VOLCANOES

What is a Volcano?A volcano is a vent in the earth’s crust

through which hot gas, ash and molten rock flows.

How do Volcanoes Form?Subduction Zones

Hot Spots

Magma/Lava

Magma: molten rock located undergroundLava: Molten rock that has reached the

surface

Viscosity

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

amount of silica present.

High Viscosity Magma High amounts of silica results in thick slow

moving magma with high gas content

Low viscosity Magma Low amounts of silica results in thin, runny

magma with low gas content

Types and Characteristics of Magma

Basaltic Andesitic RhyoliticSilica Content Least (50%) Intermediate

(60%)(73%)

Gas Content Least Intermediate HighestViscosity Lowest Intermediate HighestType of

eruptionRarely

explosiveSometimes explosive

Explosive

Melting Temp. Highest Intermediate LowestType of volcano

Shield Composite Cinder Cone

Location Rifts and oceanic hot

spots

Subduction Zones

Contential Hot Spots

Types of Volcanoes

Shield VolcanoShield 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.

Shield Volcano

Cinder Cone

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.

Cinder Cone

Composite Volcano

Composite volcanoes, sometimes called stratovolcanoes are characterized bylarge 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.

Composite Volcano

Pyroclastic Flow

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 10000 C.

Classification of Pyroclastic Material

AshDiameter less than 2 mm

LapilliDiameter from 2 mm to 64 mm

Lava Block or BombDiameter greater than 64 mm

Pyroclastic Flow

Lahars

Melted snow or ice creates a mudflow that rushes down the volcano at speeds in excess

of 100 km/h

Lahar

Caldera

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.

Caldera

Crater Lake