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Volcanoes
Chapter 6
Eruptions
• Volcano – (#34) opening in Earth’s surface
which allows gas & magma to escape
– Magma: (#35) molten rock underground
– Lava: (#36) molten rock at Earth’s surface
• Two types
– Non-explosive
• calm flows of lava such as Mt. Kiluea or sea floor spreading
• most common
• happen over long periods, all the time
– Explosive
• clouds of hot ash, debris, and gas shoot out
• rarer
• sudden and destructive
Volcano Parts
Magma
• Explosiveness of eruption depends on:
– water
– silica in magma
– gases
• Water and CO2 remain dissolved in magma until
pressure released
– Like a can of soda exploding when top opened
• Magma with high silica content very stiff and can
plug vent opening
– Builds pressure by preventing water and other gases
from escaping
Eruption Materials
• Lava (non explosive) – liquid rock
– Aa or blocky: high viscosity, stiff, slow moving, brittle, jagged or blocks
– Pahoehoe or pillow lava: low viscosity, more fluid, faster moving, smooth
More Eruption Materials
• Pyroclastic material
(explosive) –
magma hardens in
the air or shatters
existing rock
–
–
–
–
volcanic bombs
volcanic blocks
lapilli
volcanic ash
• Pyroclastic flows
(#37)
– hot ash, dust, and
gases ejected and
flow rapidly downhill
Effects
• Deaths by blast or pyroclastic flows
• Ecologically damage
• Climate change
– ash in upper atmosphere
– blocks sunlight
– Mt. Tambora in Indonesia in 1815
• 12,000 direct deaths
• 80,000 deaths from hunger and disease
• Food shortages in Europe and N. America next year due to
reduced sunlight and temperature
• Global temperatures dropped .5 degrees C
Types
• Shield
– (#39) Repeated lava flows build up over time from non-explosive
eruptions
– Gently sloping over wide area
– Mauna Kea in Hawaii
• Cinder cone
–
–
–
–
(#40) Pyroclastic material builds up from moderately explosive eruptions
Steep slopes of cinder cones
Not very stable, sometimes on sides of other volcanoes
Paricutin in Mexico
• Composite (aka stratovolcano)
–
–
–
–
Most common type
(#41) Alternating flows of lava and pyroclastic material build up
Broad bases that get steeper towards top
Mt Fuji in Japan, Mt Hood, Mt Ranier, Mt St Helens, Mt Shasta
Other Volcanic Landforms
• Craters
• Calderas (#38)
• Lava plateaus
Locations
• Divergent boundary – mid ocean ridges
– most volcanic activity on Earth
• Convergent boundary – subduction zones
– descending oceanic crust
– melting rock less dense and rises up
– 80% of all land volcanoes at convergent
• Hot spots
– mantle plumes far from plate boundaries
– Hawaiian islands
– Yellowstone