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Volcanoes Chapter 6
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Eruptions
• Volcano – (#34) opening in Earth’s surface which allows gas & magma to escape– Magma: (#35) molten rock underground– Lava: (#36) molten rock at Earth’s surface
• Two types– Non-explosive
• calm flows of lava such as Mt. Kiluea or sea floor spreading• most common• happen over long periods, all the time
– Explosive• clouds of hot ash, debris, and gas shoot out• rarer• sudden and destructive
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Volcano Parts
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Magma
• Explosiveness of eruption depends on:– water– silica in magma– gases
• Water and CO2 remain dissolved in magma until pressure released– Like a can of soda exploding when top opened
• Magma with high silica content very stiff and can plug vent opening– Builds pressure by preventing water and other gases
from escaping
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Eruption Materials
• Lava (non explosive) – liquid rock– Aa or blocky: high viscosity, stiff, slow moving, brittle, jagged or blocks– Pahoehoe or pillow lava: low viscosity, more fluid, faster moving,
smooth
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More Eruption Materials
• Pyroclastic material (explosive) – magma hardens in the air or shatters existing rock– volcanic bombs– volcanic blocks– lapilli– volcanic ash
• Pyroclastic flows (#37)– hot ash, dust, and
gases ejected and flow rapidly downhill
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Effects
• Deaths by blast or pyroclastic flows• Ecologically damage• Climate change
– ash in upper atmosphere– blocks sunlight– Mt. Tambora in Indonesia in 1815
• 12,000 direct deaths• 80,000 deaths from hunger and disease• Food shortages in Europe and N. America next year due to
reduced sunlight and temperature• Global temperatures dropped .5 degrees C
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Types• Shield
– (#39) Repeated lava flows build up over time from non-explosive eruptions
– Gently sloping over wide area– Mauna Kea in Hawaii
• Cinder cone– (#40) Pyroclastic material builds up from moderately explosive eruptions– Steep slopes of cinder cones– Not very stable, sometimes on sides of other volcanoes– Paricutin in Mexico
• Composite (aka stratovolcano)– Most common type– (#41) Alternating flows of lava and pyroclastic material build up– Broad bases that get steeper towards top– Mt Fuji in Japan, Mt Hood, Mt Ranier, Mt St Helens, Mt Shasta
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Other Volcanic Landforms
• Craters
• Calderas (#38)
• Lava plateaus
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Locations
• Divergent boundary – mid ocean ridges– most volcanic activity on Earth
• Convergent boundary – subduction zones– descending oceanic crust– melting rock less dense and rises up– 80% of all land volcanoes at convergent
• Hot spots– mantle plumes far from plate boundaries– Hawaiian islands– Yellowstone