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.


 

The presence of garnet (black), cummingtonite (colorful rhombs), hornblende (dull green) and chlorite (finely striped) minerals rich in iron, magnesium and calcium 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 650C.

 


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Amphibolite, Franklin County, Massachusetts
Crossed polarizers
 

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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