51Chapter 4Metamorphic RocksMetamorphic rocksare rocks that have undergone a change in texture and/or mineralogy due to high temperature or pressure, or through the action of chemical alteration induced by very hot and chemically aggressive, pore water. The process of alteration is termedmetamorphism, and it generally occurs along active plate tectonic boundaries or deep within the Earth's crust (some 10 to 30 km below the surface). In these areas, temperatures can exceed 800 °C and pressures can build up to 6 kbars (6000 times atmospheric pressure). Although extreme, these conditions are not quite enough to actually melt rock. Nevertheless, the effects of metamorphism can be dramatic and frequently result in the total overprintingor destruction of the rock's original mineralogy and texture. 4.1 Types of Metamorphism Geologists recognize several kinds of metamorphism, however, in the GY 111 laboratory component, we can restrict ourselves to the three major types: 1) regional, 2) contactand 3) cataclastic. Figure shows a detailed sketch of deformed and folded rock layers that are typical in regionally metamorphosed areas. From: LeConte, J., 1905. Elements of Geology. D. Appleton and Co., New York, NY, 667p.
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