2006 Final Section 3&4 Q&A

5 Pages
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Department
Earth & Planetary Sciences
Course Code
EPSC 201
Professor
Anthony Williams- Jones

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EPSC201 2006 Final Exam Review Section 3&4 1) The San Andreas Fault is a transform fault. Explain this statement. In the new idea, the relative position of the two ridges remains unchanged, but the continents (green) move further apart. In the old idea, the ridge and continents both move relative to each other. It is easier to move continents apart if the drift is due to many small slips, instead of one big slip. The main message is that the only movement occurring is in-between the ridges, because outside of the ridges, the crust is moving in the same direction at the same speed. Key Assumption – the top and bottom ridges are spreading at exactly the same rate 2) Describe the most important properties of diamond and graphite (one for each mineral) and relate these properties to molecular structure. We started talking about diamonds last class; are purely carbon. Every carbon atom is bonding to every other carbon atom. They are short and strong bonds (homogenous), which results in a super hard structure. When we say it’s hard, it’s not that we cannot break it with a hammer (we will smash it), what it means is that it cannot scratch easily. What makes diamond super strong is the fact that the carbons are covalently bonded which makes it super hard. Diamonds are ONLY formed under high temperature and high pressure (need depth of at 150-200km). If diamonds were formed on shallow depths, it would be graphite instead and not diamond. Diamonds reach the surface of the earth through volcanos; specifically magma liquid breaking pieces of rocks/mantle as it shoots up through the earth. Graphite is made up of purely carbon atoms as well, but in a layered sheet (plane) formation through covalent bonding. The atomic arrangement is the same as in diamond, but the plane sheets are held together weakly by Van der Waal’s bonding. Graphite and diamonds are not forever. But the amount of energy it takes to convert diamonds to graphite is extraordinarily high and it will take a few hundred/thousands of years before it becomes graphite. However, if you put diamond in the oven, it will fuse with oxygen and produce carbon dioxide. 3) Describe the molecular structure of pyroxene and explain its bonding. Silicate Structures Nesosilicate Inosilicate Inosilicate (Single chain)(Double chain) Pyroxene: two types (Diopside or Hypersthene) 4 - SiO 4 Si4O 116 - Pyroxene are chains of silicate tetrahedron SiO 32- glued together by cations through ionic bonds. Sheet silicate Framework silicate Key Point is to understand how chain silicate tetrahedrons need cations to balance the charge 4 - in order to form neutral structures. Si4O10 SiO 2 4) Using a sketch, describe and explain the Rock Cycle. Igneous rocks are at the beginning of the cycle. These rocks weather and compact into sedimentary rocks. If this sedimentary rock are heated and buried deep enough, they turn into metamorphic rock. If the metamorphic rock is buried deep enough, it melts into magma, and will later form into igneous rocks. 5) Describe typical strato- and shield volcanoes and explain why they differ. Volcanism is associated with spreading ridges. Iceland is a good example. Volcanoes are also associated wit hot spots, such as Hawaii. Volcanoes are forming from magma that is subjected from decompression melting. Upwelling of mantle (plume) causes melting, which in turn causes a volcano. The magma has a very low viscosity, meaning it flows easily. These volcanoes are termed shield volcanoes, which have very subtle hills because the lava flows far out from the crater. The lava is made of basalt in these types of volcanoes. At low silica content, the lava is made of isolated tetrahedrons. This flows easier then lavas made of complex silicate mixtures. Stratovolcanoes – carbonate lavas (baking soda – bicarbonates), these magmas have high silica content, which increases its explosive activity. 6) Using a temperature-depth diagram, explain decompression and hydration melting. With spreading activity, you get basalt. With subduction, you get andosite and rhyolite. At spreading centers, we have decompression melting. As the plates spread, mantle moves upwards, and releases pressure. This causes the mantle to melt. Rocks are great insulators. As the mantle moves upwards, the surrounding rocks stay hot (the temperature they were when deep). So the mantle rises up, and is under less temperature, but is at the same hot temperature. This causes the mantle to melt into magma, and is called decompression melting. Shown here is a geotherm. From this, we see that as pressure increases going deeper into the earth, higher temperatures are required to make the rock liquid. At the liquids line, everything right of the line is liquid, which shows that at higher temperatures, the rock turns to liquid. Anything on the solidus line that is moved upwards due to mantle upwelling (decompression melting) will then turn to liquid. This graph explains w
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