Volcanoes

l Evidence that we live in an active planet l The gods of the underworld l Millions of people live near active volcanoes –The greatest geological hazard

Mount Saint Helens- after

Hazard

Future hazard l Population near big, recently active volcanoes –Seattle –Mexico City –Quito –Indonesia –Japan

Lava l Molten rock that flows on the surface l Basaltic Lava –low silica, –fluid, –low gas, –not explosive l Andesitic Lava –high silica –viscous –gas-rich –explosive

Types of volcanoes l 1. Shield Volcanoes –Hawaii, Iceland –Basaltic l 2. Stratovolcanoes (or composite) –Mt. Saint Helens, Pinatubo, etc –Andesitic –Explosive

Shield Volcanoes l Huge broad cones l Gentle slopes l Made from lava flows l Basaltic l Form at hotspots l Not very dangerous l Predictable

Stratovolcanoes (composite) l Andesitic lava l Rich in dissolved gases l Violent eruptions alternate with lava flows l Steep-sided cones l Very dangerous l Unpredictable

Pyroclastic Flows l Product of explosive eruptions l Hot ash shot up into the atmosphere l Ash column collapses l Pyroclastic flow can travel 60 mi/hr l Ash can fall 100’s of miles away l Ash may affect global climate

Mt. Saint Helens before

Phreatic (vapor) eruption

Bulge

Mt. St. Helens Pyroclastic Eruption

After the eruption

Lahars (mudflows)

l Very destructive l Kill as many people as the eruption itself l Loose ash forms mud avalanches l Glaciers on top of volcano melt l Rain

Inside a volcanoe

Pyroclastic eruption

Volcanic landscape: A Caldera (Crater Lake)

Crater lake formation (caldera) l 1. Early eruptions from composite cone l 2. Great eruption empties the magma chamber l 3. Summit collapses l 4. Caldera fills with water

Plate Tectonic Settings of Volcanism

Plate tectonic settings of volcanoes