80 Chapter 3

3.4 Metamorphic Rocks

Reading StrategyOutlining Copy this outline beneath theoutline you made for Section 3.3. Complete itas you read. Include points about how each ofthese rocks form, some of the characteristics ofeach rock type, and some examples of each.

Key ConceptsWhere does mostmetamorphism take place?

How is contactmetamorphism differentfrom regionalmetamorphism?

What are three agents ofmetamorphism, and whatkinds of changes doeseach cause?

What are foliatedmetamorphic rocks, andhow do they form?

How are metamorphicrocks classified?

Vocabulary◆ metamorphism◆ contact

metamorphism◆ regional

metamorphism◆ hydrothermal

solution◆ foliated

metamorphic rock◆ nonfoliated

metamorphic rock

R ecall that metamorphic rocks form when existing rocks arechanged by heat and pressure. Metamorphism is a very appropriatename for this process because it means to change form. Rocks produced

during metamorphism often look muchdifferent from the original rocks, or parentrocks. The folds in the rocks shown inFigure 14 formed when the parent rockswere subjected to intense forces. Thesehighly folded metamorphic rocks may alsodevelop a different composition than theparent rocks had.

Formation ofMetamorphic Rocks

Most metamorphic changes occur atelevated temperatures and pressures. Theseconditions are found a few kilometersbelow Earth’s surface and extend into theupper mantle. Most metamorphism occursin one of two settings—contact metamor-phism or regional metamorphism.

Figure 14 Deformed RockIntense pressures metamorphosedthese rocks by causing them tofold as well as changecomposition.

III. Metamorphic RocksA. Foliated Rocks

1.2.

B. Nonfoliated Rocks1.2. ?

?

??

80 Chapter 3

FOCUS

Section Objectives3.11 Predict where most

metamorphism takes place.3.12 Distinguish contact

metamorphism fromregional metamorphism.

3.13 Identify the three agents ofmetamorphism and explainwhat changes they cause.

3.14 Recognize foliatedmetamorphic rocks anddescribe how they form.

3.15 Classify metamorphic rocks.

Build VocabularyParaphrase Explain vocabulary termsusing words students know. For example,contact metamorphism occurs when tworocks come into contact with oneanother. Regional metamorphism takesplace over a large region. Foliatedmetamorphic rocks have distinct layers.Nonfoliated metamorphic rocks do not.Once students are able to distinguishamong the vocabulary terms, focus onthe processes that cause the differenttypes of metamorphism and thedifferent types of metamorphic rock.

Reading StrategyA.1. rock that forms when mineralsrecrystallize at right angles to thedirection of pressureA.2. Common example of foliatedmetamorphic rock is slate.B.1. rock that does not have a bandedtextureB.2. Common example of nonfoliatedmetamorphic rock is marble.

INSTRUCT

Formation ofMetamorphic RocksUse VisualsFigure 14 Ask students to describethe rocks. (Sample answer: The rocksare folded and multicolored.) What forcecould cause the rocks to fold? (intensepressure)Visual

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Section 3.4

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Contact Metamorphism When magmaintrudes—forces its way into—rock, contact meta-morphism may take place. During contactmetamorphism, hot magma moves into rock.Contact metamorphism often produces what isdescribed as low-grade metamorphism. Such changesin rocks are minor. Marble, like that used to make thestatue in Figure 15, is a common contact metamor-phic rock. Marble often forms when magma intrudesa limestone body.

Regional Metamorphism During mountainbuilding, large areas of rocks are subjected to extremepressures and temperatures. The intense changes produced during this process are described as high-grade metamorphism. Regional metamorphismresults in large-scale deformation and high-grade metamorphism.The rocks shown in Figure 14 on page 80 were changed as the result ofregional metamorphism.

Agents of MetamorphismThe agents of metamorphism are heat, pressure, and hydrother-

mal solutions. During metamorphism, rocks are usually subjected toall three of these agents at the same time. However, the effect of eachagent varies greatly from one situation to another.

Heat The most important agent of metamorphism is heat. Heatprovides the energy needed to drive chemical reactions. Some of thesereactions cause existing minerals to recrystallize. Other reactionscause new minerals to form. The heat for metamorphism comesmainly from two sources—magma and the change in temperaturewith depth. Magma essentially “bakes” any rocks that are in contactwith it. Heat also comes from the gradual increase in temperaturewith depth. In the upper crust, this increase averages between 20°Cand 30°C per kilometer.

When buried to a depth of about 8 kilometers, clay minerals areexposed to temperatures of 150°C to 200°C. These minerals becomeunstable and recrystallize to form new minerals that are stable at thesetemperatures, such as chlorite and muscovite. In contrast, silicate min-erals are stable at these temperatures. Therefore, it takes highertemperatures to change silicate minerals.

Compare and contrast contact and regional metamorphism.

Figure 15 Statue Carved fromMarble Marble is a commonmetamorphic rock that forms as the result of contactmetamorphism of limestone.

Q How hot is it deep in thecrust?

A The deeper a person goesbeneath Earth’s surface, thehotter it gets. The deepest minein the world is the WesternDeep Levels mine in SouthAfrica, which is about 4 kilome-ters deep. Here, the tempera-ture of the surrounding rock isso hot that it can scorch humanskin. In fact, miners in this mineoften work in groups of two.One miner mines the rock, andthe other operates a large fanthat keeps the worker cool.

Build Science SkillsPosing Questions Have students readthe text about contact metamorphismand regional metamorphism. Then havethem pose questions about the conceptsthat can be answered through experi-mentation, observation, or research.A sample question might be: Duringcontact metamorphism, what causes themagma to move into the rock? (Magmais less dense than surrounding rock sopressure forces it toward the surface. As itmoves, it can come into contact with andalter surrounding rock.)Logical

Agents ofMetamorphismIntegrate PhysicsBuried rocks are subject to a force knownas confining pressure, wherein pressureis applied equally in all directions. Incontrast, differential stress is unequalforce applied in different directions.Differential stress, which occurs duringmountain-building, acts mainly alongone plane. Rocks subjected to differ-ential stress are shortened in thedirection in which pressure is appliedand lengthened in the directionperpendicular to the pressure. Havestudents observe while you squeeze aball of clay between your palms. Ask: Isthis an example of confining pressureor differential stress? (differential stress)Kinesthetic, Visual

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Answer to . . .

Both processes changeexisting rocks into

metamorphic rocks. Contact metamor-phism is caused by magma and oftenproduces slight changes in rocks.Regional metamorphism is large-scaledeformation that can result in drasticchanges to the rocks involved.

Customize for Inclusion Students

Behaviorally Disordered Minimizedistractions for students with behavioraldisorders. For example, have students sit nearthe front of the class so that they are focusedon you, rather than their classmates. Before

conducting any activities, make sure studentsclear off their desks. If necessary, providestorage space in the classroom for students’books and other materials.

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Pressure (Stress) Pressure, like temperature, also increases withdepth. Like the water pressure you might have experienced at the bottomof a swimming pool, pressure on rocks within Earth is applied in all direc-tions. See Figure 16. Pressure on rocks causes the spaces between mineralgrains to close. The result is a more compact rock with a greater density.This pressure also may cause minerals to recrystallize into new minerals.

Increases in temperature and pressure cause rocks to flow ratherthan fracture. Under these conditions, mineral grains tend to flattenand elongate.

Observing Some of the Effects of Pressure on Mineral Grains

5. Draw a top view of your rock and label it After.Include arrows to show the directions fromwhich you applied pressure.

6. Make a cut through your model rock. Sketchthis view of the rock.

Analyze and Conclude1. Comparing and Contrasting How did the

Before sketch compare with the After sketch ofyour model rock?

2. Drawing Conclusions How does pressureaffect the mineral grains in a rock?

3. Inferring Was pressure the only agent ofchange that affected your rock? Explain.

Materialssoft modeling clay; 2 pieces of waxed paper (each20 cm � 20 cm); 20–30 small, round, elongatedplastic beads; small plastic knife

Procedure1. Use the clay to form a ball about the size of a

golf ball. Randomly place all of the beads intothis model rock.

2. Make a sketch of the rock. Label the sketchBefore.

3. Sandwich the model rock between the twopieces of waxed paper. Use your weight toapply pressure to the model rock.

4. Remove the waxed paper and observe your“metamorphosed” rock.

Figure 17 Imagine thetremendous amounts of pressurethat caused these rocks to fold.

Confining pressure

Increasingconfiningpressure

Differential stress

Undeformedstrata

Deformedstrata

A

B

Figure 16 Pressure (Stress) As a Metamorphic Agent A Forces in all directions are applied equally to buried rocks.B During mountain building, rocks subjected to differentialstress are shortened in the direction that pressure is applied.

82 Chapter 3

Observing Some of theEffects of Pressure onMineral GrainsObjectiveAfter completing this activity, studentswill be able to observe the effect ofpressure on the rearrangement ofmineral grains in a model rock.

Skills Focus Modeling, Observing,Inferring

Prep Time 10 minutes toorganize materials

Class Time 15 minutes

Expected Outcome Students willobserve that the pressure from oppositedirections—from above (their pushingdown on the “rock”) and below (thetable’s pushing up on the “rock”)—willcause “minerals” to align at right anglesto the direction of stress.

Analyze and Conclude1. The model minerals were randomlydistributed throughout the rock beforepressure was applied. The mineralsaligned themselves at right angles to thedirection of stress.2. Pressure causes the minerals toreorient themselves in the rock.3. No, heat from the hand and contactwith the table also affected the modelrock.Kinesthetic, Visual

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During mountain building, horizontal forces metamorphose largesegments of Earth’s crust.This often produces intricately folded rockslike those shown in Figure 17.

Reactions in Solution Water solutions containing other sub-stances that readily change to gases at the surface play an important rolein some types of metamorphism. Solutions that surround mineral grainsaid in recrystallization by making it easier for ions to move. When solu-tions increase in temperature reactions among substances can occur ata faster rate. When these hot, water-based solutions escape from a massof magma, they are called hydrothermal solutions. These hot fluids alsopromote recrystallization by dissolving original minerals and thendepositing new ones. As a result of contact with hydrothermal solutions,a change in a rock’s overall composition may occur.

Classification of Metamorphic RocksLike igneous rocks, metamorphic rocks can be classified by texture andcomposition. The texture of metamorphic rocks can be foliatedor nonfoliated.

Foliated Metamorphic Rocks When rocks undergo contactmetamorphism, they become more compact and thus more dense. Acommon example is the metamorphic rock slate. Slate forms whenshale is subjected to temperatures and pressures only slightly greaterthan those at which the shale formed. The pressure on the shale causesthe microscopic clay minerals to become more compact. The increasein pressure also causes the clay minerals to align in a similar direction.

Under more extreme conditions, certain minerals will recrystal-lize. Some minerals recrystallize with a preferred orientation,which is at right angles to the direction of the force. The resultingalignment usually gives the rock a layered or banded appearance.This rock is called a foliated metamorphic rock. Gneiss, themetamorphic rock shown in Figure 18, is a foliated rock. Anotherfoliated metamorphic rock is schist.

Nonfoliated Metamorphic Rocks A metamorphicrock that does not have a banded texture is called a nonfoliatedmetamorphic rock. Most nonfoliated rocks contain only onemineral. Marble, for example, is a nonfoliated rock made of cal-cite. When its parent rock, limestone, is metamorphosed, thecalcite crystals combine to form the larger interlocking crystals seenin marble. A sample of marble is shown in Figure 19. Quartzite andanthracite are other nonfoliated metamorphic rocks.

Contrast foliated and nonfoliated metamorphicrocks.

Figure 18 Gneiss is a foliatedmetamorphic rock. Inferring In which directionswas pressure exerted on thisrock?

Figure 19 Marble is anonfoliated metamorphic rock.

For: Links on metamorphic rocks

Visit: www.SciLinks.org

Web Code: cjn-1033

Classification ofMetamorphic RocksBuild Reading LiteracyRefer to p.124D in Chapter 5, whichprovides the guidelines for summarizing.

Summarize Have students read thetext on page 83. In their own words,have them summarize how meta-morphic rocks are classified. (Sampleanswer: Metamorphic rocks are classifiedby texture. There are two kinds oftextures—foliated and nonfoliated.Foliated metamorphic rocks have a layeredlook; nonfoliated metamorphic rocks donot have a layered appearance.)

Verbal

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Slate is a very fine-grained foliated rockcomposed of minute mica flakes. The mostnoteworthy characteristic of slate is its excellentrock cleavage, meaning that it splits easily intoflat slabs. This property has made slate a mostuseful rock for roof and floor tiles, chalkboards,and billiard tables. Slate is most often generatedby the low-grade metamorphism of shale,

though less frequently it forms from themetamorphism of volcanic ash. Slate can bealmost any color, depending on its mineralconstituents. Black slate contains organicmaterial; red slate gets it color from iron oxide;and green slate is usually composed ofchlorite, a micalike mineral.

Facts and Figures

Answer to . . .

Figure 18 Pressure was exerted fromthe sides.

Foliated metamorphicrocks have a layered or

banded appearance. Nonfoliatedmetamorphic rocks do not have abanded texture.

Download a worksheet onmetamorphic rocks for studentsto complete, and find additionalteacher support from NSTA SciLinks.

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Section 3.4 Assessment

Reviewing Concepts1. Where does most metamorphism take

place?

2. Compare and contrast contactmetamorphism and regional metamorphism?

3. Name the agents of metamorphism andexplain how each changes a rock.

4. What are foliated rocks, and how do theyform?

5. How are metamorphic rocks classified?

Critical Thinking6. Applying Concepts What is the major

difference between igneous and metamorphicrocks?

7. Predicting What type of metamorphism—contact or regional—would result in a schist?Explain your choice.

8. Formulating Conclusions Why can thecomposition of gneiss vary but overall texturecannot?

To summarize, metamorphic rocks form when existing rocks arechanged by heat, pressure, or hydrothermal solution. Contact meta-morphism is often caused when hot magma intrudes a body of rock.Changes during this type of metamorphism are minor. Regional meta-morphism is associated with mountain building. Such metamorphicchanges can be extreme. Metamorphic rocks can be classified by tex-ture as foliated or nonfoliated, as shown in Table 3.

Explanatory Paragraph Write a shortparagraph that explains the major differ-ences and similarities among the threemajor rock groups.

Texture

Foliated

Increasing

Metamorphism

Nonfoliated

Grain Size

Very fine

Fine

Mediumto

Coarse

Mediumto

Coarse

Mediumto

coarse

Mediumto

coarse

Fine

Parent Rock

Shale,mudstone,or siltstone

Slate

Phyllite

Schist, granite,or volcanic

rocks

Limestone,dolostone

Quartzsandstone

Bituminouscoal

Comments

Smooth dull surfaces

Breaks along wavysurfaces, glossy sheen

Micaceous mineralsdominate

Banding of minerals

Interlocking calciteor dolomite grains

Fused quartz grains,massive, very hard

Shiny black organic rockthat fractures

Rock Name

Slate

Phyllite

Schist

Gneiss

Marble

Quartzite

Anthracite

Table 3 Classification of Major Metamorphic Rocks

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Use VisualsTable 3 Ask: What is the parentrock of schist? (phyllite) Which hasundergone more intensemetamorphism, slate or gneiss?Explain your answer. (Gneiss hasundergone more intense metamorphism,as indicated by the arrow in the table.)Which nonfoliated rock has the finestgrains? (anthracite)Visual

ASSESSEvaluateUnderstandingHave students examine Table 3. Ask:Generally, what can you say aboutthe relationship between texture andincreasing metamorphism that resultsin foliated rocks? (The more intense themetamorphism, the courser the texture, orlarger the grain size.)

ReteachHave students make tables that compareand contrast contact metamorphismand regional metamorphism.

Sample answer: All are solids that formand change because of Earth processes.All can be classified according to textureand/or composition. The major differenceamong the three rock types is that eachforms at different temperatures andpressures.

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Section 3.4 (continued)

3. Heat can cause existing minerals to recrys-tallize or it can cause new minerals to form.Pressure produces a more compact rock witha greater density. Pressure also causes miner-als to recrystallize. Fluids aid in recrystalliza-tion by making it easier for ions to move andby dissolving original minerals and deposit-ing new ones.4. Foliated rocks are banded metamorphicrocks that form when minerals realign as theresult of pressure from opposing sides.5. Metamorphic rocks can be classifiedaccording to composition and texture.

Section 3.4 Assessment

1. Most metamorphism takes place in a zonethat begins several kilometers below thesurface and extends into the upper mantle.2. Contact metamorphism is a processwhereby slight changes occur in rocks as theresult of an increase in temperature resultingfrom a magma body. Regional metamor-phism, which is associated with mountain-building, can result in high-grade changes inboth composition and structure.

6. While both types of rocks form as theresult of changes in temperature and pres-sure, metamorphism does not involve melting.7. Schists, as indicated in Table 3, are theresult of high-grade metamorphism that isgenerally associated with mountain-building.8. Gneiss is a banded rock that forms as theresult of pressure from opposing sides. Thisdirectional pressure results in foliation. How-ever, because the parent rocks of gneissescan vary, so can the compositions of thesemetamorphic rocks.

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Carbon Dioxide on the MoveIn the atmosphere, carbon is found mainly ascarbon dioxide. This gas absorbs much of theenergy given off by Earth. Therefore, carbon diox-ide influences the heating of the atmosphere.Carbon dioxide constantly moves into and out ofthe atmosphere by way of four major processes:photosynthesis, respiration, organic decay, andcombustion of organic material.

Carbon and Fossil FuelsSome carbon from decayed organic matter isdeposited as sediment. Over long periods of time,this carbon becomes buried. Under the right condi-tions, some of these carbon-rich deposits arechanged to fossil fuels, such as coal. When fossilfuels are burned, huge quantities of carbon dioxideenter into the air.

The Role of Marine AnimalsChemical weathering of certain rocks produce bicar-bonate ions that dissolve water. Groundwater, rivers,

and streams carry these ions to the ocean. Here,some organisms extract this substance to producebody parts—shells, skeletons, and spines—made ofcalcite. When the organisms die, these hard partssettle to the ocean floor and become the sedimen-tary rock called limestone.

The Complete CycleThe source of most CO2 in the atmosphere isthought to be from volcanic activity early in Earth’shistory. When CO2 combines with water, it formscarbonic acid. This substance reacts with rockthrough chemical weathering to form bicarbonateions that are carried by groundwater and streams tothe ocean. Here, marine organisms take over andsedimentary rock is eventually produced. If this rockis then exposed at the surface and subjected tochemical weathering, CO2 is also produced. UseFigure 20 to trace the path of carbon from theatmosphere to the hydrosphere, the geosphere, thebiosphere, and back to the atmosphere.

To illustrate the movement ofmaterial and energy in theEarth system, we can take abrief look at the carbon cycle,shown in Figure 20. Purecarbon is rare in nature. It isfound mainly as two miner-als—diamond and graphite.Most carbon is bonded toother elements to form com-pounds. Carbon dioxide(CO2), for example, is animportant gas in Earth’s atmos-phere. Calcite (CaCO3) is a mineralfound in many sedimentary and meta-morphic rocks. Hydrocarbons, such ascoal, oil, and natural gas, are compoundsmade of carbon and hydrogen. Carbon alsocombines with hydrogen and oxygen to formthe basic compounds that make up living things.This important element moves continually amongEarth’s major spheres by way of the carbon cycle.

Burningof fossil

fuels

Photosynthesisby vegetation

Weatheringof carbonate

rock

Weatheringof

granite

Volcanicactivity

Burning anddecay ofbiomass

Respirationby land

organisms

Photosynthesisand respiration

of marineorganisms

Depositionof carbonatesediments

Burial ofbiomass

CO2

dissolvesin seawater

Lithosphere

Sediment andsedimentary rock

CO2 entering theatmosphere

CO2 leaving theatmosphere

Figure 20 The Carbon Cycle

The Carbon Cycle

The Carbon CycleBackground• During photosynthesis, plants absorb

carbon dioxide from the atmosphereand use it to produce the essentialorganic compounds—complexsugars—that they need for growth.When animals consume plants orother animals that eat plants, theanimals use these organic compoundsas a source of energy. Then, throughthe process of respiration, the animalsreturn carbon dioxide to theatmosphere. Plants also return somecarbon dioxide to the atmosphere byway of respiration.

• When plants die and decay or areburned, this biomass is oxidized andcarbon dioxide is returned to theatmosphere.

• The lithosphere is by far Earth’s largestdepository of carbon. A variety ofrocks contain carbon. The mostabundant is limestone. Whenlimestone undergoes chemicalweathering, the stored carbon isreleased into the atmosphere.

Teaching Tips• Have students contrast different solids

that contain carbon, such as coal,diamond, graphite, calcite, andlimestone. Have students explain howthe carbon is released from each ofthese components of the lithosphereinto Earth’s other spheres.

• Write the chemical equations forphotosynthesis and respiration on theboard or on an overhead transparencyto reinforce the fact that the productsof one reaction are the reactants ofthe other reaction.

• Use a clean, empty 2-L bottle, plants,soil, and a thermometer to make amini-greenhouse to demonstratehow gases in the air, including carbondioxide, can absorb solar energy.Refer to the following Web site fortips on such a demonstration:http://www.bigelow.org/virtual/handson/greenhouse_make.html

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