthe deeper sediments. As the grains are pressed closer andcloser, pore space is greatly reduced. For example, when claysare buried beneath several thousand meters of material, the

volume of the clay may be reduced as much as 40 percent.Compaction is most significant in fine-grained sedimentaryrocks such as shale, because sand and other coarse sedimentscompress little.

Cementation is another important means by which sed-iments are converted to sedimentary rock. The cementingmaterials are carried in solution by water percolating throughthe pore spaces between particles. Over time, the cement pre-cipitates onto the sediment grains, fills the open spaces, andjoins the particles. Calcite, silica, and iron oxide are the mostcommon cements. Identification of the cementing material issimple. Calcite cement will effervesce (fizz) with dilute hy-drochloric acid. Silica is the hardest cement and thus pro-duces the hardest sedimentary rocks. When a sedimentaryrock has an orange or red color, this usually means iron oxideis present.

Features of Sedimentary Rocks

Sedimentary rocks are particularly important evidence ofEarth’s long history. These rocks form at Earth’s surface, andas layer upon layer of sediment accumulates, each recordsthe nature of the environment at the time the sediment wasdeposited. These layers, called strata, or beds, are the singlemost characteristic feature of sedimentary rocks (see Figure 2.16).

The thickness of beds ranges from microscopically thinto tens of meters thick. Separating the strata are bedding planes,flat surfaces along which rocks tend to separate or break. Gen-erally, each bedding plane marks the end of one episode ofsedimentation and the beginning of another.

Sedimentary rocks provide geologists with evidence fordeciphering past environments. A conglomerate, for exam-ple, indicates a high-energy environment, such as a rushingstream, where only the coarse materials can settle out. By con-trast, black shale and coal are associated with a low-energy,

Figure 2.20 White Chalk Cliffs, East Sussex,England. (Photo by Art Wolfe)


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Metamorphic Rocks: New Rock from Old 55

organic-rich environment, such as a swamp or lagoon. Otherfeatures found in some sedimentary rocks also give clues topast environments (Figure 2.24).

Fossils, the traces or remains of prehistoric life, are per-haps the most important inclusions found in some sedimen-tary rock. Knowing the nature of the life forms that existed ata particular time may help answer many questions about theenvironment. Was it land or ocean, lake or swamp? Was theclimate hot or cold, rainy or dry? Was the ocean water shal-low or deep, turbid or clear? Furthermore, fossils are impor-tant time indicators and play a key role in matching up rocksfrom different places that are the same age. Fossils are im-portant tools used in interpreting the geologic past and willbe examined in some detail in Chapter 8.

Metamorphic Rocks: New Rock from Old

Earth Materials� Metamorphhic Rockss

Recall from the discussion of the rock cycle that metamorphismis the transformation of one rock type into another. Metamor-phic rocks are produced from preexisting igneous, sedimenta-ry, or even other metamorphic rocks. Thus, every metamorphicrock has a parent rock—the rock from which it was formed.

Metamorphism, which means to “change form,” is aprocess that leads to changes in the mineralogy, texture (forexample, grain size), and often the chemical composition ofrocks. Metamorphism takes place when preexisting rock issubjected to a physical or chemical environment that is signif-icantly different from that in which it initially formed. In re-sponse to changes in temperature and pressure (stress) and tothe introduction of chemically active fluids, the rock gradu-ally changes until a state of equilibrium with the new envi-ronment is reached. Most metamorphic changes occur at theelevated temperatures and pressures that exist in the zonebeginning a few kilometers below Earth’s surface and extend-ing into the upper mantle.

Metamorphism often progresses incrementally, fromslight changes (low-grade metamorphism) to substantialchanges (high-grade metamorphism). For example, under low-grade metamorphism, the common sedimentary rock shalebecomes the more compact metamorphic rock called slate.Hand samples of these rocks are sometimes difficult to distin-guish, illustrating that the transition from sedimentary tometamorphic rock is often gradual and the changes subtle.

In more extreme environments, metamorphism causes atransformation so complete that the identity of the parent rockcannot be determined. In high-grade metamorphism, such fea-tures as bedding planes, fossils, and vesicles that may have ex-isted in the parent rock are obliterated. Further, when rocks atdepth (where temperatures are high) are subjected to directedpressure, they slowly deform to produce a variety of texturesas well as large-scale structures such as folds (Figure 2.25). Inthe most extreme metamorphic environments, the tempera-tures approach those at which rocks melt. However, during

A. Agate

B. Flint C. Jasper

D. Chert arrowhead

Figure 2.21 Chert is a name used for a number of dense, hard rocksmade of microcrystalline quartz. Three examples are shown here. A.Agate is the banded variety. (Photo by Jeffrey A. Scovil) B. The darkcolor of flint results from organic matter. (Photo by E. J. Tarbuck)C. The red variety, called jasper, gets its color from iron oxide. (Photoby E. J. Tarbuck) D. Native Americans frequently made arrowheads andsharp tools from chert. (Photo by LA VENTA/CORBIS/SYGMA)

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metamorphism the rock must remain essentially solid, for if com-plete melting occurs, we have entered the realm of igneous ac-tivity.

Most metamorphism occurs in one of two settings:

1. When rock is intruded by a magma body, contact orthermal metamorphism may take place. Here, changeis driven by a rise in temperature within the host rocksurrounding a molten igneous body.

2. During mountain building, great quantities of rock aresubjected to directed pressures and high temperaturesassociated with large-scale deformation called regionalmetamorphism.

Extensive areas of metamorphic rocks are exposed onevery continent. Metamorphic rocks are an important compo-nent of many mountain belts, where they make up a largeportion of a mountain’s crystalline core. Even the stable con-tinental interiors, which are generally covered by sedimenta-ry rocks, are underlain by metamorphic basement rocks. In allof these settings, the metamorphic rocks are usually highlydeformed and intruded by igneous masses. Indeed, signifi-cant parts of Earth’s continental crust are composed of meta-morphic and associated igneous rocks.

ID

AZ

NV

UT

Salt LakeSalt LakeCity

GreatGreatSaltSaltLake

BonnevilleSalt Flats

ORCA

ID

AZ

NV

UT

Salt LakeCity

GreatSaltLake

BonnevilleSalt Flats

ORCA

Figure 2.22 Bonneville Salt Flats, Utah. (Photo by Tom & Susan Bean, Inc.)

SWAMP ENVIRONMENT

PEAT(Partially alteredplant material;

very smoky whenburned, low energy)

LIGNITE

(Soft, brown coal;moderate energy)

Burial

Compaction

Greater burial

CompactionBITUMINOUS

(Soft; black coal;major coal used in

power generation andindustry; high energy)

METAMORPHISM

StressANTHRACITE

(Hard, black coal;used in industry;

high energy)

Figure 2.23 Successive stages in the formation of coal.

Did You Know?Some low-grade metamorphic rocks actually contain fossils.

When fossils are present in metamorphic rocks, they provide

useful clues for determining the original rock type and its de-

positional environment. In addition, fossils whose shapes have

been distorted during metamorphism provide insight into the

extent to which the rock has been deformed.


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A B

Figure 2.24 A. Ripple marks preserved in sedimentary rocks may indicate a beach or stream channel environment. (Photo byStephen Trimble) B. Mud cracks form when wet mud or clay dries and shrinks, perhaps signifying a tidal flat or desert basin.(Photo by Gary Yeowell/Getty Images Inc.—Stone Allstock)

Agents of Metamorphism

The agents of metamorphism include heat, pressure (stress),and chemically active fluids. During metamorphism, rocks areusually subjected to all three metamorphic agents simulta-neously. However, the degree of metamorphism and the con-tribution of each agent vary greatly from one environmentto another.

Heat as a Metamorphic Agent The most important agentof metamorphism is heat because it provides the energy todrive chemical reactions that result in the recrystallization ofexisting minerals and/or the formation of new minerals. Theheat to metamorphose rocks comes mainly from two sources.

First, rocks experience a rise in temperature when they areintruded by magma rising from below. This is called contactor thermal metamorphism. Here, the adjacent host rock is“baked” by the emplaced magma.

Second, rocks that formed at Earth’s surface will expe-rience a gradual increase in temperature as they are trans-ported to greater depths. In the upper crust, this increase intemperature averages between 20°C and 30°C per kilometer.When buried to a depth of about 8 kilometers (5 miles), wheretemperatures are between 150°C and 200°C, clay mineralstend to become unstable and begin to recrystallize into otherminerals, such as chlorite and muscovite, that are stable inthis environment. (Chlorite is a micalike mineral formed bythe metamorphism of iron- and magnesian-rich silicates.)

Figure 2.25 Folded and metamorphosed rocks in Anza Borrego Desert State Park, California. (Photo by A. P.Trujillo/APT Photos)


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Did You Know?The temperature of Earth’s crust increases with depth, an idea

that can be expressed as the deeper one goes, the hotter it gets. This

causes considerable problems for underground mining efforts.

In the Western Deep Levels mine in South Africa, which is 4

kilomenters (2.5 miles) deep, the temperature of the rock is hot

enough to scorch human skin. Here, the miners work in groups

of two: one to mine the rock, and the other to operate the fan

that keeps them cool.

However, many silicate minerals, particularly those found incrystalline igneous rocks—quartz, for example—remain sta-ble at these temperatures. Thus, metamorphic changes inthese minerals occur at much higher temperatures.

Pressure (Stress) as a Metamorphic Agent Pressure, liketemperature, also increases with depth as the thickness of the

overlying rock increases. Buried rocks are subjected to confiningpressure, which is analogous to water pressure, where the forcesare applied equally in all directions (Figure 2.26A). The deeperyou go in the ocean, the greater the confining pressure. Thesame is true for rock that is buried. Confining pressure causesthe spaces between mineral grains to close, producing a morecompact rock having a greater density. Further, at great depths,confining pressure may cause minerals to recrystallize intonew minerals that display a more compact crystalline form.

During episodes of mountain building, large rock bod-ies become highly crumpled and metamorphosed (Figure2.26B). The forces that generate mountains are unequal in dif-ferent directions and are called differential stress. Unlike con-fining pressure, which “squeezes” the rock equally from alldirections, differential stresses are greater in one directionthan in others. As shown in Figure 2.26B, rocks subjected todifferential stress are shortened in the direction of greateststress, and elongated, or lengthened, in the direction perpen-dicular to that stress. The deformation caused by differentialstresses plays a major role in developing metamorphic tex-tures.

A. Confining pressure

Increasingconfiningpressure

B. Differential stress

UndeformedstrataDeformed

strata

Undeformedstrata

Figure 2.26 Pressure (stress) as a metamorphicagent. A. In a depositional environment, asconfining pressure increases, rocks deform bydecreasing in volume. B. During mountainbuilding, rocks subjected to differential stress areshortened in the direction that pressure is applied,and lengthened in the direction perpendicular tothat force.


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In surface environments where temperatures are compar-atively low, rocks are brittle and tend to fracture when subject-ed to differential stress. Continued deformation grinds andpulverizes the mineral grains into small fragments. By contrast,in high-temperature environments, rocks are ductile. Whenrocks exhibit ductile behavior, their mineral grains tend to flat-ten and elongate when subjected to differential stress. This ac-counts for their ability to deform by flowing (rather thanfracturing) to generate intricate folds (Figure 2.27).

Chemically Active Fluids Fluids composed mainly of waterand other volatiles (materials that readily change to a gas atsurface conditions), including carbon dioxide, are believedto play an important role in some types of metamorphism.Fluids that surround mineral grains act as catalysts to pro-mote recrystallization by enhancing ion migration. In pro-gressively hotter environments, these ion-rich fluids becomecorrespondingly more reactive.

When two mineral grains are squeezed together, theparts of their crystalline structures that touch are the mosthighly stressed. Ions located at these sites are readily dis-solved by the hot fluids and migrate along the surface of thegrain to the spaces located between individual grains. Thus,hot fluids aid in the recrystallization of mineral grains by dis-solving material from regions of high stress and then precip-itating (depositing) this material in areas of low stress. As aresult, minerals tend to recrystallize and grow longer in a directionperpendicular to compressional stresses.

When hot fluids circulate freely through rocks, ionic ex-change may occur between two adjacent rock layers, or ionsmay migrate great distances before they are finally deposit-ed. The latter situation is particularly common when we con-sider hot fluids that escape during the crystallization of anintrusive igneous mass. If the rocks that surround the massdiffer markedly in composition from the invading fluids,there may be a substantial exchange of ions between the flu-ids and host rocks. When this occurs, a change in the overallcomposition of the surrounding rock results.

Metamorphic Textures

The degree of metamorphism is reflected in the rock’s tex-ture and mineralogy. (Recall that the term texture is used todescribe the size, shape, and arrangement of grains within a

rock.) When rocks are subjected to low-grade metamorphism,they become more compact and thus more dense. A commonexample is the metamorphic rock slate, which forms whenshale is subjected to temperatures and pressures only slight-ly greater than those associated with the compaction that lithi-fies sediment. In this case, differential stress causes themicroscopic clay minerals in shale to align into the more com-pact arrangement found in slate.

Under more extreme conditions, stress causes certainminerals to recrystallize. In general, recrystallization encour-ages the growth of larger crystals. Consequently, many meta-morphic rocks consist of visible crystals, much like coarse-grained igneous rocks.

The crystals of some minerals will recrystallize with apreferred orientation, essentially perpendicular to the direc-tion of the compressional force. The resulting mineral align-ment usually gives the rock a layered or banded appearancetermed foliated texture (Figure 2.28). Simply, foliation resultswhenever the minerals of a rock are brought into parallelalignment.

Not all metamorphic rocks have a foliated texture. Suchrocks are said to exhibit a nonfoliated texture. Metamorphicrocks composed of only one mineral that forms equidimen-sional crystals are, as a rule, not visibly foliated. For example,pure limestone, is composed of only a single mineral, calcite.When a fine-grained limestone is metamorphosed, the smallcalcite crystals combine to form larger interlocking crystals.The resulting rock resembles a coarse-grained igneous rock.This nonfoliated metamorphic equivalent of limestone is calledmarble.

Figure 2.27 Deformed metamorphic rocks exposed in a road cut inthe Eastern Highland of Connecticut. Imagine the tremendous forcerequired to fold rock in this manner. (Photo by Phil Dombrowski)

Metamorphism

Before metamorphism(Uniform stress)

After metamorphism(Differential stress)

Figure 2.28 Under the pressures of metamorphism, some mineralgrains become reoriented and aligned at right angles to the stress. Theresulting orientation of mineral grains gives the rock a foliated (layered)texture. If the coarse-grained igneous rock (granite) on the leftunderwent intense metamorphism, it could end up closely resemblingthe metamorphic rock on the right (gneiss). (Photos by E. J. Tarbuck)


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Common Metamorphic Rocks

To review, metamorphic processes cause many changes in ex-isting rocks, including increased density, growth of largercrystals, foliation (reorientation of the mineral grains into alayered or banded appearance), and the transformation oflow-temperature minerals into high-temperature minerals(Figure 2.29). Further, the introduction of ions generates newminerals, some of which are economically important.

Here is a brief look at common rocks produced by meta-morphic processes.

Foliated Rocks Slate is a very fine-grained foliated rockcomposed of minute mica flakes (Figure 2.30). The most note-worthy characteristic of slate is its excellent rock cleavage,meaning that it splits easily into flat slabs. This property hasmade slate a most useful rock for roof and floor tile, chalk-boards, and billiard tables (Figure 2.31). Slate is most oftengenerated by the low-grade metamorphism of shale, althoughless frequently it forms from the metamorphism of volcanicash. Slate can be almost any color, depending on its mineralconstituents. Black slate contains organic material; red slategets its color from iron oxide; and green slate is usually com-posed of chlorite, a micalike mineral.

Schists are strongly foliated rocks formed by regionalmetamorphism (Figure 2.30). They are platy and can be read-ily split into thin flakes or slabs. Like slate, the parent mate-rial from which many schists originate is shale, but in the caseof schist, the metamorphism is more intense.

The term schist describes the texture of a rock regardlessof composition. For example, schists composed primarily ofmuscovite and biotite are called mica schists.

Texture

Foliated

Increasing

Metamorphism

Nonfoliated

Grain Size

Very fine

Fine

Mediumto

coarse

Mediumto

coarse

Mediumto

coarse

Mediumto

coarse

Fine

Parent Rock

Shale,mudstone,or siltstone

Slate

Phyllite

Schist, granite,or volcanic

rocks

Limestone,dolostone

Quartzsandstone

Bituminouscoal

Comments

Excellent rock cleavage,smooth dull surfaces

Breaks along wavysurfaces, glossy sheen

Micaceous mineralsdominate, scaly foliation

Compositional bandingdue to segregation of

minerals

Interlocking calciteor dolomite grains

Fused quartz grains,massive, very hard

Shiny black organic rockthat may exhibit conchoidal

fracture

Rock Name

Slate

Phyllite

Schist

Gneiss

Marble

Quartzite

Anthracite

Figure 2.29 Classification of common metamorphic rocks.

Did You Know?The reason high-quality billiard tables are so heavy is that they

have surfaces made of a thick slab of the metamorphic rock,

slate. Because slate splits easily into slabs, it is highly prized for

use as billiard-table surfaces as well as building materials such

as floor and roof tiles.

Gneiss (pronounced “nice”) is the term applied to band-ed metamorphic rocks that contain mostly elongated andgranular, as opposed to platy, minerals (Figure 2.30). The mostcommon minerals in gneisses are quartz and feldspar, withlesser amounts of muscovite, biotite, and hornblende. Gneiss-es exhibit strong segregation of light and dark silicates, givingthem a characteristic banded texture. While in a plastic state,these banded gneisses can be deformed into intricate folds.

Nonfoliated Rocks Marble is a coarse, crystalline rockwhose parent rock is limestone. Marble is composed of largeinterlocking calcite crystals, which form from the recrystal-lization of smaller grains in the parent rock.

Because of its color and relative softness (hardness ofonly 3 on the Mohs scale), marble is a popular building stone.White marble is particularly prized as a stone from which tocarve monuments and statues, such as the famous statue ofDavid by Michelangelo (Figure 2.32). Often the limestone fromwhich marble forms contains impurities that color the mar-ble. Thus, marble can be pink, gray, green, or even black.


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Quartzite is a very hard metamorphic rock most oftenformed from quartz sandstone. Under moderate- to high-grade metamorphism, the quartz grains in sandstone fuse.Pure quartzite is white, but iron oxide may produce reddishor pinkish stains, and dark minerals may impart a gray color.

Foliated metamorphic rocks

Nonfoliated metamorphic rocks

Foliated metamorphic rocks

Slate

Schist

Gneiss

Nonfoliated metamorphic rocks

Marble

Quartzite

Figure 2.30 Common metamorphic rocks. (Photos by E.J. Tarbuck)

Did You Know?Because marble can be carved readily, it has been used for cen-

turies for buildings and memorials. Examples of important

structures whose exteriors are clad in marble include the

Parthenon in Greece, the Taj Mahal in India, and the Washing-

ton Monument in the United States.

Figure 2.31 Excellent rock cleavage is exhibited by the slate in thisquarry near Alta, Norway. (Photo by Fred Bruermmer/DRK Photo)Because slate breaks into flat slabs, it has many uses. In the insetphoto, it is used to roof this house in Switzerland. (Photo by E. J.Tarbuck)

Figure 2.32 Replica of the statue of David by Michelangelo. Like theoriginal, this sculpture was created from a large block of whitemarble. (Photo by Andrew Ward/Getty Images, Inc./Photo Disk)

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The Chapter in Review

1. The three rock groups are igneous, sedimentary, andmetamorphic. Igneous rock forms from magma that cools andsolidifies in a process called crystallization. Sedimentary rockforms from the lithification of sediment. Metamorphic rockforms from rock that has been subjected to great pressure andheat in a process called metamorphism.

2. Igneous rocks are classified by their texture and mineralcomposition.

3. The rate of cooling of magma greatly influences the sizeof mineral crystals in igneous rock and thus its texture. Thefour basic igneous rock textures are (1) fine-grained, (2) coarse-grained, (3) porphyritic, and (4) glassy.

4. Igneous rocks are divided into broad compositionalgroups based on the percentage of dark and light silicate min-erals they contain. Felsic rocks (e.g., granite and rhyolite) arecomposed mostly of the light-colored silicate minerals potas-sium feldspar and quartz. Rocks of intermediate composition(e.g., andesite) contain plagioclase feldspar and amphibole.Mafic rocks (e.g., basalt) contain abundant pyroxene, and cal-cium-rich plagioclase feldspar.

5. The mineral makeup of an igneous rock is ultimatelydetermined by the chemical composition of the magma fromwhich it crystallized. N. L. Bowen showed that as magmacools, minerals crystallize in an orderly fashion at differenttemperatures. Magmatic differentiation changes the composi-tion of magma and causes more than one rock type to formfrom a common parent magma.

6. Weathering is the response of surface materials to achanging environment. Mechanical weathering, the physicaldisintegration of material into smaller fragments, is accom-plished by frost wedging, expansion resulting from unloading,and biological activity. Chemical weathering involves processesby which the internal structures of minerals are altered bythe removal and/or addition of elements. It occurs when ma-terials are oxidized or react with acid, such as carbonic acid.

7. Detrital sediments originate as solid particles derivedfrom weathering and are transported. Chemical sediments aresoluble materials produced largely by chemical weatheringthat are precipitated by either inorganic or organic processes.Detrital sedimentary rocks, which are classified by particle size,contain a variety of mineral and rock fragments, with clayminerals and quartz the chief constituents. Chemical sedimen-tary rocks often contain the products of biological processes ormineral crystals that form as water evaporates and mineralsprecipitate. Lithification refers to the processes by which sed-iments are transformed into solid sedimentary rocks.

8. Common detrital sedimentary rocks include shale (themost common sedimentary rock), sandstone, and conglomerate.The most abundant chemical sedimentary rock is limestone,consisting chiefly of the mineral calcite. Rock gypsum and rocksalt are chemical rocks that form as water evaporates.

9. Some features of sedimentary rocks that are often usedin the interpretation of Earth history and past environmentsinclude strata or beds (the single most characteristic feature),bedding planes, and fossils.

10. Two types of metamorphism are (1) regional metamor-phism and (2) contact or thermal metamorphism. The agents ofmetamorphism include heat, pressure (stress), and chemicallyactive fluids. Heat is perhaps the most important because itprovides the energy to drive the reactions that result in the rec-rystallization of minerals. Metamorphic processes cause manychanges in rocks, including increased density, growth of largermineral crystals, reorientation of the mineral grains into a layeredor banded appearance known as foliation, and the formationof new minerals.

11. Some common metamorphic rocks with a foliated textureinclude slate, schist, and gneiss. Metamorphic rocks with anonfoliated texture include marble and quartzite.

Key Terms

andesitic (intermediate) com-position (p. 45)

basaltic composition (p. 44)

Bowen’s reaction series (p. 45)

chemical sedimentary rock (p. 50)

chemical weathering (p. 47)

coarse-grained texture (p. 42)

contact (thermal) metamor-phism (p. 56)

crystallization (p. 38)

crystal settling (p. 45)

detrital sedimentary rock (p. 50)

evaporite deposit (p. 53)

extrusive (volcanic) (p. 40)

felsic (p. 44)

fine-grained texture (p. 42)

foliated texture (p. 59)

fossil (p. 55)

glassy texture (p. 42)

granitic composition (p. 44)

igneous rock (p. 40)

intrusive (plutonic) (p. 40)

lava (p. 40)

lithification (p. 40)

mafic (p. 44)

magma (p. 38)

magmatic differentiation (p. 46)

mechanical weathering (p. 46)

metamorphic rock (p. 40)

metamorphism (p. 55)

nonfoliated texture (p. 59)

porphyritic texture (p. 42)

regional metamorphism (p. 56)

rock cycle (p. 38)

sediment (p. 40)

sedimentary rock (p. 40)

strata (beds) (p. 54)

texture (p. 42)

ultramafic composition (p. 45)

weathering (p. 40)


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Questions for Review

1. Explain the statement “One rock is the raw material foranother” using the rock cycle.

2. If a lava flow at Earth’s surface had a basaltic composi-tion, what rock type would the flow likely be (see Figure2.8)? What igneous rock would form from the samemagma if it did not reach the surface but instead crys-tallized at great depth?

3. What does a porphyritic texture indicate about the his-tory of an igneous rock?

4. How are granite and rhyolite different? The same? (SeeFigure 2.8.)

5. Relate the classification of igneous rocks to Bowen’s re-action series.

6. If two identical rocks were weathered, one mechani-cally and the other chemically, how would the productsof weathering for the two rocks differ?

7. How does mechanical weathering add to the effective-ness of chemical weathering?

8. How is carbonic acid formed in nature? What are theproducts when this acid reacts with potassium feldspar?

9. Which minerals are most common in detrital sedimen-tary rocks? Why are these minerals so abundant?

10. What is the primary basis for distinguishing among var-ious detrital sedimentary rocks?

11. Distinguish between the two categories of chemical sed-imentary rocks.

12. What are evaporite deposits? Name a rock that is anevaporite.

13. Compaction is an important lithification process withwhich sediment size?

14. What is probably the single most characteristic featureof sedimentary rocks?

15. What is metamorphism?16. List the three agents of metamorphism and describe the

role of each.17. Distinguish between regional and contact metamor-

phism.18. Which feature would easily distinguish schist and

gneiss from quartzite and marble?19. In what ways do metamorphic rocks differ from the ig-

neous and sedimentary rocks from which they formed?

Online Study Guide

The Foundations of Earth Science Web site uses the resourcesand flexibility of the Internet to aid in your study of the top-ics in this chapter. Written and developed by Earth scienceinstructors, this site will help improve your understandingof Earth science. Visit http://www.prenhall.com/lutgens andclick on the cover of Foundations of Earth Science 5e to find:

• Online review quizzes.• Critical thinking exercises.• Links to chapter-specific Web resources.• Internet-wide key-term searches.

http://www.prenhall.com/lutgens

GEODe: Earth Science

GEODe: Earth Science makes studying more effective by re-inforcing key concepts using animation, video, narration,

interactive exercises, and practice quizzes. A copy is includ-ed with every copy of Foundations of Earth Science 5e.


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