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A volcano is a vent in the earths crust through which hot gas, ash and molten rock flows.

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Subduction Zones

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Magma : molten rock located underground Lava: Molten rock that has reached the surface

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Viscosity is the resistance to flow of a fluid. The viscosity of magma is determined by the amount of silica present.

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High amounts of silica results in thick slow moving magma with high gas content

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Low amounts of silica results in thin, runny magma with low gas content

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BasalticAndesiticRhyolitic Silica ContentLeast (50%)Intermediate (60%) (73%) Gas ContentLeastIntermediateHighest ViscosityLowestIntermediateHighest Type of eruptionRarely explosiveSometimes explosive Explosive Melting Temp.HighestIntermediateLowest Type of volcanoShieldCompositeCinder Cone LocationRifts and oceanic hot spots Subduction Zones Contential Hot Spots

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Shield Volcano Shield volcanoes are commonly found at divergent plate boundaries and oceanic hot spots. Shield volcanoes are large with gently sloped sides. The magma is mainly composed of basaltic magma which has a low viscosity.

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Small steep sided volcano which erupts gas-rich basaltic lava. It is formed when highly viscos magma is thrown into the air from the vent, cooling and breaking into fragments upon hitting the ground.

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Composite volcanoes, sometimes called stratovolcanoes are characterized by large steep sides which are a result of repetitive explosive eruptions of andesitic and rhyolitic lava and ash. This type of volcano is mainly located along subduction zones.

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Explosive volcanoes can produce avalanches of hot gasses, ash, and rock. Pyroclastic flows can reach speed in excess of 100 km/h with temperatures greater than 1000 0 C.

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Ash Diameter less than 2 mm

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Diameter from 2 mm to 64 mm

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Diameter greater than 64 mm

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Melted snow or ice creates a mudflow that rushes down the volcano at speeds in excess of 100 km/h

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Following an eruption, the walls of a volcano are weakened and collapse into the vent and form a large crater shaped basin called a caldera.

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