Types of Volcanoes

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Volcanoes
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Types of Volcanoes

• Composite Volcanoes
• Shield Volcanoes
• Cinder Cones
• Spatter Cones

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Composite Volcanoes

• Composite volcanoes, also called strato
  volcanoes, are formed by alternating layers of
  lava and rock fragments.  This is the reason they
  are called composite.
• Strato-volcanoes often form impressive,
  snow-capped peaks which are often exceeding 2500m
  in height, 1000sq.km in surface, and 400km3 in
  volume.
• Although strato-volcanoes are usually large and
  conical, we can distinguish different shapes of
  them concave (like Agua), pyramidal (like
  Stromboli), convex-concave (like Vesuvius),
  helmet-shaped (like Mount Rainier), collapse
  caldera (like Graciosa), nested (like El Piton in
  Teide), multiple summits (like Shasta), elongated
  along a fissure (like Hekla).

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Shield Volcanoes

• Shield volcanoes are huge in size.  They are
  built by many layers of runny lava flows. Lava
  spills out of a central vent or group of vents. 
  A broad shaped, gently sloping cone is formed. 
  This is caused by the very fluid, basaltic lava
  which can't be piled up into steep mounds
• Shield volcanoes may be produced by hot spots
  which lay far away from the edges of tectonic
  plates.   Shields also occur along the
  mid-oceanic ridge, where sea-floor spreading is
  in progress and along subduction related volcanic
  arcs.
• The eruptions of shield volcanoes are
  characterized by low-explosivity lava-fountaining
  that forms cinder cones and spatter cones at the
  vent.  Famous shield volcanoes can be found for
  example in Hawaii (e.g. Mauna Loa and Kilauea).

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Cinder Cones

• A cinder cone is a steep conical hill formed
  above a vent.  Cinder cones are among the most
  common volcanic landforms found in the world. 
  They aren't famous as their eruptions usually
  don't cause any loss of life.  Cinder cones are
  chiefly formed by Strombolian eruptions.  The
  cones usually grow up in groups and they often
  occur on the flanks of strato volcanoes and
  shield volcanoes.
• Cinder cones are built from lava fragments called
  cinders.  The lava fragments are ejected from a
  single vent and accumulate around the vent when
  they fall back to earth.
• Cinder cones grow rapidly and soon approach their
  maximum size.  They rarely exceed 250m in height
  and 500m in diameter.
• The shape of a cinder cone can be modified during
  its (short) life.  When the position of the vent
  alters, aligned, twin or secant cones develop. 
  Nested,buried or breached cones are formed when
  the power of the eruption varies.
• A great example of a cinder cone is Paricutín in
  Mexico.  It was born in February 20, 1943 in a
  corn field and grew to 300 feet in 5 days.

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Spatter Cones

• When hot erupting lava contains just enough
  explosive gas to prevent the formation of a lava
  flow, but not enough to shatter it into small
  fragments the lava is torn by expanding gases
  into fluid hot clots, ranging in size from 1cm to
  50cm across, called spatter.
• When the spatter falls back to Earth the clots
  weld themselves together and solidify forming
  steep-sided accumulations. These accumulations
  focused on an individual vent are called spatter
  cones.

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Different Types of Volcanic Eruptions
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Impacts and Effects of Volcanoes

• The sudden violence of volcanic eruptions causes
  catastrophe and devastation.  Various notorious
  eruptions of volcanoes in the past, such as Mount
  Pelée, Tambora, Krakatau and Pinatubo have
  demonstrated the devastating impact of volcanic
  activity on nearby landscapes and communities. 
  Many people got killed.  A big number of people
  had to abandon their homes and land forever. 
  Even the whole world's climate was changed for a
  while as a result of an eruption!
• These effects of volcanic eruptions are mostly
  the result of certain hazards.  Volcanoes provide
  different hazards during an eruption.  Each
  hazard poses different risks affecting different
  areas. 

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How Volcanoes Erupt

• An eruption begins when pressure on a magma
  chamber forces magma up through the conduit and
  out the volcano's vents.  When the magma chamber
  has been completely filled, the type of eruption
  partly depends on the amount of gases and silica
  in the magma.  The amount of silica determines
  how sticky (level of viscosity) the magma is and
  water provides the explosive potential of steam.
• Obstacles also influence the type of eruption. 
  When the pipe is blocked by a stopple or an
  accumulation of pumice, the pressure in the pipe
  will build up very high resulting in an
  explosion.
• When magma reaches earth's surface it is called
  lava.  It may pour out in gentle streams called
  lava flows or erupt violently into the air. 
  Rocks ripped loose from the inside of the volcano
  or torn apart by the gas may be shot into the air
  with the lava.  These rocks blown out of a
  volcano are called pyroclastic rocks.  The rock
  fragments fall back to earth in many different
  shapes and sizes
• Dust - particles less than 1/100 inch in diameter
  Dust particles may be carried great distances.
  In a powerful eruption they may be carried around
  the earth several times.
• Ash - fragments less than 1/5 inch in diameter
  Most volcanic ash falls to the surface and
  cemented together by water to form a rock called
  volcanic tuff.
• Bomb - A rounded piece of newly hardened lava
  which takes shape while flying through the air.
• Block - A piece of lava that has sharp corners.
• Cinder - Bubbly rock formed by liquid lava
  cooling in the air.
• Pumice - Cinder so bubbly that it floats in
  water.  
• Volcanic activity is classified by how often a
  volcano erupts.  A volcano may be active,
  intermittent, dormant, or extinct.  Active
  volcanoes erupt constantly.  Intermittent
  volcanoes erupt fairly regularly.  Dormant
  volcanoes are inactive, but not long enough to
  determine whether they will erupt again or not. 
  Extinct volcanoes have been inactive since the
  beginning of recorded history.

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Active Volcanoes in the last 10,000 years