Volcanoes

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Volcanoes
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What is a volcano?

• A volcano is a mountain that forms when molten
  rock erupts or flows as lava from an opening in
  Earths surface.

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How do volcanoes form?

• A volcano forms when molten rock erupts or flows
  as lava from an opening in earths surface and
  builds up a volcanic cone.
• These openings are called vents.
• Volcanoes release molten rock, ash, and poisonous
  gases. All these products result from melting in
  the mantle or in the crust.

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Where do they occur?

• Like earthquakes, most volcanoes occur at plate
  boundaries.
• Along a mid-ocean rift, where two plates move
  apart, divergent boundary, a ridge of underwater
  volcanoes is formedIcelands volcanoes are part
  of this kind of ridge.
• At convergent boundaries, where an oceanic plate
  collides with a continental platethe Andes
  Mountains in South America.

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Ring of Fire
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• Not all of Earths volcanoes form at the edges of
  the plates.
• Sometimes a volcano forms when a narrow column of
  hot molten rock breaks through the lithosphere in
  the middle of a plate. This narrow column of
  magma is called a hot spot.
• A hot spot does not move. However, the plate
  above it moves forming a chain of volcanoes.

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• The island farthest from the hot spot is the
  oldest.
• The Pacific Plate is moving over the hot spot,
  which remains stationary.
• As the plate moves, the first volcano formed
  dies out and a second volcano forms over the hot
  spot, at the new location on the moving plate.
• This process continues as the plate continues to
  move over the hot spot.

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The Hawaiian Islands were formed by a hot spot
under the Pacific Plate. Today, only the island
of Hawaii, located directly over the hot spot,
has active volcanoes.
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Types of Volcanoes

• The type of eruption determines the type of
  volcano.
• Runny lava spreads far from the vent, building up
  a gentle slope.
• Thicker lava erupts more violently and produces
  more ashes and cinders that pile up to form a
  steep cone-shape.

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Types of lava

• 1. Pahoehoe hot, thin, flowing, and forms ropes
• 2. Aa cooler, much thicker, slow, forms
  jagged, sharp, blocks.

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How magma affects a volcanic eruption.

• There are three main factors that determine
  whether a volcano extrudes magma violently or
  quietly.
• The factors are
• magmas compositionsilica content
• temperature
• amount of dissolved gases it contains.

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Quiet Eruption

• In general, the hot , basaltic magmas flow
  easily and gently. It is dark-colored and
  contains a lot of water. This is rich in iron and
  magnesium and cools to form igneous rocks such as
  basalt. This lava is thin and runny and most
  tends to flow. The islands of Hawaii and Iceland
  were formed by many lava flows.

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Explosive Eruption

• Thicker, cooler magmas that contain more silica
  are more difficult to force through the vent.
  They may even plug up the vent, causing gases to
  collect in bubbles and pockets that increase in
  size and pressure. Eventually that increasing
  pressure ejects the molten rock from the volcano
  in a violent explosion. Ex. Mt. St. Helens 1980

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Composite Volcanoes
Composite volcanoes are made up of alternating
layers of ash, cinders, and lava. Their magma
is rich in silica and therefore thick. Gases are
trapped in the magma, causing eruptions that
alternate between flows and explosive activity
that produces cinders and ash. Composite
volcanoes are typically thousands of meters high,
with steep slopes.
Composite volcanoes switch between quiet
eruptions of flowing lava and violent eruptions
of thick gas-rich lava. This type of volcano
has the most powerful eruptions of all.
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• When Mount St. Helens erupted on 18 May 1980,the
  top
• 1,300 ft. disappeared within minutes.
• The blast area covered an area of more than 150
  sq. miles and sent thousands of tons of ash into
  the upper atmosphere

Mount St. Helens is a stratovolcano.
. Image taken on 10/16/94 from the Space Shuttle.
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Mt. St. Helens

• Mt. St. Helens - before 5/18/80

• Mt. St. Helens - after 5/18/80

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Shield Volcanoes

• Shield Volcanoes look like a warriors shield.
  These broad, slightly dome-shaped volcanoes are
  the worlds largest.
• Volcanoes with broad, gentle slopes and built by
  eruption of fluid basalt lava are called shield
  volcanoes.
• Basalt lava tends to build enormous, low angle
  cones because it flows across the ground easily.
• The largest volcanoes on Earth are Shield
  Volcanoes.

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Cinder Cones

• A cinder cone volcano is built up from ashes,
  cinders, and rocks that burst from Earth during
  violent eruptions.
• The rocks fall back to Earth near the opening.
• At the same time, hot ash covers a larger
  surrounding area.
• Cinder cones erupt for a very short amount of
  time.

Most cinder cones have a bowl-shaped crater at
the summit and rarely rise more than a thousand
feet or so above their surroundings.
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Volcanic Activity

• Volcanoes are rather unpredictable . Some erupt
  regularly, others have not erupted in modern
  history. Scientists classify them as active,
  dormant or extinct.

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Active Volcanoes

• An active volcano is one that erupts wither
  continually or periodically such as Mount Katmai
  in Alaska and Mount St. Helens in the Cascade
  Range.

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Dormant Volcano

• A volcano that has been known to erupt within
  modern times but is now inactive is classified as
  a dormant volcano. Mount Rainier in Washington
  state are example of dormant volcanoes in the
  United States.

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Extinct Volcano

• A volcano not known to have erupted within
  modern history is classified as an extinct
  volcano. They have been worn away almost to the
  level of their magma chamber. Scientists can be
  wrong. Mount St. Helens was considered to be
  dormant but erupted after long periods of
  inactivity.

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Predicting Eruptions

• different from predicting a flood or an
  earthquake in that volcanoes usually shows signs
  of "awakening" before they erupt, giving
  geologists time to monitor the precursory
  events. 

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Predicting Eruptions

• Geologists use sensitive instruments to monitor
  an active volcano to determine if magma is
  approaching the surface.
• Signs that a volcano is about to erupt include
• increasing temperature of hot springs near the
  volcano,
• changes in the shape of the volcano or
  surrounding land,
• an increase in hot gas and ash from the vent,
• and small earthquakes around the volcano.