Metamorphic Rocks

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Geological Sciences and Geological Engineering
GEOL 104
Michael T.Mc Bride

Metamorphic Rocks Metamorphism:  Refers to the changes that take place when a rock is subjected to temperatures or pressures that are different than conditions where the rock originated from  these changes occur in a solid state and usually occur deep below the Earth's surface Factors in Metamorphism:  Metamorphic rocks have almost identical chemical composition as its parent rock, however the texture and specific mineral makeup distinguish the transformation  For example: metamorphism of mafic igneous rock will yield metamorphic rock high in iron and magnesium minerals  Felsic igneous rock would yield metamorphic rock that is rich in feldspar and quartz  Heat derives basic chemical reactions that transform minerals into new forms, and provides mobility and reactivity for the rocks  heat can be provided by an intrusive igneous body that can locally bake the rocks around it, or by the Earth's internal heat (geothermal gradient)  different minerals are stable at different temperatures and therefore change at different times (ie. clay minerals unstable between 150-200°C and start to change into mica)  Pressure changes physical characteristics of rocks and influences the temperature at which certain minerals are stable  Confining Pressure involves the compression of a rock in all directions "confining" the material into the smallest possible volume  increases with depth due to combined weight of overlaying rocks  Directed Pressure is the unequal distribution of pressure, resulting in partial deformation of a rock body  Foliation is a result of directed pressure and forms the layered/banded feature due to preferred orientation  Three factors that influence foliation include: rotation of platy mineral grains into new orientation, changing shape of grains into long and aligned shapes, and re-crystallization of minerals to form new grains growing in the direction of preferred orientation  Chemically Active Fluids are composed mainly of water but contain some volatiles (ie. carbon dioxide)  fluids become more reactive with increasing temperatures and if they surround mineral grains, they act as catalysts to promote textural changes in rocks  minerals tend to crystallize and grow longer in a direction perpendicular to compressional stresses Metamorphic Grade and Index Minerals:  an increase in metamorphic grade (intensity o
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