Schist is a foliated regional metamorphic rock formed from basalts or slate. Schist forms at higher metamorphic grades than slate, although the protolith of both rocks may be the same parent rock stone. Regional metamorphism is caused by Earth’s internal geologic processes, including the bending and broken down of rocks during mountain uplifting, and the tremendous forces of heat and pressure exerted upon them during these geologic processes. Schists are given compound names based on their mineral content and texture, with mica schist being the most common. Other names include garnet-mica schist, hornblende-biotite schist, cordierite-mica schist, etc.
Metamorphism
The geological processes, which transform the structure of rocks in the interior of the Earth, are known as metamorphism. Plate tectonics, the formation of mountains, and earthquakes cause the bending of rocks and minerals that make up the continental and oceanic lithosphere. Earth’s internal forces, such as temperature, pressure, and the degree of deformation define the new physical and chemical characteristics of most rocks and minerals in the interior of the Earth. This process most usually occurs in igneous and sedimentary rocks. The mineral components of these types of rocks are subjected to dramatic pressures and temperatures, producing a new arrangement of atoms and molecules in their mineral components, which give them a new physical appearance.
How is schist metamorphosed?
There are various varieties of schist. Two of these variations, greenschists and blueschists, are formed under different temperature and pressure conditions. Regional metamorphosis usually occurs at subduction zones, where one tectonic plate sinks underneath another. A tectonic plate that subducts deep down into the mantle achieves both high temperature and pressure and produces greenschist; however, a subducted plate that does not sink deep enough is not affected by high pressures or temperatures and produces blueschist.
Schist structure is characterized by the parallel alignment of up to 50% of its constituent minerals, especially platy, mica and hornblende. The strong alignment of platy minerals causes schist to break into flakes. During the formation of schist, its protolith (parent rock) is heated significantly to cause the mobilization of its atoms. As it cools under anisotropic stress, the atoms acquire more stability. The metamorphic grade of schist depends on how the forces of nature have recrystallized its mineral components. Higher temperatures produce coarser crystallization, lower water content, distinctive layering and reduced schistosity. Anisotropic crystal structure gives schists their characteristic cleavage attributes.
How is schist used?
Schist can be seen as a decorative rock in the construction of houses, including walls, paving, pillars, columns, patios. Schist can be a good supportive material in the foundations of buildings. In the New York City area, many houses use schist (Yonkers Stone) to decorate their walls. Some people use them to make ornamental figures. Schist rock can be made into chalk and used as paint filler and roofing materials. In ancient times, a type of green-bluish schist slab was used in the pavement of the floors, courts and streets in various important buildings in Crete, Greece. Greenschist were used across Europe to make artifacts, including bracelets, axe heads, chisels and polishing tools, during the Neolithic period.
Where is schist found?
Most regional metamorphism occurs in the deeper levels of the Earth’s crust, along the boundaries of subducting tectonic plates. Subducting zones are often found where oceanic plates sink under continental plates. So schist can most commonly be found where new mountains are being created either in the ocean or on land. Therefore schist will most likely be found throughout the world, including the central highlands of Scotland, the mountainous western United States, the Andes in South America and many other mountainous coastal regions. According to encyclopedia.com, blueschist appears to have become more common globally, while greenschist fromation has remained constant since early history.
