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Volcanoes Molten rock reaches Earth’s surface

This article explores why volcanoes occur and the different types of lava and eruptions. Learn about basaltic and granitic lava, pyroclastic eruptions, and the destructive power of volcanoes like Mount St. Helens and Krakatoa.

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Volcanoes Molten rock reaches Earth’s surface

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

  2. 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

  3. 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

  4. Columbia River Basalts, WA & OR

  5. Devil’s Tower, WY

  6. 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

  7. 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

  8. Lava Flow, Hawaii

  9. Lava Toe, Hawaii

  10. 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

  11. 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

  12. 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

  13. Mont Pelee, West Indies 1902

  14. Nuee Ardente Pyroclastic Eruption

  15. 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

  16. Mount Saint Helens Washington Erupted May 18, 1980

  17. Mount St. Helens

  18. 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

  19. Before the eruption

  20. After the Eruption

  21. Mount Saint Helens Mud Flow

  22. 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

  23. Anak Krakatau

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