Chapter 15: Metamorphism, Metamorphic Rocks, and Hydrothermal Rocks
Metamorphic rocks develop from pre-existing rocks that have undergone solid-state
transformation due to increased temperature, increased pressure, and migrating fluids. The
word, metamorphism, comes from the Greek words, meta (change) and morph (form), so
metamorphism literally means to change form.
A key idea to appreciate about metamorphism is that of solid-state change. In other words,
we’re not melting the rock. If we did, then we’d be in the igneous realm.
Four important ways that pre-existing rocks are typically transformed as they metamorphose
include:
1. Densification (rocks become more dense)
2. Development of foliation (mineralogical alignment)
3. Recrystallization (existing minerals in the rock, including quartz and calcite, grow larger; old
minerals are destroyed and new ones created in the solid state).
4. Dehydration (as temperature and pressure increase and recrystallization occurs, water is
driven out of mineral structures).
Your book has a good summary of the factors controlling the metamorphic process, including
the composition of the parent rock, temperature, pressure, differential stress, fluids, and time.
I’ll just add a few comments about the role of pressure. In general, pressure increases with
depth below Earth’s surface. The same holds true, by the way, for depth below a water surface
(see Figure 15.2). Your book uses the term, confining pressure, to describe this trend.
An important aspect of confining pressure is that at any depth beneath Earth’s surface, the
pressure is equal in all directions. As a pre-existing rock is subjected to greater and greater
confining pressure by deep burial, for example, it compresses and becomes more dense, but it
doesn’t deform (i.e., change shape). It might not be too surprising to learn that metamorphic
rocks are typically more dense than their non-metamorphic counterparts (i.e., the parent rock
that was metamorphosed).
Pressure can be thought of as force per unit area. Typically, when increased pressures result
from geologic forces (as opposed to deep burial), the term, differential stress is used, because
the pressure is no longer equal in all directions.
Differential stress not only densifies a rock, it also deforms a rock—that is, changes the shape of
the rock and/or its constituent minerals, as shown in Figure 15.3. Differential stress can be
caused by compression and also by shearing.
When a rock mass is subjected to differential stress, it not only becomes more dense and
changes shape, but the very fabric of the rock can change as well. In particular, flat and
elongated minerals in the rock begin to align themselves roughly parallel to one another, and
new minerals begin to grow in an aligned fashion.
Mineralogical alignment caused by differential stress is called foliation (see Figures 15.4, 15.5,
15.6, and 15.7B for good examples of foliation).
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The main way we classify metamorphic rocks is by the presence or absence of foliation (Table
15.1).
If a pre-existing rock is metamorphosed under differential stress, it will develop into a foliated
metamorphic rock; if differential stress doesn’t play a role, it develops into a non-foliated rock.
Foliated metamorphic rocks are distinguished by the type of foliation they exhibit. Basically, we
name foliated metamorphic rocks based on their foliation textures. You should become familiar
with the overall characteristics of various foliation textures, including slaty, phyllitic, schistose,
and gneissic. Here’s a quick summary:
Slaty texture: Fine-grained foliation due to alignment of microscopic mica minerals; creates a
tendency for the rock to split along flat, thin sheets. Photo of Slate.
Phyllitic texture: Fine-grained foliation with a “silky/waxy sheen;” creates a tendency for the rock
to split along wavy surfaces. Photo of Phyllite.
Schistose texture: Medium- to coarse-grained foliation created by the visible alignment of mica
minerals; creates a grainy, scaly rock characterized by a wide variety of minerals that show a
strong platy alignment. Photo of Schist.
Gneissic texture: Created by compositional banding of light- and dark-colored minerals found in
separate layers or lenses. Photo of Gneiss.
A quick disclaimer: the term “phyllitic texture” is a made-up term. You won’t find it in your book.
It’s just shorthand for “intermediate between slaty and schistose texture.”
Here’s a summary classification chart for foliated metamorphic rocks:
FOLIATED METAMORPHIC ROCKS
Rock Name
(link)
Texture
Common
Minerals
Slate
Slaty
Clay, micas
Very fine grained; splits easily into
thin, flat sheets.
Phyllite
Phyllitic
Micas
Fine grained; exhibits dull, waxy
sheen; splits along wavy
surfaces.
Schistose
Variable, with
micas,
amphibole,
garnet common
Medium to coarse grained; platy
or elongated minerals visibly
aligned.
Gneissic
Feldspar, quartz,
amphibole, micas
Medium to coarse grained; light
and dark minerals occur in
separate bands or lenses that
may be folded or contorted.
Schist
Gneiss
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Identifying Characteristics
Besides allowing us to properly classify foliated metamorphic rocks, recognizing the foliation
texture allows us to say something about overall temperatures and pressures at which
metamorphism takes place.
Notice in Figure 15.14 that as depth increases, temperatures and pressures also generally
increase.
Notice, too, that the region depicted in Figure 15.14 is being compressed horizontally, and
therefore experiences differential stress. So, the combination of elevated temperature, elevated
pressure, differential stress, and migrating fluids metamorphoses the rocks in a predictable way.
For example, slate typically forms under lower temperatures and pressures than phyllite. In fact,
a pre-existing slate can be converted to phyllite under increased temperature and pressure.
In general, as temperature and pressure increase, the following sequence of foliated
metamorphic rocks may develop:
Slate
Phyllite
Schist
Gneiss
Migmatite (mixture of igneous and metamorphic
rock—develops as metamorphic rock begins to melt)
So, lower-grade metamorphic rocks like slate and phyllite can be transformed into mediumgrade metamorphic rocks like schist, and high-grade rocks like gneiss. In other words, foliation
textures are progressive in terms of temperature and pressure, with one rock type converting
into another under higher temperature/pressure conditions.
Okay, now you’re in a position to appreciate a really lame geology joke. Get ready…
Think of gneiss as a high-grade foliated metamorphic rock that’s had the schist squeezed
out of it!
Okay, on to non-foliated metamorphic rocks…
Non-foliated metamorphic rocks obviously aren’t foliated, but they have undergone changes that
have transformed the original, parent rock. Usually, such change is accomplished by increased
temperatures and migrating fluids, with increased pressure playing only a minor role.
Important non-foliated metamorphic rocks include quartzite (metamorphosed quartz sandstone)
and marble (metamorphosed limestone). Just be familiar with these two and that’ll be sufficient
for the non-foliated rocks .
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Here’s a summary classification chart for non-foliated metamorphic rocks:
NON-FOLIATED METAMORPHIC ROCKS
Rock Name
(link)
Texture
Common
Minerals
Identifying
Characteristics
Calcite
Typically medium to coarse
grained (visible crystals);
metamorphosed limestone.
Marble
Crystalline
(interlocking
crystals)
Quartzite
Crystalline
(interlocking
crystals)
Quartz
Typically medium to coarse
grained (visible crystals);
metamorphosed quartz
sandstone.
Anthracite
Glassy (highly
reflective)
No minerals
present
Very shiny; composed of
highly altered plant remains
(metamorphosed coal).
Hornfels
Microcrystalline
(small, interlocking,
non-visible
crystals)
Ferromagnesian
minerals,
plagioclase
A fine grained, dark rock
associated with contact
metamorphism.
Index Minerals
Thanks to modern laboratory testing, we can be fairly precise about the temperatures and
pressures under which various metamorphic rocks develop. The concept of index minerals is
very important in understanding metamorphic rocks (see In Greater Depth, 15.2).
Very simply, certain minerals in a metamorphic rock form only under restricted temperature and
pressure ranges and can therefore be thought of as small thermometers and pressure gauges.
Laboratory testing allows researchers to determine the approximate temperatures and
pressures under which these index minerals form. When found in metamorphic rocks, index
minerals allow researchers to infer the approximate temperatures and pressures at which the
rock formed.
Metamorphic Facies
Several decades ago, researchers developed the concept of metamorphic facies to summarize
the pressure/temperature stability fields of groups of index minerals typically found together in
various metamorphic rocks.
In the chart below, each color represents a different metamorphic facies (pressure/temperature
environment). Each facies is characterized by a different group of index minerals:
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20
18
Eclogite
16
14
Increasing Pressure
(kilobars)
12
10
Blueschist
Amphibolite
8
Granulite
6
PrehnitePumpellyite
Greenschist
4
2
Zeolite
Hornfels
0
0
100
200
300
400
500
600
Sanidinite
700
800
900 1,000
Increasing Temp ºC
Metamorphic Facies
(from Wikipedia; http://en.wikipedia.org/wiki/Metamorphic_facies)
Don’t worry, there’s no need to memorize the above diagram. Just understand that as
temperatures and pressures increase, different groups of index minerals crystallize within the
resulting metamorphic rocks, giving us different metamorphic facies. A few summary points:

Zeolite facies is an example of low-grade (i.e., mild) metamorphism characterized by relatively low
temperatures and pressures compared to other facies. Zeolites are an important group of industrial
minerals because their open crystal structures make them excellent molecular filters.

As temperatures and pressures both increase, we enter the realm of greenshist to amphibolite facies.
Greenschist facies is characterized by several greenish minerals, whereas amphibolite facies is
characterized by double-chain silicates such as hornblende (remember, the mineral, hornblende,
belongs to the amphibole mineral group).

Under very high temperatures and pressures, we enter the upper field of amphibolite facies and
granulite facies.

The hornfels facies is characterized by high temperature/low pressure conditions.

The blueschist facies is characterized by low temperature/high pressure conditions.
Taking a broader perspective, geologists have distinguished several important metamorphic
styles, including regional, contact, burial, and dynamic metamorphism. All four metamorphic
styles can be related to plate tectonic theory in an elegant way, as we’ll see…
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Regional Metamorphism
Regional metamorphism generally involves elevated temperatures (typically in the range of 300800 C), elevated pressures, and takes place at considerable depth (typically deeper than 5
km).
Regional metamorphic rocks typically contain index minerals from the greenschist to
amphibolites facies.
Regional metamorphic rocks develop deep within convergent plate boundaries (subduction and
collision zones), in the cores of mountain ranges, where confining pressure is high, differential
stress is significant, and temperatures are generally elevated.
Not surprisingly, most regional metamorphic rocks are foliated, since they develop at convergent
plate boundaries where differential stress is high.
Because plate convergence, mountain building, and regional metamorphism all occur together
in modern geologic time, regional metamorphic rocks can tell us where sites of ancient plate
convergence and mountain building occurred.
For example, consider the Appalachian Mountains, along the east coast of North America.
Throughout these ancient, deeply eroded mountains, we find foliated, regional metamorphic
rocks such as slate, phyllite, schist, and gneiss that formed at depth. Tectonic uplift and deep
erosion have exposed these rocks at the surface.
In effect, the modern Appalachians represent the deeply eroded remnants of a much older, taller
mountain range that developed during the assembly of Pangea before about 250 million years
ago. So…although a convergent plate boundary doesn’t now occur along North America’s east
coast, the presence of regional metamorphic rocks in this area is evidence that there was once
a convergent plate boundary along the east coast. This is yet another example of how our
understanding of the geologic present guides our understanding of the geologic past. Repeat
after me…Uniformitarianism!
Contact Metamorphism
In contrast to regional metamorphism, which occurs under generally elevated temperatures and
pressures, contact metamorphism occurs when increased temperature is the dominant factor
(for example, next to an igneous intrusion).
You’ve learned by now that many igneous rocks form at subduction zones from gas-charged,
water-enriched magmas created during the melting of the subducting slab and also the mantle
directly above the subducting slab.
As the magmas created in this fashion rise into the overriding (non-subducting) plate, they
“cook” the surrounding rock, turning it into contact metamorphic rock.
Hornfels-facies index minerals typically crystallize during contact metamorphism at shallow
depths.
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Hot water plays an important role in creating metamorphic rocks at divergent plate boundaries
(see Figure 15.19), where cold sea water seeps into the mid-ocean ridge, becomes hot, and
cooks the surrounding rock as it seeps back out of the ridge. This type of contact
metamorphism is sometimes described as hydrothermal metamorphism because of the role of
hot water.
Burial Metamorphism
With moderate to deep burial, slightly elevated temperatures and pressures may cause new
minerals to grow within the rock. Although foliation doesn’t develop, some existing minerals are
destroyed and new minerals such as zeolites crystallize.
Burial metamorphism grades into regional metamorphism as temperatures and pressures
increase.
Dynamic Metamorphism
Your book doesn’t cover this style of metamorphism. I’ll discuss it briefly.
Under certain conditions, for example, along faults and plate boundaries, rocks can be
plastically smeared or mechanically pulverized.
In this case, the rocks develop fabrics associated with intense shear stress. Motion along a
fault, for example, can produce a ground-up rock jumble called fault breccia. Here’s a photo of
some fault breccia within the San Jacinto Fault Zone in the Anza Borrego State Park:
Fault Breccia
Fault Breccia, Anza Borrego State Park
In the above photo, the fault breccia is the strip of rock between the dashed lines that define the
fault. It’s a jumbled mixture of gravel, sand, clay, and calcium carbonate that developed as
movement along a branch of the San Jacinto Fault Zone sheared, smeared, and pulverized the
rock on either side.
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Dynamically metamorphosed rocks like fault breccia aren’t very common, but they do help
geologists locate tectonic features like faults. In fact, that’s how I found the fault in the above
photo. I was walking along a dry stream channel and noticed a white stripe running up the
steep side of the channel. Upon closer examination I realized I’d found a fault! Whoo Hoo!
As you might expect, dynamically metamorphosed rocks are typically found right along faults
and plate boundaries, where shear stress is most intense.
Metamorphism and Plate Tectonics
Let’s connect the different metamorphic styles and facies to plate tectonic theory by developing
a simple, conceptual model, shown below.
Regional metamorphism
(high pressure/temp) beneath volcanic arc
(greenschist/amphibolite facies)
Burial metamorphism
(low pressure/low temp)
beneath/near trench
(zeolite facies)
Contact metamorphism (high temp)
surrounding shallow igneous
intrusions (hornfels facies)
Sea Surface
Dynamic metamorphism
(intense shearing) at plate
boundary interface
High pressure/low temp metamorphism
near cold, subducting slab
(blueschist facies)
Plate Convergence and Metamorphism.
The main thing to appreciate about the model shown above is that different metamorphic
conditions can be nicely related to plate tectonic processes such as subduction.
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
Deep beneath the volcanic arc, where pressure and temperature are both elevated, regional
metamorphism occurs (green region).

Contact metamorphism occurs within the country rocks surrounding hot igneous intrusions
(red region).

Burial metamorphism occurs wherever thick accumulations of sediment and sedimentary rocks
occur, such as near the trench.

Dynamic metamorphism occurs at the plate boundary interface, where one plate scrapes against the
other.

Finally, rather unique metamorphic conditions develop within the subduction zone near the boundary
between the subducting and non-subducting plates (blue region in above diagram). Because the
subducting plate is cool, temperatures remain relatively low even as pressure increases with depth
near the subducting plate. This results in the creation of a metamorphic rock called blueschist, so
named due to the presence of glaucophane, a blue amphibole mineral that grows only under low
temperature/high pressure conditions. Blueschist is a great subduction indicator. Whenever we find
blueschist, we know that the area was once a subduction zone.
The Rock Cycle
One last topic I’ve delayed covering until now is the rock cycle, which isn’t covered in this
chapter.
The reason I’ve delayed covering this topic until now is that it makes more sense in my opinion
to learn about the components of the rock cycle (i.e., the rocks themselves) before putting all
this material together into a summary diagram.
Now that we’ve learned about all three rock types, we can put the pieces of the puzzle together
and discuss how all rock types are interrelated.
My version of the rock cycle is drawn for you below.
I prefer my diagram to the one in your text (Figure 11.1) because my version is simpler and thus
easier to commit to memory.
Arguably, the rock cycle is the most important summary diagram in geology! I regularly tell my
on-campus students that if they learn nothing else in my class, I hope they take the time to
memorize the rock cycle.
The rock cycle reminds us that any one rock type can become any other rock type given the
correct set of processes acting on the rock.
Each numbered arrow below represents a set of processes listed below the diagram.
Notice that any given rock type can even be transformed back into itself.
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I won’t ask you to draw this diagram, but I may ask you some detailed questions about it on the
next exam, so you should become very familiar with it. In fact, one of the best ways to learn this
diagram is to make sure you can draw it.
Drawing the rock cycle in this way emphasizes that the same set of processes create a given
rock type regardless of which rock type we start with. That’s why, for example, all the 3’s point
toward igneous rock (because all igneous rocks form via melting of preexisting rock, cooling,
and crystallization).
On the exam, I’ll probably ask you at least one rock cycle question. Make sure you can tell me
the set of processes required to turn any one rock type into any other rock type (hint,
hint!).
3
Igneous
Rock
3
2
2
1
3
1
Metamorphic
Rock
Sedimentary Rock
1
2
THE ROCK CYCLE
1. Weathering, erosion/transport, deposition/precipitation, lithification
2. Increased temperature, increased pressure, migrating fluids.
3. Melting, cooling/crystallization.
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