Volcanoes Molten rock reaches Earths surface Depending on

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

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