Volcanoes - GEOL 114 The Earth's Dynamic Interior Lecture...

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GEOL 114 The Earth's Dynamic Interior Lecture Notes (Copyright © 2006 by Jeffrey S. Barker) 16. Volcanoes All volcanoes are caused, in one way or another, by Plate Tectonics. Active volcanism occurs at Mid-ocean ridges Most of these are underwater and invisible to us. On Iceland, however, the mid-Atlantic Ridge is above water. Here volcanic activity occurs along rift valleys. Continental rifts, such as East Africa, or the Rio Grande Rift in New Mexico. Hot spots, such as Hawaii. Hot spots beneath continents also cause volcanic activity, such as at Yellowstone. Subduction of a plate with oceanic crust beneath another plate with oceanic crust, such as Japan, the Aleutians, Indonesia, the Caribbean, etc. Subduction of a plate with oceanic crust beneath a plate with continental crust, such as the Andes, the Cascades, Mexico and Central America. GEOL 114 1 16. Volcanoes
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Magma Generation Recall that the crust and mantle of the Earth are solid; S waves propagate through both. In some locations, partial melting occurs, which is the source for nearly all magma (molten rock) erupted at the surface (volcanic rocks) and crystallized at depth (plutonic rock). Partial melting can occur when the pressure or temperature are changed locally, so that the melting point of one or more minerals is exceeded. Conversely, partial melting can also occur if the melting point of these minerals is locally depressed, such as occurs in the presence of water. We will find that the mechanism of magma generation dictates the composition of the magma, and the composition dictates its behavior. At mid-ocean ridges and continental rifts, the overlying plates are being pulled apart by subduction at the far edges of the plates. This effectively reduces the pressure immediately below the rift. The temperature remains fairly high since a continuous source of hot mantle material is available below, and there is not much time for cooling as the material ascends. Hot mantle material under reduced pressure initiates partial melting, resulting in a magma of basaltic composition. This flows to the surface relatively easily through the rift, forming new oceanic crust. At hot spots, the opposite situation occurs. Here, the overlying plate acts as a cap, holding the hot, upwelling plume material within the uppermost mantle. The pressure is maintained, but the mantle material is hot (or is heated by the rising material), so partial melting occurs once again. The composition is, again, Basalt. This flows to the surface through fractures in the overlying crust. In oceanic environments (like Hawaii), the crust is relatively thin, so the magma reaches the surface relatively unchanged (basalt). Magma beneath GEOL 114 2 16. Volcanoes melting temperature geotherm (temperature vs. depth) depth or pressure ~100 km melting temperature geotherm (temperature vs. depth) depth or pressure ~100 km
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continental hot spots may have more difficulty reaching the surface, and may reside in magma chambers at depth, where it melts and mixes with the
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Volcanoes - GEOL 114 The Earth's Dynamic Interior Lecture...

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