CEE 1030 1st Edition Lecture 6 Marble example of a metamorphic rock The rock cycle paths and processes by which rock types change at the Earth s surface and interior Metamorphic rocks changes in minerals texture and often chemical composition of a solid rock subjected to conditions very different to those in which it initially formed effects of metamorphism depend on parent rock protolith Igneous Sedminetary Metamorphic agents of metamorphism 1 heat 2 pressure 3 fluids Heat provides energy for chemical reactions Chemical reactions occur at faster rate recrystallization of existing minerals formation of new stable minerals sources of heat 1 geothermal gradient 2 local intrusion of magma Pressure also increases with depth as thickness of overlying rocks increases Confining pressure applies force equally in all directions Shape of sediments aren t changing just becoming more compressed directed pressure or differential stress forces are unequal in different look at powerpoint chemically active fluids mainly water with other volatiles Chemical reactions take place faster and at lower temperatures in the presence of water than under dry conditions Changes composition of rock sources of fluids hydrated minerals clay and micas Fluids from magma intrusions Pore waters in sedimentary rocks How we affect what the rocks look like metamorphic textures physical changes during metamorphism texture size shape and arrangement of mineral grains reorientation of rod or plate shaped crystals recrystallization of minerals resulting in fewer but larger grains creation of new stable minerals some index minerals can tell geologist about the pressure and temperature foliation any planar arrangement of mineral grains or structural features within a rock rotation of platy and or elongated minerals recrystallization of minerals in the direction of preferred orientation rock or slaty cleavage closely space planar surfaces along which rocks easily split into thin tabular sheets recrystallization and alignment of mica grains Quartz and feldspar not affected slate very fine grained rock Excellent rock cleavage Smooth dull surface schistosity higher metamorphic grade larger crystals Platy minerals are discernible with naked eye and exhibit a planar or layered structure phyllite platy minerals larger than in slate but not quite enough to see with eye schist medium to coarse grained rock Platy minerals mainly micas predominate Schist describes the rock texture Add mineral names to indicate composition garnet mica schist chlorite schist etc gneiss parent rock schist granite volcanic medium to coarse grained rock migmatite transition from high grade metamorphic rock to igneous rock felsic silicate minerals quartz feldspar have lower melting temperature than mafic silicate minerals biotite amphibole light brands igneous melt dark brands metamorphic metamorphic rocks non foliated marble parent rock limestone or dolostone coarse crystalline calcite or dolomite crystals used as a decorative and monument stone exhibits a variety of color quartzite parent rock quartz rich sandstone Quartz grains fused together v hard rock metamorphic grade index minerals certain minerals are good indications of metamorphic conditions Minerals formed depend on original rock type metamorphic environments geothermal gradient and burial metamorphism increase in temperature with depth contact or thermal metamorphism increase in heat due to intrusion of hot magma local effect zone of alteration called aureole forms in country rock around the intrusion hornfels general term for rather hard nonfoliated metamorphic rocks formed in aureole hydrothermal metamorphism chemical alteration as hot iorn0 irch fluids hydrothermal solutions circulate through fissures and cracks that develop in rock widespread along mid ocean ridge system impact metamorphism meteorites hit earth at high velocity regional metamorphism greatest quantities of metamorphic rocks occurs when rocks are squeezed between two plates as they converge during mountain building