Download metamorphism

Metamorphic Rocks
Chapter 2 Section 4
p. 44 - 49
Vocabulary:
1. foliated
2. nonfoliated
Origins of Metamorphic Rock p. 44
Rocks can also go through a process call metamorphism.
This word comes from the Greek words meta, which means
“changed” and morphos, which means “shape.” Metamorphic
rocks are rocks in which the structure, texture, or composition
of the rock have changed.
If the temperature or pressure of the new environment is
different from the one in which the rock formed, the rock will
undergo metamorphism.
It is the depth and pressure at which metamorphic rocks form
that allows the rock to heat to very high temperatures and still
keep its solid form. Large movements within the crust of the
Earth add additional pressure during metamorphism.
Contact Metamorphism p. 45
One way rock can undergo metamorphism is by being heated
by nearby magma. When magma moves through the crust, the
magma heats the surrounding rock and changes it. The
greatest change takes place where magma comes into direct
contact with the surrounding rock. The effect of heat on rock
gradually decreases as the rock’s distance from the magma
increases and as temperature decreases. Contact
metamorphism occurs near igneous intrusions.
Regional Metamorphism p. 45
When pressure builds up in rock that is buried deep below
other rock formations or when large pieces of the Earth’s crust
collide with each other, regional metamorphism occurs. The
increased pressure and temperature causes rock to become
deformed and chemically changes. Regional metamorphism
occurs over thousands of cubic kilometers deep within the
Earth’s crust. Rocks that have undergone regional
metamorphism are found beneath most continental rock
formations.
Composition of Metamorphic Rock p. 46
Metamorphism occurs when temperature and pressure inside
the Earth’s crust change. The original minerals change into
minerals that are more stable in the new conditions.
Some metamorphic minerals form only at certain temperatures
and pressures. These minerals, known as index minerals, are
used to estimate the temperature, depth, and pressure at
which a rock undergoes metamorphism.
Textures of Metamorphic Rock p. 47
All metamorphic rock has one of two textures – foliated and
nonfoliated.
Foliated Metamorphic Rock
The texture of metamorphic rock in which the
mineral grains are arranged in planes or bands is
usually called foliated. It usually contains aligned
grains of flat minerals. What you see in this picture
is an example of a sedimentary rock called shale.
When it is exposed to slight heat and pressure,
the clay minerals change into mica minerals. The
shale becomes slate. The process continues
until it becomes the
metamorphic rock known
as gneiss.
Nonfoliated Metamorphic Rock p. 46
The texture of metamorphic rock in which the mineral grains
are not arranged in planes or bands is called nofoliated.
Nonfoliated rocks are commonly made of one or only a few
minerals. During metamorphism, the crystals of these
minerals may change in size or the mineral may change in
composition in a process called recrystallization.
The marble and quartzite
shown here are examples
of sedimentary rocks that have
recrystallized during
metamorphism.
Metamorphic Rock Structures p. 49
Metamorphic rocks can tell
you something about their
history by looking at the rock’s
changes through a process
called deformation. Deformation
is a change in the shape of a
rock caused by a force placed
on it. Folds, or bends in
metamorphic rock are structures
that indicate that a rock has
been deformed.
Quiz Time!
1. What does a metamorphic rock need to form?
heat and pressure
2. Explain the difference between contact metamorphism
and regional metamorphism.
Contact metamorphism occurs near igneous intrusions
where magma comes into direct contact with surrounding
rock. Regional metamorphism occurs when large pieces
of Earth’s crust collide causing rock to become deformed
and chemically changed.
3. Why is marble considered a nonfoliated metamorphic rock?
Its mineral grains are not arranged in planes or bands.
4. What does the composition of a metamorphic rock tell you
about the rock’s origin and formation?
Different metamorphic minerals indicate the temperature and
pressure conditions that existed when the rock formed.
Science Humor…
What did the marble say to the slate?
Don’t take me for granite!