Download Volcanoes

A volcano is a vent in the earth’s crust through
which hot gas, ash and molten rock flows.
Subduction Zones
Magma: molten rock located underground
Lava: Molten rock that has reached the surface
Viscosity is the resistance to flow of a fluid.
The viscosity of magma is determined by the
amount of silica present.
High amounts of silica results in thick slow
moving magma with high gas content
Low amounts of silica results in thin, runny
magma with low gas content
Basaltic
Andesitic
Rhyolitic
Silica Content
Least (50%)
Intermediate
(60%)
(73%)
Gas Content
Least
Intermediate
Highest
Viscosity
Lowest
Intermediate
Highest
Type of eruption
Rarely explosive
Sometimes
explosive
Explosive
Melting Temp.
Highest
Intermediate
Lowest
Type of volcano
Shield
Composite
Cinder Cone
Location
Rifts and oceanic
hot spots
Subduction
Zones
Contential Hot
Spots
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.
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.
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.
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.
Ash
Diameter less than 2 mm
Diameter from 2 mm to 64 mm
Diameter greater than 64 mm
Melted snow or ice creates a mudflow that rushes
down the volcano at speeds in excess of 100 km/h
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.