GEOL 101 1nd Edition Lecture 16 Outline of Last Lecture I. OverviewII. Agents of MetamorphismIII. TexturesOutline of Current Lecture I. Nonfoliated TextureII. Metamorphic EnvironmentCurrent LectureNonfoliated 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 metamorphismo Marble Coarse, crystalline Parentlimestone 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 layerso Quartzite Parentquartz-rich sandstone Quartz grains are fused together Is usually white or pinkThese 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 conditions1) 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 magmao Aureoles can be several kilometers thick- Most easily recognized when it occurs at the surface, or in a near-surface environmento The rocks near the surface are much cooler than those at greater depths, so contact metamorphism is quite obvious- Quartziteo Parent rock is quartz sandstone- Marbleo Parent rock is limestone- Hornfelso Felsic and nonfoliatedo Parent rock may have been shale, volcanic ash, or other felsic material2) 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 rocko See it in Yellowstone with yellow rocks, rocks have been altered- It is most widespread along the axis of the mid-ocean ridge- 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 clouds3) 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 surface4) Burial Metamorphism- Associated with very thick sedimentary strata- Required depth varies from one location to another depending on the geothermal gradient5) Subduction Zone Metamorphism - Sediments along an active margin get deformed- The deformed segment is called an “accretionary wedge”6) Metamorphism along Fault Zones- Occurs at depth and high temperatures- Pre-existing minerals deform by ductile flow- Produces a rock called a mylonite7) 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
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