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Volcanoes Molten rock reaches Earth’s surface Depending on viscosity and temperature, it either flows out or explodes

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Why do volcanoes happen - PowerPoint PPT Presentation


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Volcanoes Molten rock reaches Earth’s surface Depending on viscosity and temperature, it either flows out or explodes Why do volcanoes happen? Subsurface materials heat up for various reasons Liquid rock is less dense than solid, so it rises

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slide1

Volcanoes

Molten rock reaches Earth’s surface

Depending on viscosity and temperature, it either flows out or explodes

why do volcanoes happen
Why do volcanoes happen?
  • Subsurface materials heat up for various reasons
  • Liquid rock is less dense than solid, so it rises
  • Upward force from rising magma and melting from hot rock meeting cold produce gaps in overlying rock
  • When magma reaches surface, it is more dense than air, so it stays and cools
lava types
Lava types
  • Basaltic lava -- very hot, not very viscous
    • Flood basalts - large areas covered by basaltic lava, e.g. Columbia River basalts, Deccan Traps, lunar maria
  • Granitic lava -- colder, more viscous
    • Tends to produce explosive eruptions
basaltic flows
Basaltic flows
  • Pahoehoe (means "ropy") - highly fluid lava which has thin, glassy skin under which hot lava flows
  • Aa - forms after gases have departed and cooling has begun. Skin is big and chunky -- very sharp
basaltic flows7
Basaltic flows
  • Pillow basalt -- evidence of underwater eruptions -- surface chills quickly, but flow continues
  • Bubbles -- or vesicles -- gases exist in lava but stay in solution under pressure under earth
pyroclastic eruptions
Pyroclastic Eruptions
  • Gas is trapped in magma, but magma is too viscous to flow through cracks
  • When pressure is released and gas comes out of magma, whole mountaintop can explode
  • Pyroclasts -- fire rocks
pyroclastic eruptions11
Pyroclastic Eruptions
  • Includes ash and fine material, but can be a lot bigger (one house sized piece traveled 10 km in one eruption)
  • Ash can stay aloft, entering upper atmosphere (e.g. Pinatubo)
  • If particles settle while still hot, they form tuffs -- welded together bits
pyroclastic eruptions12
Pyroclastic Eruptions
  • Pyroclastic flow -- big hazard near continental volcanoes - e.g. Japan, Mont Pelee on Martinique (1902)
  • Pyroclastic flow can be very hard to predict
    • Prof. Landes: "The Montagne Pelee presents no more danger to the inhabitants of Saint Pierre than does Vesuvius to those of Naples" -- died next day in eruption
slide14

Nuee Ardente

Pyroclastic Eruption

eruption styles
Eruption Styles
  • Lava Eruptions -- lava cone built by successive flows from central vent
  • Basalt -- creates shield volcanoes like Mauna Loa - big, broad gentle slopes
  • Rhyolite -- creates small dome in crater, plugs up areas below
  • Pyroclastic eruptions create concave cone with a summit vent
  • Stratovolcano -- alternating lava and pyroclasts (composite volcano) Fujiyama in Japan
  • Resurgent calderas
  • Phreatic explosions -- when magma meats lots of groundwater -- e.g. Krakatoa
slide16

Mount Saint Helens

Washington

Erupted

May 18, 1980

mount saint helens
Mount Saint Helens
  • Stratovolcano - mixture of lava eruptions and explosive ash eruptions
  • 1980 eruption was very explosive
  • Mountain lost its top 400 meters of elevation within minutes
krakatau august 26 1883
KrakatauAugust 26, 1883
  • Phreatic eruption of an entire island (English name is Krakatoa)
  • Loudest noise in recorded history (Heard in Australia 2000 km away)
  • Eruptive force of 100 million tons of TNT
  • 5000 times the force as the first atomic bombs
  • 36000 people drown in Tsunamis
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