111-9 - GY 111 Lab Notes D. Haywick (2008-09) 1 GY 111...

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GY 111 Lab Notes D. Haywick (2008-09) 1 GY 111 Lecture Note Series Plate Tectonics and Volcanism Lecture Goals A) Review of where volcanism occurs B) Properties of magma\lava (viscosity!) C) Types of Volcanoes Reference: Press et al. (2004) Chapters 5 and 6; Grotzinger, et al. (2007) Chapters 2, 4 and 12 A) Review of where volcanism occurs Volcanism is one of the principle consequences of plate tectonics. As we discussed in earlier lectures, there are 3 primary plate boundaries: 1) Convergent, 2) Divergent, 3) Transform Only the first 2 are sites of pervasive volcanism because this is where "cracking" occurs (as in the case of divergent plate boundaries); or subduction-induced melting occurs (e.g., convergent plate boundaries): There is also a third type of volcanic situation which we haven't really discussed yet. Consider the Hawaiian Islands. Anyone who has ever been there is aware that they are volcanic in nature (e.g., Kilauea has been erupting virtually non-stop for almost 10 years now!). But, Hawaii does not lie along a plate boundary. It is in the middle of the Pacific plate . Hawaii is known as a hotspot and we will come back to look at this type of volcanism shortly. Let's look at these volcanic settings one at a time. 1) Divergent plate boundaries : This is the most obvious type of volcanic setting. If you break the lithosphere apart, molten rock ( magma ) will fill in the gap. It isn't so straight-forward though. There is (to the best of our knowledge) no great 100 km deep cracks through which lava flows (recall that the lithosphere plates are 100 km thick). When lithosphere is cracked apart and starts to separate, molten rock starts to rise upward along a series of fractures. Instead of one great pulse of magma, you get many smaller pulses. Each takes on the form of an inverted teardrop as they rise. They are known as plutons and many underlie the divergent plate boundaries:
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GY 111 Lab Notes D. Haywick (2008-09) 2 The magma rises upward along planes of weakness (the cracks) and may either make it to the surface (where lava might erupt from a volcano; thereby forming volcanic rocks ), or, alternatively, the magma may cool and solidify within the pluton below the Earth's surface forming plutonic rocks . There are 2 important things to note: a) magma gets to the surface via a potentially complex network of fractures, caverns and pathways. (Each of these has a specific name which we will get to eventually) b) in the case of the divergent plate boundaries, the magma can move upward relatively quickly because it passes along planes of weakness. There is also no dilution of the magma with surrounding rock. Such is not the case with magma formed along convergent plate boundaries as you will soon see. In other words, lava erupted at the surface at divergent plate boundaries is more or less the same compositionally and temperature-wise as the mantle directly beneath the lithosphere. 1
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111-9 - GY 111 Lab Notes D. Haywick (2008-09) 1 GY 111...

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