Volcanic Activity

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Volcanic Activity

• Volcanoes An opening in the earth's crust
  through which molten lava, ash, and gases are
  ejected.
• All volcanoes are fueled by magma deep beneath
  Earths surface.
• Derived from Vulcan, the Roman god of fire.

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Volcanic Activity

• Magma a mixture of molten rock, suspended
  mineral grains, and dissolved gases deep beneath
  the Earths surface.
• Magma is formed when temperatures are high enough
  to melt the rocks involved.
• Temperature, pressure, and the amount of water
  present all influence whether rock will melt to
  become magma.

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Volcanic Activity

• Composition of magma
• Magma consists of the same elements found in the
  Earths crust oxygen, silicon, aluminum, iron,
  magnesium, calcium, potassium, and sodium.
• Silica (SiO2) is the most abundant compound in
  magma and has the greatest effect on its
  characteristics.

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Volcanic Activity

• Types of magma
• Magma is classified according to the amount of
  silica (SiO2) it contains.
• Basaltic 50 percent silica
• Andesitic 60 percent silica
• Rhyolitic 70 percent silica
• The amount of silica affects the melting
  temperature of magma and also impacts how quickly
  it flows.

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Volcanic Activity

• Characteristics of each type of magma
• Basaltic
• Source material upper mantle
• Viscosity (resistance to flow) low
• Gas content 1-2
• Explosiveness low
• Location of magma both oceanic and continental
  crust
• Example Hawaiian volcanoes, Kilauea and Mauna Loa

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Volcanic Activity

• Andesitic
• Source material Oceanic crust and oceanic
  sediments
• Viscosity intermediate
• Gas content 3-4
• Explosiveness intermediate
• Location of magma continental margins associated
  with subduction zones
• Example Mount St. Helens, Tambora (Indonesia)

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Volcanic Activity

• Rhyolitic
• Source material continental crust
• Viscosity high
• Gas content 4-6
• Explosiveness high
• Location of magma continental crust
• Example dormant volcanoes in Yellowstone
  National Park, Wyoming.

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Volcanic Activity

• Magma intrusion
• Since magma is less dense than the surrounding
  rocks, it wants to move upward and come in
  contact with the crust.
• Sometimes the magma intrudes into the crust above
• The intruding magma can force cracks to open in
  the crust and can fill those cracks

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Volcanic Activity

• Types of magma intrusions
• Plutons
• Intrusive igneous rock bodies formed when magma
  cools
• Batholiths
• The largest plutons
• Irregularly shaped masses of coarse-grained
  igneous rocks that cover at least 100 km2
• Common in the interiors of major mountain chains

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Volcanic Activity

• Stocks
• Irregularly shaped plutons similar to batholiths
  but smaller in size
• Both batholiths and stocks generally form 10-30
  km below the surface
• Laccoliths
• Formed when magma intrudes into parallel rock
  layers close to Earths surface.
• The rocks above bow upward
• Usually only about 10-15 km wide

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Volcanic Activity

• Sills
• A pluton formed when magma intrudes parallel to
  layers of rock.
• Can be from a few centimeters to hundreds of
  meters thick
• Dykes
• A pluton formed when magma cuts across
  preexisting rocks
• Form when magma invades cracks in surrounding
  rock areas

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Volcanic Activity

• Plutons and tectonics
• Many plutons are formed as a result of
  mountain-building processes
• Many mountain chains were formed at
  continental-continental boundaries
• Theory these plate collisions caused crust to be
  pushed down into the upper mantle. The crust
  then melted and intruded into the overlying
  rocks.
• Batholiths may form by the same process at
  oceanic-oceanic boundaries.

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Volcanoes

• Anatomy of a volcano
• Lava erupts through an opening in the crust
  called a vent.
• As lava flows out onto the surface, it cools and
  solidifies around the vent.
• Over time, lava accumulates to form a volcano.

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Volcanoes

• Anatomy of a volcano
• Crater bowl-shaped depression at the top of a
  volcano
• Volcanic craters are usually less than 1km in
  diameter
• Calderas are larger craters up to 50km in
  diameter

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Volcanoes

• Types of volcanoes
• Volcanoes classified according to two criteria
• The type of material that forms the volcano
• The type of eruptions that occur
• Three major types of volcanoes have been
  identified
• Shield volcanoes
• Cinder-cone volcanoes
• Composite volcanoes

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Volcanoes

• Shield volcanoes
• A mountain with broad, gently sloping sides and a
  nearly circular base.
• Formed when layer upon layer of basaltic lava
  accumulates during non-explosive eruptions
• Examples Hawaiian volcanoes

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Volcanoes

• Cinder-cone volcanoes
• Formed when material ejected high into the air
  falls back to Earth and piles up around the vent.
• They have steep sides and are generally small
  less than 500 meters high.
• Tend to be more explosive than shield volcanoes
  due to the higher amounts of water, silica, and
  gases.

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Volcanoes

• Composite volcanoes
• Form when layers of volcanic fragments alternate
  with lava.
• Much larger than cinder-cone volcanoes.
• High amounts of water, silica, and gases.
• Typically very explosive.
• Examples Mount St. Helens, Mount Rainier

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Volcanoes

• Volcanic material
• Tephra
• Rock fragments thrown into the air by a volcanic
  eruption
• Can be cooled lava, mineral grains, or pieces of
  the volcanic cone
• Classified by size as dust, ash, lapilli

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Volcanoes

• Pyroclastic flows
• Rapidly moving volcanic material usually clouds
  of gas, ash, and other tephra
• Can travel at speeds up to 200 km/h and can
  contain hot, poisonous gases
• The temp of pyroclastic flows can exceed 700
  degrees Celsius.

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Volcanoes

• Where volcanoes occur
• Convergent volcanism
• 80 percent of volcanoes are found along
  convergent boundaries.
• Magma forced upward through crust at a subduction
  zone
• Examples The Pacific Ring of Fire (or
  Circum-Pacific Belt) and the Mediterranean Belt

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Volcanoes

• Divergent volcanism
• 15 percent of volcanoes are found along divergent
  boundaries
• Magma forced upward into fracture formed when
  plates separate.
• Most happen underwater at ocean ridges.
• Example Volcanoes on Iceland (Iceland lies
  directly on the Mid-Atlantic Ridge)

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Volcanoes

• Hot spots
• 5 percent of volcanoes are found away from plate
  boundaries
• These volcanoes occur over hot spots unusually
  hot regions of Earths mantle where high
  temperature plumes of mantle material rise toward
  the surface.
• Examples Hawaiian volcanoes