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Climbers scaling the vertical face of El Capitan 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 this chapter, you will find it helpful to focus on the follow- ing questions:

1. What are the three groups of rocks and the geologic processes involved 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 processes associated with each?

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

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

Rocks: Materials of the Solid Earth

2 C H A P T E R

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38 Chapter 2 Rocks: Materials of the Solid Earth

W hy study rocks? You have already learned that rocks and 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, and even 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 in which it formed. For example, some rocks are composed en- tirely of small shell fragments. This tells Earth scientists that the particles making up the rock originated in a shallow ma- rine environment. Other rocks contain clues that indicate they formed from a volcanic eruption or deep in the Earth during mountain building (Figure 2.1). Thus, rocks contain a wealth of information about events that have occurred over Earth’s long history.

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

Earth as a System: The Rock Cycle

Earth Materials � The Rock Cycle

Earth is a system. This means that our planet consists of many interacting parts that form a complex whole. Nowhere is this idea 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. It helps us understand the origin of igneous, sedimentary, and metamorphic rocks and to see that each type is linked to the others 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 and many other chapters as well.

The Basic Cycle

We begin at the top of Figure 2.2. Magma is molten material that 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 magma or lava cools and solidifies, igneous rock

forms.

Magma forms when rock melts deep beneath Earth’s

surface.

Uplift, weathering,

transportation, and

deposition

Sediment is compacted and

cemented to form sedimentary rock.

When sedimentary rock is buried deep

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

metamorphic rock.

Magma

Igneous Rock

Sediment Sedimentary

Rock

Metamorphic Rock

Heat and pressure

Weathering breaks down rock that is transported and

deposited as sediment.

Uplift, weathering,

transportation, and

deposition

Lava

Melting

Heat

W e

a th

e rin

g /tran

sp o rt

M el

tin g

Crystallization

M et

am o

rp h

is 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 represent processes that link each group to the others.

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40 Chapter 2 Rocks: Materials of the Solid Earth

the surface or, following a volcanic eruption, at the surface. In either 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 influences of 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 number of erosional agents, such as running water, glaciers, wind, or waves. 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 into sedimentary rock when compacted by the weight of overly- ing layers or when cemented as percolating groundwater fills the pores with mineral matter.

If the resulting sedimentary rock is buried deep within Earth and involved in the dynamics of mountain building or intruded by a mass of magma, it will be subjected to great pressures and/or intense heat. The sedimentary rock will react to the changing environment and turn into the third rock type, metamorphic rock. If metamorphic rock is sub- jected to still higher temperatures, it will melt, creating magma, which will eventually crystallize into igneous rock, starting the cycle all over again.

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

Alternative Paths

The paths shown in the basic cycle are not the only ones that are possible. To the contrary, other paths are just as likely to be followed as those described in the preceding section. These alternatives 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 with mountain building. When this occurs, they are transformed directly into metamorphic rocks.

Metamorphic and sedimentary rocks, as well as sedi- ment, do not always remain buried. Rather, overlying layers may be eroded away, exposing the once buried rock. When this 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 cycle come from? Processes driven by heat from Earth’s interior are responsible for forming igneous and metamorphic rocks. Weathering and the movement of weathered material are ex- ternal processes powered by energy from the Sun. External processes 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 is magma and what is its source? Magma is molten rock gener- ated by partial melting of rocks in Earth’s mantle and in much smaller 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, with lesser amounts of aluminum, iron, calcium, sodium, potassi- um, magnesium, and others. Magma also contains some gases, particularly water vapor, which are confined within the magma body by the weight of the overlying rocks.

Once formed, a magma body buoyantly rises toward the surface because it is less dense than the surrounding rocks. Occasionally molten rock reaches the surface, where it is called lava. Sometimes, lava is emitted as fountains that are produced when escaping gases propel molten rock sky- ward. On other occasions, magma is explosively ejected from a vent, producing a spectacular eruption such as the 1980 eruption of Mount St. Helens. However, most eruptions are not violent; rather, volcanoes more often emit quiet outpour- ings of lava (Figure 2.3).

Igneous rocks that form when molten rock solidifies at the surface are classified as extrusive or volcanic (after the fire god Vulcan). Extrusive igneous rocks are abundant in west- ern portions of the Americas, including the volcanic cones of the Cascade Range and the extensive lava flows of the Co- lumbia Plateau. In addition, many oceanic islands, typified by the 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. Igneous rocks 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 at the surface if portions of the crust were not uplifted and the overlying rocks stripped away by erosion. Exposures of in- trusive igneous rocks occur in many places, including Mount 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

Hualalai Mauna 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 contains solids and gases. The solids are mineral crystals. The liquid portion of a magma body is composed of ions that move about freely. However, as magma cools, the random movements of the 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 crystal growth. When the crystals grow large enough for their edges to 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 a magma cools very slowly, relatively few centers of crystal growth develop. Slow cooling also allows ions to migrate over relatively great distances. Consequently, slow cooling results in the 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 all compete for the available ions. Therefore, rapid cooling results in the formation of a solid mass of small intergrown crystals.

Thus, if a geologist encounters igneous rock containing crystals large enough to be seen with the unaided eye, it means 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, there is not sufficient time for the ions to arrange themselves into a crystalline network at all. Therefore, solids produced in this manner consist of randomly distributed ions. Such rocks are called glass and are quite similar to ordinary manufactured glass. “Instant” quenching occurs during violent volcanic eruptions that produce tiny shards of glass called volcanic ash.

In addition to the rate of cooling, the composition of a magma and the amount of dissolved gases influence crystal- lization. Because magmas differ in each of these aspects, the physical 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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42 Chapter 2 Rocks: Materials of the Solid Earth

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 reveals a 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. As you might expect, the rate of cooling is slow in magma cham- bers lying deep within the crust, whereas a thin layer of lava extruded upon Earth’s surface may chill to form solid rock in a matter of hours. Small molten blobs ejected into the air during a violent eruption can solidify almost instantly.

Igneous rocks that form rapidly at the surface or as small masses within the upper crust have a fine-grained tex- ture, with the individual crystals too small to be seen with the unaided eye (Figure 2.4A). Common in many fine-grained igneous rocks are voids, called vesicles, left by gas bubbles that 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 mass of intergrown crystals, which are roughly equal in size and large enough that the individual minerals can be identified with the unaided eye. Granite is a classic example (Figure 2.4B).

A large mass of magma located at depth may require tens of thousands, even millions, of years to solidify. Be- cause all materials within a magma do not crystallize at the same rate or at the same time during cooling, it is possible for some crystals to become quite large before others even start to form. If magma that already contains some large crystals suddenly erupts at the surface, the remaining molten portion of the lava would cool quickly. The resulting rock, which has large crystals embedded in a matrix of smaller crystals, is said to have a porphyritic texture (Figure 2.4D).

During some volcanic eruptions, molten rock is ejected into the atmosphere, where it is quenched very quickly. Rapid cooling 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 are more likely than melts with a low silica content to form rocks that exhibit a glassy texture.

Obsidian, a common type of natural glass, is similar in appearance to a dark chunk of manufactured glass (Figure 2.6). Another volcanic rock that often exhibits a glassy texture is pumice. Usually found with obsidian, pumice forms when large amounts of gas escape from a melt to generate a gray, frothy mass (Figure 2.7). In some samples, the vesicles are quite noticeable, whereas in others, the pumice resembles fine shards of intertwined glass. Because of the large volume of air-filled voids, many samples of pumice will float in water.

(SiO2)

A. Fine-grained C. Glassy (pumice)

B. Coarse-grained D. Porphyritic

Intrusive igneous rocks

Extrusive igneous rocks

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 ejected into the atmosphere, a frothy glass called pumice may form. D. A porphyritic texture results when magma that already contains some large crystals migrates to a new location where the rate of cooling increases. The resulting rock consists of large crystals embedded within a matrix of smaller 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 as gas bubbles escape near the top of a lava flow. (Photo from GeoScience 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 magma from 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) are by far the most abundant constituents of igneous rocks. These two elements, plus ions of aluminum (Al), calcium (Ca), sodi- um (Na), potassium (K), magnesium (Mg), and iron (Fe), make up roughly 98 percent by weight of most magmas.

[SiO2]

As magma cools and solidifies, these elements combine to form two major groups of silicate minerals. The dark silicates are rich in iron and/or magnesium and are relatively low in silica. Olivine, pyroxene, amphibole, and biotite mica are the com- mon dark silicate minerals of Earth’s crust. By contrast, the light 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. The light silicates include quartz, muscovite mica, and the most abundant mineral group, the feldspars. The feldspars make up at least 40 percent of most igneous rocks. Thus, in addition to feldspar, igneous rocks contain some combination of the other 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 a consequence of the chemical makeup of the parent magma and the environment of crystallization.

Figure 2.6 Obsidian, a natural glass, was used by Native Americans for 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 it contains 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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44 Chapter 2 Rocks: Materials of the Solid Earth

Despite their great compositional diversity, igneous rocks can be divided into broad groups according to their proportions 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 are rocks composed almost entirely of light-colored silicates— quartz and potassium feldspar. Igneous rocks in which these are 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 quartz and feldspar, most granitic rocks contain about 10 percent dark silicate minerals, usually biotite mica and amphibole. Granitic rocks are rich in silica (about 70 percent) and are major constituents of the continental crust.

Granite is a coarse-grained igneous rock that forms where 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 of weathering and erosion strip away the overlying crust. Pikes Peak in the Rockies, Mount Rushmore in the Black Hills, Stone Mountain in Georgia, and Yosemite National Park in the Sierra Nevada are all areas where large quantities of gran- ite are exposed at the surface.

Granite is perhaps the best-known igneous rock (Figure 2.9). 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 tombstones and monuments and as building stones.

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

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

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

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

The coarse-grained, intrusive equivalent of basalt is called gabbro (Figure 2.9). Although gabbro is not commonly exposed on the surface, it makes up a significant percentage of 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

T E X T U R E

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

Obsidian (compact glass) Pumice (frothy glass)

Peridotite

Uncommon

Diorite

Andesite

Granite

Rhyolite

Quartz Potassium feldspar

Sodium-rich plagioclase feldspar

Gabbro

Basalt

Chemical Composition

Dominant Minerals

85% to 100%

Granitic (Felsic)

Andesitic (Intermediate) Ultramafic

Basaltic (Mafic)

Amphibole Sodium- and calcium-rich

plagioclase feldspar

Pyroxene Calcium-rich

plagioclase feldspar

Olivine Pyroxene

Komatiite (rare)

Figure 2.8 Classification of the major groups of igneous rocks based on their mineral composition and texture. Coarse-grained rocks are plutonic, solidifying deep underground. Fine-grained rocks are volcanic, or solidify as shallow, thin plutons. Ultramafic rocks are dark, dense rocks, composed almost entirely of minerals containing iron and magnesium. Although relatively rare on Earth’s surface, 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 after the common volcanic rock andesite. Andesitic rocks contain a mixiture of both light- and dark-colored minerals, mainly am- phibole and plagioclase feldspar. This important category of igneous rocks is associated with volcanic activity that is typi- cally confined to the margins of continents. When magma of intermediate composition crystallizes at depth, it forms the coarse-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 compositional spectrum from granitic rocks (see Figure 2.8). Because peri- dotite is composed almost entirely of dark silicate minerals, its chemical composition is referred to as ultramafic. Although ultramafic 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 may extrude lavas exhibiting quite different compositions. Data of this type led them to examine the possibility that magma might 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 by N. L. Bowen in the first quarter of the twentieth century.

Bowen’s Reaction Series In a laboratory setting, Bowen demonstrated 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 at least 200 degrees. Thus, as magma cools, certain minerals crystallize first, at relatively high temperatures (top of Figure

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

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

During the last stage of crystallization, after most of the magma 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 not usually found with quartz in the same igneous rock, because quartz crystallizes at much lower temperatures than olivine.

Evidence that this highly idealized crystallization model approximates what can happen in nature comes from the analysis of igneous rocks. In particular, we find that minerals that form in the same general temperature range on Bowen’s reaction 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 the same region of Bowen’s diagram, are typically found togeth- er as major constituents of the igneous rock granite.

Magmatic Differentiation Bowen demonstrated that different minerals crystallize at different temperatures. But how do Bowen’s findings account for the great diversity of igneous rocks? During the crystallization process, the composition of the melt (the liquid portion of magma excluding the solid crystals) continually changes because it gradually becomes depleted in those elements used to make the earlier formed minerals. This process, coupled with the fact that at one or more stages during crystallization, a separation of the solid and liquid components of magma can occur creates different mineral assemblages. One way 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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46 Chapter 2 Rocks: Materials of the Solid Earth

when the earlier formed minerals are denser (heavier) than the liquid 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 into fractures in the surrounding rocks—it will form a rock with a chemical composition much different from the parent magma (Figure 2.11). The formation of one or more secondary magmas from a single parent magma is called magmatic differentiation.

At any stage in the evolution of a magma, the solid and liquid components can separate into two chemically distinct units. Further, magmatic differentiation within the second- ary melt can generate additional chemically distinct fractions. Consequently, magmatic differentiation and separation of the solid 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 poured concrete sidewalk is smooth, but many years later, the same sidewalk will appear chipped, cracked, and rough, with peb- bles exposed at the surface. If a tree is nearby, its roots may grow 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 overlying a large body of intrusive igneous rock may be removed. This exposes the igneous rock to a whole new environment at the surface. This mass of crystalline rock, which formed deep below ground, where temperatures and pressures are high, is now subjected to very different and comparatively hostile surface 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 is what we call weathering.

In the following sections, we will discuss the two kinds of 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 different substance. Although we will consider these two processes separately, keep in mind that they usually work simultane- ously in nature. Furthermore, the activities of erosional agents—wind, water, and glaciers—that transport weathered rock particles are important. As these mobile agents move rock debris, they relentlessly disintegrate it further.

Mechanical Weathering of Rocks

When a rock undergoes mechanical weathering, it is broken into smaller and smaller pieces. Each piece retains the char- acteristics of the original material. The end result is many small pieces from a single large one. Figure 2.12 shows that breaking a rock into smaller pieces increases the surface area available for chemical attack. An example is adding sugar to water. A chunk of rock candy will dissolve much more slow-

Temperature Regimes

Igneous Rock Types

High temperature (~1200°C)

Low temperature (~ 750°C)

Olivine

Pyroxene

Amphibole

Biotite mica

D iscontinuous S

eries

of C rystallization

Ultramafic Calcium-

rich

P la

gi oc

la se

fe ld

sp ar

C on

tin uo

us S

er ie

of C

ry st

al liz

at io

Potassium feldspar

Muscovite mica

Quartz

Sodium- rich

Basaltic (Mafic)

Andesitic (Intermediate)

Granitic (Felsic)

C o

o lin

g m

a g

m a

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 the mineral 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

Host rock

Magma body

Crystallization and settling

Time

Igneous activity produces rocks having a composition of the

initial magma

Crystallization and settling changes the composition of the

remaining melt

Further magmatic differentiation results

in a more highly evolved melt

Figure 2.11 Illustration of how a magma evolves as the earlier formed minerals (those richer in iron, magnesium, and calcium) crystallize and settle to the bottom of the magma chamber, leaving the remaining melt richer in sodium, potassium, and silica A. Emplacement of a magma body and associated igneous activity generates rocks having a composition similar to that of the initial magma. B. After a period of time, crystallization and settling change the composition of the melt, while generating rocks having a composition quite different from the original magma. C. Further magmatic differentiation results in another more highly evolved melt with its associated rock types.

(SiO2).

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

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

Frost Wedging Alternate freezing and thawing of water is one 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 a very open crystalline structure. As a result, when water freezes, it expands and exerts a tremendous outward force. Here is everyday proof: Water in a car’s cooling system will freeze in winter, expanding and cracking the engine block. This is why antifreeze is added; it lowers the temperature at which the solution freezes.

In nature, water works its way into every crack or void in 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 regions in the middle latitudes where a daily freeze-thaw cycle often exists. Here, sections of rock are wedged loose and may tum- ble into large piles called talus or talus slopes that often form at the base of steep rock outcrops (Figure 2.13).

Unloading When large masses of igneous rock are exposed by erosion, entire slabs begin to break loose, like the layers of an onion. This sheeting is thought to occur because of the great reduction in pressure when the overlying rock is eroded away. Accompanying the unloading, the outer layers expand more than the rock below and thus separate from the rock body. Granite is particularly prone to sheeting.

Continued weathering eventually causes the slabs to sep- arate and spall, causing exfoliation domes. Excellent examples of exfoliation 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 the activities 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 rock by moving fresh material to the surface, where physical and chemical 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 are stable 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 will have a coating of rust (iron oxide), and if the time of exposure

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48 Chapter 2 Rocks: Materials of the Solid Earth

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 square unit

1 .5

Figure 2.12 Chemical weathering can occur only to those portions of a rock that are exposed to the elements. Mechanical weathering breaks rock into smaller and smaller pieces, thereby increasing the surface area available for chemical attack.

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

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

(H2CO3). (H2O)(CO2)

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

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

Frost wedging

Talus slope

Talus slope Talus slope

Figure 2.13 Frost wedging. As water freezes, it expands, exerting a force great enough to break rock. When frost wedging occurs in a setting such as this, the broken rock fragments fall to the base of the cliff and create a cone-shaped accumulation 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 will eventually precipitate to produce a hard, dense sedimentary rock (chert), fill pore spaces between mineral grains, or be carried to the ocean, where microscopic animals will build silica shells from it.

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

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

Table 2.1 lists the weathered products of some of the most common silicate minerals. Remember that silicate minerals make up most of Earth’s crust and are composed primarily of just eight elements (see Figure 1.14, p. 26). When chemically weathered, 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 with oxygen to produce iron-oxide compounds that give soil a red-

(SiO2)

Deep pluton

Uplift and erosion

Expansion and

sheeting

Conf ining 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 in Yosemite 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 the process of mechanical weathering. (Photo by Tom Bean/DRK Photo)

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50 Chapter 2 Rocks: Materials of the Solid Earth

dish-brown or yellowish color. The three remaining elements— aluminum, silicon, and oxygen—join with water to produce clay minerals that become an important part of the soil. Ulti- mately, the products of weathering form the raw materials for building 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 sedimentary rocks. 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 new location where they are deposited. Usually, the particles are broken down further during this transport phase. Following deposition, this sediment may become lithified, or “turned to rock.” Commonly, compaction and cementation transform the sediment 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 debris is constantly being swept from bedrock and carried away by water, ice, or wind. Eventually, the material is deposited in lakes, 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 sediment accumulate, the materials near the bottom are compacted by the weight of the overlying layers. Over long periods, these sediments are cemented together by mineral matter deposit- ed from water in the spaces between particles. This forms solid sedimentary rock.

Geologists estimate that sedimentary rocks account for only about 5 percent (by volume) of Earth’s outer 16 kilome- ters (10 miles). However, the importance of this group of rocks is far greater than this percentage implies. If you sampled the rocks exposed at Earth’s surface, you would find that the great majority are sedimentary (Figure 2.16). Indeed, about 75 percent 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 the uppermost portion of the crust. This makes sense because sediment accumulates at the surface.

It is from sedimentary rocks that geologists reconstruct many details of Earth’s history. Because sediments are de- posited in a variety of different settings at the surface, the rock layers that they eventually form hold many clues to past surface environments. They may also exhibit characteristics that allow geologists to decipher information about the method and distance of sediment transport. Furthermore, it is 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 portion of 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 sedimentary rocks are major sources of iron, aluminum, manganese, and fertilizer, plus numerous materials essential to the construc- tion industry.

Classifying Sedimentary Rocks

Materials accumulating as sediment have two principal sources. First, sediments may originate as solid particles from weathered rocks, such as the igneous rocks earlier described. These particles are called detritus, and the sedimentary rocks that 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 are called 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, clay minerals and quartz dominate. As you learned earlier, clay minerals are the most abundant product of the chemical weathering of silicate minerals, especially the feldspars. Quartz, on the other hand, is abundant because it is extremely durable 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-size particles predominate, the rock is called conglomerate if the sediment 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 magnesium hematite)

Olivine Iron minerals Silica (limonite and Ions of magnesium hematite)

(SiO2)

(SiO2) (SiO2)

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are angular (Figure 2.18B). Angular fragments indicate that the particles were not transported very far from their source prior to deposition and so have not had corners and rough edges abraded. Sandstone is the name given rocks when sand- size grains prevail (Figure 2.18C). Shale, the most common sedimentary rock, is made of very fine-grained sediment (Figure 2.18D). Siltstone, another rather fine-grained rock, is sometimes 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 the sediment was deposited. Currents of water or air sort the par- ticles by size. The stronger the current, the larger the particle size carried. Gravels, for example, are moved by swiftly flow- ing rivers, rockslides, and glaciers. Less energy is required to transport sand; thus, it is common in windblown dunes, river deposits, and beaches. Because silts and clays settle very slowly, 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 by particle size, in certain cases the mineral composition is also part of naming a rock. For example, most sandstones are pre- dominantly quartz-rich, and they are often referred to as quartz 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 can be 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, chemical sediments are derived from material that is carried in solution to lakes and seas. This material does not remain dissolved in the water indefinitely. When conditions are right, it precipi- tates to form chemical sediments. This precipitation may occur directly as the result of physical processes, or indirectly through life processes of water-dwelling organisms. Sedi- ment formed in this second way has a biochemical origin.

An example of a deposit resulting from physical processes is the salt left behind as a body of saltwater evap- orates. In contrast, many water-dwelling animals and plants extract 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 of all rock outcrops on the continents are sedimentary rocks. (Photo by Jeff Gnass)

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 sedimentary rock. It is composed chiefly of the mineral calcite Ninety percent of limestone is biochemical sediment. The rest precipitates directly from seawater.

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

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

Dissolved silica precipitates to form varieties of microcrystalline 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 precipitate from 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 in cooking and seasoning foods. Of course, it has many other uses and has been considered important enough that people have sought, traded, and fought over it for much of human history. Gypsum is the basic ingredient of plaster of Paris. This material is used most extensively in the construction in- dustry for “drywall” and plaster.

52 Chapter 2 Rocks: Materials of the Solid Earth

ClasticTexture Particle Size

Sediment Name Rock Name

Coarse (over 2 mm)

Gravel (Rounded particles)

Gravel (Angular particles)

Medium (1/16 to 2 mm)

Sand

(If abundant feldspar is present the rock is called Arkose)

Conglomerate

Breccia

Sandstone

Fine (1/16 to

1/256 mm)

Very fine (less than 1/256 mm)

Mud

Siltstone

Shale

Coquina

Chalk

Chert (light colored) Flint (dark colored)

Rock Gypsum

Rock Salt

Bituminous Coal

Calcite, CaCO3

Quartz, SiO2

Gypsum CaSO4•2H2O

Halite, NaCl

Altered plant fragments

Nonclastic: Fine to coarse

crystalline

Crystalline Limestone

Clastic: Visible shells and shell

fragments loosely cemented

Clastic: Various size shells and shell

fragments cemented with calcite cement

Clastic: Microscopic shells and clay

Nonclastic: Very fine crystalline

Nonclastic: Fine to coarse crystalline

Nonclastic: Fine-grained

organic matter

Fossiliferous Limestone

B i o c h e m i c a l

L i

m e s t o n e

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, based on their source of sediment. The main criterion for naming detrital rocks is particle size, whereas the primary basis for distinguishing among 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,”

which are harvested.

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Sedimentary Rocks: Compacted and Cemented Sediment 53

In the geologic past, many areas that are now dry land were covered by shallow arms of the sea that had only narrow connections to the open ocean. Under these conditions, water continually moved into the bay to replace water lost by evap- oration. Eventually, the waters of the bay became saturated and salt deposition began. Today, these arms of the sea are gone, and the remaining deposits are called evaporite deposits.

On a smaller scale, evaporite deposits can be seen in such places as Death Valley, California. Here, following rains or periods of snowmelt in the mountains, streams flow from surrounding mountains into an enclosed basin. As the water evaporates, 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 sedimentary rocks. 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 over extended periods (Figure 2.23).

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

A. B.

C. D.

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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54 Chapter 2 Rocks: Materials of the Solid Earth

are required for such accumulations, because dead plants nor- mally decompose when exposed to the atmosphere. An ideal environment that allows for the buildup of plant material is a 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 environments have been common. Coal undergoes successive stages of for- mation. With each successive stage, higher temperatures and pressures drive off impurities and volatiles, as shown in Figure 2.23.

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

In summary, we divide sedimentary rocks into two major groups: detrital and chemical. The main criterion for classifying detrital rocks is particle size, whereas chemical rocks are distinguished by their mineral composition. The categories presented here are more rigid than is the actual state of nature. Many detrital sedimentary rocks are a mixture of more than one particle size. Furthermore, many sedimen- tary rocks classified as chemical also contain at least small quantities of detrital sediment, and practically all detrital rocks are cemented with material that was originally dis- solved in water.

Lithification of Sediment

Lithification refers to the processes by which sediments are transformed into solid sedimentary rocks. One of the most common processes is compaction. As sediments accumulate through time, the weight of overlying material compresses the deeper sediments. As the grains are pressed closer and closer, pore space is greatly reduced. For example, when clays are 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 sedimentary rocks such as shale, because sand and other coarse sediments compress little.

Cementation is another important means by which sed- iments are converted to sedimentary rock. The cementing materials are carried in solution by water percolating through the pore spaces between particles. Over time, the cement pre- cipitates onto the sediment grains, fills the open spaces, and joins the particles. Calcite, silica, and iron oxide are the most common cements. Identification of the cementing material is simple. 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 sedimentary rock has an orange or red color, this usually means iron oxide is present.

Features of Sedimentary Rocks

Sedimentary rocks are particularly important evidence of Earth’s long history. These rocks form at Earth’s surface, and as layer upon layer of sediment accumulates, each records the nature of the environment at the time the sediment was deposited. These layers, called strata, or beds, are the single most characteristic feature of sedimentary rocks (see Figure 2.16).

The thickness of beds ranges from microscopically thin to 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 of sedimentation and the beginning of another.

Sedimentary rocks provide geologists with evidence for deciphering past environments. A conglomerate, for exam- ple, indicates a high-energy environment, such as a rushing stream, 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. Other features found in some sedimentary rocks also give clues to past 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 at a particular time may help answer many questions about the environment. Was it land or ocean, lake or swamp? Was the climate 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 rocks from different places that are the same age. Fossils are im- portant tools used in interpreting the geologic past and will be 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 metamorphism is 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 metamorphic rock has a parent rock—the rock from which it was formed.

Metamorphism, which means to “change form,” is a process that leads to changes in the mineralogy, texture (for example, grain size), and often the chemical composition of rocks. Metamorphism takes place when preexisting rock is subjected 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 to the 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 the elevated temperatures and pressures that exist in the zone beginning a few kilometers below Earth’s surface and extend- ing into the upper mantle.

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

In more extreme environments, metamorphism causes a transformation so complete that the identity of the parent rock cannot 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 at depth (where temperatures are high) are subjected to directed pressure, they slowly deform to produce a variety of textures as well as large-scale structures such as folds (Figure 2.25). In the 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 rocks made of microcrystalline quartz. Three examples are shown here. A. Agate is the banded variety. (Photo by Jeffrey A. Scovil) B. The dark color of flint results from organic matter. (Photo by E. J. Tarbuck) C. The red variety, called jasper, gets its color from iron oxide. (Photo by E. J. Tarbuck) D. Native Americans frequently made arrowheads and sharp tools from chert. (Photo by LA VENTA/CORBIS/SYGMA)

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56 Chapter 2 Rocks: Materials of the Solid Earth

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 or thermal metamorphism may take place. Here, change is driven by a rise in temperature within the host rock surrounding a molten igneous body.

2. During mountain building, great quantities of rock are subjected to directed pressures and high temperatures associated with large-scale deformation called regional metamorphism.

Extensive areas of metamorphic rocks are exposed on every continent. Metamorphic rocks are an important compo- nent of many mountain belts, where they make up a large portion 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 all of these settings, the metamorphic rocks are usually highly deformed and intruded by igneous masses. Indeed, signifi- cant parts of Earth’s continental crust are composed of meta- morphic and associated igneous rocks.

Salt LakeSalt Lake City

GreatGreat SaltSalt Lake

Bonneville Salt Flats

OR CA

Salt Lake City

Great Salt Lake

Bonneville Salt Flats

OR CA

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

SWAMP ENVIRONMENT

PEAT (Partially altered plant material;

very smoky when burned, low energy)

LIGNITE

(Soft, brown coal; moderate energy)

Burial

Compaction

Greater burial

Compaction BITUMINOUS

(Soft; black coal; major coal used in

power generation and industry; high energy)

METAMORPHISM

Stress ANTHRACITE

(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 by Stephen 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 are usually subjected to all three metamorphic agents simulta- neously. However, the degree of metamorphism and the con- tribution of each agent vary greatly from one environment to another.

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

First, rocks experience a rise in temperature when they are intruded by magma rising from below. This is called contact or 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 in temperature averages between 20°C and 30°C per kilometer. When buried to a depth of about 8 kilometers (5 miles), where temperatures are between 150°C and 200°C, clay minerals tend to become unstable and begin to recrystallize into other minerals, such as chlorite and muscovite, that are stable in this environment. (Chlorite is a micalike mineral formed by the 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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58 Chapter 2 Rocks: Materials of the Solid Earth

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 in crystalline igneous rocks—quartz, for example—remain sta- ble at these temperatures. Thus, metamorphic changes in these minerals occur at much higher temperatures.

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

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

During episodes of mountain building, large rock bod- ies become highly crumpled and metamorphosed (Figure 2.26B). 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 all directions, differential stresses are greater in one direction than in others. As shown in Figure 2.26B, rocks subjected to differential stress are shortened in the direction of greatest stress, and elongated, or lengthened, in the direction perpen- dicular to that stress. The deformation caused by differential stresses plays a major role in developing metamorphic tex- tures.

A. Confining pressure

Increasing confining pressure

B. Differential stress

Undeformed strataDeformed

strata

Undeformed strata

Figure 2.26 Pressure (stress) as a metamorphic agent. A. In a depositional environment, as confining pressure increases, rocks deform by decreasing in volume. B. During mountain building, rocks subjected to differential stress are shortened in the direction that pressure is applied, and lengthened in the direction perpendicular to that force.

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Metamorphic Rocks: New Rock from Old 59

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 and pulverizes the mineral grains into small fragments. By contrast, in high-temperature environments, rocks are ductile. When rocks 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 than fracturing) to generate intricate folds (Figure 2.27).

Chemically Active Fluids Fluids composed mainly of water and other volatiles (materials that readily change to a gas at surface conditions), including carbon dioxide, are believed to 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 become correspondingly more reactive.

When two mineral grains are squeezed together, the parts of their crystalline structures that touch are the most highly stressed. Ions located at these sites are readily dis- solved by the hot fluids and migrate along the surface of the grain 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 a result, minerals tend to recrystallize and grow longer in a direction perpendicular to compressional stresses.

When hot fluids circulate freely through rocks, ionic ex- change may occur between two adjacent rock layers, or ions may 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 an intrusive igneous mass. If the rocks that surround the mass differ 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 overall composition 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 to describe 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 common example is the metamorphic rock slate, which forms when shale 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 the microscopic clay minerals in shale to align into the more com- pact arrangement found in slate.

Under more extreme conditions, stress causes certain minerals 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 a preferred orientation, essentially perpendicular to the direc- tion of the compressional force. The resulting mineral align- ment usually gives the rock a layered or banded appearance termed foliated texture (Figure 2.28). Simply, foliation results whenever the minerals of a rock are brought into parallel alignment.

Not all metamorphic rocks have a foliated texture. Such rocks are said to exhibit a nonfoliated texture. Metamorphic rocks 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 small calcite crystals combine to form larger interlocking crystals. The resulting rock resembles a coarse-grained igneous rock. This nonfoliated metamorphic equivalent of limestone is called marble.

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

M e t a m o r p h i s m

Before metamorphism (Uniform stress)

After metamorphism (Differential stress)

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

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60 Chapter 2 Rocks: Materials of the Solid Earth

Common Metamorphic Rocks

To review, metamorphic processes cause many changes in ex- isting rocks, including increased density, growth of larger crystals, foliation (reorientation of the mineral grains into a layered or banded appearance), and the transformation of low-temperature minerals into high-temperature minerals (Figure 2.29). Further, the introduction of ions generates new minerals, 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 rock composed 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 has made slate a most useful rock for roof and floor tile, chalk- boards, and billiard tables (Figure 2.31). Slate is most often generated by the low-grade metamorphism of shale, although less frequently it forms from the metamorphism of volcanic ash. Slate can be almost any color, depending on its mineral constituents. Black slate contains organic material; red slate gets its color from iron oxide; and green slate is usually com- posed of chlorite, a micalike mineral.

Schists are strongly foliated rocks formed by regional metamorphism (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 case of schist, the metamorphism is more intense.

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

Texture

F o l i a t e d

I n c r e a s i n g

M e t a m o r p h i s m

N o n f o l i a t e d

Grain Size

Very fine

Fine

Medium to

coarse

Medium to

coarse

Medium to

coarse

Medium to

coarse

Fine

Parent Rock

Shale, mudstone, or siltstone

Slate

Phyllite

Schist, granite, or volcanic

rocks

Limestone, dolostone

Quartz sandstone

Bituminous coal

Comments

Excellent rock cleavage, smooth dull surfaces

Breaks along wavy surfaces, glossy sheen

Micaceous minerals dominate, scaly foliation

Compositional banding due to segregation of

minerals

Interlocking calcite or dolomite grains

Fused quartz grains, massive, very hard

Shiny black organic rock that 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 and granular, as opposed to platy, minerals (Figure 2.30). The most common minerals in gneisses are quartz and feldspar, with lesser amounts of muscovite, biotite, and hornblende. Gneiss- es exhibit strong segregation of light and dark silicates, giving them 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 rock whose parent rock is limestone. Marble is composed of large interlocking calcite crystals, which form from the recrystal- lization of smaller grains in the parent rock.

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

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Metamorphic Rocks: New Rock from Old 61

Quartzite is a very hard metamorphic rock most often formed from quartz sandstone. Under moderate- to high- grade metamorphism, the quartz grains in sandstone fuse. Pure quartzite is white, but iron oxide may produce reddish or 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 this quarry near Alta, Norway. (Photo by Fred Bruermmer/DRK Photo) Because slate breaks into flat slabs, it has many uses. In the inset photo, 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 the original, this sculpture was created from a large block of white marble. (Photo by Andrew Ward/Getty Images, Inc./Photo Disk)

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62 Chapter 2 Rocks: Materials of the Solid Earth

The Chapter in Review

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

2. Igneous rocks are classified by their texture and mineral composition.

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

4. Igneous rocks are divided into broad compositional groups based on the percentage of dark and light silicate min- erals they contain. Felsic rocks (e.g., granite and rhyolite) are composed 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 ultimately determined by the chemical composition of the magma from which it crystallized. N. L. Bowen showed that as magma cools, minerals crystallize in an orderly fashion at different temperatures. Magmatic differentiation changes the composi- tion of magma and causes more than one rock type to form from a common parent magma.

6. Weathering is the response of surface materials to a changing environment. Mechanical weathering, the physical disintegration of material into smaller fragments, is accom- plished by frost wedging, expansion resulting from unloading, and biological activity. Chemical weathering involves processes by which the internal structures of minerals are altered by the 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 derived from weathering and are transported. Chemical sediments are soluble materials produced largely by chemical weathering that 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 clay minerals and quartz the chief constituents. Chemical sedimen- tary rocks often contain the products of biological processes or mineral crystals that form as water evaporates and minerals precipitate. Lithification refers to the processes by which sed- iments are transformed into solid sedimentary rocks.

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

9. Some features of sedimentary rocks that are often used in the interpretation of Earth history and past environments include 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 of metamorphism include heat, pressure (stress), and chemically active fluids. Heat is perhaps the most important because it provides the energy to drive the reactions that result in the rec- rystallization of minerals. Metamorphic processes cause many changes in rocks, including increased density, growth of larger mineral crystals, reorientation of the mineral grains into a layered or banded appearance known as foliation, and the formation of new minerals.

11. Some common metamorphic rocks with a foliated texture include slate, schist, and gneiss. Metamorphic rocks with a nonfoliated 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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GEODe: Earth Science 63

Questions for Review

1. Explain the statement “One rock is the raw material for another” 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 Figure 2.8)? What igneous rock would form from the same magma 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? (See Figure 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 products of 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 the products 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 an evaporite.

13. Compaction is an important lithification process with which sediment size?

14. What is probably the single most characteristic feature of 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 resources and flexibility of the Internet to aid in your study of the top- ics in this chapter. Written and developed by Earth science instructors, this site will help improve your understanding of Earth science. Visit http://www.prenhall.com/lutgens and click 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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