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Unformatted text preview: High silica magma/lava has high viscosity because of linking and unlinking of tetrahedra (high viscosity=syrupy) Low silica (mafic) will flow fart s her Explosivity increases with silica content (more explosive eruptions with more silica) Bowens Reaction Series Starting at 1500 degrees Celsius, everything is in melt. As you cool it, things cool at different temperatures; Olivine cools first (1400 degrees C))- raisins in pudding (solid crystals in gooey melt). Viscosity changes as temperature cools because silica (and other chemicals) is being pulled from the melt). Different minerals appear at lower temperatures and zones form in chamber because minerals are forming at different times. Reaction Rim: partial reaction of already formed mineral with cooling melt Fractional Crystalization: low-temperature minerals- form at lower temperatures. Low temperature minerals are more stable at earths surface (e.g. quartz); more resistant to weathering than higher temperature minerals. Discontinuous branch called discontinuous because minerals in it are sacrificed to form one another in some cases; sometimes olivine further reacts with melt to form pyroxene which further reacts with melt to form amphibole and so on. Doesnt always entirely sacrifice itself; can have rocks with both olivine and pyroxene, for example. Continuous branch gives rise to zonation talked about earlier. Silica content increases at bottom of reaction series. Magma silica concentration increases because everything else is drawn out of melt. Silica structure becomes more complex from top to bottom (isolated tetrahedra at top, complex framework at bottom). Making Magma & Effect of Water e.g. Yellowstone; shallow magma source (when it rains, water can penetrate magma source). Subduction; ocean water pulled down with plate, and when plate melts, water is introduced into magma system. Can melt rocks not only by increasing temperature, but also by decreasing pressure at same temperature. Adding water to the system causes it to exist as a solid at a lower temperature. Partial melting Opposite of minerals solidifying at different temperatures; higher silica (felsic) minerals (e.g. quartz) melt first. Mafic minerals (e.g. olivine) melt at highest temperatures. Generates high silica melt first, and then other minerals melt in and make melt more heterogenous; silica concentration goes down as it is diluted by other minerals....
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- Fall '07