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Metamorphism - product of: • Increased Temperature • Increased Pressure • Migration of hot, chemically-active fluids Skinner, Porter, and Park, Dynamic Earth, Fig. 8.1, p. 207 Uniform pressure - typical of burial during extended deposition of sediment Lithostatic stress Chemical bonds accumulate strain In uniform pressure, bonds are shortened, and can be broken with increasing stress Two basic kinds of metamorphic rocks: Nonfoliated texture is uniform in all directions (3D) & Foliated texture has a preferred orientation * lineation * planar foliation Nonfoliated • Quartzite … from Sandstone • Marble … from limestone • Greenstone … from basalt Foliation Development of Plastic Deformation and Pressure Solution Differential pressure Development of Foliation Differential pressure Foliation Granite – crystallized under uniform stress; random orientation of crystals Gneiss – crystallized under differential stress; biotite crystals are roughly parallel resulting in foliation Foliation Skinner, Porter, and Park, Dynamic Earth, Fig. 8.1, p. 207 Shale Gneiss Clay tiny mica bigger mica breakdown of mica … Al to garnet Earth Fig. 7.17c Foliation Shale Low grade Remember: Metamorphism a) crystals get larger b) new minerals form both are form of recrystallization Thermal Metamorphism or Contact Metamorphism • Contact aureole • Hornfels • No foliation Index Minerals Skinner, Porter, and Park, 2004, Dynamic Earth, Fig. 8.8, p. 212 Metamorphic Grade as indicated by Index Minerals
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