What is the single most characteristic feature of sedimentary rocks

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.

IS B

N 0-558-71211-8

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

37

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.

IS B

N 0

-5 58

-7 12

11 -8

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, 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)

EEA RTH SCIEN

CE

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.

IS B

N 0-558-71211-8

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 eathering/transport

M el

tin g

Crystallization

M et

am or

ph 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.

39

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.

IS B

N 0

-5 58

-7 12

11 -8

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.

EEA RTH SCIEN

CE

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.

IS B

N 0-558-71211-8

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

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.

IS B

N 0

-5 58

-7 12

11 -8

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)

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.

IS B

N 0-558-71211-8

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.

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.

IS B

N 0

-5 58

-7 12

11 -8

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.

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.

IS B

N 0-558-71211-8

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)

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.

IS B

N 0

-5 58

-7 12

11 -8

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 Series

of C rystallization

Ultramafic Calcium-

rich

Pl ag

io cl

as e

fe ld

sp ar

C on

tin uo

us S

er ie

s

of C

ry st

al liz

at io

n

Potassium feldspar

Muscovite mica

Quartz

Sodium- rich

+

+

Basaltic (Mafic)

Andesitic (Intermediate)

Granitic (Felsic)

C oo

lin g

m ag

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.

EEA RTH SCIEN

CE

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.

IS B

N 0-558-71211-8

Weathering of Rocks to Form Sediment 47

Host rock

Magma body

A.

B.

C.

Crystallization and settling

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

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.

IS B

N 0

-5 58

-7 12

11 -8

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

2

2

1 .5

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.)

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.

IS B

N 0-558-71211-8

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

A.

B.

C.

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)

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.

IS B

N 0

-5 58

-7 12

11 -8

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.