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Geology 101 Lecture Outline - Metamorphic Rocks 1. Definition of Metamorphic rocks a) formed by alteration of preexisting rocks b) alteration produced by heat, pressure and/or chemically active fluids c) alteration must take place while rock remains solid (NO MELTING) d) using happens from a few kilometers deep to the crust/mantle boundary e) changes are in texture and/or mineral composition 2. Metamorphic Grade - measure of severity of conditions a) low grade - small amount of change; characteristics of original rock may still be present (e.g. bedding planes, fossils, etc.) b) high grade - rock completely altered; original rock (protolith) can only be assumed 3. Zones of Metamorphism - can occur wherever increased heat and/or pressure and/or chemically active fluids are present a) Regional Metamorphic Zone i) Convergent plate boundary ii) more of everything is here b) Contact Metamorphism i) occurs adjacent to magma chamber ii) implies and increase in heat only c) Fault Zone Metamorphism (Dynamic) i) implies additional pressure d) Hydrothermal Metamorphism i) involves hot water - particularly effective near undersea volcanic centers e) Shock Metamorphism i) the result of an instantaneous pressure wave a) from extraterrestrial impacts or possibly volcanic eruption Agents of Metamorphism a) Heat - changing temperature causes minerals to alter into forms stable at the new temperature i) clays to micas at fairly shallow depths ii) different minerals (rock types) begin to metamorphose at very different temps. (e.g. shale changes at low grade conditions, while granite won’t change until high grade conditions b) Pressure - two types, each having a different effect on the alteration i) Confining pressure (lithostatic)- caused by weight of overlying materials a) acts equally in all direction; therefore it simply tends to decrease the volume, resulting in dense minerals (garnet) ii) Directional stress - when pressure is greater in one direction than in others a) causes minerals to pack more tightly in one direction 4. Lecture 12 & 13 b) this alignment, called foliation, of grains can be used to tell which direction the pressure was coming from - perpendicular to alignment c) Chemically-active fluids - mostly just hot water, or water vapor i) What they do… a) act as pathways to allow rock to alter most quickly - like the volatiles in a cooling magma b) “foreign” elements may be present in the fluid that can be added to the protolith elements to make “new” minerals ii) Where they come from… a) released as the result of mineralogical changes; e.g. clays are water rich, mica are not b) escape from cooling magmas - may carry low melting point elements (gold, etc.) 5. Mineral changes during metamorphism: Index Minerals a) Index Mineral - one that is stable over a limited range of temperatures and pressure and can therefore be used to tell what the ultimate conditions were during metamorphism b) Example sequence; low grade to high grade conditions i) chlorite, epidote, muscovite, biotite, hornblende, stauroloite, garnet, pyroxene ii) NOTE - quartz and feldspars are not on this list, they remain stable over a wide range of temps and pressures 6. Summary of possible metamorphic alterations a) increase density - packing minerals tighter together, or packing elements tighter into a new mineral b) growth of larger crystals - more efficient packing c) reorientation of mineral grains to form alignments from directed stress d) conversion of low temp to high temp minerals e) creation of new minerals caused by injection of fluids 7. Classification of Metamorphic rocks a) Foliated - has some type of alignment of grain i) rock cleavage - aligned micas; rock is Slate ii) schistosity - aligned platy grains that are large enough to see; rock is Schist iii) gneissosity - segregation of minerals(often granular) into bands; rock is Gneiss b) Nonfoliated - no alignments i) Why no alignment? a) Created in an environment with no directed stress, or b) Minerals are equidimensionally shaped - they cannot show alignment (quartz and calcite are this way) (1) Pure quartz makes Quartzite (2) Pure calcite (limestone) makes Marble 1