Schist

Schist

Schist is a metamorphic rock consisting of mineral grains that are more or less aligned in layers. Because of this structure, schist tends to cleave into flakes or slabs.

The parent rock of a schist may be igneous (e.g., basalt, granite , syenite) or sedimentary (e.g., sandstone , mudstone, impure limestone ). The metamorphic grade of a schist depends on how thoroughly melted and recrystallized its parent rock has been; higher temperatures produce lower water content, coarser crystallization, more distinct layering, and reduced schistosity. At the high end of this scale, the schists blur into the gneisses.

The directional mineral structure of schists and gneisses arises during crystallization under anisotropic stress. Anisotropic stress is stress that does not point equally in all directions, such as would be produced by placing a block of any material on a table and leaning on it at an angle. During the formation of a schist or gneiss , the parent rock is heated sufficiently to mobilize its atoms. As it cools under anisotropic stress, its atoms assume the most stable, low-energy arrangements available to them: anisotropic crystals (plates or elongated shapes pointing in a common direction). Anisotropic crystal structure gives the schists their characteristic cleavage properties.

There are many varieties of schist. Two categories of particular importance are the greenschists and the blueschists. These have similar parent rocks but are formed under different pressure (P) and temperature (T) conditions. What minerals will crystallize during metamorphosis depend on both P and T. Greenschists form under high P and high T such as are found far below Earth's crust ; blueschists form under high P and relatively low T. Both greenschists and blueschists are found in regionally metamorphosed landscapes—that is, land masses that have been submerged entirely in Earth's interior and metamorphosed in bulk. Regional metamorphosis often occurs at subduction zones, those places where one tectonic plate is being driven edgewise into the mantle beneath another. A large, cool chunk of Earth's crust takes a long time to reach ambient T when plunged into the mantle, but is raised to high P at once; a subducted mass that stays down long enough to achieve both high T and high P produces greenschist, while one that returns to the surface relatively quickly produces blueschist. One of the great unresolved puzzles of modern geology is that blueschist formation seems to have become globally more common in the last 300 million years, while greenschist formation has remained constant throughout Earth's history.

See also Metamorphism; Partial melting; Plate tectonics