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 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 Erupted May 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

Krakatau August 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