Volcanoes Molten rock reaches Earth’s surface
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Transcript of Volcanoes Molten rock reaches Earth’s surface
Volcanoes
Molten rock reaches Earth’s surfaceDepending 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
• 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
• 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
• 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 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
• 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 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 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
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
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
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