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Process Exploring Rocks: Metamorphism
The word metamorphose means to change form. Metamorphism changes rocks
in two major ways: recrystallization makes new textures,
and chemical reactions make new minerals. Metamorphic transformations are a rock's
response to variations in pressure and temperature encountered at different
depths in Earth's crust. For example, temperature rises in a rock body intruded
by magma; pressure falls when erosion removes some of its overburden.
Despite all the changes, rocks remain solid during metamorphism.
Certain minerals grow together in metamorphic rocks. From these offspring,
geologists can - with a little luck - identify the parent rock and the physical
conditions the rock encountered.
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The presence of garnet (black), cummingtonite (colorful rhombs), hornblende
(dull green) and chlorite (finely striped) Ð minerals rich in iron, magnesium and
calcium Ð suggest that this amphibolite was formerly a volcanic rock . This
mineral combination indicates that the volcanic rock was buried to depths of 15
to 40 kilometers and cooked at temperatures between 500 to 650¡C.
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Amphibolite, Franklin County, Massachusetts
Crossed polarizers
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Kyanite (orange-pink rectangle) plus staurolite (yellow and orange blocks) plus
garnet (black blob) is a common mineral assemblage in metamorphosed
aluminum-rich sedimentary rocks. This schist experienced the same
metamorphic conditions as the amphibolite, but contains another group of
minerals because the rocks have different starting chemistries
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Pelitic Schist, Westchester County, New York
Crossed polarizers
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