Volcanoes Silica (SiO2) and water content controls viscosity of magma



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Volcanoes

  • Silica (SiO2) and water content controls viscosity of magma.

  • In order of decreasing silica:

  • Rhyolite (most viscous)- lava domes

  • Andesite

  • Basalt (most fluid)- lava flows




Volcanoes




Magma composition

  • Mafic magma – rich in Fe, Mg – low silica, low viscosity. Not explosive. Effusive.

  • Felsic composition – rich in Silica – high viscosity – explosive.

  • Trapped gases (steam 80%, CO2 10%, nitrogen) in magma



Volcano types




  • 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)




Continental flood basalts

  • Form over hot spots from mantle

  • India: Deccan Traps: ½ million sq. miles

  • Same age as dinosaur extinction (any connection?)

Pyroclastic material

  • Pyro- fire; clastic - fragments

  • Tephra

  • Blocks

  • Bombs

  • Cinder

  • Ash

  • Dust

  • Pelee’s hair: fine glass fibers




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