foliaatsiooni

Foliation is a fundamental geological term describing the development of planar fabric in metamorphic rocks. This fabric arises from the parallel alignment of platy or elongated mineral grains within the rock, or from the segregation of different minerals into distinct layers. The process is driven by directed pressure, known as stress, which forces the minerals to reorient themselves perpendicular to the direction of maximum stress.

There are several types of foliation, with slaty cleavage being a common example. In slaty cleavage, very fine-grained minerals like mica align to create a perfectly flat, sheet-like structure, allowing the rock to split easily along these planes. Phyllitic texture exhibits a slightly coarser alignment than slaty cleavage, with a satiny sheen due to the larger mica flakes. Schistosity refers to a more pronounced foliation where visible, larger mineral grains, such as micas and amphiboles, are aligned, giving the rock a layered appearance. Gneissic banding is the coarsest form of foliation, characterized by alternating layers of light-colored felsic minerals (like quartz and feldspar) and dark-colored mafic minerals (like biotite and hornblende).

Foliation is a key indicator of the metamorphic conditions, specifically the intensity and direction of stress, that a rock has experienced. Its presence and type help geologists reconstruct the tectonic history of a region and understand the processes of mountain building and rock deformation.