-
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
http://earthgeu.com
-
6.1 Formation of Sedimentary Rocks 121
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
-
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
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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
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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 partic
les
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 REVIEW6. Sequencing Sequence the processes by
which 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).
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.
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
}
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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
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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.
http://earthgeu.com
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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 REVIEW6. 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 RocksYou 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 morphē
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.
VOCABULARY
regional metamorphismcontact metamorphismhydrothermal
metamor-
phismfoliatednonfoliatedporphyroblastrock cycle
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134 CHAPTER 6 Sedimentary and Metamorphic Rocks
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
ture m
etamorp
hicrock
Sillim
anite
Biot
ite-An
dalusite
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.
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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
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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 REVIEW8. 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.
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140 CHAPTER 6 Sedimentary and Metamorphic Rocks
Interpreting Changesin Rocks
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
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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.
-
Summary
Study Guide 143
Vocabularybedding (p. 126)cementation (p. 125)clastic (p.
122)cross-bedding
(p. 126)deposition (p. 123)graded bedding
(p. 126)lithification (p. 124)sediment (p. 121)
Vocabularycontact metamor-
phism (p. 135)foliated (p. 136)hydrothermal meta-
morphism (p. 135)nonfoliated (p. 136)porphyroblast
(p. 136)regional metamor-
phism (p. 134)rock cycle (p. 138)
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.
SECTION 6.2
Types ofSedimentaryRocks
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144 CHAPTER 6 Sedimentary and Metamorphic Rocks
Test-Taking Tip
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
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 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.
erosion
recrystallization
sedimentary rock
weathering
cementation
deposition
melting
burial
heat and pressure
igneous rock
crystallization metamorphic rock
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Assessment 145
Standardized Test Practice
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?
Thinking Critically
Applying Main Ideas
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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 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
(in carats)Botswana 15 000 000Australia 13 400 000Russia 11 500
000South Africa 4 000 000Kinshasa 3 500 000Canada 2 000 000Namibia
1 990 000Angola 1 080 000Ghana 649 000Liberia 600 000
Vital Statistics
Dioptaseon panchéite
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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
formed.
GeoDigest 149
A S S E S S M E N T
Composition of Earth
Model of a water molecule
Earth Science: Geology, the Environment, and the
UniverseContents in Brief Table of ContentsUnit 1: Earth
ScienceChapter 1: The Nature of ScienceSection 1.1: Earth
ScienceSection 1.2: Methods of ScientistsSection 1.3: Communicating
in ScienceChapter 1 Study GuideChapter 1 Assessment
Chapter 2: Mapping Our WorldSection 2.1: Latitude and
LongitudeSection 2.2: Types of MapsSection 2.3: Remote
SensingChapter 2 Study GuideChapter 2 Assessment
GeoDigest: Earth Science
Unit 2: Composition of EarthChapter 3: Matter and Atomic
StructureSection 3.1: What are elements?Section 3.2: How Atoms
CombineSection 3.3: States of MatterChapter 3 Study GuideChapter 3
Assessment
Chapter 4: MineralsSection 4.1: What is a mineral?Section 4.2:
Identifying MineralsChapter 4 Study GuideChapter 4 Assessment
Chapter 5: Igneous RocksSection 5.1: What are igneous
rocks?Section 5.2: Classifying Igneous RocksChapter 5 Study
GuideChapter 5 Assessment
Chapter 6: Sedimentary and Metamorphic RocksSection 6.1:
Formation of Sedimentary RocksSection 6.2: Types of Sedimentary
RocksSection 6.3: Metamorphic RocksChapter 6 Study GuideChapter 6
Assessment
GeoDigest: Composition of Earth
Unit 3: Surface Processes on EarthChapter 7: Weathering,
Erosion, and SoilSection 7.1: WeatheringSection 7.2: Erosion and
DepositionSection 7.3: Formation of SoilChapter 7 Study
GuideChapter 7 Assessment
Chapter 8: Mass Movements, Wind, and GlaciersSection 8.1: Mass
Movements at Earth's SurfaceSection 8.2: WindSection 8.3:
GlaciersChapter 8 Study GuideChapter 8 Assessment
Chapter 9: Surface WaterSection 9.1: Surface Water
MovementSection 9.2: Stream DevelopmentSection 9.3: Lakes and
Freshwater WetlandsChapter 9 Study GuideChapter 9 Assessment
Chapter 10: GroundwaterSection 10.1: Movement and Storage of
GroundwaterSection 10.2: Groundwater Erosion and DepositionSection
10.3: Groundwater SystemsChapter 10 Study GuideChapter 10
Assessment
GeoDigest: Surface Processes on Earth
Unit 4: The Atmosphere and the OceansChapter 11:
AtmosphereSection 11.1: Atmospheric BasicsSection 11.2: State of
the AtmosphereSection 11.3: Moisture in the AtmosphereChapter 11
Study GuideChapter 11 Assessment
Chapter 12: MeteorologySection 12.1: The Causes of
WeatherSection 12.2: Weather SystemsSection 12.3: Gathering Weather
DataSection 12.4: Weather AnalysisChapter 12 Study GuideChapter 12
Assessment
Chapter 13: The Nature of StormsSection 13.1:
ThunderstormsSection 13.2: Severe WeatherSection 13.3: Tropical
StormsSection 13.4: Recurring WeatherChapter 13 Study GuideChapter
13 Assessment
Chapter 14: ClimateSection 14.1: What is climate?Section 14.2:
Climate ClassificationSection 14.3: Climatic ChangesSection 14.4:
The Human FactorChapter 14 Study GuideChapter 14 Assessment
Chapter 15: Physical OceanographySection 15.1: The OceansSection
15.2: SeawaterSection 15.3: Ocean MovementsChapter 15 Study
GuideChapter 15 Assessment
Chapter 16: The Marine EnvironmentSection 16.1: Shoreline
FeaturesSection 16.2: The SeafloorChapter 16 Study GuideChapter 16
Assessment
GeoDigest: The Atmosphere and the Oceans
Unit 5: The Dynamic EarthChapter 17: Plate TectonicsSection
17.1: Drifting ContinentsSection 17.2: Seafloor SpreadingSection
17.3: Theory of Plate TectonicsSection 17.4: Causes of Plate
MotionsChapter 17 Study GuideChapter 17 Assessment
Chapter 18: Volcanic ActivitySection 18.1: MagmaSection 18.2:
Intrusive ActivitySection 18.3: VolcanoesChapter 18 Study
GuideChapter 18 Assessment
Chapter 19: EarthquakesSection 19.1: Forces Within EarthSection
19.2: Seismic Waves and Earth's InteriorSection 19.3: Measuring and
Locating EarthquakesSection 19.4: Earthquakes and SocietyChapter 19
Study GuideChapter 19 Assessment
Chapter 20: Mountain BuildingSection 20.1: Crust-Mantle
RelationshipsSection 20.2: Convergent-Boundary MountainsSection
20.3: Other Types of MountainsChapter 20 Study GuideChapter 20
Assessment
GeoDigest: The Dynamic Earth
Unit 6: Geologic TimeChapter 21: Fossils and the Rock
RecordSection 21.1: The Geologic Time ScaleSection 21.2:
Relative-Age Dating of RocksSection 21.3: Absolute-Age Dating of
RocksSection 21.4: Remains of Organisms in the Rock RecordChapter
21 Study GuideChapter 21 Assessment
Chapter 22: The Precambrian EarthSection 22.1: The Early
EarthSection 22.2: Formation of the Crust and ContinentsSection
22.3: Formation of the Atmosphere and OceansSection 22.4: Early
Life on EarthChapter 22 Study GuideChapter 22 Assessment
Chapter 23: The Paleozoic EraSection 23.1: The Early
PaleozoicSection 23.2: The Middle PaleozoicSection 23.3: The Late
PaleozoicChapter 23 Study GuideChapter 23 Assessment
Chapter 24: The Mesozoic and Cenozoic ErasSection 24.1: Mesozoic
PaleogeographySection 24.2: Mesozoic LifeSection 24.3: Cenozoic
PaleogeographySection 24.4: Cenozoic LifeChapter 24 Study
GuideChapter 24 Assessment
GeoDigest: Geologic Time
Unit 7: Resources and the EnvironmentChapter 25: Earth
ResourcesSection 25.1: What are resources?Section 25.2: Land
ResourcesSection 25.3: Air ResourcesSection 25.4: Water
ResourcesChapter 25 Study GuideChapter 25 Assessment
Chapter 26: Energy ResourcesSection 26.1: Conventional Energy
ResourcesSection 26.2: Alternative Energy ResourcesSection 26.3:
Conservation of Energy ResourcesChapter 26 Study GuideChapter 26
Assessment
Chapter 27: Human Impact on Earth ResourcesSection 27.1:
Populations and the Use of Natural ResourcesSection 27.2: Human
Impact on Land ResourcesSection 27.3: Human Impact on Air
ResourcesSection 27.4: Human Impact on Water ResourcesChapter 27
Study GuideChapter 27 Assessment
GeoDigest: Resources and the Environment
Unit 8: Beyond EarthChapter 28: The Sun-Earth-Moon SystemSection
28.1: Tools of AstronomySection 28.2: The MoonSection 28.3: The
Sun-Earth-Moon SystemChapter 28 Study GuideChapter 28
Assessment
Chapter 29: Our Solar SystemSection 29.1: Overview of Our Solar
SystemSection 29.2: The Terrestrial PlanetsSection 29.3: The Gas
Giant PlanetsSection 29.4: Formation of our Solar SystemChapter 29
Study GuideChapter 29 Assessment
Chapter 30: StarsSection 30.1: The SunSection 30.2: Measuring
the StarsSection 30.3: Stellar EvolutionChapter 30 Study
GuideChapter 30 Assessment
Chapter 31: Galaxies and the UniverseSection 31.1: The Milky Way
GalaxySection 31.2: Other Galaxies in the UniverseSection 31.3:
CosmologyChapter 31 Study GuideChapter 31 Assessment
GeoDigest: Beyond Earth
National Geographic ExpeditionsAppendicesAppendix A:
International System of UnitsAppendix B: Safety in the
LaboratoryAppendix C: Physiographic Map of EarthAppendix D:
Topographic Map SymbolsAppendix E: Weather Map SymbolsAppendix F:
Relative HumidityAppendix G: Periodic Table of the ElementsAppendix
H: MineralsAppendix I: RocksAppendix J: Solar System ChartsAppendix
K: Start Charts
Skill HandbookThinking CriticallyPracticing Scientific
MethodsOrganizing Information
GlossaryIndexCredits
Feature ContentsActivitiesMiniLabsGeoLabsGeoLabInternet
GeoLabMapping GeoLabDesign Your Own GeoLab
Discovery LabsProblem-Solving Labs
Science ConnectionsScience & TechnologyScience &
MathScience in the NewsScience & the Environment
Student WorksheetsExploring Environmental ProblemsHow to Use
This Laboratory ManualWriting a Laboratory ReportTechnology-Based
Systems for the LabLaboratory EquipmentSafety in the
LaboratorySafety SymbolsUsing Technology to Study Environmental
ScienceCalculator-Based LabsLab 1: Calculator-Based Lab/Design You
OwnLab 2: Calculator-Based Lab/Design You OwnLab 3:
Calculator-Based LabLab 4: Calculator-Based LabLab 5:
Calculator-Based LabLab 6: Calculator-Based LabLab 7:
Calculator-Based Lab/Design You OwnLab 8: Calculator-Based LabLab
9: Calculator-Based LabLab 10: Calculator-Based LabLab 11:
Calculator-Based Lab
Global Positioning System LabsLab 12: Global Positioning System
LabLab 13: Global Positioning System LabLab 14: Global Positioning
System LabLab 15: Global Positioning System LabLab 16: Global
Positioning System Lab
GeoLab and MiniLab WorksheetsMaterials ListChapter 1Chapter
2Chapter 3Chapter 4Chapter 5Chapter 6Chapter 7Chapter 8Chapter
9Chapter 10Chapter 11Chapter 12Chapter 13Chapter 14Chapter
15Chapter 16Chapter 17Chapter 18Chapter 19Chapter 20Chapter
21Chapter 22Chapter 23Chapter 24Chapter 25Chapter 26Chapter
27Chapter 28Chapter 29Chapter 30Chapter 31
Laboratory ManualHow to Use This Laboratory ManualWriting a
Laboratory ReportLaboratory EquipmentSafety in the LaboratorySafety
SymbolsChapter 1: The Nature of Science1.1: Investigation1.2:
Design Your Own
Chapter 2: Mapping Our World2.1: Mapping2.2: Investigation
Chapter 3: Matter and Atomic Structure3.1: Design Your Own3.2:
Investigation
Chapter 4: Minerals4.1: Investigation4.2: Design Your Own
Chapter 5: Igneous Rocks5.1: Investigation5.2: Mapping
Chapter 6: Sedimentary and Metamorphic Rocks6.1:
Investigation6.2: Mapping
Chapter 7: Weathering, Erosion, and Soil7.1: Investigation7.2:
Mapping
Chapter 8: Mass Movements, Wind, and Glaciers8.1:
Investigation8.2: Design Your Own
Chapter 9: Surface Water9.1: Investigation9.2: Mapping
Chapter 10: Groundwater10.1: Investigation10.2: Design Your
Own
Chapter 11: Atmosphere11.1: Investigation11.2: Design Your
Own
Chapter 12: Meteorology12.1: Investigation12.2: Design Your
Own
Chapter 13: The Nature of Storms13.1: Investigation13.2: Design
Your Own
Chapter 14: Climate14.1: Investigation14.2: Mapping
Chapter 15: Physical Oceanography15.1: Mapping15.2:
Investigation
Chapter 16: The Marine Environment16.1: Mapping16.2:
Investigation
Chapter 17: Plate Tectonics17.1: Design Your Own17.2:
Investigation
Chapter 18: Volcanic Activity18.1: Design Your Own18.2:
Investigation
Chapter 19: Earthquakes19.1: Investigation19.2: Design Your
Own
Chapter 20: Mountain Building20.1: Investigation20.2:
Mapping
Chapter 21: Fossils and the Rock Record21.1: Investigation21.2:
Design Your Own
Chapter 22: The Precambrian Earth22.1: Investigation22.2:
Mapping
Chapter 23: The Paleozoic Era23.1: Mapping23.2:
Investigation
Chapter 24: The Mesozoic and Cenozoic Eras24.1: Mapping 24.2:
Investigation
Chapter 25: Earth Resources25.1: Design Your Own25.2:
Investigation
Chapter 26: Energy Resources26.1: Design Your Own26.2:
Investigation
Chapter 27: Human Impact on Earth Resources27.1: Design Your
Own27.2: Investigation
Chapter 28: The Sun-Earth-Moon System28.1: Investigation28.2:
Design Your Own
Chapter 29: Our Solar System29.1: Investigation29.2: Design Your
Own
Chapter 30: Stars30.1: Investigation30.2: Mapping
Chapter 31: Galaxies and the Universe31.1: Investigation31.2:
Mapping
Performance Assessment in Earth ScienceUnit 1: Earth
ScienceActivity One: The Nature of Scientific
InvestigationsActivity Two: Mapping Your School
Unit 2: Composition of EarthActivity One: Testing and
Identification of MineralsActivity Two: Rocks Are Made of
MineralsActivity Three: Igneous Rock Lab
Unit 3: Surface Processes on EarthActivity One: Assessing a Site
PlanActivity Two: Comparing Erosion Rates
Unit 4: The Atmosphere and the OceansActivity One: The Cold
PoolActivity Two: What’s your forecast?
Unit 5: The Dynamic EarthActivity One: Tracking Volcanoes and
EarthquakesActivity Two: Earthquake Preparedness
Unit 6: Geologic TimeActivity One: Dating an ArtifactActivity
Two: Exploring Taphonomy: How Fossil Formation Proceeds
Unit 7: Resources and the EnvironmentActivity One: Human Impact
on Earth ResourcesActivity Two: Surviving Together: The
Interconnectedness of LifeActivity Three: Nature and the Artistic
Impulse
Unit 8: Beyond EarthActivity One: Using the Sun and Stars to
Measure TimeActivity Two: Using the Doppler Effect
Study Guide for Content MasteryTo the StudentChapter 1: The
Nature of ScienceChapter 2: Mapping Our WorldGeoDigest 1: Earth
ScienceChapter 3: Matter and Atomic StructureChapter 4:
MineralsChapter 5: Igneous RocksChapter 6: Sedimentary and
Metamorphic Rocks
GeoDigest 2: Composition of EarthChapter 7: Weathering, Erosion,
and SoilChapter 8: Mass Movements, Wind, and GlaciersChapter 9:
Surface WaterChapter 10: Groundwater
GeoDigest 3: Surface Processes on EarthChapter 11:
AtmosphereChapter 12: MeteorologyChapter 13: The Nature of
StormsChapter 14: ClimateChapter 15: Physical OceanographyChapter
16: The Marine Environment
GeoDigest 4: The Atmosphere and the OceansChapter 17: Plate
TectonicsChapter 18: Volcanic ActivityChapter 19:
EarthquakesChapter 20: Mountain Building
GeoDigest 5: The Dynamic EarthChapter 21: Fossils and the Rock
RecordChapter 22: The Precambrian EarthChapter 23: The Paleozoic
EraChapter 24: The Mesozoic and Cenozoic Eras
GeoDigest 6 Geologic TimeChapter 25: Earth ResourcesChapter 26:
Energy ResourcesChapter 27: Human Impact on Earth Resources
GeoDigest 7: Resources and the EnvironmentChapter 28: The
Sun-Earth-Moon SystemChapter 29: Our Solar SystemChapter 30:
StarsChapter 31: Galaxies and the Universe
GeoDigest 8: Beyond Earth
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