GEOL 101 1nd Edition Lecture 16 Outline of Last Lecture I Overview II Agents of Metamorphism III Textures Outline of Current Lecture I Nonfoliated Texture II Metamorphic Environment Current Lecture Nonfoliated Texture Nonfoliated texture metamorphic rocks that lack foliation Developed in environments where deformation is minimal but heat is high Typically composed of minerals that are interlocking or mosaic like Often form during contact metamorphism o Marble Coarse crystalline Parent limestone or dolostone Composed essentially of calcite or dolomite crystals Relatively soft rock calcite s hardness 3 and so it is often used as a decorative and monument stone Easily weathered Exhibits a variety of colors May be banded hence the name marble when the limestones are interbedded with shale layers o Quartzite Parent quartz rich sandstone Quartz grains are fused together Is usually white or pink These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Has a hardness of 7 very hard Porphyroblastic texture contain large grains surrounded by smaller grains Parent rock recrystallizes to form new mineral Large new minerals are called May be in both foliated or nonfoliated rocks Metamorphic Environment The majority of environments that form metamorphic rocks occur near tectonic plate boundaries Metamorphism also commonly occurs in association with igneous activity Rare and localized metamorphism can occur under very specific conditions 1 Contact or thermal Metamorphism occurs due to rise in temperature when magma invades a host rock and bakes it A zone of alteration called a contact aureole forms in the rock surrounding the magma o Aureoles can be several kilometers thick Most easily recognized when it occurs at the surface or in a near surface environment o The rocks near the surface are much cooler than those at greater depths so contact metamorphism is quite obvious Quartzite o Parent rock is quartz sandstone Marble o Parent rock is limestone Hornfels o Felsic and nonfoliated o Parent rock may have been shale volcanic ash or other felsic material 2 Hydrothermal Metamorphism is closely associated with igneous activity which act as the heat source While magma cools ions that are not incorporated in to mineral crystals combine with left over volatiles water Chemical alteration occurs when hot ion rich fluids called Hydrothermal solutions circulate through fissures and cracks that develop in rock o See it in Yellowstone with yellow rocks rocks have been altered It is most widespread along the axis of the mid ocean ridge 3 4 5 6 7 Sea water percolates through fractures in the crust and gets heated causing it to react with metallic ions the newly formed crust iron nickel copper among others The fluids escape back out of the crust via fractures and forms black smoker clouds Regional Metamorphism produces greatest quantity of metamorphic rock Associated with convergent tectonics and mountain building As a subduction zone closes two continents collide causing deformation folding and metamorphism of the rocks The crust is thickened burying rocks deeper and deeper which results in additional metamorphism Over time the metamorphic rocks may be uplifted and eroded at the surface Burial Metamorphism Associated with very thick sedimentary strata Required depth varies from one location to another depending on the geothermal gradient Subduction Zone Metamorphism Sediments along an active margin get deformed The deformed segment is called an accretionary wedge Metamorphism along Fault Zones Occurs at depth and high temperatures Pre existing minerals deform by ductile flow Produces a rock called a mylonite Impact Metamorphism Occurs when meteorites strike Earth s surface Produces high pressure high temperature quartz polymorphs coesite and stishovite Flash melted silica beads are ejected these beads are called tektites This is also referred to as shock matemorphism
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