2004 Final Section 5&6 Q&A

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Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 2004 Final Exam Review Section 5&6 1. Describe the mineralogical changes that occur during the chemical weathering of granite. What conditions favour such weathering? Granite is much less susceptible to chemical weathering, compared to marble. This is easily observed in cemeteries, where headstones are made of both. Granite will preserve the names much longer, whereas marble weathers very easily. The reaction of marble with acid rain is widely known. Any exposure to acid will dissolve the marble. This is seen in Europe, where any buildings of marble are badly weathered due to acid rain from the early 1900s. The acid responsible for this weathering is carbonic acid. This was produced in the industrial revolution from burning coal. Water is a polar molecule. This is because the oxygen is electronegative, and has a tiny net negative charge. This gives the hydrogen’s a net positive charge. The molecule behaves like a mini bar magnet. The plus hydrogen’s are directed towards the green chlorine (negative) ions, and help it dissolve. The negative oxygen’s on water are directed towards the positive potassium ions. This is why water can dissolve salts. The charged ions from salt are able to dissolve in the polar water molecules, because the water molecules are charged and orientate themselves to stabilize individual ions. So the acid hits the limestone / marble and produces these ions, and then the water carries them away. 2. What is limestone and how does it form? Limestone is basically 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 calcium carbonate precipitates. Acid rain can dissolve rocks. Limestone that has been metamorphosed in the rock cycle forms marble. Marble is vulnerable to acid rain. Acid on marble will fizz, and carbon dioxide bubbles off. 3. Explain why recrystallization occurs and new minerals form 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. Explain the metamorphic facies concept. Name three facies. Metamorphic facies – a domain in the pressure temperature graph. The facies is dependant on the composition of the rock. There are different environments that these reactions can take place. Depending on the tectonic environment, you will take different tracks through the graph. Bluechist – low temperature, variable to high pressure Greenschist – medium temperature and medium pressure Amphibolite – high temperature and medium pressure 5. Explain the idea of paired metamorphic belts. Where do you find them and why? If erosion occurs on the surface, some time later, we can see rocks that used to be underground. These rocks exhibit these nice trends, were changes in temperature are reflected by observing nice shifts between different facies. Paired metamorphic belts – juxtaposed facies are called metamorphic belts because they are found next to each other. 6. What are horsts and grabens? Why do they form? Graben – formed from two normal faults – where things are being extended Horst - a horst is the raised fault block bounded by normal faults or graben. A horst is formed from extension of the Earth's
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