1 mt. etna, sicily. 2 parts of a volcano 3 volcano cross-section

11Mt. Etna, Sicily

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Parts Parts of a of a

VolcanoVolcano

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Volcano Cross-Volcano Cross-sectionsection

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CalderasCalderas• Created when a magma chamber is emptied, and the rock above collapses back into theempty chamber, causing a large depression in theground, which often fill up with water.

Crater Lake Caldera

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CalderasCalderas

Photograph by M. Williams, National Park Service, 1977Aniakchak Caldera

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How How Volcanoes Volcanoes

FormForm

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Plate TectonicsPlate Tectonics• Volcanoes can be located in one of threelocations:

1) A convergent plate boundary

2) A divergent plate boundary

3) A hotspot

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Convergent Convergent BoundariesBoundaries

• Plates converge, and one slides underneaththe other; oceanic crust is destroyed

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Divergent Divergent BoundariesBoundaries

Photography courtesy of Woods Hole Oceanographic Institution and members of the Adventure dive

• About 2.4 mi3

of crust is created this way

• All divergentboundaries arein the middle of the ocean atmid-oceanspreadingcenters

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Hot SpotsHot Spots• Hawaii is the mostfamous chain ofislands created byhotspot activity

• The plate slidesover the hotspot, creating volcanoesin a chain

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How Volcanoes How Volcanoes FormForm• 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 Volcano

ProductsProducts

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LavaLava

Photograph by J.D. Griggs on 13 November 1985Lava flowing on Kilauea Volcano, Hawaii

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PahoehoePahoehoe• Is Basaltic lava with a ropy, smooth or bumpy surfacetexture

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GasesGases• Mainly water, carbon dioxide, and sulfurdioxide• Toxic gases include hydrogen sulfide (rotten egg gas), sulfuric acid, and Hydrochloric acid

Photograph by K.A. McGee on 19 September 1995

Gases from Pu`u `O`o venton Kilauea Volcano, Hawaii

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Tephra: AshTephra: Ash

Photograph by D.E. Wieprecht

• Must be smallerthan 2 mm in diameter

• Commonly lessthan 0.025 mm inDiameter

• Does not dissolve in water

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Tephra: LapilliTephra: Lapilli

Lapilli from the summit of Kilauea Volcano in 1959

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Blocks and BombsBlocks and Bombs

Photograph by J.P. Lockwood on July 10, 1982

Photograph by C. Heliker on January 26, 1988

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Pyroclastic FlowsPyroclastic Flows

Photograph by C. Newhall on 15 September 1984

• 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!!!

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LaharsLahars•Volcanic mudflows, composed of volcanicmaterial, water, mud, and anything else available

Photograph by J.N. Marso on 14 August 1989

Guatemala

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TypesTypesof of

VolcanoesVolcanoes

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Cinder Cone Cinder Cone VolcanoesVolcanoes

• 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 VolcanoesCinder Cone Volcanoes

Aerial view looking west to Sunset Crater. Photography by Wendell Duffield, U.S. Geological Survey

Sunset Crater. Photograph courtesy of the National Park Service.

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Cinder Cone VolcanoesCinder Cone Volcanoes

Photograph by J.P. Lockwood on 1 December 1975Pu`u ka Pele

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Shield VolcanoesShield 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 VolcanoesShield 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 VolcanoesShield Volcanoes

Three-dimensional Space Shuttle Image of the Alcedo Shield Volcano, Galapagos. Courtesy of JPL/NASA.

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StratovolcanoesStratovolcanoes• 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 famousbecause of deadliness• Responsible for 699 / 1,511 of known eruptionsover the past 10,000 years [46.3%]

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StratovolcanoesStratovolcanoes

Photograph by R. McGimsey on 15 July 1990

Mount Mageik, Alaska

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Other Important Other Important StratovolcanoesStratovolcanoes

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 Other Important StratovolcanoesStratovolcanoes

Mt. Fuji, Japan

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Mt. St. Helens, Mt. St. Helens, 19801980

Courtesy of Don Swanson, USGS

Steam wasseen rising outHelens in thisApril 10, 1980picture

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Mt. St. Helens, Mt. St. Helens, 19801980 Shown is the

“bulge” in the Side on April 27,1980

The “bulge”had expandedmore than 100meters out!!!

Notice how large the “bulge” grew from onlyApril 10 to April 27!

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Mt. St. Helens, Mt. St. Helens, 19801980

Courtesy of USGS.

Rose to a height of more than20 kilometers!!

Lasted more than 9 Hours

Ash was carried as far away asthe great plains

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ReferencesReferences• 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/