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2. ROCKS: MATERIALS OF THE SOLID
EARTH
INTRODUCTION
Rocks: Materials of the Solid Earth introduces Earth materials as the components of the rock cycle
and how they are linked together through the rock cycle. This chapter describes how each of the
rock types—igneous, sedimentary, and metamorphic—form and are classified by their
compositions and textures.
CHAPTER OUTLINE
2.1 EARTH AS A SYSTEM; THE ROCK CYCLE
a. The Basic Cycle
b. Alternative Paths
2.2 IGNEOUS ROCKS: “FORMED BY FIRE”
a. Igneous
i. Magma and lava
ii. Volcanic
iii. Intrusive or plutonic
b. From Magma to Crystalline Rock
c. What Can Igneous Textures Tell Us?
i. Texture: size, shape, arrangement
ii. Fine-grained texture
iii. Vesicular texture
iv. Coarse-grained texture
v. Porphyritic texture
vi. Glassy texture
d. Igneous Compositions
e. Classifying Igneous Rocks
i. Granitic (felsic)
ii. Basaltic (mafic)
iii. Andesitic (intermediate)
iv. Ultramafic
f. How Different Igneous Rocks Form
i. Bowen’s reaction series
ii. Magmatic differentiation
iii. Crystal settling
2.3 WEATHERING OF ROCKS TO FORM SEDIMENT
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a. Mechanical Weathering
i. Frost wedging
ii. Sheeting
iii. Biological activity
b. Chemical Weathering
i. Oxidation
ii. Carbonic acid
iii. Products of chemical weathering
2.4 SEDIMENTARY ROCKS: COMPACTED AND CEMENTED SEDIMENT
a. Sediment
i. Compaction and cementation
ii. Sedimentary
b. Classifying Sedimentary Rocks
i. Detrital
ii. Chemical and biochemical
iii. Coal
c. Lithification of Sediment
i. Lithification
d. Features of Sedimentary Rocks
i. Strata
ii. Fossils
2.5 METAMORPHIC ROCKS: NEW ROCK FROM OLD
a. Parent Rock
b. Metamorphism
i. Contact or thermal metamorphism
ii. Regional metamorphism
c. What Drives Metamorphism?
i. Heat
ii. Confining pressure
iii. Differential stress
iv. Chemically active fluids
d. Metamorphic Textures
i. Foliation
ii. Nonfoliated textures
e. Common Metamorphic Rocks
i. Foliated rocks
ii. Nonfoliated rocks
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LEARNING OBJECTIVES/FOCUS ON CONCEPTS
Each statement represents the primary learning objective for the corresponding major heading
within the chapter. After completing the chapter, students should be able to:
2.1: Sketch, label, and explain the stages of the rock cycle.
2.2: Describe the two criteria used to classify igneous rocks and explain how the rate of cooling
influences the crystal size of minerals.
2.3: Compare and contrast mechanical and chemical weathering and give examples of each.
2.4: List and describe the two characteristics of sedimentary rocks and discuss the processes that
change sediment into sedimentary rock.
2.5: Define metamorphism, explain how metamorphic rocks form, and describe the agents of
metamorphism.
TEACHING STRATEGIES
 Students have the most trouble with the identification of igneous rocks and their textures.
Ideally, we have already exposed them to numerous samples of minerals from the previous
chapter activities – and they have already seen minerals in rock form. Now we can
introduce them to the same samples again, in a different context.
o Try to provide samples that have crystals large enough for the students to identify
minerals with the naked eye to start them off looking at the nature of minerals
interconnected in igneous rocks.
o Showing students aphanitic texture and how a rhyolite is like a granite is impossible
without aids to the naked eye. At the very least, you must be able to show the
students the samples with a hand lens. Better would be a simple binocular
microscope. Have the students try to identify the minerals they see from their
mineral unit before, to the coarse-grained phaneritic rocks, to the aphanitic rocks as
seen through the binocular microscope.
 This activity will also be useful to show students how pumice actually does
have glassy texture!
 For metamorphic rocks, have students create “family trees” that show the parent rock
transition to the metamorphic rock. Frequently there will be more than one parent rock that
can give rise to a certain metamorphic rock. Oftentimes there can be more than one
metamorphic rock along the family tree lineage. For example, you might show a shale and a
siltstone as parents to slate, which is a parent to phyllite, which is a parent to schist….
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TEACHER RESOURCES
On the Cutting Edge (SERC) resources: “Teaching petrology in the 21stC” provides several
resources that cover rocks and the rock cycle here:
http://serc.carleton.edu/NAGTWorkshops/petrology/visualizations/rock_cycle.html
ANSWERS TO QUESTIONS IN THE CHAPTER:
CONCEPT CHECKS
2.1
1. Sketch and label a basic rock cycle. Make sure your sketch includes alternative paths.
2. Use the rock cycle to explain the statement “One rock is the raw material for another.”
Any rock can become any other rock under the right conditions. A sedimentary
rock, if weathered, becomes sediment, and then, if lithified through compaction
and cementation, becomes a sedimentary rock again.
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2.2
1. What is magma? How does magma differ from lava?
Magma is molten rock occurring within Earth’s crust. Lava is molten rock at or
above the surface of Earth’s crust.
2. In what basic settings do intrusive and extrusive igneous rocks originate?
Intrusive rocks originate from cooled and crystallized magma at some depth
within the Earth. Extrusive rocks originate from cooled and crystallized lava at
the Earth’s surface.
3. How does the rate of cooling influence crystal size? What other factors influence the
texture of igneous rocks?
Slow rates of cooling allow greater time for crystals to form, so that tends to
favor larger crystal size. Fast rates of cooling do not allow time for mineral
crystals to form, so the faster the rate, the smaller the crystal size. The
composition of the magma or lava and the amount of dissolved gases present
also influence crystallization.
4. What does a porphyritic texture indicate about the history of an igneous rock?
Porphyritic texture tells a more complicated cooling history of the magma/lava.
A melt that has already formed large crystals at some depth will continue to cool
and crystallize more rapidly if it then erupts at surface. The end product is a
rock with two distinct crystal sizes in it, which is porphyritic texture.
5. List and distinguish among the four basic compositional groups of igneous rocks.
Felsic composition (granitic) is the richest in silica and lightest in color, it is
affiliated with continental crust. The name derives from feldspar and silica. Mafic
(basaltic) is richer in iron and magnesium, the elements from which the word
“mafic” is derived. It tends toward black in color and is typical oceanic crust
composition. Composition between felsic and mafic is termed intermediate, or
andesitic. Rocks extremely rich in iron and magnesium, and poor in silica, are
termed ultramafic and are associated with Earth’s mantle.
6. How are granite and rhyolite different? In what way are they similar?
Granite is an intrusive igneous rock with phaneritic texture. Rhyolite is an
extrusive igneous rock with aphanitic texture. Both are felsic in composition,
more or less chemically identical.
7. What is magmatic differentiation? How might this process lead to the formation of
several different igneous rocks from a single magma?
As crystals form from a cooling melt, they selectively remove certain elements
from the magma; this is magmatic differentiation. As this occurs, it leaves the
remaining liquid portion of the melt depleted in the elements that have been
removed by the formation of certain minerals. Should those first minerals that
crystallize settle or the remaining melt inject into a fracture or erupt, the
minerals that crystallize from that remaining melt will be of a different
composition than the first minerals to form, resulting in different igneous rock
originating from the same initial melt.
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2.3
1. What are the two basic categories of weathering?
Mechanical and chemical weathering.
2. When a rock is mechanically weathered, how does its surface area change? How does
this influence chemical weathering?
When a rock is mechanically weathered, it increases its surface area. This
creates a greater contact area upon which chemical weathering can act. Thus,
as physical weather progresses, it allows for an increased effect of chemical
weathering as well.
3. Explain how water can cause mechanical weathering. Chemical weathering?
Water can cause mechanical weathering by freezing in pore spaces and
fractures within rock. When it freezes, it expands, gradually pushing apart the
space or fracture, which allows for more water to infill and freeze and continue
the effect. This is called frost wedging. Water is the most important agent of
chemical weathering. Oxygen dissolved in water can oxidize (rust) materials—
this is oxidation. Carbon dioxide dissolved in water creates carbonic acid, which
readily dissolves some rock—this is dissolution. Further, carbonic acid reacts
with some rock and replaces ions, most often potassium ions, with hydrogen
ions, transforming rock into clay—this is hydrolysis.
4. How does biological activity contribute to weathering?
Roots can break apart fractures in rock by growing deeper and “searching” for
water. Burrowing animals mix up rock matter and soil, and bring fresher material
to the surface where it can be broken down more readily by chemical and
physical processes.
5. How is carbonic acid formed in nature?
Carbonic acid is formed in nature by falling rain dissolving carbon dioxide out of
the atmosphere. Groundwater also dissolves carbon dioxide from decaying
organic matter.
6. What products result when granite is chemically weathered by carbonic acid?
When granite is chemically weathered by carbonic acid, the potassium ions in
orthoclase are removed and replaced with hydrogen ions. This changes the
mineral’s crystal structure, and results in clay minerals. Most frequently, this clay
is kaolinite. Silica can also be removed by dissolution, and eventually the silica is
precipitated out as chert.
2.4
1. Why are sedimentary rocks important?
Sedimentary rocks comprise about 75 percent of the Earth’s outcrops. It is from
these rocks that geologists determine much of Earth’s history—from
depositional environments, climate, and life. Many sedimentary rocks are also
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economic. Coal, iron, aluminum, manganese, and fertilizer are all mined from
sedimentary deposits.
2. What minerals are most abundant in detrital sedimentary rocks? In which rocks do
these sediments predominate?
Quartz and clay minerals are most abundant in sedimentary rocks. Clay is the
most abundant product of chemical weathering, and quartz is the mineral that is
most resistant to weathering. These sediments are most common in sandstone,
shale, and siltstone.
3. Distinguish between conglomerate and breccia.
Conglomerate is an aggregate of rounded detrital gravel-sized particles. Breccia
is an aggregate of angular detrital fragments, also gravel-sized.
4. What are the two categories of chemical sedimentary rock? Give an example of a rock
that belongs to each category.
Chemical sedimentary rock can be inorganic chemical precipitate or made from
bio-chemical sediments. Chemical sedimentary rocks include chert and
travertine. Bio-chemical rocks include coquina, chalk, and diatomite.
5. How do evaporates form? Give an example.
Evaporation causes minerals to precipitate from solution, especially in deserts
and tidal flats at the ocean. Halite precipitation results in rock salt, whereas
gypsum precipitation results in rock gypsum.
6. Compaction is an important lithification process with which sediment size?
Compaction is the most important lithification process for fine-grained
sedimentary rocks.
7. What is the single most characteristic feature of sedimentary rocks?
Bedding is the single most characteristic feature of sedimentary rocks. Beds,
separated by bedding planes, are also referred to as strata (layers).
2.5
1. Metamorphism means to “change form.” Describe how a rock may change during
metamorphism.
Metamorphism most frequently results in a change of texture through crystal
size and shape, or a change of mineralogy through recrystallization. Sometimes
there can be a change of chemical composition due to the interaction of
chemically reactive fluids.
2. Briefly describe what is meant by the statement “Every metamorphic rock has a parent
rock.”
A metamorphic rock results from the transformation of another rock by a change
in heat, pressure, and/or chemically active fluids. The rock that is being
changed by these environmental conditions is the “parent rock,” or original rock
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that is undergoing metamorphosis.
3. List the three agents of metamorphism and describe the role of each.
1) Heat is the most important agent of metamorphism. It causes
recrystallization of minerals into new minerals that are stable under the
conditions of metamorphism.
2) Pressure squeezes the rock, compacts it, makes it denser, and can
cause recrystallization as well.
3) Chemically active fluids can act as catalysts for recrystallization. The
hotter the fluids, the more they influence recrystallization. The ions they
contain can also result in changing the chemical composition of the rock
during recrystallization.
4. Distinguish between regional and contact metamorphism.
Regional metamorphism results from mountain building. It affects large volumes
of rock and does so gradationally. The interior-most rock within the area
undergoing orogenesis will be the most highly metamorphosed, whereas the
edges will be the least altered. Contact metamorphism arises from the intrusion
of a magma body that essentially “cooks” the country rock that it is intruding
into. This metamorphism is also gradational, lessening the farther away from
the intrusion. Contact metamorphism is driven by heat and regional
metamorphism is driven by pressure.
5. Which feature would easily distinguish schist and gneiss from quartzite and marble.
Schist and gneiss both have foliated textures, whereas quartzite and marble
both are nonfoliated.
6. In what ways do metamorphic rocks differ from the igneous and sedimentary rocks
from which they formed?
Metamorphic rocks differ from their parent rocks in crystal shape, crystal size,
mineral composition, and sometimes a foliated texture. For example, a
phaneritic granite can become a foliated gneiss after metamorphism. A muddy
limestone will become a coarse-crystalled marble after metamorphism.
CONCEPTS IN REVIEW
2.1: Sketch or explain the next stage or possible changes that could occur to the igneous rock
shown in this diagram.
If the igneous rock were buried deep in Earth’s crust, it could be exposed to high enough
pressures and temperatures that it beings to deform and recrystallize without melting,
becoming a metamorphic rock. If it does melt again, it could eventually recrystallize
again as an igneous rock. If it goes in the other direction altogether and is uplifted to
Earth’s surface, it would be exposed to processes of weathering and erosion and
eventually be broken down into sediment that could be deposited and lithified into a
sedimentary rock.
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2.3: How are the two main categories of weathering represented in this image that shows
human-made objects?
The two main categories of weathering are mechanical and chemical. Mechanical
weathering is the physical breakdown of material into smaller components; this is shown
in the photo with the pieces of shattered glass bottles. Chemical weathering is the
breakdown of the minerals and elements that comprise a rock by the chemical reaction
of these components with water in some way. This process is represented by the rusted
cans in the photo, as rust is a product of chemical weathering of iron-bearing materials.
2.4: This photo was taken in the Grand Canyon, Arizona. What name(s) do geologists use for
the characteristic feature found in the sedimentary rocks shown in this image?
This photo shows horizontal bedding planes of sedimentary rock layers.
2.5: Examine the photograph. Determine whether this rock is foliated or nonfoliated, then
determine whether it formed under confining pressure or differential stress. Which of the pairs of
arrows shows the direction of maximum stress?
This metamorphic rock is foliated; this texture is created through the alignment of dark
color minerals. This texture forms under differential stress, where greater pressure from
one direction forces the mineral crystals to rotate and recrystallize in the other direction.
The arrows labeled B show the direction of principle stress.
GIVE IT SOME THOUGHT
1. Refer to Figure 2.1. How does the rock cycle diagram, in particular the labeled arrows,
support the fact that sedimentary rocks are the most abundant rock type on Earth’s
surface?
“Uplift, weathering, erosion, and deposition” brings sediment from all rock types
to Earth’s surface. This process is followed by “Lithification (compaction,
cementation),” which transforms this sediment into sedimentary rock.
2. Would you expect all of the crystals in an intrusive igneous rock to be the same size?
Explain why or why not.
Most of the crystals in an intrusive igneous rock can be the same size. However,
the mineral composition also drives the crystal size, so some minerals simply do
not grow the same size as others even when they are all formed under the same
conditions.
3. Apply your understanding of igneous rock textures to describe the cooling history of
each of the igneous rocks pictured here.
A. Aphanitic texture (fine grained), means there was little time for mineral crystals to
form, so the lava cooled rapidly at Earth’s surface.
B. Porphyritic (two distinct grain sizes), means the magma started cooling at depth,
slowly, where large crystals formed. But then it began cooling rapidly, not
allowing the other minerals time to grow, resulting in a finer-grained matrix
around the phenocrysts.
C. Phaneritic (coarse crystals), means the magma cooled slowly at depth, allowing
all the minerals to crystallize slowly and form crystals big enough to discern with
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the naked eye.
D. Pumice (vesicular glassy texture), forms from extruded lava that cooled very
rapidly, allowing no time for minerals to crystallize at all, instead volcanic glass
threads form from instantaneous cooling. The spongy texture arises from all the
volcanic gases escaping the melt as it is cooling during eruption.
4. Is it possible for two igneous rocks to have the same mineral composition but be
different rocks? Use an example to support your answer.
Yes, two igneous rocks can have identical mineral composition and be
completely different rocks. If we take an intermediate composition volcano, for
example, and dissect it, we might find the following: Pumice chunks from a
violent eruption on the surface of an andesitic flow and a diorite volcanic core.
Because the violent eruption sent some lava spewing into the air in molten blobs
that cooled almost instantaneously, this resulted in pumice. Meanwhile, some of
the lava flowed down the sides of the volcano, cooling rapidly, but not to the
extent that the flying blobs did—this resulted in the andesitic rock. The magma
that didn’t make it out of the volcano and remained insulated within the cone
cooled much more slowly, allowing crystals to grow within the melt. This resulted
in the diorite pluton.
5. Use your understanding of Bowen’s reaction series (Figure 2.11) and the process of
magmatic differentiation to explain how partial melting can generate magmas with
different compositions.
As crystals form from a cooling melt, they selectively remove certain elements
from the magma; this is magmatic differentiation. The minerals that are removed
from the melt first are those that crystallize at the highest temperatures, such as
anorthite and olivine. These minerals would be followed by pyroxenes,
amphiboles, and plagioclase. As this occurs, it leaves the remaining liquid
portion of the melt depleted in the elements that have been removed by the
formation of certain minerals. Should those first minerals that crystallize settle or
the remaining melt inject into a fracture or erupt, the minerals that crystallize
from that remaining melt will be of a different composition from the first minerals
to form, resulting in a different igneous rock originating from the same initial
melt. The last minerals to crystallize are those happier at the coolest
temperatures, such as orthoclase, biotite, and quartz. If this process goes on for
a long time from a very large volume of magma, volatile elements that normally
do not get incorporated into mineral crystals end up getting forced together,
resulting in rare gemstone pegmatites.
6. Dust collecting on furniture is an everyday example of a sedimentary process. Provide
another example of a sedimentary process that might be observed in or around where
you live.
Snow falling and accumulating on the lawn in winter is a similar sedimentary
process to the collection of dust on furniture. The buildup of residue on the tub or
inside the kettle is an example that represents chemical precipitation.
7. Describe two reasons why sedimentary rocks are more likely to contain fossils than
igneous rocks.
Igneous rocks form from molten rock; if there were fossils present prior to
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melting, they would be destroyed by the melting, preventing there from ever
being a fossil in an igneous rock. Metamorphic rocks are formed by
recrystallizing and squeezing pre-existing rocks. If fossils are present in the
parent rock, they will at least be deformed by squeezing if not be completely
destroyed by recrystallization. Sedimentary rocks form by the compaction and
cementation of particles together, a process that favors the preservation of fossil
material—if there is any present to preserve.
8. If you hiked to a mountain peak and found limestone at the top, what would that indicate
about the likely geologic history of the rock there?
Limestone forms by precipitation of calcium carbonate from ocean water.
Limestone atop a mountain tells us that rock formed at the base of an ocean,
was lithified and uplifted such that it is now at the top of a mountain.
9. The accompanying photos each illustrate either a typical igneous, sedimentary, or
metamorphic rock body. Which do you think is a metamorphic rock? Explain why you
ruled out the other rock bodies.
Photo B represents the metamorphic rock as it exhibits folding, a texture that
forms due to the deformation of rock under heat and pressure. Rock A is
sedimentary, as it has bedding and is clearly made of layers of very fine
sediment grains and very coarse sediment grains that have all lithified together.
Rock C is igneous, as it shows two distinct compositions of rocks, the phaneritic
crystal texture (indicating slow cooling from a melt) of the host rock that has been
intruded by an aphanitic texture (indicating rapid cooling from a melt) dike.
10. Examine the accompanying photos, which show the geology of the Grand Canyon.
Notice that most of the canyon consists of layers of sedimentary rocks, but if you were to
hike down into the inner gorge, you would encounter the Vishnu schist, a metamorphic
rock.
a. What process might have been responsible for the formation of the Vishnu
schist? How does this process differ from the processes that formed the
sedimentary rocks that are atop the Vishnu schist?
The Vishnu schist is a folded metamorphic rock, it certainly was
transformed by heat and a tremendous amount of pressure. The
sedimentary rocks that lie more or less horizontally above it were
deposited, compacted, and cemented without any tectonic disruption and
virtually no heat.
b. What does the Vishnu schist tell you about the history of the Grand Canyon
prior to the formation of the canyon itself?
The Vishnu schist indicates a very long history of Grand Canyon rocks
long before canyon development! There were rocks that were deposited,
deformed, metamorphosed, and intruded long before the deposition of
many layers of sedimentary rock…all of which were sliced through by the
Colorado River well after they were lithified and later uplifted.
c. Why is the Vishnu schist visible at Earth’s surface?
The Vishnu schist is visible at the Earth’s surface both because it has
been uplifted by tectonic forces and because it has been exposed at the
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canyon bottom by the incision of the Colorado River.
d. Is it likely that rocks similar to the Vishnu schist exist elsewhere but are not
exposed at Earth’s surface? Explain.
Yes, it is very likely that other rocks like the Vishnu schist exist but are not
exposed. The Vishnu formed under great heat and pressure, which exist
at great depth in the continental crust. In order for us to see it at surface,
it must be brought there by uplift at least. Uplift does not occur
everywhere on Earth at all times.
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