CEE 1030 1st Edition Lecture 6Marble- 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 metamorphism1. heat2. pressure3. 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 sedimentsaren’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