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Chapter 7
Metamorphism and Metamorphic Rocks
Notes
(3 classes notes + 1 class Xword & review + 1 class test)
Objectives:
1. Define the term metamorphism. p. 193
2. Given a metamorphic rock, identify the parent rock
from which it came. p. 193, 199-207
3. Explain how the type of stress affects the foliation of a
metamorphic rock. pp. 194-196
4. Identify and describe the three (3) agents (causes or
controlling factors) in metamorphism. pp. 193-196
5. Identify and describe the two (2) general types of
metamorphism. pp. 198-207
Def n
Metamorphism is the term to describe what takes place
when a rock is subjected to temperatures and/or pressures
that are very different from those which formed it
originally.
Metamorphic Rocks - These are formed as a result of
sedimentary, igneous or other metamorphic rocks being altered
by chemistry, heat, and pressure to form a new kind of rock.
Some rocks that have been altered from sedimentary rocks are
slate, mica, schist, gneiss, quartzite and marble. Some altered
forms of igneous rocks are actinolite, chlorite, and amphibolite.
The difficult name for this class of rocks comes from two Greek
words: meta which means "after" and morphe meaning "shape" a rock which has changed its shape or composition. There are
several causes for rocks changing in this way. An increase in
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temperature and pressure caused by geological movement such
as earthquakes or volcanic eruptions can change the rocks.
Water carrying chemicals can be the cause of new minerals in
the rocks. A very long period of time is necessary for these
changes to occur. Keep in mind however, that the rock cannot
melt while these changes are occurring.
Completely new minerals can be formed that were not in the
original rock or original mineral crystals. Original mineral
crystals can be rearranged in shape, though not chemically
altered. Water and carbon dioxide are frequently added or
removed during metamorphism.
There are two types of metamorphism, depending on the setting
in which it occurs.
Contact Metamorphism
Thermal or contact metamorphism happens when igneous rocks
are intruded or force their way up into the "country rock".
Because the igneous rocks are hot (lava or magma) they heat
surrounding rock and add pressure. In this way, new minerals
are formed and recrystallization takes place. (The surrounding
rocks are baked!). The crystals in the baked rock are not forced
to line up, so contact metamorphism typically produces
nonfoliated rocks. Chemically active fluids may also produce
new rock during contact metamorphism. See animation here and
pic here.
Regional Metamorphism
Regional metamorphism is caused by large movements of the
Earth's crust such as during mountain building, creating great
pressure and heat on the rocks forming the crust. The rocks
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becomes folded over each other, twisted and doubled. These
metamorphic rocks often compose large masses of some of the
oldest known rocks - up to 3500 million years old. As with
contact metamorphism, chemically active fluids may also
produce new rock during regional metamorphism. See
animations here and here.
Objectives:
1. Contrast contact and regional metamorphism. pp. 198-207
2. Describe the features of and identify selected metamorphic
rocks. pp. 199, 201-205, (At least slate, phyllite, schist,
gneiss, quartzite from regional metamorphism and marble and
hornfels from contact metamorphism.)
3. Relate the types and characteristics of metamorphic rocks to
parent rock, temperature, pressure, and chemical conditions.
pp. 193-196
The term metamorphism means to "change form". Rocks which
undergo metamorphism can change in size, shape, texture and
mineral content. All three types of rocks (igneous, sedimentary,
and metamorphic) can undergo metamorphism. The agents
(causes) of metamorphism include heat, pressure, and
chemically active fluids.
Two (2) Grades of Metamorphism
Low-grade metamorphism - rocks are only slightly changed.
An example is the metamorphism that changes shale into slate.
The slate is more compacted than the shale, but samples of
these rocks may still be difficult to distinguish.
High-grade metamorphism - rocks are greatly changed and it
may be impossible to identify the parent rock. During highgrade metamorphism, temperatures may approach those at
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which the rocks may melt. However, if the rocks do melt and
reform, they are igneous rocks. For example, the high grade
metamorphism of slate may produce gneiss.
Two (2) Settings in Which Metamorphism Occurs
Regional metamorphism - this type of metamorphism occurs
during mountain building and in subduction zones when rocks
are subjected to intense stress and high temperatures. This type
of metamorphism creates slate, phyllite, schist, gneiss, and
quartzite. Pressure is the main cause of metamorphism here
although chemically active fluids and heat can also be involved
in the process.
Contact metamorphism - this type of metamorphism occurs
when rock is near, or in contact with, a mass of magma, beneath
lava flows, and around dykes and sills. The changes to the rock
are caused mostly by high temperatures and the rock is "baked".
This type of metamorphism creates marble from limestone (see
Figure 7.14B, p. 202), quartzite (see Figure 7.14A, p. 202), and
hornfels (see Figure 7.12 p. 201 text). High-grade
metamorphism is caused by contact and the grade of
metamorphism decreases as you move away from the magma
body. Heat is the main cause of metamorphism here although
chemically active fluids and pressure can also be involved in
the process.
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Activity: On the image below, identify areas where regional and
contact metamorphism could occur.
The Three (3) Agents (Causes) of Metamorphism
Heat - this is the most important cause of metamorphism.
Heat provides the energy to drive the chemical reactions that
cause minerals to recrystallize. Heat increases with depth.
Pressure - as rocks become buried, they come under increasing
pressure (confining (uniform) pressure) because of the weight
of the material above them. This pressure acts equally in all
directions (see Figure 7.3A p. 194 text and Figure 7.4A p. 195
text). Rocks are also subjected to directed (differential) stress
during mountain building. This stress does not act equally in all
directions and is responsible for folds and bends in
metamorphic rock (see Figure 7.3A p. 194 text and Figure 7.4B
p. 195 text).
Chemically active fluids - the most common chemically active
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fluid is the water that is contained in the pores of rocks. This
water acts as a catalyst (it helps speed up reactions) by allowing
ions to move through the rocks more easily. These ions may
cause the minerals in the rock to recrystallize into more stable
crystals or to change into completely different minerals.
Changes in Texture Caused by Metamorphism
The degree of metamorphism (how much the parent rock is
changed) can be determined by the texture of the rock and by
the minerals it contains. For example, when shale is changed to
slate, the pressure exerted on the shale causes the minerals in
the shale to align in a more compact arrangement. However, the
types of minerals in the slate remain exactly the same as those
in the shale and the texture of the rock is changed only slightly.
This explains why shale and slate are sometimes difficult to
distinguish.
If conditions are more extreme, the pressure may cause the
minerals to recrystallize, usually into larger crystals. In this
case, the rock will appear to have visible crystals (and a coarse
grain like granite (an igneous rock)). Some minerals will
recrystallize in a direction perpendicular to the direction of the
compressional force (see Figure 7.4B p. 195 text) and cause the
rock to have a layered or banded appearance. This type of
texture is called foliated. Gneiss is an example of a foliated
metamorphic rock (see Figure 7.16 p. 204 text), as are slate,
phyllite, and schist. See animations here, here, and here.
A foliated texture does not occur in all metamorphic rocks. A
rock made of only one mineral will generally have a nonfoliated
texture. For example when fine-grained pure limestone
undergoes metamorphism, the small calcite crystals combine to
form larger crystals. The new rock (marble) has a coarse grain
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but is not foliated. Quartzite is another non-foliated
metamorphic rock.
New materials can be introduced as metamorphism occurs. For
example, rock near a large body of magma can be altered by
hydrothermal (hot water) solutions released by the magma as it
cools. These solutions may contain ions such as those of
valuable minerals, and these minerals may become part of the
nearby rock during metamorphism. See here.
Classifying Metamorphic Rocks
Indicators that a rock may be metamorphic are high density,
large crystals, foliation, and the presence of economically
important minerals. Keep in mind that these properties do not
guarantee that the rock is metamorphic.
Common Metamorphic Rocks
Common Foliated Metamorphic Rocks
i. Slate - this rock is formed from the low-grade
metamorphism of shale. It can be easily split into flat slabs.
It is used for roof tile, chalkboards, and pool tables (see
Figure 7.16 p. 204 text).
ii. Schists - these are strongly foliated rocks, formed from
regional metamorphism (mountain-building). It can also be
split into thin flakes or slabs. Schists are also created from
shale but the metamorphism is more intense (see Mica
schist Figure 7.16 p. 204 text).
iii.
Gneiss (pronounced "nice") - is a visibly banded
metamorphic rock that contains elongated and granular
minerals (see Figure 7.16 p. 204 text).
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Common Nonfoliated Metamorphic Rocks
i. Marble - this rock is a coarse grained rock created from
limestone (see Figure 7.14B p. 202 text).
ii. Quartzite - this rock is very hard and is formed from quartz
sandstone which undergoes moderate to high grade
metamorphism (see Figure 7.14A p. 202 text).
Sample Exam Questions
1. Which statement is TRUE about the rock cycle?
(A) Igneous rocks lithify to form sediments.
(B) Igneous, sedimentary, and metamorphic rocks are all
subject to metamorphism.
(C) Magma crystallizes to form both igneous and
metamorphic rocks.
(D) Weathering affects only sedimentary rocks.
2. Which rock was subjected to the highest temperature and
pressure conditions?
(A) gneiss
(B) marble
(C) shale
(D) slate
3. Using examples, distinguish between foliated and
nonfoliated metamorphic rocks.
Core Lab 4: Igneous, Sedimentary, and Metamorphic Rock,
Part III, Metamorphic Rock
Read items 1-6, 8 p. 213
Do #'s 1-6, 8-12, 17, 18 p. 214 text.
Read pp. 217-224 for next day
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