Unit Seven Notes

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Volcanoes
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Introduction to volcanoes

• Volcano
• an opening in the earths surface through which
  lava, hot gases, and rock fragments erupt

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Origin of Volcanoes

• Magma 50-100 miles below the earths surface
  slowly begins to rise to the surface
• As the magma rises it melts gaps in the
  surrounding rock
• As more magma rises a large reservoir forms as
  close as 2 miles below the surface (magma chamber)

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Origin of Volcanoes

• Pressure from the surrounding rock causes the
  magma to blast or melt a conduit (channel) to the
  surface where magma erupts onto the surface
  through a vent (opening)

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Origin of Volcanoes

• The magma, now called lava, builds up at the vent
  forming a volcano

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Origin of Volcanoes

• 6. Often the volcano sides will be higher than
  the vent forming a depression called a crater

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• Crater

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• Caldera
• an unusually large crater or the remains when
  the cone collapses into its own magma chamber

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The Birth of a volcano
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Anatomy of a Volcano

• Cone or volcano structure
• the above ground structure built from lava
  and/or tephra

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• Conduit (also known as the Pipe)
• the path that magma takes from the magma chamber
  to the vent

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• Magma Chamber
• the reservoir located under the volcano where
  magma collects and becomes the supply of
  magma/lava to build the volcano

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• Parasitic Cone or Side Vent
• a smaller secondary volcano built on the side of
  or near the main volcano, but sharing the same
  conduit to the magma chamber

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• Fumarole
• a secondary vent that emits only gases

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• Fissure
• a long fissure (crack) from which lava flows

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• Vent opening of the volcano, through which lava,
  ash and gases flow

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Take a minute to label the parts on the diagram
(not all parts are shown)
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Ash Cloud/Gases
Vent
Parasitic Cone or Side Vent
Lava Flow
Conduit or Pipe
mantle
Magma chamber
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Mafic/Basaltic Magma

• Contains low gas/water content.
• Contains low amounts of silica (combination of
  silicon and oxygen)
• Has low viscosity (which means it flows easily).
• High temperature.
• Creates basalt, pumice, and pillow rock.
• Creates effusive (quiet) eruptions.

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• There are 3 types of lava that is produced when
  Mafic/Basaltic Magma reaches the surface

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• Pahoehoe lava
• Hot, thin, fast flowing
• harden with a relatively smooth surface
• Often has a ropy or wrinkled appearance

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• Pahoehoe lava

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• Aa lava
• Cooler, thicker, slow moving
• Hardens with a rough, jagged, sharp edge surface

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• Pillow Lava
• Lava suddenly cooled by water
• shows sack-like segments (stuffed pillows)

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Can you identify the kinds of lava from the
pictures? Circle your choice.
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1
AA
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2
Pahoehoe
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3
Pillow
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4
AA PahoeHoe
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Felsic/Rhyolitic Magma

• Contains extremely high gas/water content
• High silica content
• High Viscosity (very resistant to flow like
  honey)
• Cools to form Rhyolite and Granite rock.
• (Light- Colored)
• Can create very explosive eruptions!!!
• Found more with continental crust

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Intermediate Magma

• This type of magma contains BOTH Mafic and Felsic
  properties of magma.
• Two Types of Intermediate
• Magma
• Andesitic Magma creates fairly larger
  explosive eruptions than Mafic Magma
• Dacitic Magma creates very explosive eruptions,
  but not quite as large as Felsic Magma.
• Order of Explosivity
• Mafic-gtAndesitic-gtDacitic-gtFelsic

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• Tephra (pyroclastic, rock fragments)
• Volcanic Dust Smallest particles and carried
  by atmosphere circulation

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• Volcanic Ash
• 0.25-0.5 cm diameter
• Generally settles out within miles of the cone
  but can be carried greater distances by stronger
  winds.
• Forms a mudflow when mixed with water

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• Bomb
• Smaller bombs (gravel, pea size) are called
  cinders.
• Walnut size bombs are called lapilli.
• Larger fragments up to 4 feet in diameter are
  called bombs.

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• Lahar (mudflow)
• mixture of ash, eroded land, and water flowing
  down river valleys

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• Lahar (mudflow)

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• Gases
• water vapor, carbon dioxide, nitrogen, sulfur
  dioxide, hydrogen sulfide, chlorine

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Locations of Volcanoes

• Divergent Boundaries
• As the plate move long cracks (rifts) form and
  lava builds up forming
• volcanoes.

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• If the boundary is on the ocean floor, volcanoes
  can grow tall enough to break the surface of the
  ocean and become islands (Iceland)

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• Convergent Boundaries
• Places where plates are moving toward each other
  forming a subduction zone.
• One plate melts under the other and the magma
  moves upward to form volcanoes.

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• Example Pacific Ring of Fire

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• Example Cascade Volcanoes

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Hot Spots

• Magma that may originate in the mantle or outer
  core will move upward, breaking the surface and
  forming a volcano, they are independent of plate
  boundaries and a chain of volcanoes may form as
  the plate moves across a hot spot.

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Hot Spots

• (Examples Hawaiian Islands and Yellowstone
  National Park)

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Oceanic Hotspots Formation of Hawaiian Islands
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Types of Volcanic Eruptions

• Two factors determine the type of eruption
• Amount of water vapor other gases in the magma
• The chemical composition of the magma (how much
  silica and water it contains)
• The higher the silica the more eruptive it
  becomes.

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Explosive Eruptions

• Trapped gases under high pressure will violently
  explode when the magma reaches the lower pressure
  of the surface.
• Has granitic magma is very thick and plugs the
  vent causing the pressure to build until it blows
  violently out the vent
• The high water content of the magma produces more
  water vapor which when mixed in granitic magma
  produces explosive eruptions

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Explosive Eruptions
Mt. Pinatubo
Montserrat
Mt. St. Helens
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Quiet (Effusive) Eruptions

• Low pressure gas
• Has basaltic magma (is more fluid and will flow
  instead of explode)
• And has low water content
• Low Silica

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A Typical Hawaiian Eruption
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Types of Volcano Mountains

• Cinder Cones
• Small base, steep-sided, loosely consolidated
• Up to 1000 feet tall
• Life span of a few years
• Commonly built from gravel size lava rock
  fragments call cinders
• Has violent eruptions, dangerous when close.

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Cinder Cone Volcanoes
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Types of Volcano Mountains

• Cinder Cones

Cinder Cones
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• High pressure gas bubbles causes thick lava to
  explode into the air, lava begins to cool as it
  rises and falls becoming very sticky
• When lava hits the ground it sticks rather than
  flows
• This builds a steep cone with a small base

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Types of Volcano Mountains

• Shield Volcanoes
• Large base, gentle slope, lava rock layers
• A few miles high
• Life span of a million years or more
• The lava is hot, thin, very fluid, often
  basaltic.
• Example Hawaiian Islands

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Shield Volcanoes
Take a look at these examples http//www.volcano.
si.edu/world/tpgallery.cfm?categoryShield20Volca
noes
The Mauna Loa volcano in Hawaiithe largest
volcano on Earthhas the broad expanse
characteristic of shield volcanoes. It spreads
across half the island of Hawaii.
Shield volcano on Mars Taken from space
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• Shield Volcanoes

Mauna Kea
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Types of Volcano Mountains

• Composite or Stratovolcanoes
• Large mountain volcano often snow capped, a few
  miles high
• Life span of million years or more
• Have alternating eruptions of tephra (air-borne)
  and lava. The tephra adds height to the volcano
  and the lava cements the tephra together and adds
  to the base.
• Found mostly in subduction zones and have violent
  eruptions.
• Examples Mt Rainier, Mt Fuji, Mt Kilimanjaro

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• Composite or Stratovolcanoes

Mt. Fuji
Mt. Rainier
Mt. Kilimanjaro
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Sketch a composite Volcano
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Composite/Stratovolcanoes
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Volcano Activity Levels (Stages)

• Active (awake)
• Has erupted within recent time and can erupt
  again at any time.
• Pre-eruption activities
• Increase in earthquake activity under the cone
• increase in temperature of cone,
• melting of ice/snow in the crater
• swelling of the cone
• steam eruptions
• minor ash eruptions

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Mt St. Helens
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• Dormant (sleeping)
• No eruption within recent times, but there is
  record of past eruptions
• Can become active and erupt again after a wake
  up period
• Example Mt. Rainier

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• Extinct
• No eruption within recorded history
• Not expected to ever erupt again
• Example Mount Mazama (Crater Lake)

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Crater Lake
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• Mount Rainier
• The most dangerous volcano in the US
• The danger is mostly from lahars traveling down
  river valleys at a speed of 25mph and destroying
  everything in its path
• 100,000 people live on the solidified mudflows of
  previous eruptions

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• Mount Rainier
• The mountain is dangerously unstable, a tall,
  steep heap of loose rock held together by the
  force of gravity and a cubic mile of glacier ice
  that could be melted or shaken loose
• Lahar flows average every 500 years and have gone
  as far as the Puget Sound lowlands (1 in 7 chance
  of it happening during your lifetime)
• Mount Rainier has erupted 4 times in the last
  4000 years with the last eruption 200 years ago

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What about the most dreaded type of volcano???
The SUPERVOLCANO!!!
http//www.youtube.com/watch?vVR1bg_Yf0T4
http//www.youtube.com/watch?v4ggNyQozurw