Also found in: Dictionary, Thesaurus, Wikipedia.
the ability of rocks to split relatively easily parallel to a certain plane when struck. This mechanical anisotropy of rocks is caused by dynamic metamorphism in which the platy and rodlike grains of micas, hornblende, chlorites, and other minerals that make up a rock take on an identical orientation as a result of recrystallization or rotation. In irregularly shaped grains of minerals, orientation of the optical axes of the grains and other crystallographic characteristics may be observed. Petrotectonic analysis is used to study the rules of orientation and the rules for decoding the motions that cause the orientation.
Foliation may be regional, manifesting itself over large areas, or local, related to shifts along tectonic displacements. It frequently occurs during rock folding, with the foliation usually subparallel to the axial planes of the folds. Foliation also forms in cleavage: the crystals are flattened in a plane perpendicular to the axis of compression and the rock acquires a plane-parallel oriented internal structure. Some geologists use the term “cleavage” as a synonym for “foliation.”
REFERENCESPek, A. V. Treshchinnaia tektonika i strukturnyi analiz. Moscow-Leningrad, 1939.
Fairbairn, H. W. Strukturnaia petrologiia deformirovannykh gornykh porod. Moscow, 1949. (Translated from English.)
A. V. PEK