111-11 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111 Lecture Note Series Crystallization of Magma and Lava Lecture Goals A) The chemical composition of the crust B) Crystallization of molten rock C) Bowen's Reaction Series Reference: Press et al., 2004, Chapters 5 and 6; Grotzinger et al., 2007, Chapter 4 A) The chemical composition of the Earth: In an earlier lecture, we discussed the nature of the interior of the Earth. Although it is unlikely that we will be able to visit the Earth's deep interior any time soon, we can still resolve chemical and mineralogical changes on the basis of seismic wave behavior. Volcanoes also give us some idea of the materials that compose the Earth's interior (after all, the lava that they erupt has to come from somewhere!). So what exactly is the composition of the Earth's interior? It depends upon the depth that you are interested in. The mantle is different from the core and the core is different from the crust. Since we live on the crust, it is perhaps best to focus on this layer rather than others. At this point in our course, it is preferable not to use minerals as the basis of composition. They are too highly variable. Besides, there are two general types of crust anyway: 1) continental crust and 2) oceanic crust. Instead, we'll just look at the elemental composition of the Earth's crust. The following table lists the 8 most common elements (in weight percent and in % of atoms) of average crust: Element Wt% % of atoms Oxygen 46.6 60.5 Silicon 27.7 20.5 Aluminum 8.1 6.2 Iron 5.0 1.9 Calcium 3.6 1.9 Sodium 2.8 2.5 Potassium 2.6 1.8 Magnesium 2.1 1.4 All other elements 1.5 3.3 If you ever wondered why quartz (SiO 2 ) is so common in crustal rocks or why there are so many different silicate minerals, these data should answer your questions. The more of a particular element that you have to work with, the more that element will form minerals. The equivalent
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GY 111 Lecture Notes D. Haywick (2008-09) 2 number of atoms is particularly useful for estimating mineral composition as it allows you to predict mineral formulas and hence mineral percentages. Anyway, the gist of the matter is that the crust contains a lot of Si and O and there is a lot of SiO 4 4- available for minerals. Now what we need to do is to discuss how these minerals actually grow in molten rock. B) Crystallization of molten rock Picture a container full of magma that you collected from a pluton deep in the Earth's interior: As the magma starts to cools, it will start to grow crystals. This process is called crystallization and it is analogous to precipitation
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This note was uploaded on 02/04/2012 for the course GLY 111 taught by Professor Haywick during the Fall '11 term at S. Alabama.

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111-11 - GY 111 Lecture Notes D. Haywick (2008-09) 1 GY 111...

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