Transcript Slide 1

Mt. Etna, Sicily
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Parts
of a
Volcano
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Volcano Cross-section
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Calderas
• Created
when a magma chamber is emptied,
and the rock above collapses back into the
empty chamber, causing a large depression in the
ground, which often fill up with water.
Crater Lake Caldera
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Calderas
Photograph by M. Williams, National Park Service, 1977
Aniakchak Caldera
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How
Volcanoes
Form
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Plate Tectonics
• Volcanoes can be located in one of three
locations:
1) A convergent plate boundary
2) A divergent plate boundary
3) A hotspot
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Convergent Boundaries
• Plates converge, and one slides underneath
the other; oceanic crust is destroyed
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Divergent Boundaries
• About 2.4 mi3
of crust is
created this way
Photography courtesy of Woods Hole Oceanographic
Institution and members of the Adventure dive
• All divergent
boundaries are
in the middle of
the ocean at
mid-ocean
spreading
centers
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Hot Spots
• Hawaii is the most
famous chain of
islands created by
hotspot activity
• The plate slides
over the hotspot,
creating volcanoes
in a chain
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How Volcanoes Form
• Weaknesses exist at these boundaries
• Magma lighter than surrounding rock
• Finds a weakness and pushes upwards to the
surface, causing a volcano
Sketch by B. Myers
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Volcano
Products
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Lava
Photograph by J.D. Griggs on 13 November 1985
Lava flowing on Kilauea Volcano, Hawaii
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Pahoehoe
• Is Basaltic lava with a ropy,
smooth or bumpy surface
texture
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Gases
• Mainly
water, carbon dioxide, and sulfur
dioxide
• Toxic gases include hydrogen sulfide (rotten
egg gas), sulfuric acid, and Hydrochloric acid
Gases from Pu`u `O`o vent
on Kilauea Volcano, Hawaii
Photograph by K.A. McGee
on 19 September 1995 15
Tephra: Ash
• Must be smaller
than 2 mm in
diameter
• Commonly less
than 0.025 mm in
Diameter
• Does not
dissolve in water
Photograph by D.E. Wieprecht
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Tephra: Lapilli
Lapilli from the summit of Kilauea Volcano in 1959
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Blocks and Bombs
Photograph by J.P. Lockwood on July 10, 1982
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Photograph by C. Heliker on January 26, 1988
Pyroclastic Flows
• Are dangerous movements of ash,
rock fragments, pumice and gas
• Move very rapidly: up to 65 mph!!
• Usually more than 5000 F!
Pyroclastic flows are far and
away the most deadly volcanic
occurrences. If you see one,
get as far away as fast as
possible!!!
Photograph by C. Newhall on 15 September 1984
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Lahars
•Volcanic mudflows, composed of volcanic
material, water, mud, and anything else available
Guatemala
Photograph by
J.N. Marso on
14 August 1989
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Types
of
Volcanoes
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Cinder Cone Volcanoes
• They are the most common type, yet are
the smallest type as well
• Have the steepest sides of the three
types, and the largest summit craters
• Responsible for 648 / 1,511 of known
eruptions over the past 10,000 years [42.9%]
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Cinder Cone Volcanoes
Sunset Crater. Photograph courtesy
of the National Park Service.
Aerial view looking west to Sunset Crater.
Photography by Wendell Duffield, U.S. Geological Survey
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Cinder Cone Volcanoes
Photograph by J.P. Lockwood on 1 December 1975
Pu`u ka Pele
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Shield Volcanoes
• They are very wide, broad volcanoes
• The magma that is released flows very easily,
which creates their wide profile
• Responsible for 164 / 1511of known eruptions
over the past 10,000 years [10.8%]
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Shield Volcanoes
View of the NNW flank of Mauna Loa Volcano from the south
side of Mauna Kea Volcano, Hawaii; both are shield volcanoes.
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Shield Volcanoes
Three-dimensional Space Shuttle Image of the Alcedo
Shield Volcano, Galapagos. Courtesy of JPL/NASA.
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Stratovolcanoes
• Also known as composite volcanoes
• Are the “most picturesque”, but also the most
deadly because of the material they eject
• Have gentle lower slopes, and steep upper
Slopes
• Not the most common, but are most famous
because of deadliness
• Responsible for 699 / 1,511 of known eruptions
over the past 10,000 years [46.3%]
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Stratovolcanoes
Photograph by R. McGimsey on 15 July 1990
Mount Mageik, Alaska
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Other Important
Stratovolcanoes
Mt. St, Helens,
Washington. A view to
the north of the "two
tone" mountain - an
appearance produced
by prevailing easterly
winds during the initial
activity of Mount St.
Helens.
Photo by C. Dan Miller
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Other Important
Stratovolcanoes
Mt. Fuji, Japan
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Mt. St. Helens, 1980
 Steam was
seen rising out
Helens in this
April 10, 1980
picture
Courtesy of Don Swanson, USGS
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Mt. St. Helens, 1980
 Shown is the
“bulge” in the
Side on
April 27,1980
 The “bulge”
had expanded
more than 100
meters out!!!
 Notice how large the “bulge” grew from only
April 10 to April 27!
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Mt. St. Helens, 1980
 Rose to a height of more than
20 kilometers!!
 Lasted more than 9 Hours
 Ash was carried as far away as
the great plains
Courtesy of USGS.
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References
• Most images from USGS :
http://volcanoes.usgs.gov/Products/Pglossary/pglossary.html
• Most
information and data from:
http://www.geology.sdsu.edu/how_volcanoes_work/
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