metamorphic rocks and conclusion of sedimentary rocks
TRANSCRIPT
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Metamorphic Rocks and Conclusion of Sedimentary
Rocks
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The deal with sedimentary rocks
• The bigger the grains, the higher the energy of the depositional environment in which it was formed.
• The smaller the grains, the quieter the environment in the rock formed.
• SEDIMENTARY ROCKS ARE SO USEFUL BECAUSE THEY RECORD THE EARTH’S HISTORY. THEY TELL US WHAT KIND OF ENVIRONMENT WAS WHERE AND WHEN.
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Stratification
• Since sedimentary rocks are formed by deposition of sediment, they form layers of rock.
• Stratification occurs when you get distinct layering in rocks (you deposit something different on top of something else).
• Principle of Uniformitarianism and Suposition – each layer is uniform in composition, older layers are on bottom, newer layers on top.
• If there is discontinuity in the layers, geologists term it an unconformity.
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How can you get an unconformity?
• Erosion of a layer just deposited.
• Faulting (normal or reverse)
• For reverse faulting, your older rocks would get pushed on top and mess up the order.
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Cool Sed rock features
• Cross-bedding – formed from changes in wind direction, or change in direction of water current.
• Ripple marks – you can definitely tell which way water or wind moved
• Mudcracks – formed when mud dries up and shrinks
• Geodes- water within sediments leaches into cavities of rocks precipitating calcite or quartz crystals
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Metamorphic Rocks
• Metamorphism (in the geological sense)- the changing of one rock into another by inducing heat and/or pressure.
• Two types of metamorphism: contact, and regional
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Contact Metamorphism
• When rocks are changed by heat
• How this happens:
1.) rocks that are subducted and come in contact with magma
2.) rocks happen to come into contact with magma that is migrating towards the surface (some new volcanism)
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Regional Metamorphism
• Rock change because they undergo some kind of process that induces a lot of pressure
• How this happens:
1.) rock in a subduction zone, the deeper you go, the more pressure you encounter
2.) rock in a fault zone (lots of pressure there)
3.) compressional forces via plate tectonics, isostatic adjustment, remember folding?????? Ductile deformation?
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Chemical Alteration
• Hydrothermal – pretty much water eruptions- water comes into contact with magma, the magma boils it an sends it back to the surface enriched with chemicals from the subsurface and magma chamber
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How you can tell if a rock is formed by contact or regional
metamorphism.• One word: FOLIATION• Foliated rocks have visible parallel banding. A lot
of the time these bands are wavy, warped, because after all, they were formed under pressure. So, FOLIATED ROCKS ARE FORMED BY REGIONAL METAMORPHISM.
• Non-foliated – most of the time are rocks formed by contact metamorphism
• An example: marble – it’s a non foliated rock formed by regional metamorphism