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10/19/2012 1 Chapter 8 10/19/2012 Introduction Metamorphism - The transformation of rocks, usually beneath Earth's surface, as the result of heat, pressure, Chapter 8 and/or fluid activity, produces metamorphic rocks During metamorphism, rocks are subjected to sufficient heat, pressure and/or fluid activity to change their mineral composition or texture, or both. All this occurs below the melting point in the solid state. Metamorphism and Metamorphic Rocks Introduction Introduction Distribution of Metamorphic Rocks 1. Shields – oldest part of the continental crust 2. Cores of large mountain ranges Metamorphism is also responsible for producing a number of economically valuable materials, like slate, used as flooring and roofing material for hundreds of years. Metamorphism is an important process that is closely related to plate tectonics, the growth of continents, and even climate change. Fig. 8.10, 190 The Agents of Metamorphism The Agents of Metamorphism The principal agents of metamorphism are heat, pressure, fluid activity and time. Heat Intrusive magmas, lava flows or deep burial provide Heat is an important agent of metamorphism heat which causes metamorphism. Heat Increases the rate of reactions Pressure is produced by overlying rocks (lithostatic) or is differential pressure produced by various stresses. Sources of heat include: Extrusive lava Fluid activity increases the rate of metamorphism Intrusive magma and brings new elements. Deep burial Time is also important, as the more time involved, the greater the degree of metamorphism produced. Temperature increase with depth. The geothermal gradient averages about 25 degrees C/ km (greater under hot spots and rifts). 1 10/19/2012 The Agents of Metamorphism The Agents of Metamorphism Pressure Pressure What are lithostatic and differential pressures, What are lithostatic and differential pressures, and why are they important? and why are they important? Lithostatic pressure is a uniform Differential pressure is a field of pressure experienced by nonuniform field of pressure; the most rocks beneath Earth’s pressure acting on a rock in some surface. Like the hydrostatic directions is stronger than it is in pressure experienced by divers others. Many metamorphic rocks underwater, the pressure acting form under conditions of differential on a rock embedded in the crust pressure, which influences the “feels” the same from all development of metamorphic directions. structures and textures in significant ways. Fig. 8.2, p. 184 http://geophysics.ou.edu/geol1114/notes/met_rx/diff_stress.jpg The Agents of Metamorphism The Agents of Metamorphism Fluid Activity Time Fluids within sedimentary rocks or issuing from Time makes a large difference in the final metamorphic magmas can accelerate chemical changes rock product. More time under lower pressures and which occur during metamorphism and can temperatures can produce a large amount of cause new minerals to form. How? By metamorphism. Less time, even under high bringing new elements to the host material and temperatures and pressures, may not produce as by allowing ion exchange to occur through the much change. Time is always important in altering fluid medium. or changing rocks. The Agents of Metamorphism The Seven Types of Metamorphism The type of metamorphism that results largely depends on which of the three main agents was dominant. Regional metamorphism Contact Dynamic Hydrothermal Burial High Pressure and Ultra-High Pressure Shock Metamorphism Fig. 8.4, p. 186 2 10/19/2012 The Seven Types of Metamorphism The Seven Types of Metamorphism Regional metamorphism Regional metamorphism is the most Contact Metamorphism common type of metamorphism. Contact metamorphic rocks form under conditions of Regional metamorphism occurs when high high temperature and lower pressure. They are temperatures and pressures are imposed over a arrayed in aureoles, or metamorphosed zones, around broad area of the crust. plutons and other intrusive igneous bodies. They also Old igneous, sedimentary and metamorphic occur at the base of lava flows. textures are destroyed. Volcanic arcs (High T, Moderate to Low P), Subduction Zones (Moderate T, High P), and Continent-Continent Collision zones (Very High P, High T). Fig. 8.5, p. 186 Fig. 8.6, p. 187 The Seven Types of Metamorphism The Seven Types of Metamorphism Factors Affecting Contact Metamorphism Dynamic metamorphism Dynamic metamorphism is associated with faults The Size, Temperature and Composition of the and areas where high pressure builds up in the Intrusion. crust, but the temperature is low, such as in the The Composition of the Country Rock. accretionary wedges at convergent plate boundaries. The amount of Fluids available for metamorphic alteration. Fig. 8.7, p. 188 The Seven Types of Metamorphism The Seven Types of Metamorphism Dynamic metamorphism Hydrothermal metamorphism The type of pressure is differential, in other words Occurs through the interaction of hot fluids with the amount is different in different directions. country rock. On the sea floor, this happens when basalts are altered to serpentine. http://ic.ucsc.edu/~casey/eart150/Lectures/Foliations&Lineations/ http://www.absoluteastronomy.com/topics/Mid-ocean_ridge http://www.nvcc.edu/home/cbentley/geoblog/2008_03_01_archive.html http://www.geol.umd.edu/~jmerck/geol100/lectures/16.html 3 10/19/2012 The Seven Types of Metamorphism The Seven Types of Metamorphism High Pressure or Ultra-High Pressure Burial metamorphism metamorphism Deep burial of sedimentary rocks and the Occurs at depths greater than 30 km. increased pressure and temperatures – makes At 120+ km and 800+ degrees diamonds can form. non-layered metamorphic rocks. Brought to surface in narrow pipes called kimberlites. Sedimentary structures can be preserved, although the minerals change. Used in oil industry to determine if temperatures were too high to form petroleum (different minerals form under different conditions). http://www.kgs.ku.edu/Publications/GeoRecord/2000/vol6.1/Page1.html The Seven Types of Metamorphism Metamorphic Zones and Facies Metamorphic facies – refers to a group of rocks containing a distinctive mineral assemblage formed Shock metamorphism under similar conditions of temperature and pressure. Occurs when a meteorite impacts the Earth. Results in fracturing, shatter cones and new minerals. http://ottawa-rasc.ca/wiki/index.php?title=Odale-Articles-Introduction Fig. 8.19, p. 194 Metamorphic Zones and Facies Metamorphic Zones and Facies Index Minerals and Metamorphic Grade Index Minerals and Metamorphic Grade Metamorphic grade – the degree of metamorphic Index minerals typically don’t form in carbonate or change a rock has undergone. quartz-rich parent rocks. Index minerals – certain minerals are known to only form under specific temperatures and pressure. Fig. 8.8 p. 188 Fig. 8.8 p. 188 4 10/19/2012 Metamorphic Zones and Facies Metamorphic Zones and Facies Metamorphic zone – a belt of rocks showing roughly How do metamorphic zones and metamorphic the same degree of metamorphism facies differ? Metamorphic zones show the gradational metamorphic change within a single rock composition. Metamorphic facies are groups of many different rock compositions whose mineral contents all indicate common temperature and pressure conditions during metamorphism. Fig. 8.18, p. 194 How are Metamorphic How are Metamorphic Rocks Rocks Classified? Classified? Metamorphic rocks are classified principally according to texture. Classification of Common Metamorphic Rocks Foliated - Foliated texture is produced by the preferred orientation of platy minerals. Nonfoliated - Nonfoliated textures do not exhibit preferred orientation of minerals. Fig. 8.9a, p. 189 Table 8.1, p. 190 How are Metamorphic Rocks Classified? How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Foliated Metamorphic Rocks Foliated texture is produced by the preferred orientation of platy minerals. Schist and Gneiss Foliated metamorphic rocks form a graded series of grain size and/or development of foliation, from fine grained slate, to phyllite and coarser grained schist, to gneiss, with segregated bands of minerals. Amphibolite is another fairly common coarse grained foliated metamorphic rock. Fig. 8.9d, p. 189 8.10a, p. 190 Figs 8.11-8.13, p. 191 5 10/19/2012 How are Metamorphic Rocks Classified? How are Metamorphic Rocks Classified? Foliated Metamorphic Rocks Migmatite – contain streaks of granite Nonfoliated Metamorphic Rocks Nonfoliated textures do not exhibit preferred orientation of minerals. Common nonfoliated metamorphic rocks are marble, quartzite, greenstone, and hornfels. Fig. 8.16, p. 193 Fig. 8.17, p. 193 Fig. 8.14, p. 192 Plate Tectonics and Metamorphism Plate Tectonics and Metamorphism Metamorphic rocks formed near the surface and within an Metamorphism can occur along all types of plate oceanic-continental convergent plate boundary zone result boundaries, but is most common and extensive along convergent boundaries. from low temperature and high pressure conditions. Blueschist facies Fig. 8.20, p. 195 Fig. 8.20, p. 195 Plate Tectonics and Metamorphism Metamorphism and Natural Resources Higher temperatures and pressures existing at depth Economic mineral resources which are associated with within such plate boundaries produce higher grades of metamorphic rocks include tin, copper, iron, lead, metamorphism in a subducting oceanic plate. tungsten and zinc (fluids during Contact Metamorphism; Hydrothermal systems = Cyprus-type deposits) Copper Ore Tin Ore Fig. 8.20, p. 195 http://library.thinkquest.org/06aug/00425/Natural_Recousces.html
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