Foliation Definition (Geology)

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What does foliation mean? How does foliation occur? Information about foliation in geology.

FOLIATION, is a structural property of some rocks that causes them to break or separate along nearly parallel surfaces, particularly along surfaces related to the layered arrangement of crystals in the rocks. Schists and gneisses are typical foliated rocks; the gneisses are banded because their mineral layers are of different composition.

Foliation may be a primary feature of rocks, as when fragments of platy minerals such as mica are deposited in sedimentary beds. More commonly, the term refers to the structure produced in slates and shales by the reorganization of their minerals into parallel layers under conditions o( regional metamorphism. Heat, pressure, and the effects of shearing and flow movements in the earth produce the foliation.

The structure is dependent in part on the cleavage of the minerals involved. (In crystallography, cleavage is the property related to the molecular structure within a crystal that enables it to split along parallel planes.) Cleavage planes are rather smooth however; foliation planes tend to be rough and irregular.

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