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Unformatted text preview: 51 Chapter 4 Metamorphic Rocks Metamorphic rocks are 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 termed metamorphism , 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 overprinting or 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) contact and 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. Chapter 4: Metamorphic Rocks 52 4.1-1. Regional Metamorphism The most widespread and easily recognizable metamorphic rocks are those that form through regional metamorphism. This is a large scale process involving both heat and directed pressure, and generally occurs as part of the mountain building process. It is important to recognize that regionally metamorphosed rocks are important elements of all major mountain chains, including the Rockies and the Appalachians. Rocks generally undergo changes in their texture when they experience regional metamorphism. Most of the time, this is a completely destructive process and little remains of the rock's original texture. Marble , which was originally limestone, is composed of crystalline calcite. The exact variety of limestone from which the marble formed (e.g., oolitic, micritic, fossiliferous), is usually impossible to tell because nothing was preserved of the original texture following metamorphism. In rare cases, regional metamorphism may not be so pervasive, and some of the original texture of a rock may be preserved. Metamorphosed sedimentary rocks containing bedding can retain vestiges of sedimentary structures. Look for evidence of preserved bedding or cross-stratification in some of the samples in your specimen tray. 4.1-2. Contact Metamorphism This type of metamorphism, as the name implies, involves localized alterations induced by contact with an igneous intrusion. The major agent of contact metamorphism is heat....
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.
- Fall '11