Volcanoes and Volcanism

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Volcanoes and Volcanism
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Introduction
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• volcano - from the little island of Vulcano in
  the Mediterranean Sea off the coast of Sicily.
• centuries ago the people believed that Vulcano
  was the chimney of the forge of Vulcan - the
  blacksmith of the Roman gods
• volcanoes are mountains but not created by
  folding or faulting or erosion
• volcanoes are built by the accumulations of their
  own eruptive products - lava, ashflows, and
  tephra (airborne ash and dust)
• a volcano is most commonly a conical hill or
  mountain built around a vent that connects with
  reservoirs of molten rock below the surface of
  the earth

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• driven by buoyancy and gas pressure, the molten
  rock (called magma), which is less dense
  (lighter) than the surrounding solid rock, forces
  its way upward
• ultimately breaks through zones of weaknesses in
  the Earths crust and erupts onto the earths
  surface
• once magma is erupted onto the Earths surface it
  is called lava

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• as the rising magma nears the Earths surface,
  pressure decreases, which causes the gases that
  are dissolved in the magma to expand
• depending on the amount and nature of gases in
  the magma, the molten rock may pour from the vent
  as a non-explosive lava flow or it may shoot
  violently into the air as dense clouds of lava
  fragments
• larger fragments fall back around the vent and
  accumulate, giving the volcano its shape
• finer ash particles may be injected miles into
  the atmosphere and can be carried many times
  around the world by stratospheric winds before
  settling

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Types of Volcanoes
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Volcanoes can be categorized based on what sorts
of features they are or how they are created. The
following list is a combination of these two.
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• 1. Calderas
• the largest and most explosive volcanic eruptions
• eject tens to hundreds of cubic kilometers of
  magma onto the Earths surface
• when such a large volume of magma is removed for
  beneath a volcano, the ground subsides or
  collapses into the emptied space, to form a huge
  depression called a caldera

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http//www.btinternet.com/sa_sa/amsterdam/images/
caldera.jpg
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Amsterdam and St Paul Islands are located in the
southernmost Indian Ocean at 3750' South and
7735' East, and are amongst the most isolated in
the world.
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• 2. Cinder Cones
• built from particles and blobs of congealed lava
  ejected from a single vent
• as the gas-charged lava is blown violently into
  the air, it breaks into small fragments that
  solidify and fall as cinders around the vent to
  form a circular or oval cone

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http//earthsci.org/teacher/basicgeol/igneous/cind
ercone.gif
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A Cinder Cone Volcano
http//volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/3
0424305-084_large.JPG
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• 3. Shield Volcanoes
• shield volcanoes are the largest
• the Hawaiian volcanoes are shield volcanoes
• made from the build up of success lava flows
  eruptions are not explosive
• because of this they are not steep, their slopes
  are very gradual

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http//volcanoes.usgs.gov/Products/Pglossary/Shiel
dVolcano.html
Shield Volcano - Mauna Loa Volcano, Hawaii
http//mac.usgs.gov/mac/visitors/pictures/volc1.jp
g
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• 4. Composite Volcanoes
• composite (stratovolcanoes) volcanoes are the
  most common (60) of the Earths volcanoes
• typically symmetrical with steep sided
• built from alternating layers of lava and cinders
• can be very explosive
• most have a crater at the top which contains a
  central vent that the magma flows up
• lava flows through breaks in the crater wall or
  issue from fissures on the flanks of the cone
• Mount Fuji in Japan and Mount St. Helens are
  composite volcanoes

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A cross section of a composite volcano
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• 5. Lava Plateaus
• do not form mountains
• lava flows very slowly, often from long fissures
  instead of central vents
• it can flood the surrounding countryside with
  lava flow upon lava flow
• forms broad plateaus
• Iceland is a good example

http//vulcan.wr.usgs.gov/Imgs/Gif/ColumbiaPlateau
/Maps/map_columbia_river_flood_basalts.gif
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http//englishriverwebsite.com/LewisClarkColumbiaR
iver/Images/wallula_gap_basalts_right_bank_2003_me
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http//www.gly.fsu.edu/salters/GLY1000/9Volcanoes
/Slide17.jpg
http//vulcan.wr.usgs.gov/Imgs/Gif/ColumbiaPlateau
/Maps/map_columbia_river_flood_basalts.gif
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• 6. Hot Spot Volcanoes
• most volcanic rocks are generated at plate
  boundaries
• there are a few exceptionally active sites of
  volcanism within the plate interiors
• intraplate regions of volcanism are called
  hotspots
• most hotspots are over a large plume of
  exceptionally hot mantle
• mantle plumes appear to be generated in the lower
  mantle and rise slowly through the mantle by
  convection
• hot spots often create a series or chain of
  volcanic islands
• eg, Hawaii

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• if we can assume that such a hot spot is
  stationary, then we can calculate the absolute
  velocity of the Pacific Plate as it has moved
  over the hot spot

http//www.tulane.edu/sanelson/images/hotspot.gif
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• mantle plumes are largely unaffected by plate
  motions (they dont move)
• as lithospheric plates move across stationary
  hotspots, volcanism will generate volcanic
  islands that are active above the mantle plume,
  but become inactive and progressively older as
  they move away from the mantle plume in the
  direction of plate movement
• thus, a linear belt of inactive volcanic islands
  and seamounts will be produced
• classic example of this mechanism is demonstrated
  by the Hawaiian and Emperor seamount chains.

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• the "Big Island" of Hawaii lies above the mantle
  plume
• it is the only island that is currently
  volcanically active
• the seven Hawaiian Islands become progressively
  older to the northwest
• the main phase of volcanism on Oahu ceased about
  3 million years ago, and on Kauai about 5 million
  years ago
• this trend continues beyond the Hawaiian Islands,
  as demonstrated by a string of seamounts (the
  Hawaiian chain) that becomes progressively older
  toward Midway Island
• Midway is composed of lavas that are
  approximately 27 million years old.

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• northwest of Midway, the volcanic belt bends to
  the north-northwest to form the Emperor seamount
  chain
• here, the seamounts become progressively older
  until they terminate against the Aleutian trench
• the oldest of these seamounts near the trench is
  gt70 million years old
• this implies that the mantle plume currently
  generating basaltic lavas on the Big Island has
  been in existence for at least 70 million years!

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http//squall.sfsu.edu/courses/geol102/ex3.html
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How does a chain of islands over a hot spot get
formed?
Ocean
Oceanic crust
Magma chamber
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Volcanic mountain/island is created.
Magma chamber fills magma flows to the surface,
erupts.
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Magma chamber empties eruption stops
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Volcanic island moves away from the vent of the
magma chamber. It is now an extinct volcano.
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A new volcanic/mountain island is created.
Magma chamber fills again and erupts.
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Magma chamber empties eruptions stops.
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The new island moves away as the plate keeps
moving.
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And so on . This creates an arc of islands.
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Other Volcanic Features
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• a volcano formed under water it is called a
  submarine volcano or a seamount.
• volcanoes formed under ice or a glacier are
  called tuyas.
• if the lava is very sticky and does not flow far
  from its vent, it creates steep mounds called
  lava domes.

http//vulcan.wr.usgs.gov/Imgs/Jpg/Katmai/novarupt
a.jpg
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A crater lake formed in a caldera
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Volcanic Arcs - A range of volcanoes created when
an oceanic plate subsides under a continental
plate. The old volcanoes on the west coast of
North America are of this type. The Pacific Ring
of Fire is made of these volcanoes.
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Volcanic Explosivity
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• a great range in the explosivity of volcanic
  eruptions.
• some are quiet, characterized by the calm,
  nonviolent extrusion of lava flows on the earth's
  surface
• other eruptions are highly explosive and are
  characterized by the violent ejection of
  fragmented volcanic debris, called
  tephrar(material that solidifies in the air)
• can extend tens of kilometers into the atmosphere
  above the volcano.
• the type of volcanic eruption depends on a
  variety of factors, which are ultimately linked
  to the composition of the magma (molten rock)
  underlying the volcano.

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• factors controlling explosivity are viscosity,
  temperature, and the amount of dissolved gases in
  the magma.

• Non-explosive eruption
• eruption of effusive basalt from the Pu'u O'o
  volcano on the east rift zone of the larger
  Kilauea volcano, Hawaii.

http//www.geology.sdsu.edu/how_volcanoes_work/Thu
mblinks/Puuoo84_page.html
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• Explosive Eruption
• eruption of a voluminous plume of tephra is
  typical of explosive (also called Plinian)
  eruptions, as demonstrated in the 1980 eruption
  of Mt. St. Helens
• eruptive tephra plumes can travel tens of
  kilometers into the stratosphere.

http//www.geology.sdsu.edu/how_volcanoes_work/Thu
mblinks/msh5_page.html
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• How does the amount of dissolved gases affect the
  explosivity of volcanoes?
• the dissolved gases in the magma provides the
  force for explosive eruptions
• as magma rises toward the surface, dissolved
  gases in the liquid rock begin to come out of
  solution (called exsolution)
• bubbles begin to form in the magma magma gets
  frothy
• not all magma has a lot of dissolved gases

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• explosive eruptions, which get started by
  exsolution of gases, can be made more dramatic by
  sudden decompression, which lowers the confining
  pressure on the magma
• similar to what can happen to a bottle of pop
  when the cap is removed the sudden release of
  pressure can cause the CO2 to come out of
  solution explosively and rapidly, resulting in
  the pop to spray out of the bottle
• happens when a volcanic mountain suddenly breaks
  open or the magma plug from a previous eruption
  gets pushed out of the way
• Mt St. Helens is an example

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http//www.kidscosmos.org/kid-stuff/mars-trip-grap
hics/mt-st-helens-before.jpg
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• How does viscosity affect explosivity?
• viscosity the ability of a substance to resist
  flow viscosity is the inverse of fluidity
• the higher the temperature, the more fluid a
  substance becomes, thus lowering its viscosity
• lava in this case is thin and runny and gases
  escape easily
• magma's resistance to flow is a function of its
  "internal friction" due to the nature of the
  chemical bonds (called polymerization) within the
  liquid
• magma with a higher silica concentration has a
  higher viscosity - it resists flow up through the
  vent - lava in this case is thick and sticky and
  gases cannot escape
• silica rich rocks are typically found in granitic
  rocks

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• as magma cools and silica minerals begin to
  crystallize, the magma becomes increasingly
  viscous
• the left over (residual) liquid rock has a lot of
  gas in it
• this can lead to an explosive volcanic eruption
  if confining pressure of the surrounding rock is
  suddenly released
• an earthquake or tremour can open up the ground
  releasing this pressure and a violent eruption
  occurs

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• the rapid eruption of expanding gases results in
  the obliteration and fragmentation of magma and
  rock
• the greater the explosivity, the greater the
  amount of fragmentation
• individual eruptive fragments are called
  pyroclasts ("fire fragments")
• tephra (Greek, for ash) is a term for any
  airborne pyroclastic accumulation
• if the explosivity of the magma is low, the
  liquid rock tends to flow out onto the surface as
  lava
• basaltic rock tends to be low in silica and is
  not explosive

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Volcanoes and Plate Tectonics
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• What is the relationship of volcanic eruptions to
  plate tectonics?
• volcanos tend to be highly concentrated near
  convergent plate boundaries (also called
  subduction zones)
• as one oceanic crustal plate descends under the
  other plate, it gets very hot as it enters the
  asthenosphere in the upper mantle and the rock
  melts
• most of this rock was formed from the
  asthenosphere millions of years before at the
  divergent plate boundaries

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• a (comparatively) small amount of sedimentary
  rock accumulates on top of the oceanic crust over
  the millions of years
• the sediments are from organic life in the ocean,
  run-off into the oceans, and wind deposited
  material, all of which settle to the bottom

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• this sedimentary rock has a different mineral
  composition (it is mainly granitic) it is less
  dense than the surrounding liquid asthenosphere
  rock (mainly basaltic)
• therefore, after it melts in the mantle, it rises
  slowly toward the surface, breaking through the
  crust and erupting onto the surface

Island arc formed by oceanic-oceanic subduction.
Volcanic arc formed by oceanic-continental
subduction
http//www.geology.sdsu.edu/how_volcanoes_work/sub
ducvolc_page.html
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This type of granitic magma has a high silica
concentration and is therefore more viscous. It
also tends to have high level of dissolved
gases. This accounts, therefore, for the more
violent eruptions found near subduction zones,
like the so-called Pacific Ring of Fire. Mt. St.
Helens, Krakatoa, Pompeii are examples of this
type of volcanic activity.
http//www.geology.sdsu.edu/how_volcanoes_work/Thu
mblinks/ring_page.html
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Products of volcanic eruption
http//www.geology.sdsu.edu/how_volcanoes_work/Thu
mblinks/products_page.html
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• a pyroclastic flow is a fluidized mixture of
  solid to semi-solid fragments and hot, expanding
  gases that flows down the flank of a volcano
• awesome features are heavier-than-air emulsions
  (mixtures) that move much like a snow avalanche,
  except that they are fiercely hot, contain toxic
  gases, and move at phenomenal, hurricane-force
  speeds, often over 100 km/hour
• are the most deadly of all volcanic phenomena
• sometimes the hot magma and pyroclastic flow
  melts the ice on high mountains creates a lahar,
  which is categorized as a mudflow
• extremely dangerous because a lahar not only
  carries with it the lava from the eruptions but
  with the added water, part of the mountain goes
  with it as well

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THE END!