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Volcanoes Volcanoes Evidence that we live in an active planet The gods of the underworld Millions of people live near active volcanoes – The greatest geological hazard Mount Saint Helens- after Hazard Year 1991 1985 1902 1883 1815 1792 Volcano Pinatubo Ruiz Pelee Krakatau Tambora Unzen Country Philippines Colombia Martinique Indonesia Indonesia Japan Deaths 300 (200,000 evaq.) 23,000 28,000 36,000 92,000 14,500 Future hazard Population near big, recently active volcanoes – Seattle – Mexico City – Quito – Indonesia – Japan Lava Molten rock that flows on the surface Basaltic Lava – low silica, – fluid, – low gas, – not explosive Andesitic Lava – high silica – viscous – gas-rich – explosive Types of volcanoes 1. Shield Volcanoes – Hawaii, Iceland – Basaltic 2. Stratovolcanoes (or composite) – Mt. Saint Helens, Pinatubo, etc – Andesitic – Explosive Shield Volcanoes Huge broad cones Gentle slopes Made from lava flows Basaltic Form at hotspots Not very dangerous Predictable Stratovolcanoes (composite) Andesitic lava Rich in dissolved gases Violent eruptions alternate with lava flows Steep-sided cones Very dangerous Unpredictable Pyroclastic Flows Product of explosive eruptions Hot ash shot up into the atmosphere Ash column collapses Pyroclastic flow can travel 60 mi/hr Ash can fall 100’s of miles away Ash may affect global climate Mt. Saint Helens before Phreatic (vapor) eruption Bulge Mt. St. Helens Pyroclastic Eruption After the eruption Lahars (mudflows) Lahars (mudflows) Very destructive Kill as many people as the eruption itself Loose ash forms mud avalanches Glaciers on top of volcano melt Rain Inside a volcanoe Pyroclastic eruption Volcanic landscape: A Caldera (Crater Lake) Crater lake formation (caldera) 1. Early eruptions from composite cone 2. Great eruption empties the magma chamber 3. Summit collapses 4. Caldera fills with water Plate Tectonic Settings of Volcanism Plate tectonic settings of volcanoes