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

Columbia River Basalts, WA & OR

Devil’s Tower, WY

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

Lava Flow, Hawaii

Lava Toe, Hawaii

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

Mont Pelee, West Indies 1902

Nuee Ardente

Pyroclastic 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

Washington

EruptedMay 18, 1980

Mount St. 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

Before the eruption

After the Eruption

Mount Saint Helens Mud Flow

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

Anak Krakatau