Chapter 9 - Volcanoes

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Chapter 9 - Volcanoes

• Volcano An opening in the Earths surface
  through which magma and volcanic gases pass.
• Magma Molten rock, gases and water vapor inside
  the Earth.
• Lava Molten rock that flows out onto the
  surface of the Earth.
• Eruption Types
• Nonexplosive The most common
  eruption where huge amounts of lava are released.
• Explosive A much rarer but
  destructive type of eruption with hot debris, ash
  and gases rapidly shooting out from the volcano.
• Ash Dust-sized particles of rock that get
  thrown into the atmosphere.

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ia

• Magma Chamber The underground body of magma
  that feeds the volcano.
• Vent The opening in the Earths surface through
  which magma is released.
• The explosive potential of the volcano depends on
  the silica, water and gas content of the magma.
  The more water, the more explosive due to the
  underground pressure build up of the steam.
  Silica rich magma is thicker and can build up
  more pressure.
• What erupts from a volcano
• Lava Liquid magma that flows out
  through the vent.
• Blocky Flows slowly
  oozes and doesnt travel very far.
• Pahoehoe Flows slowly
  has a glassy surface and resembles dripping
  wax.
• aa Flows quickly forms a
  brittle crust.
• Pillow lava Flows
  quickly forms underwater.
• Pyroclastic Material Magma that has
  been blasted into the air and hardens before
  landing.

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Types of Pyroclastic Material

• Bombs Blobs of magma that cool as they are
  thrown high in the air.
• Lapilli Pebble-sized bits of magma that harden
  before hitting the ground.
• Ash Most of the material ejected during a
  volcanic eruption.
• Blocks House-sized pieces of rock that are
  sheared from the vent throat and thrown into the
  air.
• Pyroclastic Flows Dangerous eruptions in which
  huge amounts of hot ash, dust and gases blast
  from the volcano with temperatures of 1300F and
  speeds of 130 m.p.h.

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Types of Volcanoes

• The effects of a volcanic eruption are not only
  dangerous immediately but the enormous amounts of
  ash and gases ejected into the upper atmosphere
  can change the climate worldwide resulting in
  famine and disease.
• Shield Cones - can be enormous but are usually
  built on layers of lava in non-explosive
  eruptions.
• Cinder Cones smaller and formed from moderately
  explosive eruptions that last a short time and
  are composed of pyroclastic materials such as
  ash.
• Composite Cones one of the most common types
  formed from alternating explosive/non-explosive
  eruptions and lava flows.

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Volcanic Landforms

• Crater A funnel shaped pit at the top of many
  volcanoes.
• Caldera A large depression formed after the
  eruption and much larger than the crater. A
  crater with collapsed walls.
• Lava Plateaus Formed by repeated eruptions with
  massive outpourings of lava spreading out over a
  large area. These are usually formed by rift
  zones (huge cracks in the surface of the Earth).
• Causes of Volcanic Eruptions The rock of the
  mantle is hot enough to melt but the pressure
  prevents the molecules from liquefying. However,
  when the pressure is decreased, magma forms and
  begins to rise because it is less dense than the
  surrounding rock.
• Where Volcanoes form 80 of the worlds volcanoes
  form at convergent plate boundaries and 15 form
  at divergent boundaries.
• The Ring of Fire The area of the Pacific plate
  perimeter where many volcanoes are found.

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Tectonic Plate Boundaries

• Divergent Boundary Areas where tectonic plates
  separate forming rift zones (deep cracks). The
  pressure rapidly decreases allowing the rock to
  melt and form magma which rapidly rises to fill
  in the rift zones.
• Mid-Oceanic Ridges Undersea rift zones that
  produce volcanoes and mountains by producing new
  crust from erupted lava.
• Convergent Boundary An area where tectonic
  plates collide.
• Subduction Zones Areas where one tectonic plate
  is pulled underneath (subducted) under another.
  Oceanic crust will subduct under continental
  crust because it is denser than continental
  crust. This process generates tremendous heat and
  pressure that helps in the formation of magma and
  new volcanoes.

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• Hot Spots Volcanically active places that are
  far from plate boundaries. These usually will
  form volcanic island chains.
• A. Mantle Plumes Areas with rising
  magma that form hot spots.
• B. Cracks in the Earths crust that
  permit magma to rise upward.
• Volcanic Activity
• A. Extinct Volcano has not erupted
  within recorded history and most likely never
  will.
• B. Dormant Volcano is currently not
  erupting but may again.
• C. Active Volcano is currently erupting
  or showing signs of erupting in the near future.
• Eruption Prediction
• 1. Watch for an increase in
  earthquakes.
• 2. Observe the ratio of certain gases.
• 3. Tiltmeters and GPS detect changes in
  the volcano's slope.
• 4. Infrared satellite images watch for
  an increase in temperature.