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WhereShape of volcanoesViolenceVolcanic HazardsVOLCANOESWhereMost of the extrusive volcanism in the Earth occurs at mid ocean ridges. Here, moltenmaterial from the Earth's interior is erupted onto the ocean floor, forming characteristic pillow lavas(squished out like toothpaste) as the erupted lava cools in contact with the cool sea water.The other major zone of volcanism occurs at destructive continental margins beyondsubduction zones.The Morphology of Volcanoes:Shape of volcanoesLava: Lava is molten rock (magma) that has erupted onto the Earth's surface:Lava can be: Basalt--80% of all lava. Basalt is 50% silica and has a low viscosity. It can flowas fast as 16 km/h (e.g. Kilauea, Mauna Loa). Andesite--10% of all lava, 60% silica(e.g. Mt. St.Helens, Krakatau). Rhyolite--10% of all lava, 70% silica, is highly viscous and does not flow well(e.g. Yellowstone), Lava is frequently 1000 to 1200C hot, but may be as hot as 1400C.The gases dissolved in magma make up as much as 0.2-0.3 wt% of the magma, and arecomposed mostly of H2O and CO2 (98%).Violence The violence of volcanic eruptions is directly coupled to the silica content of the lava.Violent eruptions occur because the gas held within the lava as it erupts cannot easily escape to theair. As it expands in response to the low pressures at the Earth's surface and the lower solubility ofthe gas in the minerals, the gas may forcibly push its way out of the lava, blasting the surface layers ofthe lava into the air.Obviously, if the gas can escape easily, as in low viscosity lavas such as basalt, there will belittle or no violence. Fountaining occurs when you have rapid gas escape from fresh basalt.If the gas is trapped in the lava and cannot escape until the pressure builds very high, as inmore viscous lavas, such as rhyolites, the explosions may be exceedingly violent.The viscosity of the lava also defines the shape of the volcano.Types of Volcanoes 1. Shield Volcanoes- low viscosity lava comes out of a central vent and spreads easily out over a large area. Thesevolcanoes are mostly basaltic. In the extreme case of low viscosity we have flood basalts.- these volcanoes have low profiles with gently sloping walls (mostly 2-10o)- Hawaii is a series of shield volcanoes stacked up on each other (Kilauea, Taupo).As basalts cool, the lava solidifies and contracts, putting the surface under tension. Thistension may open regular cracks, such as in a dry mud pool. The characteristic six-sided columnsoften found in basaltic lava flows result from this cooling.2. Cinder Cones- the cone is made up of pyroclastic material ejected from the volcanic vent which stacks up on theslope (about 30o, but always less than 33o)- cinder cones are much smaller than shield volcanoes- they require gas build up in the lava to produce the splattering which gives the fragments making upthe cone- although the lava may be of any type, it is more likely to be more silica rich (andesite, rhyolite)- because of the nature and size of these volcanoes, they are relatively short lived3. Composite cones- made up of alternating layers of pyroclasts and lava flows (slopes 10 - 30o)- the lava stabilizes the cone, so that it can continue to grow over a long period(e.g. the Andes mountains are mostly composite volcanoes)- the lavas are mostly intermediate in nature, constituting andesite and sometimes rhyolite(the more explosive ones are more silica rich, e.g. Mt. St. Helens)- these volcanoes are mostly found at destructive plate margins.4. Volcanic Domes- not really a type of volcano, domes are formed from very silica rich lava which cannot move fastenough to form a flow. Generally, they form after an eruption when most of the gas has escapedfrom the magma. Such domes are not that common. A lava dome is currently forming in the calderaof Mt. St. Helens.Calderas are volcanic craters that have collapsed, usually when blocks of the volcanic cone fall backinto the magma chamber to produce a much larger depression.e.g. Crater lake.Nuees Ardents or Pyroclastic Flows are spectacular eruptions that occur when trapped gas issuddenly released from a lava body. The hot gas, heavy with dust and hot ash flows down themountain side at up to supersonic speeds (as high as 700 km/h), burning and covering everything inits path. Some pyroclastic flows have travelled as far as 100 km. e.g. Mt St. HelensVolcanic Hazards 1. Pyroclastic Flows (e.g. Mt Pelee, Martinique, 1902 (29,000 dead); Mt St. Helens, 1980)2. Tephra and Hot Poisonous Gases (e.g. Mt Vesuvius, 79 AD (Pompeii))3. Tephra plus Water gives Mudflows (e.g. Nevada del Ruiz volcano, Columbia, 1985 (20,000 dead))4. Tidal Waves / Tsunamis (e.g. Krakatau, 1883 (36,000 dead on Java))5. Famine from Destroyed Crops, Dead Livestock (e.g. Tambora, Indonesia, 1815 (12,000 die fromPyroclastic flow, 80,000 from


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PSU GEOSC 001 - VOLCANOES

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