Chap06 Sedimentary and Metamorphic Rocks

8/10/2019 Chap06 Sedimentary and Metamorphic Rocks

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120

What Youll Learn

How sedimentary rocksare formed.

How metamorphic rocksare formed.

How rocks continuously

change from one type toanother in the rockcycle.

Why Its Important

Sedimentary rocks pro-vide information aboutsurface conditions andorganisms that existed inEarths past. In addition,

mineral resources arefound in sedimentary andmetamorphic rocks. Therock cycle further pro-vides evidence that Earthis a dynamic planet, con-stantly evolving andchanging.

SedimentaryandMetamorphic

Rocks

SedimentaryandMetamorphic

Rocks

66

Mount Kidd, Alberta, CanadaMount Kidd, Alberta, Canada

To find out more aboutsedimentary and metamor-phic rocks, visit theEarth Science Web Siteat earthgeu.com
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6.1 Formation of Sedimentary Rocks 1

Sedimentary rocks are usuallyfound in layers. How do these layers

form? In this activity, you will investi-gate how layers form from particlesthat settle in water.

. Obtain 100 mL of soil from a loca-tion specified by your teacher.Place the soil in a tall, narrow, jar.

. Add water to the jar until it isthree-fourths full. Put the lid onthe jar so that it is tightly sealed.

. Pick up the jar with both handsand turn it upside down severaltimes to mix the water and soil.

. Quickly turn the jar upright andset it on a flat surface.

In your science journal,draw a diagram of what you observe.What type of parti-cles settled out first?What type of parti-cles form the topmostlayers? How is thisactivity related to thelayering that occursin sedimentary rocks?

DiscoveryLabDiscoveryLab

OBJECTIVES

Sequence the formationof sedimentary rocks.

Explain the formationand classification ofclastic sediments.

Describefeatures ofsedimentary rocks.

VOCABULARY

sediment bedding

You learned in Chapter 5 that igneous rocks are the most commrocks in Earths crust, yet when you look at the ground, you may nsee igneous rocks. In fact, you usually dont see any solid rock at aWhy is this? Much of Earths surface is covered with sedimenSediments are pieces of solid material that have been deposited Earths surface by wind, water, ice, gravity, or chemical precipitatioWhen sediments become cemented together, they form sedimentarocks. The formation of sedimentary rocks begins when weatheriand erosion produce sediments.

WEATHERINGWherever Earths crust is exposed at the surface, it is continuoubeing worn away by weathering, a set of physical and chemiprocesses that break rock into smaller pieces. Chemical weatherioccurs when the minerals in a rock are dissolved or otherw

Formation of

Sedimentary Rocks

6.16.1

clasticdepositionlithificationcementation

gradedbedding

cross-bedding
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chemically changed. Study Figure 6-1. What happens to more-resistant minerals during weathering? While the less-stable mineralsare chemically broken down, the more-resistant grains are brokenoff of the rock as smaller grains. During physical weathering, onthe other hand, minerals remain chemically unchanged. Rock frag-ments simply break off of the solid rock along fractures or grainboundaries.

Weathering produces rock and mineral fragments known as clasticsediments. The word clastic comes from the Greek word klastos,meaning broken. Clastic sediments range in size from huge boul-ders to microscopic particles. Table 6-1 summarizes the classificationof clastic sediments based on size. Clastic sediment particles usuallyhave worn surfaces and rounded corners caused by physical abrasion

during erosion and transport.

EROSION ANDTRANSPORTAfter rock fragments have been weathered out of outcrops, theyare transported to new locations. The removal and movement ofsurface materials from one location to another is called erosion.

122 CHAPTER 6 Sedimentary and Metamorphic Rocks

Table 6-1 Classification of Clastic Sediments

Particle Size Sediment Rock

> 256 mm Boulder25664 mm Gravel Cobble Conglomerate642 mm Pebble

20.062 mm Sand Sandstone

0.0620.0039 mm Silt Siltstone


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Figure 6-2 shows the four main agents of erosion: wind, movingwater, gravity, and glaciers. Visible signs of erosion are all around

you. For example, water in streams becomes muddy after a stormbecause silt and clay particles have been added to it. The dust thatcollects on shelves in your home is another indication of erosion.Where do you think this dust comes from? How is it carried and howdoes it eventually settle on the shelves?

Eroded materials are almost always carried downhill. Althoughwind can sometimes carry fine sand and dust to higher elevations,particles transported by water are almost always moved downward.Eventually, even wind-blown dust and fine sand are pulled downhillby gravity. You will learn more about this in the next chapter.

Deposition When sediments are laid down on theground or sink to the bottoms of bodies of water depositionoccurs. During the Discovery Lab at the beginning of thischapter, what happened when you stopped moving the jar

full of sediment and water? The sediment sank to the bot-tom and was deposited. Similarly, in nature, sediments are


Figure 6-2 Winds blowsand into dunes on theNavajo Indian Reservationin Arizona (A). The riverLethe cuts through theValley of Ten ThousandSmokes in Katmai NationaPark, Alaska (B). This land

slide in Papua, New Guinecarried the entire hillside300 m into the canyon (CThis terminal moraine wabuilt up by the AthabascaGlacier in Jasper NationalPark, Alberta, Canada (D)

A

B

C

D
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deposited when transport stops. Perhaps the wind stops blowing or a

river enters a quiet lake or the ocean. In each case, the particles beingcarried will settle out, forming layers of sediment. You observed thiswhen you completed the Discovery Labat the beginning of this chap-ter, that the sediment formed layers as it settled to the bottom of the

jar. The sediment formed a layered deposit with the largest, grains atthe bottom and the smallest particles at the top. As the water in the jarslowed down, the largest particles settled out first. Why? Faster-moving water can transport larger particles. As water slows down, thelargest particles settle out first, then the next-largest, and so on,so thatdifferent-sized particles are sorted into layers. Such deposits are char-acteristic of sediment transported by water and wind.Wind, however,can move only small grains. For this reason, sand dunes, such as theones shown in Figure 6-3A, are commonly made of fine, well-sortedsand like the sand in Figure 6-3B.

Not all sediment deposits are sorted. Glaciers, for example, moveall materials with equal ease. Large boulders, sand, and mud are allcarried along by the ice and dumped in an unsorted pile at the endof the glacier. Landslides create similar deposits when sedimentmoves downhill in a jumbled mass.

Burial Most sediments are ultimately deposited on Earth in

depressions called sedimentary basins. These basins may containlayers of sediment that together are more than 8 km thick. As moreand more sediment is deposited in an area, the bottom layers aresubjected to increasing pressure and temperature. These conditionscauselithification, the physical and chemical processes that trans-form sediments into sedimentary rocks. Lithify comes from theGreek word lithos, which means stone.

A

BB

Figure 6-3 This large sanddune(A) in Algeria, Africa,

is made up of fine sandsuch as this from Kalahari,South Africa (B).


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LITHIFICATIONLithification begins with compaction. The weightof overlying sediments forces the sediment grainscloser together, causing the physical changesshown in Figure 6-4. Layers of mud may containup to 60 percent water, and these shrink as excess

water is squeezed out. Sand, however, is usuallywell compacted during deposition, and resistsadditional compaction during burial. Grain-to-grain contacts in sand form a supporting frame-work that helps maintain open spaces between thegrains. Groundwater, oil, and natural gas are com-monly found in these spaces in sedimentary rocks.

The temperature in Earths crust increases withdepth by about 30C per kilometer. Sediments that are buried 3 to 4 kmdeep experience temperatures that are high enough to start thechemical and mineral changes that cause cementation. Cementationoccurs when mineral growth cements sediment grains together intosolid rock. There are two common types of cementation. The firsttype occurs when a new mineral, such as calcite (CaCO3) or ironoxide (Fe2O3) grows between sediment grains as dissolved mineralsprecipitate out of groundwater. The second type occurs when exist-ing mineral grains grow larger as more of the same mineral precipi-tates from groundwater and crystallizes around them. These twotypes of cementation are shown in Figure 6-5.


50%60% H2O

10%20% H2O

Grain to grain contac

prevent compactio

Mud

Sand

Figure 6-4 Pressure andweight from overlyingsediments causes flat clayparticles to compact (A).The irregular shape of sangrains prevents similaramounts of compaction (B

Quartz sandgrains

Calcite cement

between grainsQuartz crystallized

on quartz sand grains

Quartz sandgrains

Figure 6-5 Cementation occurs in one of twoways. Either a new mineral, such as the calciteshown in A, grows between the grains (B) or,the same mineral grows between and over thegrains in a process called overgrowth (C).

B C

AQuartz

Calcite

A

B


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FEATURES OFSEDIMENTARYROCKSThe primary feature of sedimentary rocksis horizontal layering, called bedding.Bedding can range from a millimeter-thicklayer of shale to sandstone deposits several

meters thick. The type of bedding dependsupon the method of transport, while thesize of the grains and the material withinthe bedding depend upon many factors.

Bedding in which the particle sizesbecome progressively heavier and coarsertowards the bottom layers is called graded

bedding. Graded bedding is often observedin marine sedimentary rocks that weredeposited by underwater landslides. As thesliding material slowly came to rest underwa-ter, the largest and heaviest material settledout first and was followed by progressivelyfiner material.

Another characteristic feature of sedi-mentary rocks is cross-bedding. As you cansee in Figure 6-6A,cross-beddingis formedas inclined layers of sediment move forwardacross a horizontal surface. Small-scalecross-bedding can be observed at sandybeaches and along sandbars in streams and

rivers. Most large-scale cross-bedding, suchas that shown in Figure 6-6B, is formed bymigrating sand dunes. Small sedimentaryfeatures, such as the ripple marks shown

Current direction

Current direction

Sandpart

icles

Partic

lemovement

What happenedhere?

Interpret animalactivity from pat-

terns of fossil foot-

prints.

Procedure

1. Study the photograph of a set of foot-

prints that has been preserved in sedi-

mentary rocks.

2. Write a description of how these tracks

might have been made.

3. Draw your own diagram of a set of fos-silized footprints that record the interac-

tions of organisms in the environment.

4. Give your diagram to another student and

have them interpret what happened.

Analyze and Conclude

1. How many animals made the tracks

shown?

2. What types of information can be inferred

from a set of fossil footprints?

3. Did other students interpret your diagramthe same way? What might have caused

any differences?


A

B

Figure 6-6 Cross-bedding is formed as sedimentis carried forward across a layer of sediment, andcascades down the front face of the layer (A).Large-scale cross-bedded sandstones are commonin Zion National Park (B).


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in Figure 6-7, are also preserved in sedimentaryrocks. Ripple marks form when sediment is movedinto small ridges by wind or wave action, or by a rivercurrent. The back-and-forth movement of waves cre-ates ripples that are symmetrical, while a currentflowing in one direction, such as in a river or stream,produces asymmetrical ripples. If a rippled surface isburied gently by more sediment without being dis-turbed, it might later be preserved in solid rock.

Evidence of Past Life Probably the best-known features of sedi-

mentary rocks are fossils. Fossils are the preserved remains, impres-sions, or any other evidence of once-living organisms. When anorganism dies, it may be buried before it decomposes. If its remains arefurther buried without being disturbed, it might be preserved as a fos-sil. During lithification, parts of the organism can be replaced by min-erals and turned into rock, such as fossilized shells. Fossils are of greatinterest to Earth scientists because fossils provide evidence of the typesof organisms that lived in the distant past, the environments thatexisted in the past, and how organisms have changed over time. Youwill learn more about fossils and how they form in Chapter 21. By

doing theMiniLab on the previous page, you can learn first-hand howfossils can be used to interpret past events.


1. How are clastic sediments formed, andhow do scientists classify them?

2. Why do sediment deposits tend to formlayers?

3. As sediments are buried, what two factorsincrease with depth? How do these fac-tors cause lithification?

4. Compare and contrast graded beddingand cross-bedding.

5.Thinking Critically Is it possible for a layerof cross-bedded strata to show gradedbedding as well? Explain.

SKILLREVIEW6.Sequencing Sequence the processes by

which a sedimentary rock is formed fromclastic sediments. For more help, refer totheSkill Handbook.

Figure 6-7 Given the right sedimentaryconditions, these modern sand ripples(A) could become preserved like theseripple marks in Capitol Reef NationalPark, Utah (B).

A

B

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6.26.2 Types of Sedimentary Rocks

The classification of sedimentary rocks is based on how they wereformed. There are three main groups of sedimentary rocks: clastic,chemical, and organic. Table 6-2 summarizes the classification sys-

tem for sedimentary rocks.

CLASTICSEDIMENTARYROCKSThe most common type of sedimentary rocks,clastic sedimentaryrocks, are formed from the abundant deposits of loose sedimentsfound on Earths surface. Clastic sedimentary rocks are further clas-sified according to the sizes of their particles. This classification sys-tem was shown in Table 6-1 on page 122.

Coarse-Grained Clastics Sedimentary rocks consisting ofgravel-sized rock and mineral fragments are classified as coarse-grained clastics, as shown in Figure 6-8. What differences betweenthese two rocks do you notice? Conglomerates are coarse-grainedsedimentary rocks that have rounded particles,whereas breccias con-tain angular fragments. How are these different-shaped particlesformed? Because of its relatively large mass, gravel is transported byhigh-energy flows of water, such as those generated by mountainstreams, flooding rivers, some ocean waves, and glacial meltwater.During transport, gravel becomes abraded and rounded as the parti-cles scrape against one another. This is why beach and river gravelsare often well rounded. Conglomerates provide evidence that this

type of transport occurred in the past. In contrast, the angularity of


OBJECTIVES

Describe the types ofclastic sedimentary rocks.

Explain how chemicalsedimentary rocks form.

Describe organic sedi-mentary rocks.

Recognize the impor-tance of sedimentaryrocks.

VOCABULARY

clastic sedimentary rockporosityevaporite

Table 6-2 Classification of Sedimentary Rocks

Rock Type Rock Name Method of Formation

ClasticCoarse-grained Conglomerate or breccia Lithification ofMedium-grained Sandstone clastic sedimentsFine-grained Shale

Chemical Limestone

Calcite Rock salt Precipitation of Halite Rock gypsum dissolved mineralsGypsum from water

OrganicCalcium carbonate Accumulation and

shells Limestone lithification of remainsplant matter Coal of living things

}


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CHEMICALSEDIMENTARYROCKSWhat happens when you allow a glass of saltwater to evaporate?Eventually, the water disappears and a layer of salt accumulates onthe bottom of the glass. A similar process occurs in nature whenchemical sedimentary rocks are formed. During chemical weather-ing, minerals can be dissolved and carried into lakes and oceans. As

water evaporates from the lakes and oceans, the dissolved mineralsare left behind. In arid regions, high evaporation rates can increasethe concentration of dissolved minerals in bodies of water. The GreatSalt Lake, shown in Figure 6-10A, is a well-known example of a lakethat has high concentrations of dissolved minerals.

Rocks Formed from Evaporation When the concentration ofdissolved minerals in a body of water reaches saturation, which is thepoint at which no more minerals can be dissolved in the water, crys-tal grains precipitate out of solution and settle to the bottom. Thelayers of chemical sedimentary rocks that form as a result are calledevaporites. Evaporites most commonly form in arid regions, inoceans and in drainage basins on continents that have low water

flow. Because little freshwater flows into these areas,the concentration of dissolved minerals remains high.

As more dissolved minerals are carried into thebasins, evaporation continues to remove freshwaterand maintain high mineral concentrations. Over time,thick layers of evaporite minerals can accumulate onthe basin floor, as illustrated in Figure 6-10B.

Figure 6-10 Evaporation ofwater from the Great SaltLake, Utah, has resulted insalt precipitation on theseboulders(A). The processof evaporite formation isillustrated in B.


Evaporation

Evaporating shallow basin

(high salinity)

Crystals of gypsum

or halite settle to bottomEvaporite sediment:

gypsum and halite

Barrier bar

or otherflow restriction

Replenishment

from open ocean

Freshwater

inflow (small)

Ocean

A

B


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The three most common evaporite minerals are calcite (CaCO3),

halite (NaCl), and gypsum (CaSO4). Layers of these minerals areoften mined for their chemical content.

Organic Sedimentary Rocks Organic sedimentary rocks areformed from the remains of once-living things. The most abundantorganic sedimentary rock is limestone, which is composed primarilyof calcite. Some organisms that live in the ocean use the calcium car-bonate dissolved in seawater to make their shells. When these organ-isms die, their shells settle to the bottom of the ocean and can formthick layers of carbonate sediment. During burial and lithification,calcium carbonate precipitates out of the water, crystallizes betweenthe grains of carbonate sediment, and forms limestone. Limestone iscommon in shallow water environments such as those in theBahamas, where coral reefs thrive in 15 to 20 m of water just off-shore. The skeleton and shell materials that are currently accumulat-ing there will someday become limestone as well. Many types oflimestone contain evidence of their biologic origin in the form ofabundant fossils. As shown in Figure 6-11, these fossils range fromlarge corals to microscopic unicellular organisms. Other organismsuse silica to make their shells. These shells form sediment that isoften referred to as siliceous ooze because it is rich in silica.

Another type of organic sedimentary rock, coal, forms from theremains of plant material. Over long periods of time, thick layers ofvegetation slowly accumulate in swamps and coastal areas. Whenthese layers are buried and compressed, they are slowly lithified intocoal. Coal is composed almost entirely of carbon and can be burnedfor fuel. You will learn more about coal as an energy source inChapter 25.

6.2 Types of Sedimentary Rocks 1

Figure 6-11 Fossils in organic sedimentary rocksmay range in size from corals such as these in a lime-stone from South Florida (A), to these Nummulitesmicrofossils(B) preserved in the limestones that wereused to build the pyramids in Egypt.

Magnification:

AA

B

Topic: Coral ReefsFor an online update ofcoral reefs in the Bahamavisit the Earth ScienceWeb Site at earthgeu.com

Activity: Discuss the causand significance of themajor bleaching event of1997-1998 in coral reefsaround the world.
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IMPORTANCE OFSEDIMENTARYROCKSThe characteristic textures and features of sedimentary rocks, such ascross-bedding, ripple marks, layering, and fossils, provide a geologicsnapshot of surface conditions in Earths past. Fossils, for example,provide information about animals and plants that existed in thepast. Other sedimentary features indicate the location and direction

of flow of ancient rivers, the wave or wind direction over lakes anddeserts, and ancient shoreline positions. Rock fragments found inconglomerates and breccias are large enough to easily identify whattypes of bedrock they were eroded from. By considering all of thisinformation, geologists can reconstruct the nature of Earths surfaceat various times in the past. Thus, they can better understand howgeologic changes occur over time.

Energy Resources The study of sedimentary rocks providesinformation about Earths past, but it also has great practical value.Many of the natural resources used by humans come from sedimen-tary rocks. For example oil, natural gas, and coal are found in sedi-mentary rocks. Uranium, which is used for nuclear power, is oftenmined from sandstone. Large deposits of phosphate, which is usedfor fertilizer, and iron, which is used to make steel, are also found insedimentary rocks. Limestone is processed to make cement for theconstruction industry. Sandstone and limestone are often cut intoblocks for use in walls and buildings. Were any sedimentary rocksused to construct your school? What sedimentary rocks were used inthe construction of your home?


EnvironmentalConnection

1. Compare and contrast the main types ofclastic sedimentary rocks.

2. Why do chemical sedimentary rocks formprimarily in areas that have high rates ofevaporation?

3. Why is coal an organic sedimentary rock?

4. What are some of the commercial valuesof sedimentary rocks?

5.Thinking Critically The original concentra-tion of dissolved minerals in a restricted

ocean basin was enough to form only athin evaporite layer. How, then, is it possi-ble that thick evaporite layers formedthere?

SKILLREVIEW6.Comparing and Contrasting Make a data

table to compare and contrast the forma-tion of the three types of sedimentaryrock. For more help, refer to the SkillHandbook.

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6.36.3 Metamorphic Rocks

You have learned that increasing pressure and temperature duringburial cause recrystallization and cementation of sediments. Whathappens when rocks are buried at even greater depths?

CAUSES OFMETAMORPHISMPressure and temperature increase with depth. When temperatureor pressure becomes high enough, rocks melt and form magma. Butwhat happens if the rocks do not quite reach the melting point?When high temperature and pressure combine to alter the texture,mineralogy, or chemical composition of a rock without melting it, ametamorphic rock forms. The word metamorphism is derived fromthe Greek words meta, meaning change, and morphe meaningform. During metamorphism, a rock changes form while remain-ing solid.

The high temperatures required for metamorphism ultimatelyare derived from Earths internal heat, either through deep burial orfrom nearby igneous intrusions. The high pressures required formetamorphism can be generated in two ways: from vertical pressurecaused by the weight of overlying rock, or from the compressiveforces generated as rocks are deformed during mountain building.

TYPES OFMETAMORPHISMDifferent combinations of temperature and pressure result indifferent types of metamorphism, shown in Figure 6-12. Each com-

bination produces a different group of metamorphic minerals and

6.3 Metamorphic Rocks 1

Regional Metamorphic Grades

1000

800

600

Pressure(MPa)

400

10

20

Depth(km)

30

200

Lithification

Low grade

Intermediate

gradeHigh grade

Partial melting

of granites

0

200 400 600

Temperature (C)

800 1000

Figure 6-12 The grade ometamorphism, whetherit is low, medium or high,is dependent upon thepressure on the rocks,the temperature and thedepth below the surface.

OBJECTIVES

Compare andContrasthe different types and

causes of metamorphis

Distinguish amongmetamorphic textures.

Explain how mineraland compositionalchanges occur duringmetamorphism.

Understand how rockscontinuously change

from one type to anothin the rock cycle.

VOCABULARY

regional metamorphismcontact metamorphismhydrothermal metamor-phism

foliatednonfoliated

porphyroblastrock cycle


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textures. When high temperature and pressure affect large regions ofEarths crust, they produce large belts ofregional metamorphism.Regional metamorphism can be low grade, intermediate grade, andhigh grade. The grade of regional metamorphism reflects the relativeintensity of temperature and pressure, with low-grade metamor-phism reflecting the lowest temperature and pressure. Figure 6-13shows the regional metamorphic belt that has been mapped in the

northeastern United States. Geologists have divided the belt intozones based upon the mineral groups found in the rocks. Some ofthe key minerals used to map metamorphic zones are listed in Figure6-14. Knowing the temperatures that certain areas experienced whenrocks were forming can help geologists locate economically valuable

Unmetamorphosed

Biotite zone

Staurolite zone

Sillimanite zone

Granitic plutons

Garnet zone

0 200 km100

N

Prince Edward

Island

Nova Scotia

New

Brunswick

Canada

United States

MA

CT

NH

ME

Metamorphic Rock BeltsFigure 6-13 The northeastportion of North Americahas undergone severalepisodes of regional meta-morphism. The results canbe seen in the distributionof metamorphic rocks.

Lithification

Chlorite

White mica (mainly muscovite)

Biotite

Garnet

Staurolite

Kyanite

Albite (sodium plagioclase feldspar)

Low grade Intermediate grade High grade

Sillimanite

Minerals in Metamorphosed Shale

Figure 6-14 Mineralchanges in shale (A) andbasalt(B), as a result ofmetamorphism follow aspecific path. The grade ofmetamorphism (low, inter-mediate, or high) determineswhich minerals will form.

Lithification

Chlorite

Zeolites

Epidote

Amphibole

Garnet

Pyroxene

(Sodium-rich) Plagioclase feldspar (Calcium-rich)

Low grade Intermediate grade High grade

Minerals in Metamorphosed BasaltA B


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metamorphic minerals such as garnetand talc. An interesting connectionbetween talc and asbestos, another meta-morphic mineral, is described in theScience in the News feature at the end ofthis chapter.

When molten rocks, such as those in anigneous intrusion, come in contact withsolid rock, a local effect called contactmetamorphism occurs. High temperatureand moderate-to-low pressure form themineral assemblages that are characteristicof contact metamorphism. Figure 6-15shows zones of different minerals sur-rounding an intrusion. Why do you thinkthese zones occur? Because temperature decreases with distance froman intrusion, metamorphic effects also decrease with distance. Recall

from Chapter 4 that minerals crystallize at specific temperatures.Minerals that crystallize at high temperatures are found closest to theintrusion, where it is hottest. Contact metamorphism from extrusiveigneous rocks is limited to thin zones. Normally, lava cools tooquickly for the heat to penetrate very far into surface rocks.

When very hot water reacts with rock and alters its chemistry andmineralogy hydrothermal metamorphism occurs. The wordhydrothermal is derived from the Greek words hydro, meaningwater, and thermal, meaning heat. Hydrothermal fluids can dis-solve some minerals, break down others, and deposit new minerals.

These types of changes caused the yellow color of the cliffs shown inFigure 6-16. Hydrothermal metamorphism is common aroundigneous intrusions and near active volcanoes.


High

-tem

perat

ureme

tamorp

hicroc

k

Sillima

nite

Bio

tite-

Andal

usite

Chlo

rite-M

usc

ovi

te

Granitebatholith

Temperaturedecreasing

1 km

Figure 6-15 Contact metmorphism results in large-scale mineral changes wit

little deformation. Geolo-gists can follow the occurrence of metamorphicminerals to locate theigneous intrusion.

Figure 6-16 YellowstoneNational Parks name comfrom the hydrothermallychanged rocks of the areaThese rocks are beautifullexposed in the GrandCanyon of the YellowstonWyoming.


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METAMORPHICTEXTURESMetamorphic rocks are classified into twotextural groups: foliated and nonfoliated.Wavy layers and bands of minerals charac-terize foliated metamorphic rocks. Highpressure during metamorphism causes

minerals with flat or needlelike crystals toform with their long axes perpendicular to the pressure, as shown inFigure 6-17. This parallel alignment of minerals creates the layersobserved in foliated metamorphic rocks. The two most commontypes of foliated metamorphic rock are schist, which is derived fromshale, and gneiss, which is derived from granite. Some common foli-ated metamorphic rocks are compared in Figure 6-18.

Unlike foliated rocks,nonfoliated metamorphic rocks lack min-eral grains with long axes in one direction. Nonfoliated rocks arecomposed mainly of minerals that form with blocky crystal shapes.Two common examples of nonfoliated rocks, shown in Figure 6-19,are quartzite and marble. Quartzite is a hard, light-colored rockformed by the metamorphism of quartz-rich sandstone. Marble isformed by the metamorphism of limestone. Some marbles have verysmooth textures that are formed by interlocking grains of calcite.Such marbles are sought by artists for sculptures.

Porphyroblasts Under certain conditions, new metamorphicminerals can grow quite large while the surrounding mineralsremain small. The large crystals, which can range in size from a fewmillimeters to a few centimeters, are called porphyroblasts.Porphyroblasts are found in areas of both contact and regional

metamorphism. These crystals resemble the very large crystalsfound in porphyritic igneous rocks but form not from magma but

Figure 6-17 Compressionalpressure causes elongateminerals to line up perpen-dicular to the pressuredirection(A). This photo-micrograph of a mica schist(B) shows the resulting foli-ation.

Magnification: 7

A

B

A

Figure 6-18 These commonfoliated rocks are arrangedin order of increasing meta-morphic grade: slate(A),phyllite(B), gneiss(C), andschist(D).


B

D

C


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in solid rock by the reorganization of atoms during metamorphism.Garnet, shown in Figure 6-20, is a mineral that commonly formsporphyroblasts.

MINERALCHANGESHow do minerals change without melting? Think back to the conceptof fractional crystallization, discussed in Chapter 5. Minerals are sta-ble at certain temperatures and crystallize from magma at differenttemperatures. Scientists have discovered that these stability rangesalso apply to minerals in solid rock. During metamorphism, the min-erals in a rock change into new minerals that are stable under the newtemperature and pressure conditions. Minerals that change in thisway are said to undergo solid-state alterations. Scientists have con-ducted experiments to identify the metamorphic conditions that cre-ate specific minerals. When these same minerals are found in rocks,scientists are able to interpret the conditions inside the crust duringthe rocks metamorphism. Recall from page 134 that these conditions

are temperature- and pressure-related, with low temperatures andpressures resulting in low-grade metamorphism. You will comparethe changes in mineralogy as a result of high- and low-grade meta-morphism in the Problem-Solving Labon the next page.

COMPOSITIONALCHANGESMost metamorphic rocks reflect the original chemical compositionof the parent rock. Gneiss, for example, has the same general chemi-cal composition as granite. In some instances, however, the chem-istry of a rock can be altered along with its minerals and texture. This

occurs because hot fluids migrate in and out of the rock duringmetamorphism, which can change the original composition of therock. Chemical changes are especially common during contact meta-morphism near igneous intrusions. Hydrothermal fluids invade thesurrounding rocks and change their mineralogy, textures, and chem-istry. Valuable ore deposits of gold, copper, zinc, tungsten, and leadare formed in this manner.


Figure 6-19 Despite theextreme temperature andpressures, cross-beddingand ripple marks are oftepreserved in quartzite (A)Fossils, on the other handare never preserved inmarble(B).

Figure 6-20 This garnetmica schist comes froman exposure in RoxburyFalls, Connecticut.

A B


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THEROCKCYCLEMetamorphic rocks are formed by the changing of other rocks. Whattypes of rocks can metamorphic rocks be changed into? The threetypes of rock igneous, sedimentary, and metamorphicaregrouped according to how they form. Igneous rocks crystallize frommagma; sedimentary rocks form from cemented sediments; and meta-

morphic rocks form by changes in temperature and pressure. Once arock forms, does it remain the same type of rock forever? Maybe, butprobably not. Heat and pressure may change an igneous rock into ametamorphic rock.A metamorphic rock may be changed into anothermetamorphic rock or melted to form an igneous rock. Or, the meta-morphic rock may be weathered and eroded into sediments that maybecome cemented into a sedimentary rock. In fact, any rock can bechanged into any other type of rock. This continuous changing andremaking of rocks is called the rock cycle. The rock cycle is summa-rized in Figure 6-21. The arrows represent the different processes thatchange rocks into different types. Essentially, rocks are recycled intodifferent rock types much like glass is recycled into different types of

jars and bottles. You will learn more about the similarities and differ-ences between rock types when you complete the GeoLabat the end ofthis chapter.


Determine which metamorphicminerals will form The types of min-erals found in metamorphic rocks depends

on metamorphic grade and composition

of the original rock. In this activity, you

will compare how these factors affectmetamorphic minerals.

Analysis

1. Study Figure 6-14 on page 134, show-ing the different mineral groups that

are created under different metamor-phic conditions. What mineral is

formed when shale and basalt are

exposed to low-grade metamorphism?

2. What mineral is formed when shale isexposed to high-grade metamorphism

that is not found in basalts under the

same conditions?

Thinking Critically

3. Compare the mineral groups youwould expect to form from intermedi-

ate metamorphism of shale, basalt andlimestone.

4. What are the major compositional dif-

ferences between shale and basalt?

How are these differences reflected

in the minerals formed during meta-morphism?

5. When limestones are metamorphosed,there is very little change in mineral-

ogy, and calcite is still the dominant

mineral. Explain why this happens.

Interpreting Scientific Illustrations


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Interpreting Changes

in RocksAs the rock cycle continues, and rocks change from onetype to another, more changes occur than meet the eye.Color, grain size, texture and mineral composition are easily

observed and described visually. Yet, with mineral changes

come changes in crystal structure and density. How can these

be accounted for and described? Studying pairs of sedimentary

and metamorphic rocks can show you how.

ProblemHow do the characteristics of sedimen-tary and metamorphic rocks compare?

Materialssamples of sedimentary rocks and their

metamorphic equivalentsmagnifying glass or hand lenspaperpencilbeam balance100-mL graduated cylinder or beaker

large enough to hold the rock sampleswater

Objectives

In this GeoLab, you will: Describe the characteristics of sedi-

mentary and metamorphic rocks. Determine the density of different

rock types.

Infer how metamorphism changesthe structure of rocks.

Safety PrecautionsAlways wear safety goggles and anapron in the lab.

Preparation


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GeoLab 1

1. Prepare a data table similar to the

one shown below.2. Observe each rock sample. Recordyour observations in the data table.

3. Recall that density = mass/volume.Make a plan that will allow you to

measure the mass and volume of a

rock sample.4. Determine the density of each rocksample and record this informationin the data table.

1. Compare and contrast a shale anda sandstone.

2. How does the grain size of a sand-stone change during metamorphism?

3. What textural differences do youobserve between a shale and a slate?

4. Compare the densities you calculatedwith other students. Does everybodyhave the same answer? What are someof the reasons that answers may vary?

1. Why does the color of a sedimentaryrock change during metamorphism?

2. Compare the density of a slate and aquartzite. Which rock has a greater

density? Explain.

3. Compare the densities of shale andslate, sandstone and quartzite, andlimestone and marble. Does densityalways change in the same way?

Explain the results that you observed.

Procedure

Analyze

Conclude & Apply

Sample Rock Specificnumber Type characteristics Mass Volume Density

1

2

3

4

DATATABLE


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Research the use of talc in other products.Why is talcum powder no longer recom-

mended for infants? Does the use of talc in

make-up expose teens to the same

asbestos risk that crayons posed for

younger children? Present your findings

in a written report.

Activity

point out, though, that if the talc fibers look so

much like asbestos fibers, they are likely to have

the same affect in the lungs.

The risk to children from the asbestos and

asbestos-like fibers in crayons is extremely

small. The amount of fibers present is very low,

and, because the talc is embedded in wax, there

is little likelihood that the fibers will be inhaled.

The Consumer Product Safety Commission did a

test that simulated one half-hour of hard coloring

and found no airborne fibers. Still, crayon manu-

facturers understand that, when it comes to chil-dren, any slight risk is too much. All of the major

crayon makers have decided to change their

crayon formulas to eliminate all use of talc. The

use of talc in childrens chalk, clay, and sand is

also being investigated. The use of asbestos was

phased out in the United States in 2001.

What do crayons have to do with rocks? A

metamorphic mineraltalcis used in the man-

ufacture of crayons. In fact, its the properties of

talc that led to the asbestos scare.

l

Talc is a soft white mineral with a hardness of

1. It is used in many cosmetics and art supplies.

Talc is often found in association with serpentine,

which is the parent rock of asbestos. Asbestos

comes from the mineral chrysotile, which breaks

into tiny, hairlike fibers. Asbestos has been usedin insulation and in fireproof fabrics and building

products. However, in the 1960s and 1970s, it

was discovered that inhaling asbestos fibers leads

to lung cancer, asbestosis, and mesothelioma

each of which can be fatal. Beginning in the

1980s, virtually all uses of asbestos were banned

in the United States and much of Europe.

u , r tGround up talc is used as a hardener in

crayons, which are basically a mixture of pig-

ments, hardeners, and wax. Without talc, crayons

would get too sticky to handle. However, when

talc is ground, fibers that resemble asbestos are

created. Testers arent sure whether the tiny

amount of asbestos found in crayons comes

from these asbestos-like fibers, from chrysotile

contamination of the talc, or from both. They

Good NewsCrayons SafeThe Consumer Products Safety Commission, after extensive testing,declared that crayons are safe for children to use. Although trace

amounts of asbestos and asbestos-related fibers were found in many

of the crayons tested, the amounts were not considered to be dan-

gerous. No recall was issued, but crayon manufacturers were urged to

create a new recipe for crayons to exclude the asbestos fibers.


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Summary

Study Guide 1

Vocabulary

bedding (p. 126)cementation (p. 12clastic (p. 122)cross-bedding

(p. 126)deposition (p. 123graded bedding

(p. 126)lithification (p. 124sediment (p. 121)

Vocabulary

contact metamor-phism (p. 135)

foliated (p. 136)hydrothermal met

morphism (p. 13nonfoliated (p. 13porphyroblast

(p. 136)regional metamor

phism (p. 134)rock cycle (p. 138)

Main Ideas

The processes of weathering, erosion, deposition, burial, andlithification form sedimentary rocks.

Clastic sediments are rock and mineral fragments produced byweathering and erosion. They are classified based on particlesize.

Sediments are lithified into rock by the processes of compactionand cementation.

Sedimentary rocks can contain depositional features such ashorizontal bedding, cross-bedding, and ripple marks.

Fossils are the remains or other evidence of once-living thingsthat are preserved in sedimentary rocks.

Main Ideas

Metamorphic rocks are formed when existing rocks are sub-jected to high temperature and pressure, which cause changesin the rocks textures, mineralogy, and composition.

The three main types of metamorphism are regional, contact,and hydrothermal.

Metamorphic rocks are divided into two textural groups: foliatedand nonfoliated.

During metamorphism, minerals change into new minerals thatare stable under the conditions of temperature and pressure atwhich they formed.

The rock cycle is the set of processes whereby rocks continu-ously change into other types of rock.

SECTION6.1

Formation of

SedimentaryRocks

SECTION6.33

MetamorphicRocks

Vocabularyclastic sedimentar

rock (p. 128)evaporite (p. 130)porosity (p. 129)

Main Ideas There are three main classes of sedimentary rocks: clastic, which

are formed from clastic sediments; chemical, which are formedfrom minerals precipitated from water; and organic, which areformed from the remains of once-living things.

Clastic sedimentary rocks are classified by particle size andshape.

Evaporites are chemical sedimentary rocks that form primarily inrestricted ocean basins in regions with high evaporation rates.

Limestone, composed primarily of calcite, is the most abundantorganic sedimentary rock. Coal is another organic sedimentary

rock. Sedimentary rocks provide geologists with information about

surface conditions that existed in Earths past.

SECTION6.2Types ofSedimentaryRocks

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Test-Taking Tip

1. What are solid particles that have been depositedon Earths surface called?a. porphyroblasts c. schistsb. sediments d. quartzites

2. What process breaks solid rock into smaller pieces?a. deposition c. weatheringb. cementation d. metamorphism

3. What agent of erosion can usually move onlysand-sized or smaller particles?a. landslides c. waterb. glaciers d. wind

4. Which of the following is an example of amedium-grained clastic sedimentary rock?a. conglomerate c. evaporiteb. breccia d. sandstone

5. Which of the following are formed by thechemical precipitation of minerals from water?a. sandstones c. salt bedsb. coal beds d. shale

6. Which of the following would you expect to havethe greatest porosity?a. sandstone c. shaleb. gneiss d. quartzite

7. Which of the following is a common mineral foundin both organic and chemical sedimentary rocks?a. calcite c. garnetb. quartz d. biotite

8. By what process are surface materials removedand transported from one location to another?a. weathering c. depositionb. erosion d. cementation

9. What mineral commonly forms porphyroblasts?a. quartz c. talcb. garnet d. calcite

Understanding Main Ideas 10. What are the two primary causes of lithification?

11. Why is the term clasticappropriate for particles

weathered from solid rock?12. Describe the two main types of cementation.

13. How are the three types of sedimentary rocksclassified?

14. Rearrange the terms below to create a conceptmap of the rock cycle.

15. What are the two most common types of foliatedmetamorphic rocks?

16. What are porphyroblasts, and how do they form?

17. What parts of the rock cycle occur at Earthssurface?

WORDS ARE EASY TO LEARN Wheneveryou hear or read a word that you cannot define,

jot it down on an index card. Then look it up inthe dictionary and write the definition on theback of the card. Try to write a sentence or drawa picture using the word, too. Practice saying theword aloud until you are comfortable with it.

erosion

recrystallization

sedimentary rock

weathering

cementation

deposition

melting

burial

heat and pressure

igneous rock

crystallization metamorphic rock

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Matter and Atomic StructureElements Atoms are the basic building blocks ofmatter. They are made of protons, which have posi-

tive electrical charges; electrons, which have negative

electrical charges; and neutrons, which are neutral.

Protons and neutrons make up the nucleus of an

atom; electrons surround the nucleus in energy lev-

els. An element is a substance consisting of atoms

with a specific number of protons in their nuclei.

Examples of elements include hydrogen, neon, gold,

carbon, and uranium. Isotopes of an element differ

by the number of neutrons in their nuclei. All ele-

ments are mixtures of isotopes. The number of elec-

trons in the outermost energy levels of atoms

determines their chemical behavior. Elements with

the same number of electrons in their outermost

energy levels have similar chemical properties.

How Atoms Combine Atoms of differentelements combine to form compounds. Molecular

compounds are formed when atoms are held

together by the sharing of electrons in covalent

bonds.Atoms also combine ionically. Ions are elec-

trically charged atoms or groups of atoms. Positive

and negative ions attract each other and form ionic

compounds. Acids are solutions containing hydro-

gen ions. Bases are solutions containing hydroxideions. Acids and bases can neutralize each other.

A mixture is a combination of components that

retain their identities and can still be distinguished.

A solution is a mixture in which the components

can no longer be distinguished as separate. Solu-

tions can be liquid, solid, or gaseous.

146 UNIT 2

States of Matter On Earth, matter exists inthree physical states: solid, liquid, and gas. The uni-

verse also contains a fourth state of matter: plasma.

Most solids have crystalline structures. Atoms, ions,

or molecules in crystals are arranged in regular geo-

metric patterns. Most rocks are polycrystalline

materials. Liquids are densely packed arrangements

of mobile particles. Gases are widely separated,

individual particles. Plasmas are hot, highly ionized,

electrically conductive gases. Changes of state

involve thermal energy. Melting and evaporation

absorb thermal energy, whereas freezing and con-

densation release thermal energy.

Minerals

A mineral is a naturally occurring, inorganic solidwith a specific chemical composition and a definite

crystalline structure. There are at least 3000 known

minerals in Earths crust. Minerals form from

Composition of Earth

For a preview of Earths composition, study this GeoDigest before you read the chapters.

After you have studied the chapters, you can use the GeoDigest toreview the unit.

Sodium and chlorine reaction


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magma or from solution. Most minerals are formed

from the eight most common elements in Earths

crust. Oxygen readily combines with other elementsto form a diverse group of minerals, including sili-

cates, carbonates, and oxides. Minerals are virtually

everywhere. Your body needs many of the elements

found in minerals to survive, such as iron, calcium,

and sodium. Some minerals are found as ores. A

mineral is an ore if it contains a useful substance

that can be mined at a profit. Ores from deep

within Earth are removed by underground mining.

Ores close to Earths surface are obtained from

open-pit mines. If responsible procedures are not

followed, mining can cause environmental damage.

Gems, such as diamonds and rubies, are valuable

minerals that are prized for their rarity and beauty.

The presence of trace elements can make one vari-

ety of a mineral more colorful and thus more prized

than other varieties of the same mineral. For exam-

ple, amethyst which contains traces of manganese,

is a gem form of quartz.

Identifying Minerals Minerals can be identi-fied based on their physical and chemical properties.The most reliable way to identify a mineral is to use

a combination of tests of color, hardness, and den-

sity, among other characteristics.A minerals color is

generally the result of trace elements within the

mineral. Texture describes how a mineral feels.

Luster describes how a mineral reflects light. Cleav-

age and fracture describe how a mineral breaks.A

minerals streak, its color in powdered form, its

hardness, and its density are also methods of identi-

fication. Special properties of minerals, such as mag-netism, can also be used for identification purposes.

Igneous RocksFormation and Types Igneous rocks, formedby the cooling and crystallization of magma, may

be intrusive or extrusive. Intrusive rocks form inside

Earths crust; extrusive rocks form at or near Earths

surface. Minerals crystallize from magma in a

sequential pattern known as Bowens reactionseries. Different minerals melt and crystallize at dif-

ferent temperatures in the processes of partial melt-

ing and fractional crystallization. Igneous rocks are

classified as felsic, intermediate, mafic, and ultra-

mafic, depending upon their mineral compositions.

Igneous groups are further identified by crystal size,

also called texture. For example, extrusive rocks,

which cool more rapidly than intrusive rocks, are

generally more fine grained meaning they have

small crystals. Early forming minerals may have

well-shaped crystals, while later-forming minerals

have irregular shapes. Porphyritic textures

contain both large and small crystals.

Igneous Rock Resources Igneousrocks are often used as building materials

because of their strength, durability, and

beauty. Valuable ore deposits and gem

crystals are often associated with

igneous intrusions. For example, dia-

monds are found in rare types of igneousintrusions known as kimberlite pipes.

Composition of Eart

GeoDigest 1

Top Ten Diamond-Mining CountriesCountry Mine Production

(in carats)Botswana 15 000 000Australia 13 400 000Russia 11 500 000

South Africa 4 000 000Kinshasa 3 500 000Canada 2 000 000Namibia 1 990 000Angola 1 080 000Ghana 649 000Liberia 600 000

Vital Statistics

Dioptaseon panch


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Sedimentary andMetamorphic RocksFormation and Types Sedimentary rocks areformed by weathering, erosion, deposition, burial,

and lithification. Clastic sediments are rock and

mineral fragments produced by weathering and

erosion. Lithification occurs through the processesof compaction and cementation. Sedimentary rocks

can be identified by depositional features such as

horizontal bedding, cross-bedding, and ripple

marks. Sedimentary rocks often contain the remains

or evidence of once-living things: fossils. Sedi-

mentary rocks also provide geologists with infor-

mation about surface conditions that existed in

Earths past. Clastic sedimentary rocks form from

sediments and are classified by particle size and

shape. Chemical sedimentary rocks are formed fromminerals precipitated from water. Such rocks include

evaporites, which form primarily in restricted ocean

basins in regions of high evaporation. Organic sedi-

mentary rocks are formed from the remains of

once-living things. Limestone and coal are organic

sedimentary rocks.

Metamorphism and the Rock CycleMetamorphic rocks are formed when existing rocks

are subjected to high temperature and pressure,which cause changes in the rocks texture, mineral-

ogy, and composition. The three main types of

metamorphism are regional, contact, and hydrother-

mal. The two textural groups of metamorphic rocks

are foliated and nonfoliated. During metamorphism,

minerals change into new minerals that are stable

for the temperature and pressure conditions under

which they formed. Geologists use the stability

ranges for these minerals to infer the history of

Earths crust. Metamorphism is part of the rockcycle, whereby rocks continuously change into other

types of rock. Any type of rock can be changed into

any other type of rock.

148 UNIT 2

Composition of Earth

F O C U S O N C A R E E R S

ulptor

Sculptors use rocks, minerals, andother Earth materials to create

works of art. Many sculptors cast

in bronze, an alloy of copper and

tin. Others carve the metamor-

phic rock marble. Sculptors usu-

ally refine their talents in art

schools or in the art departments

of universities. A good under-

standing of the materials used is

critical to creating a sculpture.The sculptor must know which

tools to use on rocks of different

hardnesses, how a material will

fracture, and how a material in

an outdoor sculpture will hold up

in weather.

Metamorphic rock, Ontario, Canada


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Understanding Main Ideas1. How are atoms best described?

a. negatively charged

b. the building blocks of matter

c. isotopes

d. energy levels surrounded by nuclei

2. Hydrogen, neon, gold, carbon, and uranium

are examples of what?

a. elements c. protons

b. energy levels d. nuclei

3. What is a combination of components that

retain their identities called?

a. an ionic solution c. hydroxide ions

b. acids and bases d. a mixture

4. How are atoms, ions, or molecules in crystals

arranged?

a. as widely separated particles

b. as densely packed mobile particles

c. in regular geometric patterns

d. in solution

5. What is a useful substance that can be

mined at a profit called?

a. calcium c. an ore

b. hematite d. a mineral

6. Which of the following tests is the most reli-able means of identifying a mineral?

a. hardness

b. streak

c. density

d. a combination of tests

7. Where do intrusive igneous rocks form?

a. on Earths surface

b. inside Earths crust

c. in the ocean

d. in Bowens reaction series

8. What is a kimberlite pipe?

a. an igneous intrusion

b. an igneous extrusion

c. a durable gem

d. an extrusive igneous rock

9. How are clastic sedimentary rocks classified?

a. by particle color

b. by ripple marks

c. by the presence of fossilsd. by particle size and shape

10. What is the process whereby rocks continu-

ously change into other types of rocks?

a. the rock cycle

b. metamorphism

c. porphyry

d. erosion

Thinking ritically

1. Why would a tossed salad be classified as a

mixture?

2. Compare and contrast texture and luster.

3. Describe how clastic sedimentary rocks are

formed.

A S S E S S M E N T

Composition of Eart

Model of a water molecule

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Chap06 Sedimentary and Metamorphic Rocks

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