Volcanoes

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Volcanoes
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Learning Targets

• Describe how plate tectonics influences the
  formation of volcanoes
• Locate major zones of volcanism
• Identify the parts of a volcano
• Differentiate between volcanic landforms

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Volcanism

• All the processes associated with the discharge
  of magma, hot fluids, and gases.
• In any given year, 60 volcanic eruptions will
  occur.

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Volcanism

• The majority of volcanoes are found at plate
  boundaries.
• Most are found at convergent boundaries or
  divergent boundaries.

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Convergent Volcanism

• Most volcanoes on land occur because oceanic
  plates subduct under continental plates.
• As the oceanic plate descends, magma forms.
• Eventually the magma moves up because it is less
  dense.
• These volcanoes have explosive eruptions.

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Convergent Volcanism

• Most convergent volcanoes are found in two major
  geographic belts
• Pacific Ring of Fire (Circum-Pacific)
• Mediterranean Belt

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Pacific Ring of Fire
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Mediterranean Volcanic Belt
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Divergent Volcanism

• At divergent boundaries in the ocean, new ocean
  floor is produced as magma fills up the new gaps.
• These volcanoes are rarely explosive.

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Hot Spots

• Some volcanoes form far away from plate
  boundaries.
• These are called hot spot volcanoes.
• It is theorized that hot spots form in unusually
  hot regions of Earths mantle
• Most are formed under the ocean and form islands
  (Hawaii).

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Learning Target

• Describe how plate tectonics influences the
  formation of volcanoes.
• Where are the major zones of volcanism? What is
  significant about their locations?

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Anatomy Of A Volcano
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Anatomy of a Volcano

• Conduit tube structure through which lava
  travels to reach the surface
• Vent an opening in the conduit to allow lava to
  leave the conduit
• Crater a bowl shaped depression formed around
  the vent
• Caldera a larger depression that can form later

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Types of Volcanoes

• Shield Volcano
• Mountain with broad sloping sides and circular
  base
• Largest volcano
• Usually not very explosive

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Types of Volcano

• Cinder Cones
• Steep sides generally small
• Form when small pieces of magma pile up around
  the vent
• Often form around larger volcanoes

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Types of Volcanoes

• Composite Volcanoes
• Formed from hardened chunks of lava from very
  violent eruptions
• Cone shaped but are larger than cinder cones
• Very explosive generally

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Learning Targets

• List the parts of a volcano and then define each.
• Differentiate between volcanic landforms.

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Magma Formation

• Magma can be formed by
• Melting the crust
• Melting in the mantle
• Four main factors are involved
• Temperature
• Pressure
• Water Content
• Mineral content of crust

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Magma Formation

• Temperature increases with depth in Earths crust
• Pressure also increases with depth due to weight
  of overlying rocks
• Water content rocks often have water in them
  which changes melting point of rocks as water
  content increases, melting point decreases

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Magma Formation

• Mineral Content
• Different minerals have different melting points
• Basalt has a high melting point
• Granite is lower
• Generally rocks with high iron and magnesium melt
  at higher temperatures

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Magma Formation

• Viscosity
• The physical property that describes a materials
  resistance to flow
• Cooler magma has a higher viscosity (its
  thicker) it resists flowing

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Formation of Magma
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Types of Magma

• Balsaltic magma
• Forms when rocks in upper mantle melt
• Less than 50 silica
• Low viscosity
• Found in quiet eruptions
• Kilauea in Hawaii

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Types of Magma

• Andesitic magma
• 50-60 silica
• Found along oceanic-continental subduction zones
• Intermediate viscosity
• Intermediate explosivity

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Types of Magma

• Rhyolitic magma
• More than 60 silica
• High viscosity
• Very explosive

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Explosive Eruptions

• When lava is too viscous to flow freely, pressure
  builds up until the volcano explodes. This throws
  lava and rock into the air.
• Tephra is the pieces of lava or pieces of crust
  thrown into the air

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Explosive Eruptions

• Pyroclastic flows
• Tephra that is rapidly moving mixed with hot
  (over 700oC) suffocating gases
• Mt. Pelee in the Carbbean Sea killed 29,000.
• Death caused by suffocation or burned to death

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Learning Targets

• Explain how magma type influences volcanic
  activity.
• Discuss the role of temperature, pressure and
  dissolved gases in eruptions.
• Recognize classifications of material ejected by
  eruptions.

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Mount Rainer, Washington 4392 m.

• Mount Rainer is potentially the most dangerous
  volcano in the Cascades because it is very steep,
  covered in large amounts of ice and snow, and
  near a large population that lives downhill.

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Mount Erebus, Antarctica 3794 m

• -Southernmost historically active glacier
• -Capped by an elliptical 500 x 600 m. wide. 110
  m. deep summit crater with an active lava lake
  continuous activity since 1972
• -Occasional strombolian eruptions eject lava
  bombs onto crater rim

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Devils Tower, Wyoming
http//volcano.und.nodak.edu/vwdocs/kids/legends.h
tml
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Caldera

• A large depression created by the collapse of a
  volcano
• Magma chamber drains
• Loss of structural support
• Collapse of overlying material

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Volcanoes are good!

• Produced our atmosphere
• Volcanic soil very fertile
• Beautiful scenery
• Mineral resources
• -Diamonds, copper, gold, lead, zinc, etc.
• -Pumice, BBQ rocks, Aquarium stones
• Geothermal heat
• Geologic time markers
• Eruptions make for a good story line!

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