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Types of Volcanoes
Chapter 11
Section 2
Types of Eruptions:
• Quiet
• Explosive
• Determined by:
– Amount of water vapor/trapped gases in the
magma.
– Amount of silica (silicon and oxygen) in the
magma.
Quiet Eruptions
• Contain basaltic magmas
– Low in silica
– Flows very easily
• Contain gases that are easily released due
to the high viscosity (ability to flow) of the
magma.
Quiet Eruptions
• Pahoehoe
– Lava cooled in a rope-like structure
• Aa
– Forms at lower temperatures than pahoehoe
• Pillow lava
– Igneous rock structures shaped like baloons,
tubes, or pillows.
Quiet Eruptions
• Examples:
– Hawaii
– Rift zones (Iceland)
Explosive Eruptions
• Contain granitic magma
– High in silica
– Thick
– Low in viscosity (ability to flow)
– Contains many trapped gases
• May also contain andesitic magma
– Intermediate silica content
Explosive Eruptions
• Examples
– Convergent boundaries
Forms of Volcanoes
• Dependent upon type of magma and type
of eruption.
• 3 basic forms:
– Shield volcanoes
– Cinder cone volcanoes
– Composite volcanoes
Shield Volcano
• Broad volcano with gently sloping sides.
• Result of basaltic magma and quiet
eruptions.
• Example: Hawaii
Flood Basalts
• Occur when basaltic magma flows onto the
surface of the earth through large cracks called
fissures.
– When the magma cools, it covers large areas with
thick igneous rock.
– Accounts for largest volume of erupted volcainc
material on Earth.
• Examples:
– Columbia Plateau
– New ocean floor
Cinder Cone Volcano
• Form when tephra fall to the ground and
form a loosely-packed steep-sided
structure.
– Tephra consist of bits of rock or solidified lava
drpped from the air.
• Includes volcanic ash, cinders, and larger rocks
called bombs and blocks
• Example:
– Paricutin – Feb 20th, 1943
Types of Tephra
• Volcanic Ash
– Tephra particles less than 2 mm in
diameter.
• Volcanic Dust
– Particles less than 0.25 mm in
diameter.
• Lapilli
– Larger tephra particles, less than 64
mm in diameter.
• Generally fall near the vent.
Types of Tephra
• Volcanic Bombs
– Tephra particles with round or spindle shape.
• Volcanic Blocks
– Solid rocks blasted from a fissure.
Composite Volcano
• Formed from alternating quiet and explosive
eruptions, depending on the amount of trapped
gases and silica content at the time of eruption.
• Result is alternating layers of tephra and lava.
• Examples:
– Convergent boundaries – Mount St. Helen’s, Mount
Raineer
Krakatau
• Island in the Sunda Straits near
Indonesia.
• Erupted in August of 1883, causing the
“island” to collapse into the empty
magma chamber (caldera-forming).
• More than 36,000 people were killed
(mainly by a tsunami)
• Global temps dropped as much as 1.8
degrees Celsius for five years.
Effects of Eruptions
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Lava flows
Collapse of buildings from ash fall
Air pollution (lung disease/stress)
Acid rain
Pyroclastic flows
– Huge, hot, fast rush of ash and debris
down the mountain (can result in fires)
• Permanent/Temporary Evacuation