GEOL 101 : Metamorphic Rocks and Processes
GEOL 101 : Metamorphic Rocks and Processes 101-017
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This 4 page Class Notes was uploaded by Natalee Stanton on Wednesday February 3, 2016. The Class Notes belongs to 101-017 at University of South Carolina taught by in Winter 2016. Since its upload, it has received 21 views. For similar materials see Geology 101-017 in Geology at University of South Carolina.
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Date Created: 02/03/16
GEOL 101 Metamorphic Rocks and Processes B The Rock Cycle Any rock can be subject to metamorphism Many metamorphic rocks originate as sedimentary rocks Diagenesis – chemical and physical processes that change sediments to sedimentary rocks Compaction – removal of pore space and pore fluid (physical) o Reduces pore space Cementation – precipitation of new minerals between grains (Chemical) o Chemical precipitation of mineral material between grains binds sediment into hard rock Metamorphism and Metamorphic Rocks Metamorphism – change mineralogy and texture affected by pressure, heat, and water that results in a more compact and more highly crystalline condition Metamorphic Rock – rock hat has undergone changes in texture, mineralogical, and or chemical composition in the solid state Factors in Metamorphism Main factors driving metamorphism: o Internal heat of the Earth o Pressures within the Earth o Fluids within the Earth How Do Rocks Metamorphose? Through partial or complete recrystallization of minerals in the rocks over long periods of time Rocks remain in solid state during metamorphism Role of Pressure Greater pressure tends to decrease the space available for mineral growth: increase in density Increased pressure can come from any directed stress (burial, collision) – confining pressure Stress causes preferred orientation of minerals – differential stress (directed pressure) Role of temperature Heat – greatly affects a rock’s mineralogy and texture , increases mobility of elements Geothermal gradient – measure of variation of temperature with depth ( average 30C/km) Metamorphic Reactions Prograde – mineral changes that take place during am increase in temperature Retrograde – mineral changes that take [lace during a decrease in temperate Role of Fluids Fluids (H20) acts as a catalyst during metamorphism Aid the exchange of ions between growing crystals Metasomatism – change in a rocks bulk composition by fluid transport of chemical substance into or out of rock Metamorphic Grade Refers to the intensity of metamorphism Low grade – low temperature and pressure High grade – high temperature and pressure The Amount of Change During Metamorphism Depends on: The grade of metamorphism The duration of metamorphism The composition of the rock Changes in Texture are Due to Recrystallization Grain size can increase or decrease Shape of grains can change Preferred orientation/ arrangement of mineral Directed stress will orient minerals in two ways o Lineation o Foliations A set of flat or wavy parallel planes produced by deformation Metamorphic textures Classification of foliated rocks: o Size of crystals o Nature of foliation o Degree to which minerals are segregated into bands o Metamorphic grade 23 Stability of Minerals Most minerals are stable over relatively narrow range of pressure and temperature The stability range of different mineral sometimes overlap and provide insights into the metamorphic history of rocks Metamorphic facies A given set of metamorphic conditions Each facies is characteristic of particular tectonic environments and has certain minerals that are indicate of those conditions The minerals in a rock can therefore be clues to the (pressure and temperature) history of rock Different kinds of metamorphic rocks are formed o Form parent rocks of different composition at the same grade of metamorphism o Form parent rocks of the same composition under different grades of metamorphism Plate Tectonics and Metamorphism Continent ocean collision Continent – continent collision Metamorphism is a dynamic and not a static process Sample Exam Questions Which of the following tectonic settings will be coolest at 30 km depth? a. Regions of continental extension b. Stable continental lithosphere c. Volcanic arcs d. Temperature in all three tectonic settings above will be the same at 30kn depth Metamorphic rocks result from the recrystallization of existing rocks while they are still in a solid state. a. True b. False Which of the following rocks represents the highest metamorphic grade? a. Gneiss b. Phyllite c. Schist d. Slate A rock exhibiting high grade metamorphism has experienced high temperatures and/ or pressures. a. True b. False The parent rock of a quartzite is ______. a. Sandstone b. Shale c. Granite d. Limestone