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© 2014 Pearson Education, Inc. Metamorphism and Metamorphic Rocks Chapter 8 Lecture Tarbuck and Lutgens Earth: An Introduction to Physical Geology Eleventh Edition
© 2014 Pearson Education, Inc. Metamorphism
© 2014 Pearson Education, Inc. Rock Environments
© 2014 Pearson Education, Inc. Metamorphic Environments
© 2014 Pearson Education, Inc. What Is Metamorphism? Metamorphism –Changes rock via temperatures and/or pressures unlike those in which it initially formed –All metamorphic rocks have a parent rock (the rock from which it formed) –Parent rocks can be igneous, sedimentary, or other metamorphic rocks.
© 2014 Pearson Education, Inc. Change occurring during metamorphism Progresses from low grade (low temperatures and pressures) to high grade (high temperatures and pressures) During metamorphism, the rock must remain essentially solid Metamorphic Grade
© 2014 Pearson Education, Inc. Metamorphic Grade
© 2014 Pearson Education, Inc. What Drives Metamorphism? Heat –Most important agent –Two sources of heat: Geothermal gradient: an increase in temperature with depth (about 25 o C per kilometer) Contact metamorphism: rising mantle plumes
© 2014 Pearson Education, Inc. Confining Pressure –Forces are applied equally in all directions –Causes the spaces between mineral grains to close What Drives Metamorphism?
© 2014 Pearson Education, Inc. Differential Stress –Forces are unequal in different directions –Compressional stress Rocks are squeezed as if in a vice Shortened in one direction and elongated in the other direction What Drives Metamorphism?
© 2014 Pearson Education, Inc. Confining Pressure and Differential Stress
© 2014 Pearson Education, Inc. Chemically Active Fluids –Enhances migration of ions –Aids in recrystallization of existing minerals What Drives Metamorphism?
© 2014 Pearson Education, Inc. Rock or slaty cleavage Split into thin slabs Low-grade metamorphism Metamorphic Textures - Foliated
© 2014 Pearson Education, Inc. Metamorphic Textures -Foliated Schistosity Platy minerals (mica) are visible Exhibit a planar or layered structure Rocks having this texture are referred to as schist
© 2014 Pearson Education, Inc. Gneissic texture High-grade metamorphism, segregation of minerals into light and dark bands Metamorphic rocks with this texture are called gneiss Gneiss does not split as easily as slates and schists Metamorphic Textures - Foliated
© 2014 Pearson Education, Inc. Nonfoliated composed of minerals that lack layering/foliation Metamorphic Textures - Nonfoliated
© 2014 Pearson Education, Inc. Porphyroblastic textures Large grains (porphyroblasts) surrounded by a fine-grained matrix of other minerals Metamorphic Textures - Porphyroblastic
© 2014 Pearson Education, Inc. Common Metamorphic Rocks Foliated Rocks –Slate Very fine-grained Excellent rock cleavage Most often generated from low-grade metamorphism of shale, mudstone, or siltstone
© 2014 Pearson Education, Inc. Foliated Rocks –Phyllite Degree of metamorphism between slate and schist Platy minerals are larger than slate but not large enough to see with the unaided eye Glossy sheen and wavy surfaces Exhibits rock cleavage Common Metamorphic Rocks
© 2014 Pearson Education, Inc. Foliated Rocks –Schist Medium- to coarse-grained Parent rock is shale that has undergone medium- to high-grade metamorphism The term schist describes the texture Platy minerals (mainly micas) predominate Can also contain porphyroblasts Common Metamorphic Rocks
© 2014 Pearson Education, Inc. Foliated Rocks –Gneiss Medium- to coarse-grained metamorphic rock with a banded appearance The result of high-grade metamorphism Composed of light-colored, feldspar-rich layers with bands of dark ferromagnesian minerals Common Metamorphic Rocks
© 2014 Pearson Education, Inc. Increasing Metamorphic Grade
© 2014 Pearson Education, Inc. Common Metamorphic Rocks Nonfoliated Rocks –Marble Crystalline rock from limestone or dolostone parent rock Main mineral is calcite –Calcite is relatively soft (3 on the Mohs scale) –Used as a decorative and monument stone Impurities in the parent rocks provide a variety of colors
© 2014 Pearson Education, Inc. Non-foliated Marble
© 2014 Pearson Education, Inc. Marble Quarries in Italy
© 2014 Pearson Education, Inc. Taj Mahal
© 2014 Pearson Education, Inc. Lincoln Memorial
© 2014 Pearson Education, Inc. Nonfoliated Rocks –Quartzite Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together Pure quartzite is white –Iron oxide may produce reddish or pink stains –Dark minerals may produce green or gray stains Common Metamorphic Rocks
© 2014 Pearson Education, Inc. Quartzite
© 2014 Pearson Education, Inc. Common Metamorphic Rocks
© 2014 Pearson Education, Inc. Metamorphic Environments Contact or Thermal Metamorphism –Results from a rise in temperature when magma invades a host rock –Occurs in the upper crust (low pressure, high temperature) –The zone of alteration (aureole) forms in the rock surrounding the magma
© 2014 Pearson Education, Inc. Contact Metamorphism
© 2014 Pearson Education, Inc. Contact Metamorphism
© 2014 Pearson Education, Inc. Hydrothermal Metamorphism –Chemical alteration caused by hot, ion-rich fluids circulating through pore spaces and rock fractures –Typically occurs along the axes of mid-ocean ridges Metamorphic Environments
© 2014 Pearson Education, Inc. Hydrothermal Metamorphism Along a Mid-Ocean Ridge
© 2014 Pearson Education, Inc. Burial Metamorphism –Associated with very thick sedimentary strata in a subsiding basin Gulf of Mexico is an example Subduction Zone Metamorphism –Sediments and oceanic crust are subducted fast enough that pressure increases before temperature Metamorphic Environments
© 2014 Pearson Education, Inc. Regional Metamorphism –Creates the most metamorphic rock –Associated with mountain building and the collision of continental blocks Metamorphic Environments
© 2014 Pearson Education, Inc. Regional Metamorphism
© 2014 Pearson Education, Inc. Impact Metamorphism –Occurs when meteorites strike Earth’s surface Product of these impacts are fused fragmented rock plus glass-rich ejecta that resemble volcanic bombs – Called impactiles Metamorphic Environments
© 2014 Pearson Education, Inc. Impact Metamorphism
© 2014 Pearson Education, Inc. Textural Variations Caused by Regional Metamorphism
© 2014 Pearson Education, Inc. Rocks that have been partially melted Represent the highest grades of metamorphism Transitional to igneous rocks Migmatites
© 2014 Pearson Education, Inc. Metamorphic Zones and Index Minerals
© 2014 Pearson Education, Inc. End of Chapter 8
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