Rocks

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Rocks
Anthony Isinta Offline
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Rocks

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 Please help in metamorphic rocks


 
08-16-2016 01:40 PM
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Nelly kessy Offline
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RE: Rocks

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Metamorphic rocks arise from the
transformation of existing rock types, in a
process called metamorphism , which means
"change in form". [1] The original rock
( protolith) is subjected to heat (temperatures
greater than 150 to 200 °C) and pressure
(1500 bars ), [2] causing profound physical and/
or chemical change. The protolith may be a
sedimentary rock , an igneous rock or another
older metamorphic rock.
Metamorphic rocks make up a large part of
the Earth's crust and are classified by texture
and by chemical and mineral assemblage
( metamorphic facies ). They may be formed
simply by being deep beneath the Earth's
surface, subjected to high temperatures and
the great pressure of the rock layers above it.
They can form from tectonic processes such
as continental collisions, which cause
horizontal pressure, friction and distortion.
They are also formed when rock is heated up
by the intrusion of hot molten rock called
magma from the Earth's interior. The study of
metamorphic rocks (now exposed at the
Earth's surface following erosion and uplift)
provides information about the temperatures
and pressures that occur at great depths
within the Earth's crust. Some examples of
metamorphic rocks are gneiss , slate , marble,
schist , and quartzite.
Metamorphic minerals
Metamorphic minerals are those that form
only at the high temperatures and pressures
associated with the process of
metamorphism. These minerals, known as
index minerals , include sillimanite , kyanite ,
staurolite , andalusite , and some garnet .
Other minerals, such as olivines , pyroxenes ,
amphiboles , micas , feldspars, and quartz , may
be found in metamorphic rocks, but are not
necessarily the result of the process of
metamorphism. These minerals formed during
the crystallization of igneous rocks. They are
stable at high temperatures and pressures and
may remain chemically unchanged during the
metamorphic process. However, all minerals
are stable only within certain limits, and the
presence of some minerals in metamorphic
rocks indicates the approximate temperatures
and pressures at which they formed.
The change in the particle size of the rock
during the process of metamorphism is called
recrystallization. For instance, the small
calcite crystals in the sedimentary rock
limestone and chalk change into larger
crystals in the metamorphic rock marble, or in
metamorphosed sandstone, recrystallization of
the original quartz sand grains results in very
compact quartzite, also known as
metaquartzite, in which the often larger quartz
crystals are interlocked. Both high
temperatures and pressures contribute to
recrystallization. High temperatures allow the
atoms and ions in solid crystals to migrate,
thus reorganizing the crystals, while high
pressures cause solution of the crystals within
the rock at their point of contact.

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10-03-2016 12:55 PM
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