Geology 101 Outline of Last Lecture Test corrections Metamorphic Textures Outline of Current Lecture Common Metamorphic Rock Metamorphic Environment Metamorphic Zones Metamorphic Facies Metamorphism and Plate Tectonics Structural Geology Deformation Deformation Features Common Metamorphic Rock o Foliated Slate Very fine grained Excellent rock cleavage Most often generated from low grade metamorphism Phyllite Gradational between slate and schist Platy minerals not large enough to be identified with the unaided eye Break along wavy surfaces Glossy sheen Schist Medium to coarse grained Platy minerals mainly micas predominate The term schist describes the texture To indicate composition mineral names are used Gneiss Medium to coarse Banded experience Often composed of light colored feldspar rich layers with bands of dark ferromagnesian minerals o Non foliated Marble Medium to coarse grained crystalline Parent rock was limestone or dolostone Used as a decorative and monument stone Geology 101 Exhibits a variety of colors Quartzite Formed from a parent rock of quartz rich sandstone Quartz grains are fused together Metamorphic Environment o Contact or thermal metamorphism Result from a rise in temperatures when magma invades a host rock The zone of alteration aureole forms in the rock surrounding the magma Most easily recognized when it occurs near Earth s surface Non foliated rocks are common Shallow no differential stress Volcanoes are examples o Hydrothermal Metamorphism o Chemical alteration occurs when hot ion rich fluids circulate through fissure and cracks that develop in rock o Most wide spread along the axis of the mid ocean ridge Black smoke hot metal rich seawater Hot mineral rich water rises to the seafloor Cold seawater perculates in the hot newly formed crust Block smoker spews hot mineral rich seawater o Regional Metamorphism Produces the greatest quantity of metamorphic rock Associated with mountain building along convergent plate boundaries continental collisions Deep burial Foliated rocks are common Differential stress o Other Burial metamorphism Associated with very thick sedimentary strata Required depth varies depending on the geothermal gradient Subduction zone metamorphism Burial Differential stress Metamorphism along fault zones Shallow depth o Brittle fault breccias and gouge Fragmented Great depth and high temperatures o Ductile deformation Mylonite Impact metamorphism Occurs when meteorites strike earth s surface o Heat Metamorphic Zones Geology 101 o Systematic variation in the mineralogy and textures of metamorphic rocks Variations in the degree of metamorphism o Textual variations Start from shale Increase in metamorphic intenisty o Grain size increases resulting in a sequence of rock Slate Phyllite schist gneiss o Index minerals and metamorphic grade Grade refer to the mainly temperature From low grade to high grade new minerals forms Certain minerals called index minerals are good indicator of the metamorphic conditions in which they form Metamorphic Facies o Metamorphic rocks that contain the same assemblage of minerals Form in similar environment o Common faces Hornfels Low Pressure High temperature thermal Blueschist High Pressure Low Temperature Eclogite High Pressure High Temperature Metamorphism and Plate Tectonics o Divergent Hydrothermal Low pressure High temperature o Convergent Contact and hydrothermal Volcanic activities o Low pressure Subduction zone Blueschist o High pressure o low temperature Eclogite o High pressure o High temperature Regional metamorphism o Mountain belts Structural Geology o Study of the architecture and processes responsible from deformation of Earth s crust Geometry and shape o Tectonic or rock structure Folds Geology 101 Faults Joints Foliation and cleavage o Reconstruct historical events find resources engineering site selection Deformation o Changes in position orientation shape and or size of a rock of body o Occurs mostly along plate boundaries Deformation Features o Undeformed Flat lying beds Spherical grains Clay flaks bedding o Deformed outcrop Contorted beds Flatted grains Clay flaks not bedding Deformation Kinds o Translation Change in location o Rotation Change in orientation o Distortion Change in shape o Strain with distortion only Stretching Elongation Shortening Contraction Shear o Stress Cause of strain Def force per unit area Kinds Compression Tension Shear stress Pressure Different from everyday usage How rocks deform o Elastic deformation If stresses are within limit of their strength rocks deform elastically recoverable If subjected to stresses greater than the strength they deform by fracturing or flowing failure Brittle fracturing breaking apart Ductile solid flow change in shape Geology 101