120 What You’ll Learn • How sedimentary rocks are formed. • How metamorphic rocks are formed. • How rocks continuously change from one type to another in the rock cycle. Why It’s Important Sedimentary rocks pro- vide information about surface conditions and organisms that existed in Earth’s past. In addition, mineral resources are found in sedimentary and metamorphic rocks. The rock cycle further pro- vides evidence that Earth is a dynamic planet, con- stantly evolving and changing. Sedimentary and Metamorphic Rocks Sedimentary and Metamorphic Rocks 6 6 Mount Kidd, Alberta, Canada Mount Kidd, Alberta, Canada To find out more about sedimentary and metamor- phic rocks, visit the Earth Science Web Site at ear thgeu.com
Transcript
120
What You’ll Learn• How sedimentary rocks
are formed.
• How metamorphic rocksare formed.
• How rocks continuouslychange from one type toanother in the rockcycle.
Why It’s ImportantSedimentary rocks pro-vide information aboutsurface conditions andorganisms that existed inEarth’s 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.
SedimentaryandMetamorphicRocks
SedimentaryandMetamorphicRocks
66
Mount Kidd, Alberta, CanadaMount Kidd, Alberta, Canada
To find out more aboutsedimentary and metamor-phic rocks, visit the Earth Science Web Site at earthgeu.com
Sedimentary rocks are usuallyfound in layers. How do these layersform? In this activity, you will investi-gate how layers form from particlesthat settle in water.
1. Obtain 100 mL of soil from a loca-tion specified by your teacher.Place the soil in a tall, narrow, jar.
2. Add water to the jar until it isthree-fourths full. Put the lid onthe jar so that it is tightly sealed.
3. Pick up the jar with both handsand turn it upside down severaltimes to mix the water and soil.
4. Quickly turn the jar upright andset it on a flat surface.
Observe 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?
Model Sediment LayeringDiscovery LabDiscovery Lab
OBJECTIVES
• Sequence the formationof sedimentary rocks.
• Explain the formationand classification ofclastic sediments.
• Describe features ofsedimentary rocks.
VOCABULARY
sediment bedding
You learned in Chapter 5 that igneous rocks are the most commonrocks in Earth’s crust, yet when you look at the ground, you may notsee igneous rocks. In fact, you usually don’t see any solid rock at all.Why is this? Much of Earth’s surface is covered with sediments.Sediments are pieces of solid material that have been deposited onEarth’s surface by wind, water, ice, gravity, or chemical precipitation.When sediments become cemented together, they form sedimentaryrocks. The formation of sedimentary rocks begins when weatheringand erosion produce sediments.
WEATHERINGWherever Earth’s crust is exposed at the surface, it is continuouslybeing worn away by weathering, a set of physical and chemicalprocesses that break rock into smaller pieces. Chemical weatheringoccurs when the minerals in a rock are dissolved or otherwise
Formation ofSedimentary Rocks
6.16.1
clasticdepositionlithificationcementation
graded bedding
cross-bedding
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 abrasionduring erosion and transport.
EROSION AND TRANSPORTAfter rock fragments have been weathered out of outcrops, they are 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 Boulder256–64 mm Gravel Cobble Conglomerate64–2 mm Pebble
2–0.062 mm Sand Sandstone
0.062–0.0039 mm Silt Siltstone
<0.0039 mm Clay Mudstone or shale
}
Chemical weatheringattacks less resistant minerals
Resistant grains are releasedas weaker grains break down
Resistant grainssuch as quartz
Figure 6-1 The process of chemical weathering isillustrated in (A). Granitebreaks apart easily under the effects of chemicalweathering (B).
A
B
Figure 6-2 shows the four main agents of erosion: wind, movingwater, gravity, and glaciers. Visible signs of erosion are all aroundyou. 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 jarfull of sediment and water? The sediment sank to the bot-tom and was deposited. Similarly, in nature, sediments are
6.1 Formation of Sedimentary Rocks 123
Figure 6-2 Winds blowsand into dunes on theNavajo Indian Reservationin Arizona (A). The riverLethe cuts through theValley of Ten ThousandSmokes in Katmai NationalPark, Alaska (B). This land-slide in Papua, New Guinea,carried the entire hillside 300 m into the canyon (C).This terminal moraine wasbuilt up by the AthabascaGlacier in Jasper NationalPark, Alberta, Canada (D).
A
B
C
D
124 CHAPTER 6 Sedimentary and Metamorphic Rocks
deposited when transport stops. Perhaps the wind stops blowing or ariver 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 Lab at the beginning of this chap-ter, that the sediment formed layers as it settled to the bottom of thejar. 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 indepressions 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 conditionscause lithification, 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 sand such as this from Kalahari,South Africa (B).
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 excesswater 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 Earth’s crust increases withdepth by about 30°C 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.
6.1 Formation of Sedimentary Rocks 125
50%–60% H2O
10%–20% H2O
Grain to grain contactsprevent compaction
Mud
Sand
Figure 6-4 Pressure andweight from overlying sediments causes flat clayparticles to compact (A).The irregular shape of sandgrains prevents similaramounts of compaction (B).
Quartz sandgrains
Calcite cementbetween grains
Quartz crystallizedon 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
FEATURES OFSEDIMENTARY ROCKSThe primary feature of sedimentary rocksis horizontal layering, called bedding.Bedding can range from a millimeter-thicklayer of shale to sandstone deposits severalmeters 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 gradedbedding. 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-bedding is formedas inclined layers of sediment move forwardacross a horizontal surface. Small-scalecross-bedding can be observed at sandybeaches and along sandbars in streams andrivers. 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
Sand particles
Particle movement
What happenedhere?
Interpret animalactivity from pat-terns of fossil foot-prints.
Procedure1. Study the photograph of a set of foot-
prints that has been preserved in sedi-mentary rocks.
2. Write a description of how these tracksmight 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 andhave them interpret what happened.
Analyze and Conclude1. 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 diagram
the same way? What might have causedany differences?
126 CHAPTER 6 Sedimentary and Metamorphic Rocks
A
B
Figure 6-6 Cross-bedding is formed as sediment is 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).
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. Bydoing the MiniLab on the previous page, you can learn first-hand howfossils can be used to interpret past events.
6.1 Formation of Sedimentary Rocks 127
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.
SKILL REVIEW
6. Sequencing Sequence the processes bywhich a sedimentary rock is formed fromclastic sediments. For more help, refer tothe Skill 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).
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.
CLASTIC SEDIMENTARY ROCKSThe most common type of sedimentary rocks, clastic sedimentaryrocks, are formed from the abundant deposits of loose sedimentsfound on Earth’s 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 thistype of transport occurred in the past. In contrast, the angularity of
128 CHAPTER 6 Sedimentary and Metamorphic Rocks
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 of Medium-grained Sandstone clastic sedimentsFine-grained Shale
Chemical LimestoneCalcite Rock salt Precipitation ofHalite Rock gypsum dissolved mineralsGypsum from water
OrganicCalcium carbonate– Accumulation and
shells Limestone lithification of remainsplant matter Coal of living things
}
particles in breccias indicates that the sediments from which theyformed did not have time to become rounded. This suggests that theparticles were transported only a short distance and deposited closeto their source. Under what kinds of circumstances might this typeof transport occur?
Medium-Grained Clastics In what types of environments issand found? Stream and river channels, beaches, and deserts oftencontain abundant sand-sized sediments. Sedimentary rocks thatcontain sand-sized rock and mineral fragments are classified asmedium-grained clastic rocks. When these medium-sized sedi-ments are buried and lithified, sandstone is formed. Sandstoneusually contains several features of interest to scientists. For exam-ple, because ripple marks and cross-bedding indicate the directionof current flow, geologists find sandstone layers particularly usefulin mapping old stream and river channels.
Another important feature of sandstone is its relatively highporosity. Porosity is the percentage of open spaces between grainsin a rock. Loose sand can have a porosity of up to 40 percent; someof its open spaces are maintained during the formation of sand-stone. The incomplete cementation of mineral grains can result inporosities as high as 30 percent. When pore spaces are connected toone another, fluids can move through sandstone. This feature makessandstone layers valuable as underground reservoirs of oil, naturalgas, and groundwater.
Fine-Grained Clastics Sedimentary rocks consisting of silt andmud are called siltstone and mudstone. Siltstone is mostly composedof silt-sized grains, while shale is composed mostly of silt and clay-sized particles. Shale often breaks along thin layers, as shown inFigure 6-9. Unlike sandstone, this fine-grained sedimentary rock hasvery low porosity. It often forms barriers that hinder the movementof groundwater and oil.
Figure 6-9 The thin bed-ding that is characteristic of shale is clearly seen in this outcrop fromOntario, Canada.
129
Using PercentagesAssume that the volume of a layer ofmud will decrease by35 percent duringburial and com-paction. If the origi-nal sediment layer is30 cm thick, what willbe the thickness of the shale layer aftercompaction andlithification?
Figure 6-8 This coarse-grained breccia (A) hasangular fragments, and the conglomerate (B) hasrounded fragments.
B
A
CHEMICAL SEDIMENTARY ROCKSWhat 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. Aswater 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 process of evaporite formation isillustrated in B.
130 CHAPTER 6 Sedimentary and Metamorphic Rocks
Evaporation
Evaporating shallow basin(high salinity)
Crystals of gypsumor halite settle to bottom
Evaporite sediment:gypsum and halite
Barrier baror other
flow restriction
Replenishmentfrom open ocean
Freshwaterinflow (small)
Ocean
A
B
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 131
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: 12�
AA
B
Topic: Coral ReefsFor an online update ofcoral reefs in the Bahamas,visit the Earth Science Web Site at earthgeu.com
Activity: Discuss the causeand significance of themajor bleaching event of1997-1998 in coral reefsaround the world.
IMPORTANCE OF SEDIMENTARY ROCKSThe characteristic textures and features of sedimentary rocks, such ascross-bedding, ripple marks, layering, and fossils, provide a geologic“snapshot” of surface conditions in Earth’s past. Fossils, for example,provide information about animals and plants that existed in thepast. Other sedimentary features indicate the location and directionof 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 Earth’s 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 Earth’s 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?
132 CHAPTER 6 Sedimentary and Metamorphic Rocks
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?
SKILL REVIEW
6. Comparing and Contrasting Make a datatable to compare and contrast the forma-tion of the three types of sedimentaryrock. For more help, refer to the SkillHandbook.
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 OF METAMORPHISMPressure 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 meaning“form.” During metamorphism, a rock changes form while remain-ing solid.
The high temperatures required for metamorphism ultimatelyare derived from Earth’s 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 OF METAMORPHISMDifferent combinations of temperature and pressure result in different types of metamorphism, shown in Figure 6-12. Each com-bination produces a different group of metamorphic minerals and
6.3 Metamorphic Rocks 133
Regional Metamorphic Grades
1000
800
600
Pre
ssu
re (
MP
a)
400
10
20
Dep
th (k
m)
30
200
Lithification
Low grade
Intermediategrade
High gradePartial melting
of granites
0
200 400 600
Temperature (°C)
800 1000
Figure 6-12 The grade ofmetamorphism, whether it is low, medium or high, is dependent upon the pressure on the rocks, the temperature and thedepth below the surface.
OBJECTIVES
• Compare and Contrastthe different types andcauses of metamorphism.
• Distinguish amongmetamorphic textures.
• Explain how mineraland compositionalchanges occur duringmetamorphism.
• Understand how rockscontinuously change from one type to anotherin the rock cycle.
textures. When high temperature and pressure affect large regions ofEarth’s crust, they produce large belts of regional 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 thenortheastern 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
UnmetamorphosedBiotite zoneStaurolite zoneSillimanite zoneGranitic plutonsGarnet zone
0 200 km100
N
Prince EdwardIsland
Nova Scotia
NewBrunswick
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 a specific 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
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. Recallfrom 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, meaning“water,” 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.
6.3 Metamorphic Rocks 135
High
-tem
pera
turemetamorphic rock
Sillim
anite
Biot
ite-Andalusite
Chlo
rite-
Mus
covit
e
Granitebatholith
Temperaturedecreasing
1 km
Figure 6-15 Contact meta-morphism results in large-scale mineral changes withlittle deformation. Geolo-gists can follow the occur-rence of metamorphicminerals to locate theigneous intrusion.
Figure 6-16 YellowstoneNational Park’s name comesfrom the hydrothermallychanged rocks of the area.These rocks are beautifullyexposed in the GrandCanyon of the Yellowstone,Wyoming.
METAMORPHIC TEXTURESMetamorphic rocks are classified into twotextural groups: foliated and nonfoliated.Wavy layers and bands of minerals charac-terize foliated metamorphic rocks. Highpressure during metamorphism causesminerals with flat or needlelike crystals to
form 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 regionalmetamorphism. 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).
136 CHAPTER 6 Sedimentary and Metamorphic Rocks
B
D
C
in solid rock by the reorganization of atoms during metamorphism.Garnet, shown in Figure 6-20, is a mineral that commonly formsporphyroblasts.
MINERAL CHANGESHow 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 conditionsare 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 Lab on the next page.
COMPOSITIONAL CHANGESMost 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. Thisoccurs 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.
6.3 Metamorphic Rocks 137
Figure 6-19 Despite theextreme temperature andpressures, cross-beddingand ripple marks are oftenpreserved in quartzite (A).Fossils, on the other hand,are never preserved in marble (B).
Figure 6-20 This garnetmica schist comes from an exposure in RoxburyFalls, Connecticut.
A B
THE ROCK CYCLEMetamorphic 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 metamorphic—aregrouped 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 ofjars and bottles. You will learn more about the similarities and differ-ences between rock types when you complete the GeoLab at the end ofthis chapter.
138 CHAPTER 6 Sedimentary and Metamorphic Rocks
Determine which metamorphicminerals will form The types of min-erals found in metamorphic rocks dependson metamorphic grade and compositionof the original rock. In this activity, youwill compare how these factors affectmetamorphic minerals.
Analysis1. Study Figure 6-14 on page 134, show-
ing the different mineral groups thatare created under different metamor-phic conditions. What mineral isformed when shale and basalt areexposed to low-grade metamorphism?
2. What mineral is formed when shale isexposed to high-grade metamorphism
that is not found in basalts under thesame conditions?
Thinking Critically3. Compare the mineral groups you
would 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 dominantmineral. Explain why this happens.
Interpreting Scientific Illustrations
OTHER POSSIBLE PATHSThere is more than one path in therock cycle. Sandstone might just aseasily become uplifted and weath-ered back into sediments, therebybypassing the metamorphic andigneous stages. Another possibilityis that sandstone could becomeintruded by magma and melted,thereby directly becoming anigneous rock and bypassing meta-morphism. Can you think of otherpossible paths? How might ametamorphic rock become a sedi-mentary rock?
The rocks of Earth’s crust areconstantly being recycled fromone type to another. At any given time, magma is crystallizing, sed-iments are being cemented, and deeply buried rocks are metamor-phosing. These processes take place underground, where theycannot be easily observed. However, as you have learned, not allphases of the rock cycle occur beneath Earth’s surface. Theprocesses that help shape Earth’s landscapes are also part of therock cycle. You will learn more about these surface processes in thenext few chapters.
6.3 Metamorphic Rocks 139
1. How can the chemical composition of arock be changed during metamorphism?
2. What are the three main types ofmetamorphism? Compare and contrastthe factors that cause each type.
3. How does quartzite differ from schist?
4. What causes foliated metamorphictextures?
5. How are the three types of rocks classified?
6. What parts of the rock cycle occur deep inEarth’s crust?
7. Thinking Critically Which would you
expect to cause the greatest amount ofcontact metamorphism: an intrusion ofbasaltic magma or an intrusion of rhy-olitic magma?
SKILL REVIEW
8. Interpreting Photos Study Figure 6-18C.If this sample is in the exact position itwas when it formed, from which directiondid the compressional forces originate?What group of metamorphic mineralswould you expect during high-grademetamorphism of shale? For more help,refer to the Skill Handbook.
Sediments
Deposition, burial,lithification
Heat and pressureMelting
Cooling andcrystallization
Uplift
Uplift
Heatand
pressure
External processes
Internal processes
Weatheringand erosion
Magma
Sedimentaryrocks
Metamorphicrocks
Igneousrocks
Figure 6-21 The rocks ofEarth, whether at the sur-face or below the crust, arealways positioned some-where on the rock cycle.
As 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 easilyobserved and described visually. Yet, with mineral changescome changes in crystal structure and density. How can thesebe accounted for and described? Studying pairs of sedimentaryand 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
ObjectivesIn this GeoLab, you will:• Describe the characteristics of sedi-
mentary and metamorphic rocks.• Determine the density of different
rock types.• Infer how metamorphism changes
the structure of rocks.
Safety PrecautionsAlways wear safety goggles and anapron in the lab.
Preparation
GeoLab 141
1. Prepare a data table similar to theone 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 arock sample.
4. Determine the density of each rocksample and record this informationin the data table.
1. Compare and contrast a shale and a 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 greaterdensity? 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
DATA TABLE
142 CHAPTER 6 Sedimentary and Metamorphic Rocks
Research the use of talc in other products.Why is talcum powder no longer recom-mended for infants? Does the use of talc inmake-up expose teens to the sameasbestos risk that crayons posed foryounger children? Present your findings in a written report.
Activity
point out, though, that if the talc fibers look somuch like asbestos fibers, they are likely to havethe same affect in the lungs.
The risk to children from the asbestos andasbestos-like fibers in crayons is extremelysmall. The amount of fibers present is very low,and, because the talc is embedded in wax, thereis little likelihood that the fibers will be inhaled.The Consumer Product Safety Commission did atest that simulated one half-hour of hard coloringand 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 majorcrayon makers have decided to change theircrayon formulas to eliminate all use of talc. Theuse of talc in children’s chalk, clay, and sand isalso being investigated. The use of asbestos wasphased out in the United States in 2001.
What do crayons have to do with rocks? Ametamorphic mineral—talc—is used in the man-ufacture of crayons. In fact, it’s the properties oftalc that led to the asbestos scare.
Talc and AsbestosTalc 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. Asbestoscomes from the mineral chrysotile, which breaksinto tiny, hairlike fibers. Asbestos has been usedin insulation and in fireproof fabrics and buildingproducts. However, in the 1960s and 1970s, itwas discovered that inhaling asbestos fibers leadsto lung cancer, asbestosis, and mesothelioma—each of which can be fatal. Beginning in the1980s, virtually all uses of asbestos were bannedin the United States and much of Europe.
Blue, green, and asbestos?Ground up talc is used as a hardener in
crayons, which are basically a mixture of pig-ments, hardeners, and wax. Without talc, crayonswould get too sticky to handle. However, whentalc is ground, fibers that resemble asbestos arecreated. Testers aren’t sure whether the tinyamount of asbestos found in crayons comesfrom these asbestos-like fibers, from chrysotilecontamination of the talc, or from both. They
Good News—Crayons SafeThe Consumer Products Safety Commission, after extensive testing,declared that crayons are safe for children to use. Although traceamounts of asbestos and asbestos-related fibers were found in manyof the crayons tested, the amounts were not considered to be dan-gerous. No recall was issued, but crayon manufacturers were urged tocreate a new “recipe” for crayons to exclude the asbestos fibers.
Main Ideas• The processes of weathering, erosion, deposition, burial, and
lithification form sedimentary rocks.• Clastic sediments are rock and mineral fragments produced by
weathering 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 changes in 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.
SECTION 6.1
Formation ofSedimentaryRocks
SECTION 6.33
MetamorphicRocks
Vocabularyclastic sedimentary
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 sedimentaryrock.
• Sedimentary rocks provide geologists with information aboutsurface conditions that existed in Earth’s past.
1. What are solid particles that have been depositedon Earth’s 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 the chemical 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 clastic appropriate for particlesweathered 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 Earth’s surface?
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.
1. What initiates the process that turns sedi-ments into sedimentary rocks?a. bedding c. cementationb. burial d. compaction
2. Which sedimentary rock is used to makecement for the construction industry?a. shale c. phosphateb. sandstone d. limestone
3. Which of the following are rocks composedof minerals that form with blocky crystalshapes?a. foliated c. porphyroblastsb. nonfoliated d. phenocrysts
INTERPRETING SCIENTIFIC ILLUSTRATIONSUse the illustration below to answer question 4.
4. Which rocks are most likely to metamorphosefrom a lava flow?a. only the rocks in the crater of the volcano,
where the lava is hottestb. rocks in the crater and rocks along the top
half of the mountainc. all the rocks on the mountaind. all the rocks reached by the lava flow
18. How can chemical weathering assist physicalweathering?
19. Glaciers move very slowly, yet they are able tocarry large particles with ease. Why?
20. How does graded bedding form?
21. Geologists have uncovered a mudstone layer con-taining mud cracks and ripple marks. This layerlies beneath a layer of sandstone. Explain howthese structures and layers might have formedduring the deposition of the sediments.
22. What information about sedimentary environ-ments can be interpreted from a breccia?
23. What is the source of the calcite found in organi-cally formed limestone?
24. What type of rock is marble? What characteristicsmake it well suited for sculptures?
25. How might a sedimentary rock become anothersedimentary rock without first changing intoanother rock type?
26. Why are sand dunes commonly composed of fine,well-sorted sand?
27. Why do muds lose more volume during com-paction than sands do?
28. How could you tell the difference between sedi-mentary rocks formed from an underwater land-slide and sedimentary rocks formed from alandslide on Earth’s surface?
29. Sand is often found between the larger grains ofconglomerates, but large particles are seldomfound in sandstone. Why is this?
30. Would you expect foliated metamorphic texturesin rocks that have undergone contact metamor-phism? Why or why not?
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 negativeelectrical charges; and neutrons, which are neutral.Protons and neutrons make up the nucleus of anatom; electrons surround the nucleus in energy lev-els. An element is a substance consisting of atomswith a specific number of protons in their nuclei.Examples of elements include hydrogen, neon, gold,carbon, and uranium. Isotopes of an element differby 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 atomsdetermines their chemical behavior. Elements withthe same number of electrons in their outermostenergy levels have similar chemical properties.
How Atoms Combine Atoms of differentelements combine to form compounds. Molecularcompounds are formed when atoms are heldtogether by the sharing of electrons in covalentbonds. Atoms also combine ionically. Ions are elec-trically charged atoms or groups of atoms. Positiveand negative ions attract each other and form ioniccompounds. 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 thatretain their identities and can still be distinguished.A solution is a mixture in which the componentscan 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 polycrystallinematerials. Liquids are densely packed arrangementsof mobile particles. Gases are widely separated,individual particles. Plasmas are hot, highly ionized,electrically conductive gases. Changes of stateinvolve thermal energy. Melting and evaporationabsorb thermal energy, whereas freezing and con-densation release thermal energy.
MineralsA mineral is a naturally occurring, inorganic solidwith a specific chemical composition and a definitecrystalline structure. There are at least 3000 knownminerals in Earth’s crust. Minerals form from
Composition of Earth
For a preview of Earth’s composition, study this GeoDigest before you read the chapters.After you have studied the chapters, you can use the GeoDigest to review the unit.
Sodium and chlorine reaction
magma or from solution. Most minerals are formedfrom the eight most common elements in Earth’scrust. Oxygen readily combines with other elementsto form a diverse group of minerals, including sili-cates, carbonates, and oxides. Minerals are virtuallyeverywhere. Your body needs many of the elementsfound in minerals to survive, such as iron, calcium,and sodium. Some minerals are found as ores. Amineral is an ore if it contains a useful substancethat can be mined at a profit. Ores from deepwithin Earth are removed by underground mining.Ores close to Earth’s surface are obtained fromopen-pit mines. If responsible procedures are notfollowed, mining can cause environmental damage.Gems, such as diamonds and rubies, are valuableminerals 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 prizedthan 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 usea combination of tests of color, hardness, and den-sity, among other characteristics. A mineral’s color isgenerally the result of trace elements within themineral. Texture describes how a mineral feels.Luster describes how a mineral reflects light. Cleav-age and fracture describe how a mineral breaks. Amineral’s streak, its color in powdered form, itshardness, 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, maybe intrusive or extrusive. Intrusive rocks form inside
Earth’s crust; extrusive rocks form at or near Earth’ssurface. Minerals crystallize from magma in asequential pattern known as Bowen’s reactionseries. Different minerals melt and crystallize at dif-ferent temperatures in the processes of partial melt-ing and fractional crystallization. Igneous rocks areclassified 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, aregenerally more fine grained meaning they havesmall crystals. Early forming minerals may havewell-shaped crystals, while later-forming mineralshave irregular shapes. Porphyritic texturescontain both large and small crystals.
Igneous Rock Resources Igneousrocks are often used as building materialsbecause of their strength, durability, andbeauty. Valuable ore deposits and gemcrystals are often associated withigneous intrusions. For example, dia-monds are found in rare types of igneousintrusions known as kimberlite pipes.
Composition of Earth
GeoDigest 147
Top Ten Diamond-Mining CountriesCountry Mine Production
Sedimentary andMetamorphic RocksFormation and Types Sedimentary rocks areformed by weathering, erosion, deposition, burial,and lithification. Clastic sediments are rock andmineral fragments produced by weathering anderosion. Lithification occurs through the processesof compaction and cementation. Sedimentary rockscan be identified by depositional features such ashorizontal bedding, cross-bedding, and ripplemarks. Sedimentary rocks often contain the remainsor evidence of once-living things: fossils. Sedi-mentary rocks also provide geologists with infor-mation about surface conditions that existed inEarth’s past. Clastic sedimentary rocks form fromsediments and are classified by particle size andshape. Chemical sedimentary rocks are formed fromminerals precipitated from water. Such rocks includeevaporites, which form primarily in restricted oceanbasins in regions of high evaporation. Organic sedi-mentary rocks are formed from the remains ofonce-living things. Limestone and coal are organicsedimentary rocks.
Metamorphism and the Rock CycleMetamorphic rocks are formed when existing rocksare subjected to high temperature and pressure,which cause changes in the rocks’ texture, mineral-ogy, and composition. The three main types ofmetamorphism are regional, contact, and hydrother-mal. The two textural groups of metamorphic rocksare foliated and nonfoliated. During metamorphism,minerals change into new minerals that are stablefor the temperature and pressure conditions underwhich they formed. Geologists use the stabilityranges for these minerals to infer the history ofEarth’s crust. Metamorphism is part of the rockcycle, whereby rocks continuously change into othertypes of rock. Any type of rock can be changed intoany other type of rock.
148 UNIT 2
Composition of Earth
F O C U S O N C A R E E R S
SculptorSculptors use rocks, minerals, andother Earth materials to createworks of art. Many sculptors castin bronze, an alloy of copper andtin. Others carve the metamor-phic rock marble. Sculptors usu-ally refine their talents in artschools or in the art departmentsof universities. A good under-standing of the materials used iscritical to creating a sculpture.The sculptor must know whichtools to use on rocks of differenthardnesses, how a material willfracture, and how a material inan outdoor sculpture will hold upin weather.
Metamorphic rock, Ontario, Canada
Understanding Main Ideas1. How are atoms best described?
a. negatively chargedb. the building blocks of matterc. isotopesd. energy levels surrounded by nuclei
2. Hydrogen, neon, gold, carbon, and uraniumare examples of what?a. elements c. protonsb. energy levels d. nuclei
3. What is a combination of components thatretain their identities called?a. an ionic solution c. hydroxide ionsb. acids and bases d. a mixture
4. How are atoms, ions, or molecules in crystalsarranged?a. as widely separated particlesb. as densely packed mobile particlesc. in regular geometric patternsd. in solution
5. What is a useful substance that can bemined at a profit called?a. calcium c. an oreb. hematite d. a mineral
6. Which of the following tests is the most reli-able means of identifying a mineral?a. hardnessb. streakc. densityd. a combination of tests
7. Where do intrusive igneous rocks form?a. on Earth’s surfaceb. inside Earth’s crustc. in the oceand. in Bowen’s reaction series
8. What is a kimberlite pipe?a. an igneous intrusionb. an igneous extrusionc. a durable gemd. an extrusive igneous rock
9. How are clastic sedimentary rocks classified?a. by particle colorb. by ripple marksc. 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 cycleb. metamorphismc. porphyryd. erosion
Thinking Critically1. Why would a tossed salad be classified as a
mixture?2. Compare and contrast texture and luster.3. Describe how clastic sedimentary rocks are
