DEFINITION OF PETROLOGY

The branch of geology that attempts to describe

and understand the origin, occurrence, structure, and evolution

of rocks. It is similar to but an extension of petrography

that describes the minerals and textures in rock bodies. The

petrologic classification of rocks recognizes three main categories

with different modes of origin and histories. Igneous

rocks crystallized from magma, and include plutonic and volcanic

varieties that cooled below and at the surface, respectively.

Metamorphic rocks are those that have been changed

in some way, such as the growth of new minerals or structures,

during heating and pressure from being subjected to

tectonic forces. Sedimentary rocks include clastic varieties

that represent the broken down, transported, deposited, and

cemented fragments of older rocks, as well as chemical and

biochemical varieties that represent chemicals that precipitated

from a solution.

See also IGNEOUS ROCKS; PETROGRAPHY; SEDIMENTARY

ROCKS.

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