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Volcanoes

Volcanoes. A Hot Topic. What is a volcano?. A mountain formed by lava and/or pyroclastic material. Nature of Volcanoes. Explosive vs. Effusive. (Erupts violently). (Quiet eruptions). Nature of Eruption Controlled by Viscosity. A material’s resistance to flow.

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Volcanoes

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  1. Volcanoes A Hot Topic

  2. What is a volcano? • A mountain formed by lava and/or pyroclastic material

  3. Nature of Volcanoes Explosive vs. Effusive (Erupts violently) (Quiet eruptions)

  4. Nature of Eruption Controlled by Viscosity • A material’s resistance to flow • Factors controlling viscosity: • Composition • Temperature • Dissolved gases

  5. Factors Controlling Viscosity • Composition • ↑ SiO2; ↑ Viscosity • ↓ SiO2;↓ Viscosity • Temperature • ↑ temperature; ↓ Viscosity • Dissolved Gases • ↓ pressure; Gases come out of solution • How easily gases escape determine eruptive style

  6. Factors Controlling Viscosity Andesite Rhyolite Basalt

  7. What is Extruded? • Lava • Dissolved Gases • Pyroclastic Material

  8. Types of Lava • Aa • Rough, jagged surface • Higher viscosity • Lower temperature • Pahoehoe • Smooth, ropy surface • Lower viscosity • Higher temperature

  9. Dissolved Gases - Volatiles • H2O • steam • CO2 • Greenhouse gas • SO2 • “Rotten egg” smell • HCl • Acid rain

  10. Pyroclastic Material • Ground-hugging avalanche that rushes down the side of a volcano • Material consists of: • Hot ash • Pumice • Rock fragments • Dust

  11. Volcano Anatomy • Vent • Opening near surface • Crater • Steep-walled depression • Conduit • Narrow pipe • Magma Chamber • Stored magma

  12. Types of Volcanoes • Mauna Loa – the largest volcano in Hawaii • Shield Volcanoes • Composite Cone (Stratovolcanoes) • Compare Mauna Loa with Mt. Rainier • Cinder Cone • May be found on the flanks of shield and composite cones

  13. Shield Volcanoes • Broad, gently sloping - “Warrior’s Shield” • Covers large area • Erupts mainly basalt in a relatively quiet eruption

  14. Mauna Loa, Hawaii Shield Volcano Hawaiian Islands

  15. Cinder Cones • Cone built by cinders – lava fragments • Explosive, but small • One eruptive event

  16. Cinder Cones California Cima Volcanic Field Amboy Crater

  17. Composite (Stratovolcano) • Associated with subduction zones • Alternating layers of lava and pyroclastic material • Large, classic-shaped • Violent eruptions

  18. Composite VolcanoCascade Ranges Mount Lassen, California

  19. Deadly VolcanoesNuée Ardentes • Pyroclastic flow of hot ash and gas • Also known as “glowing avalanche” • Move at speeds up to 200 km/hr Montserrat, 1996

  20. Deadly VolcanoesLateral Blasts • An explosion of rock, ash, and gas released at side of volcano • May exceed speed of sound St. Pierre, 1902; 29,000 killed

  21. Deadly VolcanoesLahars • Volcanic mudflow • Causes: • Consist of water, rock, ash, and mud • May travel >80 km distance at 35-60 km/hr Nevado del Ruiz, Colombia In 1985, 23,000 people died • Heat melts snow and ice • Rainfall eroding loose volcanic debris • Landslides of water-saturated debris

  22. before During After Mount St. Helens, Washington • Lateral Blast

  23. Mount St. Helens, Washington • Lateral Blast • Pyroclastic flow • Lahar

  24. Predicting Eruptions • Seismicity • Ground shaking • Geomorphology • Changes in ground surface shape • Geophysical properties • Electrical conductivity, gravity anomaly • Gas content • ↑SO2

  25. Mount Pinatubo, PhilippinesSuccessful Prediction • Erupted in 1991 • Prediction saved 1,000’s of lives • Cloud circled entire equator • Global temperatures decreased

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