13 - Nats 101 Volcanoes and Other Mountains(part deux...

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Nats 101 13/02/2008 16:56:00 Volcanoes and Other Mountains (part deux) Cascade Mountains Volcanic arc in Pacific Northwest Major Cities within 100 km of the volcano Volcanoes formed above subduction zone where Juan de Fuca plate slides  beneath North America Mt. St. Helens is most active volcano in conterminous US Measuring the Eruption Volcanic Explosivity Index (VEI) measures volume of erupted material Low Viscosity Basaltic  Allow gases to escape easily Produce lavas that flow rapidly Single lave flows can travel tens of km Form shield volcanoes High viscosity adesitic and rhyolitic magmas Gas pressures can build up  Violent eruptions (e.g. widespread ash clouds) Produce lavas that flow slowly cover small areas Forms stratovolcanoes (Mt. St. Helens)  Higher viscosity lava remains within volcano crater
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13 - Nats 101 Volcanoes and Other Mountains(part deux...

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