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TRANSCRIPT
Volcano :
An opening or vent in the
Earth’s crust through which
molten rock, gases, ash, and
pyroclastic materials erupt
A landform or mountain built
from the products of eruption
that develops around the
opening
Magma Composition
The amount of Silica present
Controls Volcanoes!
Effects the character of the
eruption (mild – explosive)
Determines the shape of the
volcanic landform produced
Viscosity : resistance to flow
• Influenced by temperature and
composition of magma
• Composition : % of Silica
Why do Volcanoes Erupt??
1. MAGMA IS BUOYANT
• Molten rock is less dense
than solid rock, so it rises
2. MAGMA CONTAINS GASES
• If pressure is lowered quickly,
the gas is released rapidly,
causing violent eruptions
Dissolved Gases in Magma
Water Vapor
Carbon Dioxide
Sulfur Dioxide
Lesser Amounts of
Carbon Monoxide, Hydrogen Gas,
Methane, Hydrogen Sulfide,
Mercury, Helium , and others
Types of Magma
1. Basaltic Magma :
Mafic – low in silica
Low viscosity – thin and runny
High Temperature
Contains least amount of gases
Mild / “gentle” eruptions
Found most often at rifts and
oceanic hotspots
Ex. Kilauea , Hawaii
2. Andesitic Magma :
Intermediate silica content - @ 60%
Intermediate gas content
Intermediate viscosity
Intermediate temperature
Eruptions – sometimes explosive
Found mostly at subduction
boundaries
Ex. Mount St. Helens (Washington),
Mt. Fuji (Japan), Mt. Rainier
(Washington)
3. Rhyolitic Magma
Felsic – high in silica
High viscosity – thick , resists flow
High gas content – trapped,
pressure builds
Violent eruptions
Lowest temperature
Found mostly where hot spots
underlie continental plates
Ex. Yellowstone
Where do Volcanoes Occur??
** Plate Boundaries and Hot Spots
Hot Spots
Areas of volcanic activity that
result from plumes (rising
streams) of hot materials that
originate deep in the mantle
Hawaii
• Pacific Plate moved Northwest
over a Hot Spot
• Magma breaks through the
crust forming a volcano
• Hot Spot remains in the same
spot
Plate Boundaries
1. Convergent – 2 lithospheric
plates move toward each
other
Denser plate is subducted
2 oceanic plates
1 oceanic and 1 continential
2. Divergent – 2 plates move apart
most along ocean floor
Creates deep rift valleys
Ex. Iceland – Mid Atlantic Ridge
rises above sea level