What are the causes of metamorphism? Metamorphism is alteration in the mineralogy, texture, or chemical composition of solid rock. It is caused by increases in pressure and temperature and by reactions with chemical components introduced by hydrothermal solutions. As rocks are pushed deep within the crust by plate tectonic processes and exposed to increasing temperatures and pressures, the chemical components of the parent rock rearrange themselves into a new set of minerals that are stable under the new conditions. Metamorphic rocks that form at relatively low temperatures and pressures are referred to as low-grade metamorphic rocks; those that form at high temperatures and pressures are high-grade metamorphic rocks. Chemical components may be added to or removed from a rock during metamorphism, usually by hydrothermal solutions.
What are the various types of metamorphism? The three most common types of metamorphism are regional metamorphism, during which rocks over large areas are metamorphosed by high pressures and temperatures generated during mountain building; contact metamorphism, during which country rock close to an igneous intrusion is transformed by the heat of the intruding magma; and seafloor metamorphism, during which hot fluids percolate through and metamorphose oceanic crust. Less common types are burial metamorphism, during which deeply buried sedimentary rocks are altered by pressures and temperatures higher than those that result in diagenesis; high-pressure and ultra-high-pressure metamorphism, which occur at great depths, as when sediments are subducted; and shock metamorphism, which results from meteorite impacts.
What are the chief kinds of metamorphic rocks? Metamorphic rocks fall into two major textural classes: foliated rocks (displaying foliation, a pattern of parallel cleavage planes resulting from a preferred orientation of crystals) and granoblastic, or nonfoliated, rocks. The kinds of rocks produced depend on the composition of the parent rock and the grade of metamorphism. Regional metamorphism of shale leads to zones of foliated rock of progressively higher grade, from slate to phyllite, schist, gneiss, and finally migmatite. Among granoblastic rocks, marble is derived from the metamorphism of limestone, quartzite from quartz-rich sandstone, and greenstone from basalt. Hornfels is the product of contact metamorphism of fine-grained sedimentary rocks and other types of rock containing an abundance of silicate minerals. Regional metamorphism of mafic volcanic rocks progresses from zeolite facies to greenschist facies and then to amphibolite and granulite facies.
What do metamorphic rocks reveal about the conditions under which they were formed? Zones of metamorphism can be mapped with isograds defined by the first appearance of an index mineral. The presence of an index mineral can indicate the temperature and pressure under which the rocks in the zone were formed. According to the concept of metamorphic facies, rocks of the same metamorphic grade may differ because of variations in the chemical composition of the parent rock, whereas rocks metamorphosed from the same parent rock may vary because they were subjected to different grades of metamorphism.
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How are metamorphic rocks related to plate tectonic processes? During subduction and continent-continent collision at convergent plate boundaries, rocks and sediments are pushed to great depths in Earth’s crust, where they are subjected to increasing pressures and temperatures that result in metamorphism. The shapes of metamorphic P-T paths provide insight into the plate tectonic settings where these rocks were metamorphosed. In the case of ocean-continent convergence, P-T paths indicate rapid subduction of rocks and sediments to environments with high pressures and relatively low temperatures. In continent-continent collision zones, rocks are pushed down to depths where pressures and temperatures are both high. In both settings, the P-T paths show that after the rocks experience the maximum pressures and temperatures, they are returned to shallow depths. This process of exhumation may be driven by weathering and erosion at Earth’s surface as well as by plate tectonic processes.