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A Closer Look: Metamorphic Rocks
What are metamorphic rocks?
Metamorphic rock is rock that has physically
and chemically changed, or "morphed," into new rock. The word "metamorphic" has
its origins in classical Greek and means "to change form." Rock
of any type (sedimentary, igneous, or metamorphic) that is
subjected to high pressure, high temperatures, and/or reactions
with chemical
solutions can be converted to metamorphic rock. This transformation
can involve changes in a rock’s texture (grain size and shape),
fabric (how the grains are oriented relative to one another),
chemical composition, and mineral content. The rock remains
solid as these changes
occur. Through the process of metamorphism, the original rock,
or protolith, changes into a new metamorphic rock.
Metamorphic rock (left) compared with
sedimentary rock (right).
How are
these high pressures and temperatures generated?
The answer
lies in the processes of plate tectonics. Plates that move against
each other produce huge forces that create
high pressures and temperatures that can deform rock by bending
or breaking it. Rock
can also be buried and metamorphosed when plates collide. Temperatures
and pressure within the Earth increase with depth, so that rock
deep in the crust will experience extreme heat and pressure.
Rock can
also be
subjected to high temperatures in regions of volcanism as well
as in places beneath the Earth where magma intrudes into the
rock above it.
What types of metamorphism exist?
Regional Metamorphism: Many metamorphic
rocks form by regional metamorphism, named for the large areas
of the crust that are
affected. Regional metamorphism usually results from mountain
building processes,
which are caused by the collision of tectonic plates. These collisions
compress and thicken the crust and cause considerable rock deformation.
High Pressure Metamorphism: Some metamorphic rock forms
at high pressures but at temperatures that are relatively low. This type
of metamorphism occurs at subduction zones. Here, high pressures
result when one plate
is submerged under the mantle. Temperatures remain relatively
low because the crust that forms the upper part of the subducting
plate is cool, having
been close to the Earth's surface. As the plate subducts, it
actually cools the mantle. The subducting plate reaches high
pressures faster than
it heats to high temperatures, and this pressure is enough to
cause metamorphism.
High Temperature Metamorphism: Some metamorphic
rock forms at
high temperatures but without high pressures. This occurs near
hot intrusions of magma from the mantle into the crust. Rock
that is in contact with
these intrusions undergoes contact, or thermal, metamorphism.
This heat causes minerals to react with each other, which produces
new minerals.
Hydrothermal Metamorphism: This process is associated
with contact metamorphism. When very large masses of magma — called
plutons — intrude
from the mantle into the crust, a great amount of heat is generated.
This huge body of hot magma creates a heat source that can cause fluids
in
the crust to circulate. Chemical reactions occur as a result
of this circulation. This type of metamorphism is common near mid-oceanic
ridges and around
large plutonic intrusions in the crust.
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