Metamorphic Rocks
Lesson 6
etamorphic rocks are rocks of change. The term metamorphic comes from the Greek
word meta meaning change and morpho meaning shape. Igneous, sedimentary, and
metamorphic rocks change their crystal structure and shape as the heat and pressure
increases rocks. Heat and pressure usually work together in the metamorphic process causing
minerals to destabilize and then recrystallize. The minerals that were stable in rocks at the
surface of the Earth become unstable deeper in the crust. The minerals within the rocks
recrystallize into new minerals that are stable at the new heat and pressure without melting. The
new metamorphic rocks have changed in both mineral composition and texture. The amount of
heat and pressure applied to the rocks will determine the type of minerals that form within the
rocks. Fluids speed up the process and add new elements to the rocks that combine with the
existing minerals as they recrystallize.
M
Movement along the San Andreas Fault near Palmdale, California created these
folded rocks.
Contact metamorphism occurs when hot magma rising up through the crust of the Earth heats the
country rocks (local rocks in the area) through which it moves. Where there is a crack or joint in
the rock the hot magma will flow into it. Dikes and sills form in the rocks as these small
intrusions of magma fill the spaces in the rock. Sometimes the cracks or joints are only a meter
or two wide. In these areas, only a thin circle of rocks around the intrusion will recrystallize.
Rocks a few centimeters away from the dike will not recrystallize even though heated by the
molten magma.
Large intrusions of magma moving upward can be a kilometer or more in size. Scientists have
found these large intrusions of magma create circles of recrystallized rock. Each circle is like the
rinds of an orange. Large bodies of magma will have three circles of rock with different types of
minerals in each circle. The circle closest to the large magma chamber will have rocks that
contain minerals that crystallize at high temperatures like garnets. Epidote and other minerals in
the middle circle crystallize at cooler temperatures. The outer circle of metamorphosed rock
contains mica minerals that crystallize at even lower temperatures.
Large bodies of magma affect a much wider area of rocks. Rocks sometimes metamorphose one
hundred meters away from the intruding magma. Hot magma containing large amounts of fluids
affects an even larger area of rocks.
Each time a large intrusive body of magma invades country rock the type of metamorphic rock
that forms depends on the following:
♦ Minerals contained in the hot magma.
♦ Amount of fluids contained in the magma and country rocks.
♦ Type of country rock the magma intrudes.
Regional metamorphism occurs over large
areas forming the large volumes of
metamorphic rock. When two continental
plates collide, the rocks between the
converging plates are compressed creating
folded mountains like the Himalayas. The
deepest rocks in folded mountains are
subjected to the greatest amount of heat
and pressure.
Gneiss
The rocks begin the series with distinct
foliation and rock cleavage. As the pressure
and heat increases the foliation and rock cleavage decreases and the grain size increases. Slate,
phyllite, schist and gneiss form a metamorphic series in folded mountains. Slate undergoes the
least amount of change and gneiss the greatest.
The Himalayan Mountains are growing as the continental plates of Asia and India collide.
Beneath these mountains, igneous, sedimentary and metamorphic rocks are changing as the
minerals in the rocks adjust to the increasing heat and pressure caused by the two continents
colliding.
Oceanic plates subducting beneath continental plates create areas where regional metamorphism
is occurring. The Cascadia Subduction Zone located off the coast of Northern California, Oregon
and Washington is where the Juan de Fuca Plate is subducting beneath the North American Plate.
Metamorphic rocks produced in this subduction zone include serpentinites, greenschists, and
amphibolites.
Rocks just beginning to metamorphose are low-grade metamorphic rocks. Deeper rocks in folded
mountains become intermediate-grade metamorphic rocks and those deep in the roots of the
mountains turn into high-grade metamorphic rocks. Here are some examples of metamorphic
rocks that recrystallize and the parent rocks from which they are derived.
Foliated metamorphic rocks
♦ Shale changes into slate.
♦ Slate changes into phyllite.
♦ Phyllite changes into schist.
♦ Schist changes into gneiss.
♦ Gabbro or peridotite changes into amphibolite.
Non-foliated or massive metamorphic rocks
♦ Sandstone changes into quartzite.
♦ Limestone changes into marble.
♦ Peridotite changes into serpentinite
♦ Basalt changes into greenschist or greenstone.
Carbon rocks
♦ Bituminous coal changes into anthracite coal.
♦ Anthracite coal changes into graphite.
♦ Graphite, a pure carbon mineral, changes into diamonds.
All metamorphic rocks recrystallize when they are in a solid state. Migmatites are metamorphic
rocks that almost melted and are combination of metamorphic gneisses and igneous rocks. If the
rocks are heated enough to cause them to completely melt they become igneous rocks.
Common Metamorphic Rocks
Metamorphic Rock
Slate
Schist
Gneiss
Quartzite
Marble
Anthracite coal
Graphite
Serpentinite
Low-grade
X
X
Medium-grade
High-grade
X
X
X
X
X
X
X
X
X
X
X
Lesson summary
♦
♦
♦
♦
♦
Heat, pressure, and fluids transform igneous, sedimentary and metamorphic rocks into
new metamorphic rocks without melting.
Contact metamorphism occurs when hot magma rising up through the crust of the Earth
heats the country rocks it intrudes.
Regional metamorphism occurs when continental plates collide.
Slate, phyllite, schist and gneiss form a metamorphic series.
Subduction zones produce serpentinites, greenschists, and amphibolites in areas of
regional metamorphism.
Name __________________________
Date ___________________________
Quiz 6
Metamorphic Rocks
Fill in the blanks using words from the Word Bank
1. Anthracite is a metamorphic type of
.
2. Heat, pressure, and
into metamorphic rocks.
turn igneous and sedimentary rocks
3. Fluids in rocks
up metamorphism.
4.
rocks recrystallize to form metamorphic rocks.
5. Minerals are just
recrystallize in low-grade metamorphic rocks.
6. Minerals
and pressure is exerted on rocks.
to
in metamorphic rocks when heat
7. Metamorphic rocks are rocks of
.
8. Rocks that
become igneous rocks.
due to heat and pressure
9. Heat and pressure usually work
form new rocks.
to
10. Graphite and
pure carbon.
are both made of
Word Bank
speed
liquids
recrystallize
diamonds
change
beginning
together
sedimentary
coal
melt
Modeling Gneiss
Activity 6
Introduction
Gneiss is a high-grade metamorphic rock that forms when continental plates collide.
Metamorphic rocks recrystallize as heat and pressure increases when crustal plates collide. Great
mountain chains forms creating a whole series of foliated rocks depending on the depth the rocks
are buried. This activity demonstrates the geologic forces that form gneiss in the roots of
mountains.
Materials
♦
Modeling clay the type that
never dries out (red, blue orange
and green)
♦ Rolling pin
♦ Plastic wrap
♦ Knife
Directions
Step 1 Setting up and conducting your activity
1. Take the red stick of clay out of its wrapper and begin compressing the stick with your
fingers.
2. Flatten the red stick of clay so it forms a rectangle. As you work the clay it will get softer
and easier to handle.
3. Place the red clay layer on a piece of plastic wrap on a sturdy table.
4. Repeat steps 1, 2, and 3 with the green, orange and blue clay.
5. The four layers of clay should look like the picture on the next page when you have
finished.
6. Place a second piece of plastic wrap on top of the top layer of clay.
7. Press down very hard with a rolling pin in the center of the clay.
8. Roll the rolling pin away from the center of the clay just like you would if this were a pie
crust.
9. Repeat this step so the clay layers are pressed together without any holes between the
layers.
10. Pull the plastic wrap off the top and bottom of the clay.
11. Grasp the clay at both ends and begin to compress the clay toward the center.
12. Notice how the layers begin to move up and down as the pressure increases.
13. Continue forcing the layers of clay together and until they are not any spaces between the
compressed layers.
14. Cut the clay with a knife so the inside of the layers of your “clay rock” are exposed.
15. Notice how the layers form a wavy line.
16. Gneiss rocks, which are subjected to high heat and pressure sometimes have wavy layers
of minerals.
Step
1.
2.
3.
2 Recording your activity
Place the title and date of the activity at the top of a sheet of notebook paper.
List the materials used in the activity.
Draw two pictures of your activity.
a. First, draw a picture of the flattened layers of clay.
b. Next, draw a picture of the inside of the “clay rock” after you have cut across the
layers with a knife.
4. Write a short summary of how heat and pressure deforms metamorphic rocks that
recrystallize into gneiss.
Extensions
♦
♦
Increase the number of clay layers from three to seven.
Use several sticks of each color of clay. Make your rectangle about 18 inches long. See
what happens when these layers are compressed.