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Unformatted text preview: Lecture #9 notes; Geology 3950 2010; CR Stern Magma types and types of eruptions Volcanic eruptions release magma, which consists of melted rocks and dissolved gases. Magmas are hot (800-1200°C) and are derived from deep (50 to 150 km) below the earth’s surface in the mantle or lower part of the crust. There is no permanent reservoir of liquid magma inside the earth. Magmas are generated by perturbations of the earth’s mantle and lower crust due to plate tectonics. The outer core of the earth is liquid iron, but magmas are rock, not iron, and the outer core is too deep to be tapped by volcanism Magmas are generated in 3 different tectonic environments. The main two (figure 1) are: 1) at extensional or rifting/spreading plate boundaries, such as the mid-oceanic spreading ridge and rift system, below which convectively upwelling hot mantle moving melts due to decompression as it approaches closer to the earths surface. As it melts, the lower density liquid magma escapes from the mantle and erupts through the normal fault systems that are associated with extension at this type of plate boundary. 2) above subduction zones, in collision or compressional plate boundaries, such as in the northwest USA and many other localities around the Pacific Ocean “rim of fire” due to the fact that subducting lithosphere tectonically carries water and crust into the mantle which lowers the melting temperature of the mantle. As crust into the mantle which lowers the melting temperature of the mantle....
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- Spring '08
- Geology, Volcano, Mantle, magmas