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Volcanoes
Volcano Vocabulary
– Magma: molten (liquidy) rock, underground
– Lava: Liquid rock that has escaped out of the ground, it is
above ground
– Crater: Circular hole near top of volcano, where Lava comes
out
– Tephra: Rock and hardened lava that pops/explodes out of
volcano
– Pyroclastic Flow: avalanche of hot rocks that flows on a
cushion of intensely hot gasses. Travels up to 200 km/hr.
– Viscosity: Resistance to flow
High Viscosity = Thick, Slow flow
Low Viscosity = Thin, Fast flow
Why Volcanoes Erupt
-Pressure builds as gases in Magma
try to escape.
-Enough pressure and Volcano erupts
3 Types of Lava
–Basaltic: Low Silica, Low Viscosity, thin, flows easily.
–Andestic: Medium Silica, Medium Viscosity.
–Rhyolitic: High Silica, High Viscosity, thick, traps
gasses creating violent eruptions.
Shield Volcanoes
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Basaltic lava – Lava oozes out – Gentle Eruption
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Buildup of basaltic layers forms wide flat volcano
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Layers of Cooled Lava
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Ex.: Mauna Loa, HI
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Found at weak spots in crust called “Hot Spots”
Shield Volcano
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Cinder Cone Volcanoes
 Andesitic Lava
 Builds pressure and Lava pops out
 Lava cools in air making Tephra
 Piles of Tephra, Short with steep sides.
 Found At Fault lines like in Nevada and Arizona
Tephra Pictures
Cinder Cone Volcano
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Composite Volcanoes
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Rhyolitic Lava – High Pressure
Forms giant Mountain-like Volcanoes
Alternates between violent and non-violent
eruptions:
When high pressure = violent eruptions
But, when the pressure is released, the next eruption
will be non – violent (lava flow).
• Has layers of tephra covered by lava
• Occurs at Subduction Zones
• Also known as a Stratovolcano
– Ex. Mt. St. Helens, Soufriere Hills
Composite Volcano
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