The Nature of Volcanic Eruptions

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The Nature of Volcanic Eruptions

• Chapter 10, Section 1

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Factors Affecting Eruptions

• The primary factors that determine whether a
  volcano erupts violently or quietly include magma
  composition, magma temperature, and the amount of
  dissolved gases in the magma
• Viscosity a substances resistance to flow
• Something that is more viscous flows more slowly
  as a lava flow cools, it becomes more viscous as
  the lava slows down
• The more silica there is in a lava, the more
  viscous that lava is
• During explosive eruptions, the gases trapped in
  magma provide the force to eject molten rock from
  the vent, an opening to the surface

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Factors Affecting Eruptions
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Volcanic Material

• The less silica content there is in a basaltic
  lava, the faster it will flow, lava flows are
  common in Hawaii
• Magma contains varied amounts of dissolved gases
  held in the molten rock by confining pressure, it
  only is about 1 to 6 percent of the total weight
  of the magma
• It is important to study the composition of
  volcanic gases, because they are the gases that
  formed the atmosphere in the past
• Pyroclastic material name given to particles
  produced in volcanic eruptions
• The fragments ejected during eruptions range in
  size from very fine dust and volcanic ash (less
  than 2 millimeters) to pieces that weigh several
  tons (blocks and lava bombs)

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Volcanic Material
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Concept Check

• What is a volcanic bomb?
• A large streamlined chunk of pyroclastic material
  that is larger than 64 mm in diameter.

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Anatomy of a Volcano

• The three main volcanic types are shield
  volcanoes, cinder cones, and composite cones
• Volcanic activity often starts with a fissure, or
  crack, develops and magma is forced through it
• Volcano a mountain built by repeated eruptions
  of lava or pyroclastic material often separated
  by long inactive periods
• Crater steep-walled depression located at the
  top of many volcanoes
• The form of the volcano is largely determined by
  the composition of the magma

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Anatomy of a Volcano
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Shield Volcanoes

• Shield Volcano produced by the accumulation of
  fluid basaltic lavas
• Shape broad, slightly domed structure that
  resembles a warriors shield
• Most have grown from the ocean-floor to form
  islands (Hawaii and Iceland)

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Shield Volcanoes
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Cinder Cones

• Cinder Cone built by ejected lava fragments the
  size of cinders, which harden in the air
• Product of relatively gas-rich basaltic magma
• Shape determined by the steep-sided slope that
  loose pyroclastic material maintains as it comes
  to rest
• Usually the product of a single eruption that
  lasts only a few weeks, rarely a few years
• The magma in the pipe solidifies after the
  eruption, and the volcano doesnt erupt again
• Cinder cones are small, 30-300 meters and less
  than 700 meters in height
• There are thousands of cinder cones around the
  world

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Cinder Cones
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Composite Cones

• Composite Cone (Stratovolcano) large, nearly
  symmetrical structure composed of layers of both
  lava and pyroclastic deposits
• The silica-rich magmas typical of composite cones
  generate viscous lavas that can only travel short
  distances
• May generate the most explosive eruptions that
  eject huge quantities of pyroclastic materials
• About 50 of these have erupted in the U.S. in the
  last 200 years
• The most dangerous results of one of these
  eruptions is a pyroclastic flow (hot gases and
  rock fragments), coming at a speed of 200 km/hr
• Lahars destructive mudflow created when
  volcanic debris becomes saturated with water and
  rapidly moves down the volcano, often following
  stream valleys

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Composite Cones
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Concept Check

• What is a lahar?
• A mudflow down the slope of a volcano.

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Profiles of Volcanic Landforms
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Other Volcanic Landforms

• Caldera a large depression in a volcano
• Forms by (1) collapse of the top of a composite
  volcano after an explosive eruption, or (2)
  collapse of the top of a shield volcano after the
  magma chamber is drained
• Most volcanoes are fed magma through conduits,
  called pipes, connecting a magma chamber to the
  surface
• When the volcano has been eroded, especially
  cinder cones, the harder solidified magma will
  remain and become a volcanic neck
• Lava Plateau the greatest volume of volcanic
  material doesnt build volcanoes, but is extruded
  through fissures and flows over a large area

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Formation of Crater Lake, Oregon
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Volcanic Areas in the Northwestern United States
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Assignment

• Read Chapter 10, Section 1 (pg. 280-288)
• Do Section 10.1 Assessment 1-7 (pg. 288)