2006 Final Section 5&6 Q&A

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 2006 Final Exam Review Section 5&6 1) Name three sedimentary structures and explain how they form. How do we go from broken down material to rocks? Cementation. There is a large number of grains present, for instance quartz sand. We can cement sand into sandstone. The weight of water forces the sediment to pack together. The warm groundwater moves in-between the spaces of the sediment, and cools as it moves through the cold sediment. The ions in the solution will precipitate (solidfy out of solution) as the water cools. The ions are only dissolved because the water is warm. For example, SiO2 (quartz) will precipitate out as the groundwater is cooled in-between the sediment. This precipitated silica joined together all the claustic sediments. Many sediments are cemented by silica. Another type of sediments is glacial moraines. These are extremely variable in size, and poorly sorted. Lots of angular fragments. Once the glacier picks up the material, it is just moved along, and not subjected to weathering. Dried mud causes mudcracks. This sandy material was exposed to the atmosphere, and was dried out. Similar formations can be proven to have been exposed to the environment. The mudcracks on rocks show which end of the rocks was on the surface, and that the rock itself was on the surface. The mudcracks are usually formed after a rain. 2) Name a rock-type formed mainly by biological activity and explain its formation. Some sedimentary rocks are exploited commercially. Coal is a sediment formed from biologically activity, such as swamps or forests. If the sea level rises, the swamp may get covered with sediments, and get buried. The coal is formed from biological material being subjected to heat and pressure, which happens underground. 3) Explain why and how recrystallization occurs during metamorphism. The first change rocks can undergo is recrystallization. Here we see sandstone, a fine-grained sedimentary rock that has been packed together. If sandstone undergoes recrystallization, it turns into quartzite. The recrystallization forces the sandstone, which has no molecular order, to organize its structure and become quartzite. The crack represents a split in the crystal structure. There is no molecular organization on the crack. Because of that, the crack is a region of instability. When things are heated up and subjected to pressure, the rocks are trying to form the most stable form. This is why things crystallize. The heat gives them the energy to reorganize themselves once cooled. The energy of the crystal rock is at a minimum. The higher the energy, the less stable the state is going to be. Key point – Crystals are in a low state of energy – therefore it is stable 4) Using a sketch, explain why slate forms. Slaty cleavage – the plane along which the rock will preferentially break. This is the product of differential stress at low metamorphic grade shale. Slaty Cleavage is a type of foliation expressed by the tendency of a rock to split along parallel planes. This should not be confused with bedding planes, which are a sedimentary structure. Slaty cleavage results from the parallel orientation of microscopic platy minerals. 5)Explain why the aluminosilicate minerals (Al SiO 2, kya5ite, andalusite and sillimanite are important to metamorphic pet
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