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Lecture 17

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Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC 201 Lecture 17 Notes Why to stalactites form? They are made from calcium carbonate. Water drops from the ceiling, and calcium carbonate precipitates. Carbon dioxide is dissolved in the water. When CO2 is in contact with water, you get carbonic acid. When the water drops, it loses its CO2, which means the water is no longer acidic, and the calci- um carbonate precipitates. The metamorphic rocks are formed through heat and pressure. Igneous rocks are eroded, and form layers of sedimentary rocks. Metamorphic rocks derive their name from metamorph, Latin for change. If these rocks are in an environment of heat and pressure, they are able to undergo change. If igneous rocks on the surface are buried and heated up, they change into metamorphic rock. Layers of rock have different composition. Sedimentary rock subjected to heat and pressure changes the com- position. Often layering effects will be observed, because the sedimentary rocks have pre-existing layers. Specific sedimentary rocks give rise specific metamorphic rocks. The first change rocks can undergo is re- crystallization. Here we see sandstone, a fine-grained sedimentary rock that has been packed together. If sandstone under- goes 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 organiza- tion 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 Any constact between two crystals represents a zone of stability. Well all crystals can be broken down into a combination of crystals. Crystals want to minimize their surface area, which represents instability. So the crystal grows, making current crystals bigger. The crystals want to minimize the surface area. This minimizes the zones of instability. Well isn’t the perfect shape a sphere? Yes, a sphere does have the minimum surface area to vol- ume ratio. But the molecules also need to be mini- mized with respect to each other. To make sure there is no space in-between space crystal, they take polygonal shape. This allows for crystals to connect to each other, and minimize surface area. Limestone – CaCo3 – calcium carbonate Limestone is transformed into marble by recrystal- lization. Here we see that marble formed bigger crystals to minimize surface area. So we are going from something fine grained to some- thing course grained. There can sometimes be tiny films of water left at the surface of crystals. In a continent-continent collision there are stresses. Lets imagine there is shale, which forms in layers. Clay min- erals tend to be sheet silica’s, which prefer to be in horizontal layers. These sheets can easily be broken in certain directions. Crystals grow at right angles (90 de- grees) to stresses. In particular, sheet crystals grow at 90 degrees to the stress, to minimize directed stress. Roofs are made from slate. The slates brake at when a differential stress is ap- plied. Slaty cleavage – the plane along which the rock will preferentially break. This is the product of differential stress at low metamorphic grade shale. Slaty Cleav- age is a type of foliation expressed by the tendency of a rock to split along par- allel planes. This should not be confused with bedding planes, which are a sedi- mentary structure. Slaty cleavage results from the parallel orientation of microscopic platy miner- als. When a stress is applied, the new crystals will grow at right angles to the stress. The pressure would be applied over a long period of time, because of the geological time scale. If the stress is applied too rapidly, the rocks will break. Think of squeezing a pack of cards. The cards bend into a U shape. If you take a flexible book and push on the ends, the book will fold, and your hands will get closer. Same concept with rock, but slower timescale. If chlorite and muscovite are present in the same rock under metamorphic conditions, they will form garnet. The important note for this reaction is that the formation of minerals does not change the chemical composition.
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