Lake usually forms Phreatic explosions hot magma meets sea or ground water

Lake usually forms phreatic explosions hot magma

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Lake usually forms.Phreatic explosions– hot magma meets sea or ground water, forming superheated steam and causing violent explosions. In Krakatoa, Indonesia in 1883, explosion heard in Australia, thousands of km away and it killed 30,000 people (tsunami)Fissure EruptionsOn landform flood basalts. Many cover ~1 million sq. km., apparently “erupted” quickly.At spreading centers– form entire sea floor, topped by pillow lavas.Ash flow deposits– huge eruptions of pyroclasts, cover large areas and ~1 km thick. Never seen by man, but in past occurred at Yellowstone.Other phenomenaLahar– torrential mudflow of wet volcanic debris—very dangerous.1.Mix water/rain/ice with pyroclasts or lava. Forms “slurry”--rolls down fast, wiping out towns/villages. Unpredictable.2.Nevado del Ruiz (1985)--killed ~10,000.Volcanic gases– generally made up of water vapor, carbon dioxide, sulfur dioxide, oxides of nitrogen. 1.If mainly gases heavier than air, can form thick carpet in countryside. 2.Silent killer—asphyxiation.Hydrothermal activity– interaction between circulation ground-water and buried magma.1.Geysersformed—Old Faithful.2.Can use for geothermal energy—clean system.Global Pattern of Volcanism“Ring of Fire” around the Pacific Ocean. Caused by subducted plates leading to (mainly) andesitic volcanism.
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In areas, volcanic islands formed—Japan, Philippines (two oceanic plates converge).In other areas, mountain chain on continent formed, e.g. Andes (continental plate—oceanic plate convergence).Some volcanoes located at/near divergent plate boundaries (Iceland).Wednesday—February 14Many intraplate volcanoes and flood basalt areas thought to be linked to hot spots(plumes), thin jet of molten material rising “rapidly” from core-mantle boundary.As Pacific plate has moved over the (Hawaiian) hot spot, it has formed a chain of volcanic islands and seamounts.Other island chains in Pacific also formed in same way.Can calculate plate velocities from such “hot spot tracks”--controversial. Volcanism and Human AffairsGenerally cannot predict eruptions accuratelyPyroclastic flows and lahars most dangerousMany (major) cities in danger—Tacoma(WA) and Naples (Italy)Collapse of Minoan civilization caused by volcanic eruption in Aegean.Explosions at sea can trigger tsunamis—waves that travel at 7500km/hr across open sea and break ~20m high at coastline. Little time for warning to coastal areas.(Violent) eruptions throw lots of dust/ash and gases into the atmosphere. Can cause plane engines to “seize up.” Fine dust leads to spectacular sunsets for many years. Can also cause (minor) global climatic changes—cooling. 1.Examples seen in 1810s following big eruptions in Indonesia.
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