GSU GEOL 1122K - Chapter 7 Textbook Annotations (14 pages)

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Chapter 7 Textbook Annotations



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Chapter 7 Textbook Annotations

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Pages:
14
School:
Georgia State University
Course:
Geol 1122k - Introductory Geosciences Ii
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7 1 Introduction James Hutton o 18th century Metamorphic rock o Meta Change o Morphe Form o Metamorphic rock A rock that forms when a preexisting rock Protolith o Undergoes a solid state change in response to the modification of the environment This process of change is called metamorphism o By Solid state Metamorphic rock does NOT from by the solidification of magma That is an igneous rock o By change Metamorphism produces new minerals that did not occur in the protolith and or produces a new texture arrangement of mineral grains that is distant to that protolith o By modification of environment Metamorphism takes place when a protolith endures a rise or fall in temperature and or pressure undergoes compress and shear or reacts with hydrothermal fluids very hot water solutions o Forming of metamorphic rocks 1 Some metamorphic rocks are adjacent to igneous intrusions So metamorphism can take place when heat from an intrusion cooks the tock which it intrudes 2 Some metamorphic rocks occur over broad regions in the absence of intrusions So metamorphism can also take place when rocks end up at great depth beneath Earth s surface 7 2 Consequences and Causes of Metamorphism What is a Metamorphic Rock o First Can possess metamorphic minerals New minerals that grow in place within the solid rock only under metamorphic temperatures and pressures o Can produce a group of minerals that make up metamorphic mineral assemblage o Second Can have a metamorphic texture Defined by distinctive arrangement of mineral grains that are NOT found in other rocks o Textures results in a metamorphic foliation The parallel alignment of platy minerals and or the presents of alternating light colored and dark colored layers o When metaphorical minerals and or textures develop a metamorphic rock becomes different from its protolith This takes place very slowly Thousands to millions of years o Involves serval processes which can occur alone or together The most common processes Recrystallization o Changes the shape or size of grains without changing the identity of the mineral making up the grains Phase change o Transforms one mineral into another mineral with the same composition BUT a different crystal structure o Phase change involves the rearrangement of atoms Metamorphic reaction or neocrystallization o New o The growth of new mineral crystals that differ from the protolith o During neocrystallization chemical reactions digest minerals of the protolith that produces new minerals Pressure solution o When a wet rock is squeezed strongly in one direction more than another o Mineral grains dissolved where the surfaces are pressed against other grains o This produces ions that migrate through to water to from a precipitate Plastic deformation o When a rock is squeezed or sheared at elevated temperatures and pressures o Grains behave like a soft plastic and changes shape without breaking Rocks undergo metamorphism when they are subjected to heat pressure compression and shear and or very hot water Metamorphism Due to Heating o When you heat a rock it transforms into a metamorphic rock Heat causes the atoms to vibrate rapidly stretching and bending chemical bonds that lack atoms to their neighbors If the bonds stretch too far and break then the atoms detach from their neighbors more slightly and from new bonds with other atoms Repetition of this process leads to the rearrangement of atoms within the grains or the migration of atoms into and out of the grains Called solid state diffusion o Therefore Recrystallization and or neocrystallization takes place enabling a metamorphic material assemblage to grow in a solid rock Takes place at temperatures between though at which diagenesis occurs and those that causes melting You can find metamorphic rocks in outcrops on continents formed at temperatures between 250 and 850 C Metamorphism Due to Pressure o Pressure can cause a material to collapse inward Near the earth s surface minerals with relatively open crystal structures can be stable If you subject these minerals to extreme pressure the atoms pack more closely together and dense minerals tend to form o This transformation involves phase changes and or neocrystallization Changing Both Pressure and Temperature o In the earth pressure and temperature change together with increasing depth Experiments and calculations how that the stability of certain minerals The ability of a mineral to form and survive o Depends on both pressure and temperature When pressure and temperature increase the original mineral assemblage in a rock becomes unstable and a new assemblage forms out of minerals that are stable For example a rock formed at 8km does not contain the same minerals as one formed at 20km Compression Shear and Development of Preferred Orientation o Compression Flattens a material o Shear Moves one part of a material sideways relative to another o When rocks are subjected to compression and shear at elevated temperatures and pressures they can change shape without breaking As it changes shape the internal texture of a rock changes Platy pancake shaped grains become parallel to one another Elongated cigar shaped grains align in the same direction o Inequant grains meaning that the dimensions of a grain is not the same in all directions o Equant grains have roughly the same dimensions in all directions The alignment of Inequant minerals in a rock results in a preferred orientation The Role of Hydrothermal Fluids o Reactions comply take place in the presences of hydrothermal fluids Very hot water solutions Where does this fluid come from o Originally bonded to minerals in the protolith Reactions release water into its surroundings o May seep up into the protolith from a nearby igneous intrusion or down from overlying groundwater reservoirs o Under extremely high pressure and temperatures The water is neither a gas nor a liquid but a supercritical state It has the characteristic of both gas and liquids o Some hydrothermal fluids chemically react with rock They accelerate reactions because atoms involve in the reactions can migrate faster through a fluid then they can through a solid Hydrothermal fluids can provide water that can be absorbed by minerals during metamorphic reactions o Fluids passing through a rock may pick up some dissolved ions and drop off others Changes the chemical composition of a rock This is called metasomatism Take Home Message o Metamorphism takes place in response to changes in temperature


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