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Unformatted text preview: Volcanoes • Magma that reaches the surface is called lava Eruptions • Not all volcanic eruptions are the same E.g. Hawaiian volcanism isn’t explosive But other volcanoes are: • Mt. St. Helens, May 18, 1980 Why are some volcanoes so explosive? • Two factors: Magma viscosity Magma gas content • Viscosity determines how resistant a fluid is to flow Honey is about 10,000 times more viscous than water Viscosity • Three things affect viscosity in lava 1. Which of these would slide more? Pyramids or spheres? 2. Silica content—more felsic means more viscous a. Silica tetrahedral stick together 3. Lava temperature – higher temperature means less viscous 4. Gas content – more dissolved means more viscous • Pressurized gas explodes outward when the magma comes to the surface 1. No gas, no explosion 5. Felsic means sticky, explosive 6. Mafic means runny, non-explosive Volcanic Gases • Outgassing: gases given off by volcanic activity • Present day volcanoes: mostly water vapor and carbon dioxide (co2) with small amounts of so2, xo, s, cl, n, h, etc. • Most gas released is water vapor • Also carbon dioxide and other gases—our atmosphere was built by ancient volcanoes Explosive volcanoes • Need viscous and gas filled lava o Heated gas becomes explosive o Viscous lava keeps the gas from escaping Hawaii • In the middle of the ocean •...
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This note was uploaded on 04/09/2008 for the course GEOL 1030 taught by Professor Abolins during the Fall '08 term at Middle Tennessee State University.
- Fall '08