Metamorphism Physical Geology Metamorphism: Causes, Essay

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The effects of liquids are not a factor in regional metamorphism as fluids cannot circulate due to the effects of pressure upon the rocks, which seal in the circulation of moisture. Regional metamorphism occurs in areas of heavy tectonic activity, near the places where the earth's plates rub up against one another. "Regional metamorphism can be subdivided into different pressure-temperature conditions based on observed sequences of mineral assemblages. It may include an extreme condition, where partial melting occurs" (Jessey & Tarman 2010). Regional metamorphism is commonly found in mountain regions (hence the name regional metamorphism), consisting of foliated rocks developed under medium to high temperatures. "The accompanying pressures vary from low to high. Geothermal gradients, which are likewise moderate to high, produce Buchan and Barrovian Facies series. Because the pressures of Buchan and Barrovian Facies series are commonly higher than are those of Contact Facies Series, they may contain different sequences of minerals" in their observed composition (Regional, 2010, Pomona).

For example, the "Buchan Facies Series of regional metamorphism is characterized by the presence of andalusite, and sometimes cordierite, in intermediate grade mineral assemblages indicating that the conditions of metamorphism were at lower pressure and along a higher metamorphic field gradient than that recorded in Barrovian Facies Series metamorphic rocks" (Nelson 2003). For a Buchan Facies "the critical sequence of aluminum silicates is kaolinite -->pyrophyllite -->andalusite --> sillimanite" (Regional, 2010, Pomona). For a Barrovian Facies Series, "the resulting aluminum silicate mineral sequence is kaolinite -->pyrophyllite -->kyanite --> sillimanite…the presence of either andalusite or kyanite on metamorphosed shales and siltstones at the middle grades of metamorphism is one feature that distinguishes these facies series from one another" (Regional, 2010, Pomona).

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Other, less common types of metamorphism include dynamic metamorphism, which occurs only along faults zones due to pressure, and is not due to heat or fluids. "Textures produced by such adjustments range from breccias composed of angular, shattered rock fragments to very fine-grained, granulated or powdered rocks with obvious foliation and lineation termed mylonites" (Jessey & Tarman 2010).

Rocks produced by metamorphism are usually classified into two distinct types, based upon the processes that generate them. Foliated rocks "contain linear or planar features," and "form in response to active pressure during regional metamorphism" (Jessey 2010). Foliated rocks include fine-grained slate, phyllite, schist, and gneiss. Non-foliated rocks are generated from contact metamorphism, not due to active pressure. They include marble, quartzite, and spotted rock hornfels produced by "large crystals in a fine-grained matrix" (Jessey 2010).

Further criteria employed in the classification of metamorphic rocks include other aspects of the rock's apparent texture, the rock's mineralogical and chemical composition, and the degree to which the rock has changed from its natural state. For example, "if a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite. In addition to these conventions, certain non-foliated rocks with specific chemical compositions and/or mineral assemblages are given specific names" (Nelson 2004)......

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