Download Volcanic rocks

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Volcanoes
Silica (SiO2) and water content controls viscosity of magma.
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In order of decreasing silica:
Rhyolite (most viscous)- lava domes
Andesite
Basalt (most fluid)- lava flows
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Volcanoes
Associated with plate boundaries and hot spots
Mt. St Helens- Cascadia subduction
Hawaii – Pacific plate hot spot
Iceland – mid-Atlantic ridge
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Magma composition
Mafic magma – rich in Fe, Mg – low silica, low viscosity. Not explosive. Effusive.
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Felsic composition – rich in Silica – high viscosity – explosive.
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Trapped gases (steam 80%, CO2 10%, nitrogen) in magma
Volcano types
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Shield volcanoes- low angle slopes e.g Hawaii
Strato volcanoes- high angle slopes – Mt. St. Helens
Caldera – collapsed crater
Lava dome – high viscosity dome
Cinder cone – small volcano of pyroclastic material (volcanic fragments)
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Continental flood basalts
Form over hot spots from mantle
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India: Deccan Traps: ½ million sq. miles
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Same age as dinosaur extinction (any connection?)
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Columbia rivers basalt- Oregon, Washington
Pyroclastic material
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Pyro- fire; clastic - fragments
Tephra
Blocks
Bombs
Cinder
Ash
Dust
Pelee’s hair: fine glass fibers
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