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Natural Disasters

Natural Disasters. Volcanoes: Materials & Effects. Nature of Volcanic Activity. Volcanic eruptions are caused by magma ( liquid rock, crystals and dissolved gas ) expelled onto the Earth’s surface. Types of Magma. Basalt Dark color SiO 2 – 45-55 wt.% 1,000-1,200 °C. Rhyolite

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Natural Disasters

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  1. Natural Disasters Volcanoes: Materials & Effects

  2. Nature of Volcanic Activity • Volcanic eruptions are caused by magma(liquid rock, crystals and dissolved gas) expelled onto the Earth’s surface

  3. Types of Magma Basalt Dark color SiO2 – 45-55 wt.% 1,000-1,200°C Rhyolite Light color SiO2 – 65-75wt.% 650-800°C Andesite Intermediate color SiO2 – 55-65 wt.% 800-1,000°C

  4. Magma Viscosity • A fluid’s resistance to flow • Depends primarily on: • Composition • Temperature • Composition • ↑ SiO2 ↑ viscosity • Temperature • ↑ T  ↓ viscosity

  5. Magma Summary

  6. Volcanic Behavior • The nature of volcanic eruptions depends on composition; hence, its viscosity • Viscosity is an important property in determining the eruptive behavior of magmas • ↑ viscosity  explosive eruption • ↓ viscosity  quiet eruption

  7. Primary Effects of Volcanic Eruptions Lava Pyroclastic Material Poisonous Gas Emissions

  8. Lava Flows • Aa • Rough, jagged • High Viscosity • Low temperature • Pahoehoe • Ropy, smooth • Low viscosity • High temperature Hawaii Eruption

  9. Pyroclastic Material • Ash falls • Clouds of gas and volcanic ash rise in eruption column • Ash picked up by wind and falls back to surface • Pyroclastic flows • If eruption column collapses, gas and ash rush down flanks of volcanoes at high speeds • Ash become fused and become welded ash

  10. Pyroclastic Material • If gas pressure is directed outward instead of upward, produce a lateral blast or surge – a high-speed ash-rich shock wave

  11. Poisonous Gas Emissions • Carbon Dioxide (CO2) • More dense than oxygen • Lethal if concentration >10% • As magma rises, gas comes out of solution

  12. Secondary & Tertiary Effects Mudflows Debris Avalanches & Flows Atmospheric Effects Famine and Disease

  13. Mudflows • Mudflows, also called lahars, is a mixture of water and sediment, which moves rapidly down slope along existing stream valleys

  14. Debris Avalanche and Flows • Ground-hugging avalanche or flow composed of hot ash, pumice, rock fragments and dust

  15. Atmospheric Effects • Volcanic ash may stay suspended in atmosphere for years • Ash may reflect solar radiation and cause global cooling • Gas, like SO2, also reflects sunlight

  16. Famine and Disease • Ash falls destroy crops and animals (including humans) that feed on them • Decaying organisms may contaminate water and spread disease

  17. Case Histories • Versuvius, (79 A.D.)

  18. Case Histories • Mt. Pelée, Martinique (1902)

  19. Case Histories • Mount St. Helens (1980)

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