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Lecture Outlines Physical Geology, 15/e Plummer, Carlson & Hammersley © McGraw-Hill Education. Permission required for reproduction or display. Metamorphism and Metamorphic Rocks Physical Geology 15/e, Chapter 7 © McGraw-Hill Education. Permission required for reproduction or display. Metamorphism Metamorphism refers to solid-state changes to rocks in Earth’s interior • produced by increased heat, pressure, or the action of hot, reactive fluids • old minerals, unstable under new conditions, recrystallize into stable ones Rocks produced from pre-existing or parent rocks in this way are called metamorphic rocks Metamorphic rocks common in the old, stable cores of continents, known as cratons © McGraw-Hill Education. Permission required for reproduction or display. Metamorphism Texture and mineral content of metamorphic rocks depend on: • parent rock composition • temperature and pressure during metamorphism • effects of tectonic forces • effects of fluids, such as water Parent rock composition • usually no new material is added to rock during metamorphism • resulting metamorphic rock will have similar composition to parent rock © McGraw-Hill Education. Permission required for reproduction or display. Factors Controlling the Characteristics of Metamorphic Rocks Temperature • heat comes from Earth’s deep interior • all minerals stable over finite temperature range, if range exceeded, new minerals result • if temperature gets high enough, melting will occur © McGraw-Hill Education. Permission required for reproduction or display. Factors controlling metamorphic rock characteristics Pressure • confining pressure applied equally in all directions • pressure proportional to depth within the Earth • high-pressure minerals more compact/dense • Differential Stress – created by forces that are not equal in all directions • compressive stress causes flattening perpendicular to stress • shearing causes flattening by sliding parallel to stress • Foliation - planar rock texture of aligned minerals produced by differential stress © McGraw-Hill Education. Permission required for reproduction or display. Factors controlling metamorphic rock characteristics Composition of the Parent Rock •Composition may remain the same or change during metamorphism depending upon the parent rock’s composition and whether new chemicals are added during the process. Fluids •hot water (as vapor) is most important •rising temperature causes water to be released from unstable minerals •hot water very reactive; acts as rapid transport agent for mobile ions Time •metamorphism, particularly from high pressures, may take millions of years •longer times allow newly stable minerals to grow larger and increase foliation © McGraw-Hill Education. Permission required for reproduction or display. Classification of Metamorphic Rocks Nonfoliated Rocks • non-foliated rocks are named based on composition • Marble – coarse grained rock composed of interlocking calcite crystals • Quartzite – produced when grains of quartz sandstone are welded together • Hornfels – fine grained rock typically composed of microscopically visible micas formed from the clay particle in shale. Foliated Rocks • foliated rocks named based on the type of foliation (slaty, schistose, gneissic) • Slate, Phyllite, Schist and Gneiss © McGraw-Hill Education. Permission required for reproduction or display. Types of Metamorphism Contact Metamorphism • high temperature is dominant factor • produces non-foliated rocks • occurs adjacent to magma bodies intruding cooler country rock • occurs in narrow zone (~1-100 m wide) known as contact aureole • rocks may be fine- (e.g., hornfels) or coarse-grained (e.g., marble, quartzite) © McGraw-Hill Education. Permission required for reproduction or display. Types of Metamorphism Hydrothermal Metamorphism– rocks precipitated from or altered by hot water • common at divergent plate boundaries Hydrothermal processes: • metamorphism • metasomatism Hydrothermal Processes and Ore Deposits • water passes through rocks and precipitates new minerals on walls of cracks and in pore spaces • metallic ore deposits often form this way (veins) © McGraw-Hill Education. Permission required for reproduction or display. Metallic Ore Vein deposit Types of Metamorphism Regional metamorphism • high pressure is dominant factor • results in rocks with foliated textures • prevalent in intensely deformed mountain ranges • may occur over wide temperature range Shock metamorphism is produced by rapid application of extreme pressure • • meteor impacts produce this shocked rocks are found around and beneath impact craters © McGraw-Hill Education. Permission required for reproduction or display. Metamorphic Grade Minerals present in a metamorphic rock indicate its metamorphic grade. Prograde Metamorphism – as a rock is buried to greater depths it is subject to greater temperatures and pressures causing recrystallization into higher grade rocks. Slate Phyllite Lower Grade Schist Gneiss Higher Grade Migmatites (partial melting) exhibit both intrusive igneous and foliated metamorphic textures Pressure and Temperature Paths in Time Index minerals can be used to approximate temperature and pressure conditions © McGraw-Hill Education. Permission required for reproduction or display. Migmatite Plate Tectonics and Metamorphism Foliation and Plate Tectonics • Regional metamorphism associated with convergent plate boundaries • Differential stress • Gravitational collapse and spreading Pressure-Temperature Regimes • Temperature varies laterally at convergent boundaries • isotherms bow down in sinking oceanic plate and bow up where magma rises • wide variety of metamorphic facies © McGraw-Hill Education. Permission required for reproduction or display. Plate Tectonics and Metamorphism Hydrothermal Metamorphism and Plate Tectonics • Particularly important at mid-oceanic ridges as seas water moves downward into cracks in the sea floor • Hydrothermal vents such as “black smokers” occur as the water returns to the ocean • Dissolved metals and sulfur precipitate to create mounds around the vents © McGraw-Hill Education. Permission required for reproduction or display. End of Chapter 7 © McGraw-Hill Education. Permission required for reproduction or display.