Unformatted text preview: Geology 1001
Chapter 12 Volcanoes Mt. Saint Helens: Pre May 1980 Volcanoes Volcano: Topographic Feature constructed from the accumulation of lavas and debris. Plumbing: Conduits to the surface. Central and Side Vents 1 What drives volcanic eruptions: Buoyant magma rising to the surface Release of gases trapped in magmas Volcanic System Input: magma from the asthenosphere Products Lava Flows Pyroclasts Landforms Debris (Pyroclastic) Flows (Pyroclastic) Gases 2 Input: mafic magma Basaltic Lavas: most common type of melt Lavas: Mid-ocean ridges Mid Rift valleys Hot spots Basaltic Lavas Low Viscosity: fast-moving flows fast- High T at eruption Low silica content 3 Basaltic Lavas: Cooling on land "Cold" cooling: Cold" Thick skin "Hot" cooling: Hot" Thin skin Basaltic Lavas: Cooling in water Pillow lavas Input: intermediate magmas Andesitic lavas: primarily above subduction zones. More viscous than basaltic lavas (a.k.a. sticky lavas): Lower eruption temperature Higher silica content Commonly explosive: contain trapped gases. 4 Mt Saint Helens, prior to 1980 Early eruption, 1980 5 May, 1980 Input: Felsic magmas Rhyolitic Lavas: melting at the base of continental crust Most viscous lavas (a.k.a. stickiest lavas): Lowest eruption temperature Highest silica content Explosive: Most destructive eruptions. 6 Yellowstone National Park Yellowstone N.P., USA 7 Pyroclasts: products of explosive eruptions Pyroclasts: Ash can reach 10's of miles 10' Collapse of ash column Travel at 60 mph 10's of miles 10' Mt Pinatubo, Philippines 8 Mt Saint Helens, 1980 Pyroclasts
Obsidian Bomb Pumice Eruptions & Landforms: Shield Volcanoes Broad, low relief features Produced by fluid basaltic lavas Rarely explosive: very little trapped gas 9 Shield Volcano: Mauna Loa, Hawaii Eruptions & Landforms: Volcanic Domes Cooling lava plugs the central vent Produced by felsic lavas - commonly explosive 10 Lava Dome: Mount St. Helens Eruptions & Landforms: Cinder Cones Steep sided: large fragments near vent, fine ash around base Produced by pryoclastic eruptions Cerro Negro, Nicaragua 11 Eruptions & Landforms: Stratovolcanos Produced by alternating pryoclastic and lava eruptions Common above subduction zones. Stratovolcano: Mt. Fuji, Japan Stratovolcano: 12 Stratovolcano: Mt. Rainier, USA Stratovolcano: Eruptions & Landforms: Craters Found at the summit of most volcanoes. Post-eruption feature: lava sinks back into Postthe vent and solidifies. Volcanic Crater: Nyiragongo, Tanzania Nyiragongo, Shallow feature 13 Eruptions & Landforms: Calderas Post-eruption feature: collapse of the roof Postabove an empty magma chamber. Caldera: Crater Lake, USA Deep Feature Caldera: Yellowstone N.P., USA 14 Caldera: Yellowstone N.P., USA Eruptions & Landforms: Fissure Eruptions Largest eruptions: associated with a long fissure or crack, not a central vent. Produces widespread, low relief features Fissure Eruptions: Laki, Iceland Laki, 15 Fissure Eruptions: Kilauea, Hawaii Fissure Eruptions: Flood Basalts Global Volcanic Patterns 80% of volcanoes at convergent margins 15% at divergent margins 5% intraplate 16 Mid-Ocean Ridges: lava production Mid- Produce 3X the amount of volcanic rock produced at convergent margins. Intraplate Volcanism: Hot Spots Hot Spots: Oceanic Crust 17 Hot Spot: Continental Crust
1.8 2.0 0.8 Ma Montana Washington Yellowstone Caldera Chain 15.5 Oregon Idaho Wyoming 13.7 16.1 California Nevada Utah 13.8 12.5 13.8 11 10.3 6.2 4.3 Yellowstone National 6.6 Park Hot spot PACIFIC OCEAN 15.6 14.7 Columbia River Basalts 18 Snake River Basalts Hot Spot Controversy Hot Spots may not be stationary. Contradicted by other evidence! Hot Spot Controversy Active hot spots may not be able to produce flood basalts. New model: The plume head 19 Volcanic Hazards Volcanic Hazards 20 Pyroclastic flows: #1 threat to human life 21 Lahar: volcanic "mudflow" Lahar: mudflow" Volcanic material suspended in water Travel for 100's of 100' miles, up to 90 mph Carry giant boulders in suspension 22 ...
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- Spring '07
- Geology, Volcano, Fissure Eruptions, eruptions, basaltic lavas