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6.3 Metamorphic Rocks Metamorphic Rock • Meta means change • Morphe means form • A metamorphic rock forms when high temperature and pressure combine and change the texture, mineral composition or chemical composition of a rock without melting it AGENTS OF METAMORPHISM heat pressure chemically-active fluids Figure 4.18 Metamorphism occurs at high temperature within Earth although not high enough temperatures (at least 600oC for some rocks) to cause melting. Earth’s Geothermal Gradient HEAT is the most essential agent of metamorphism Increasing temperatures -break chemical bonds, -increase reaction rates -and lead to formation of: -New Minerals stable at higher temperatures -Larger mineral grains Figure 8.4A Increasing confining pressures produce denser minerals and new minerals with more “compact” crystal structures. Figure 8.4B DIRECTED PRESSURE Much of the pressure exerted on rocks does not push in on the rocks evenly from all directions. Figure 8.3 Directed Pressures develop at Convergent Plate Boundaries DIRECTED PRESSURE Development of Foliation due to DIRECTED PRESSURE Metamorphic Rock Foliation Foliation is different from original bedding planes Figure 8.8 Figure 8.12 Metamorphic Textures • Metamorphic Rocks are classified into two textural groups 1) Foliated - layers and bands (left) 2) Nonfoliated - blocky crystal shapes (right) Grades of Metamorphism • Low-grade metamorphism is associated with low temperatures and pressures • High-grade metamorphism is associated with high temperatures and pressures Types of Metamorphic Rock 1) Regional Metamorphism 2) Contact Metamorphism 3) Hydrothermal Metamorphism Regional Metamorphism • When a large belt regionally has metamorphed • Results include changes in mineral and rock types, plus folding and deformation of rock layers • Mountains can be an example of regional metamorphism Regional metamorphism occurs where rocks are squeezed between two converging tectonic plates during mountain building Contact Metamorphism • When molten material (like an igneous intrusion) comes into contact with solid rock contact metamorphism has occurred • Lava cools so contact metamorphism from lava occurs most near the lava • As the distance from an intrusion increases it is harder for contact metamorphism to occur since the temperature has dropped Figure 8.19 Contact Metamorphism along Mid-ocean Ridge around pluton Hydrothermal Metamorphism • When hot water reacts with rock and alters its chemical and mineral composition hydrothermal metamorphism has occurred • Ore deposits of gold, copper, zinc, tungsten and lead are formed in this manner Economic Importance of Metamorphic Rocks and Minerals • Metamorphism of ultrabasic igneous rocks produce the minerals talc and asbestos • Graphite can be formed by the metamorphism of coal The Rock Cycle • The three types of rocks: igneous, sedimentary, and metamorphic are grouped based on how they form • Igneous = crystallize from magma • Sedimentary = cemented or precipitated sediments • Metamorphic = changes in temperature and pressure The Rock Cycle • Once a rock forms does it remain the same? Maybe but quite possibly it changes into a different rock. • Heat and pressure can change an igneous rock into a metamorphic rock • A metamorphic rock can break into sediments that become cemented into a sedimentary rock