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earths_interior - Earth's Interior Formation of Magmas EENS...

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This document last updated on 17-Jan-2012 EENS 2120 Petrology Tulane University Prof. Stephen A. Nelson Structure of the Earth and the Origin of Magmas Magmas do not form everywhere beneath the surface of the Earth. This is evident from looking at the world distribution of volcanoes. Thus, magmas must require special circumstances in order to form. Before we talk about how and where magmas form, we first look at the interior structure of the Earth. The Earth's Internal Structure Evidence from seismology tells us that the Earth has a layered structure. Seismic waves generated by earthquakes travel through the Earth with velocities that depend on the type of wave and the physical properties of the material through which the waves travel. Types of Seismic Waves z Body Waves - travel in all directions through the body of the Earth. There are two types of body waves: { P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel. V p = [(K + 4/3 μ )/ ρ Where, V p is the velocity of the P-wave, K is the incompressibility of the material, μ is the rigidity of the material, and ρ is the density of the material. P-waves are the same thing as sound waves. They move through the material by compressing it, but after it has been compressed it expands, so that the wave moves by compressing and expanding the material as it travels. Thus the velocity of the P- wave depends on how easily the material can be compressed (the incompressibility), how rigid the material is (the rigidity), and the density of the material. P-waves have the highest velocity of all seismic waves and thus will reach all seismographs first. { S-Waves - Secondary waves, also called shear waves, travel with a velocity that depends only on the rigidity and density of the material through which they travel: V p = [( μ )/ ρ ] S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel. The resistance to shearing of a material is the property called the rigidity. It is notable that liquids have no rigidity, so that the velocity of an S-wave is zero in a liquid. (This point will become important later). Note that S-waves travel slower than P-waves, so they will reach a seismograph after the P-wave. Earth's Interior & Formation of Magmas 1/17/2012 Page 1 of 12
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z Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth. Thus they can give us information about the properties of rocks near the surface, but not about the properties of the Earth deep in the interior.
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