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typesmetamorph - Types of Metamorphism EENS 2120 Petrology...

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This document last updated on 16-Feb-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Types of Metamorphism Metamorphism is defined as follows: The mineralogical and structural adjustment of solid rocks to physical and chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under which the rocks in question originated. The word " Metamorphism " comes from the Greek: meta = change, morph = form, so metamorphism means to change form. In geology this refers to the changes in mineral assemblage and texture that result from subjecting a rock to conditions such pressures, temperatures, and chemical environments different from those under which the rock originally formed. z Note that Diagenesis is also a change in form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200 o C and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3 kilobars of pressure (1kb = 100 MPa). z Metamorphism, therefore occurs at temperatures and pressures higher than 200 o C and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they are buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction. z The upper limit of metamorphism occurs at the pressure and temperature where melting of the rock in question begins. Once melting begins, the process changes to an igneous process rather than a metamorphic process. Grade of Metamorphism As the temperature and/or pressure increases on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. Types of Metamorphism 2/16/2011 Page 1 of 7
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z Low-grade metamorphism takes place at temperatures between about 200 to 320 o C, and relatively low pressure. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals . With increasing grade of metamorphism, the hydrous minerals begin to react with other minerals and/or break down to less hydrous minerals. z High-grade metamorphism takes place at temperatures greater than 320 o C and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H 2 O, and non-hydrous minerals become more common. Types of Metamorphism Contact Metamorphism Contact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion . Since only a small area surrounding the
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