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3.3.
Metamorphic rocks
Rocks that are located deep in the crust are subjected to the pressure created by
the rocks above them. In addition, if these rocks are near magma, they are also
exposed to high temperatures. Pressure and high temperature cause changes in
the minerals that compose a rock and a new rock forms.
Metamorphic rocks are classified according to their texture, in two groups: foliated
and non-foliated.
❚
In
foliated metamorphic rocks
the minerals are arranged to form parallel
layers. This layered structure is easily observed in many rocks of this type.
Foliated metamorphic rocks are identified from low to high grades of
metamorphism: slate, schist, gneiss and migmatite.
❚
In
non-foliated metamorphic rocks
the minerals are not arranged in layers.
The most common are marble and quartzite.
Slate
results from the
metamorphism of clay.
It is composed of small
crystals visible under the
microscope. Slate breaks into
thin sheets.
Schist
comes from clay,
after a higher metamorphism
than in the case of slate.
Its crystals are visible without
a microscope.
Gneiss
originates from
detrital sedimentary rocks or
from granite. Its crystals are
bigger than those in schist.
They are arranged in wavy
bands.
Migmatite
is a rock halfway
between igneous and
metamorphic rocks. They have
experienced a very intense
metamorphism that partially
melts the original rock.
Marble
originates
from the metamorphism
of limestone.
Its minerals are
transformed due
to exposure to high
temperatures.
Quartzite
originates from
the metamorphism of
sandstone. Like marble,
it forms due to the exposure
of the original rock to high
temperatures.
❚
Metamorphism
is the process that changes a rock into a different type of
rock.
❚
Rocks that form in this way are called
metamorphic rocks
.
Analyse
24.
Compare the igneous andmetamorphic rock processes.What conditions
are needed for a rock to transform into metamorphic rock and not
igneous when subjected to high temperatures?
