Volcanoes

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Volcanoes Volcanic Landforms
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Mini Quiz To Begin Volcanoes

• 1. What is the difference between magma and lava
  ?
• 2. What kind (type) of rocks are formed from the
  cooling of this magma or lava?
  
• 3. Why does magma force its way up to the surface
  of the earth ? (hint what does it want to do?)
  

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Volcano Background

• Volcano a term borrowed from the Romans Vulcan
  was the Roman God of fire that forged metal the
  vent (or chimney) of his furnace was an island
  named Volcano this was a simple attempt by
  people to try to understand the complex processes
  that cause volcanic activity

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Volcano A Definition

• Volcano is the modern term used to describe
  vents in the earths surface through which molten
  rock, heated rock debris, gases and water vapor
  are expelled from beneath the earths surface in
  igneous processes

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Volcanic Materials

• The stuff that results from volcanic activity can
  be in the form of solids, liquids and gases some
  of these materials include
• water vapor (the major gas expelled)
• rock (lava or magma)
• pyroclastic materials

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Pyroclastic Material

• Anything that is ejected from a volcano is termed
  Pyroclastic...
• Pyro fire Clastic particles
• Pyroclastic materials that are ejected and go
  airborne are termed Tephra

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Tephra

• Classified according to size
• ASH less than two millimeters in size
• LAPILLI 2 mm to 64 mm in size
• BLOCKS gt64mm ejected as solid
• BOMBS gt64mm ejected as liquid

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Tephra

• Although pyroclastic materials are born of fire
  (igneous), as they fall to the ground and settle
  they become sedimentary!

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Igneous Rocks

• Igneous rocks fall into two categories
• Intrusive (plutonic) that cool slowly
• and
• Extrusive (volcanic) that cool quicker

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Magma Compositions

• There are three general types of volcanic magma
  compositions
• Basaltic
• Andesitic
• Rhyolitic

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Magma Compositions

• These magmatic compositions are classified
  according to the amount of silica present
• the less silica present, the more fluid the
  magma/lava will be
• the more silica present, the more viscous the
  magma/lava will be

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Magma Compositions

• Magma compositions classified by the percent
  silica present turn out like this
• basaltic 50 silica
• andesitic 60 silica
• rhyolitic 70 silica

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Explositivity of Magmas

• The more silica that is present, the more viscous
  (stickier) that magma is
• Basaltic 50 Si, fluid, not explosive
• Andesitic 60 Si, regular explosions
• Rhyolitic 70 Si, really big bangs !

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Land Shapes via Magma Type

• The magmatic composition not only controls the
  color and explositivity, but it also controls the
  external volcanic land features those magmas
  that are fluid when they erupt tend to run like
  molasses while the really sticky magmas dont
  really erupt at all they just blow up.

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Land Shapes via Magma Type

• Basaltic 50 Si , fluid.
• Basaltic eruptions tend to flow out on the
  earths surface like molasses initially it will
  mound up but will settle out under the influence
  of gravity forming a broad, low shield-like
  volcano called.
• You guessed it - a shield volcano

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Land Shapes via Magma Type

• Andesitic 60 Si, kind of viscous
• Andesitic eruptions are what most people
  typically associate with volcanic eruptions
  there are lava flows, ash clouds, and nuee
  ardentes. Andesitic eruptions can form two
  volcanic landforms
• composite cones and cinder cones

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Land Shapes via Magma Type

• Composite volcanoes are volcanoes built of
  alternating flows of lava and pyroclastic
  material composite cones are larger than cinder
  cones but arent as steep the composition of
  these volcanoes is mainly andesitic most large
  active volcanoes around the world are composite
  volcanoes

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Land Shapes via Magma Type

• Cinder cones are volcanoes that have steep slopes
  (30 degrees) and a large crater at the top
  these volcanoes are composed entirely of
  fragments of volcanic rock and material these
  fragments are generally basaltic or andesitic in
  composition

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Land Shapes via Magma Type

• Rhyolitic 70 Si, very viscous!
• Rhyolitic eruptions generally dont produce any
  distinctive landform at all the stuff is so
  viscous (sticky) that it pressurizes until it
  just blows up creating a big hole in the ground
  all that remains are usually basins that are
  hotbeds of volcanic activity

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So, where are these landforms?

• The low-slung broad landforms that characterize
  basaltic shield volcanoes are nicely demonstrated
  by some well-known island chains
• Hawaiian Islands
• Galapagos Islands

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So, where are these landforms?

• The steep sloped sides of the cinder cone
  volcanoes can be found throughout Central and
  South America and many other places world-wide
  just take, for example, Paracutin in Mexico...

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So, where are these landforms?

• The classic volcano shape that we all think of is
  demonstrated well by Mt Ranier in Washington
  State, or the most photographed volcano in the
  world Mt Fuji in Japan

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So, where are these landforms?

• Rhyolitic landforms are noted by their absence
  there is usually nothing left after a rhyolitic
  eruption but a hole in the ground that is active
  with geothermal energyjust like Iceland or
  Yellowstone National Park...