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

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

• Associated with plate boundaries and hot spots

• Mt. St Helens- Cascadia subduction

• Hawaii – Pacific plate hot spot

• Iceland – mid-Atlantic ridge

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?)

• Columbia rivers basalt- Oregon, Washington

Pyroclastic material

• Pyro- fire; clastic - fragments

• Tephra

• Blocks

• Bombs

• Cinder

• Ash

• Dust

• Pelee’s hair: fine glass fibers