Process

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) 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.

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