Metamorphic Rock Identification Page 2
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4Temperature and Pressure
When rocks undergo high temperatures and intense
pressure, they have properties similar to plastic. The rocks
can bend and fold without melting. A permanent change in
shape by bending and folding is called plastic deformation. It is one
way that a rock’s texture changes during metamorphism.
Plastic deformation is common during uplifting events when
tectonic plates collide and form mountains. Changes in
composition and structure are clues that a rock has
metamorphosed.
The parent rock is the rock that changes during
metamorphism. The parent rock’s composition determines
the temperature at which the rock will metamorphose. Some
metamorphic rock forms at temperatures between 150°C and
200°C. Like temperature, pressure increases with depth. The
relationship between pressure and depth in Earth’s crust
and mantle is shown in the graph below. Pressure is
Key Concept Check
measured in kilobars (kb).
2. Describe
Under what
Pressure v. Depth
conditions do metamorphic
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rocks form?
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Math Skills
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The line graph represents
90
pressure below Earth’s
100
surface. What is the pressure
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12
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at a depth of 50 km?
Pressure (kb)
a.
Read the title of the
graph to determine what
Metamorphic Rock Identification
data are represented.
Changes in temperature, pressure, or the addition of
b.
Read the labels on the
chemical fluids can result in the rearrangement of minerals
x-axis and y-axis to
or the formation of new minerals in a metamorphic rock.
determine the units.
Geologists study the texture and composition of minerals to
c.
Move horizontally from
identify metamorphic rocks.
50 km to the diagonal line.
Move vertically from the
Metamorphic rocks are classified into two groups based
diagonal line to the x-axis.
on texture. Pressure can cause minerals to align and form
The pressure is 14 kb.
layers in metamorphic rocks. The layers look similar to the
3. Use Graphs
At what
layers in clastic sedimentary rock. The mineral content helps
depth is the pressure 20 kb?
geologists tell the difference between metamorphic and
sedimentary rock.
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Rocks
Reading Essentials
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