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Transcript 
Volcanoes
Volcanoes



Evidence that we live in an active planet
The gods of the underworld
Millions of people live near active volcanoes
– The greatest geological hazard
Mount Saint Helens- after
Hazard
Year
1991
1985
1902
1883
1815
1792
Volcano
Pinatubo
Ruiz
Pelee
Krakatau
Tambora
Unzen
Country
Philippines
Colombia
Martinique
Indonesia
Indonesia
Japan
Deaths
300 (200,000 evaq.)
23,000
28,000
36,000
92,000
14,500
Future hazard

Population near big, recently active volcanoes
– Seattle
– Mexico City
– Quito
– Indonesia
– Japan
Lava



Molten rock that flows on the surface
Basaltic Lava
– low silica,
– fluid,
– low gas,
– not explosive
Andesitic Lava
– high silica
– viscous
– gas-rich
– explosive
Types of volcanoes


1. Shield Volcanoes
– Hawaii, Iceland
– Basaltic
2. Stratovolcanoes (or composite)
– Mt. Saint Helens, Pinatubo, etc
– Andesitic
– Explosive
Shield Volcanoes







Huge broad cones
Gentle slopes
Made from lava flows
Basaltic
Form at hotspots
Not very dangerous
Predictable
Stratovolcanoes
(composite)






Andesitic lava
Rich in dissolved gases
Violent eruptions alternate with lava flows
Steep-sided cones
Very dangerous
Unpredictable
Pyroclastic Flows






Product of explosive eruptions
Hot ash shot up into the atmosphere
Ash column collapses
Pyroclastic flow can travel 60 mi/hr
Ash can fall 100’s of miles away
Ash may affect global climate
Mt. Saint Helens before
Phreatic (vapor) eruption
Bulge
Mt. St. Helens
Pyroclastic
Eruption
After the eruption
Lahars (mudflows)
Lahars (mudflows)





Very destructive
Kill as many people as the eruption itself
Loose ash forms mud avalanches
Glaciers on top of volcano melt
Rain
Inside a volcanoe
Pyroclastic eruption
Volcanic landscape:
A Caldera (Crater Lake)
Crater lake formation
(caldera)




1. Early eruptions from composite cone
2. Great eruption empties the magma chamber
3. Summit collapses
4. Caldera fills with water
Plate Tectonic Settings of Volcanism
Plate tectonic settings of volcanoes
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