volcanism & magmatism

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    A vocano is generally a conical shaped h il l o r mountain bu il t by accumulations of lava flows ,tepra , and volcanic ash . About 95 % of active volcanoes occur at the plate subduction zone and

    at the mid-oceanic ridges (Figure 10n-1). The other 5 % occur in areas associated wi thlithospheric hot spots . These hot spots have no direct relationships with areas of crustalcreation o r subduction zones. It is believed that hot spots are caused by plumes of risingmagma that have their origin wi thin the asthenosphere.

    Figure 10n-1: Location of the Earth's major volcanoes. Most occur along tectonic plateboundaries where plate subduction creates rising plumes of magma. The volcanoesthat do not occur along plate boundaries are the result of localized asthenosphere hot

    spost that melt through the Earth's crust. The Hawaiian Island chain of volcanoes wascreate by a hot spot.

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    Over the last 2 million years, volcanoes have been depositing lava, tephra,and ash in particular areas of the globe ( Figure 10n-2 ). These areas occur at

    hot spots , rift zones , and along plate boundaries where tectonic subduction istaking place.

    Figure 10n-2 : Location of major volcanic deposits laid down during the last

    2 million years.

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    Figure 10n-3: Low viscosi ty basaltic lava flow from an activevolcano on one of the Hawaiian Islands

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    Figure 10n-4 : Cinder cone volcano high viscosity magma. Note howthe vegetation near the mound has been burnt by lava flows

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    Figure 10n-5: Mount St.Helens eruption on May 18,1980. (Source: U . S .G e o l o g i c a l Su r v e y ,p h o t o g r a p h b y A u s t i n P o s t ).

    Figure 10n-6: The above image is a post-eruption computer rendering of Mount St.Helens from a U.S.Geological Survey digitalelevation model (DEM). The lateral eruptionremoved 2.8 cubic kilometers of rock and

    sediment from from the volcano and loweredits height by 400 meters. Detectable amountsof ash were spread over 50,000 squarekilometers of area surrounding the volcano.The large crater created by the explosiveeruption is about 600 meters deep and can beseen in the center of the image above

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    The most explosive type of volcano is the caldera . The cataclysmic explosion of

    these volcanoes leaves a huge circular depression at the Earth's surface. Thisdepression is usually less than 40 kilometers in diameter. These volcanoesform when " wet" granitic magma quick ly r ises to the surface of the Earth. Whenit gets to with in a few kilometers of the surface the top of the magma cools toform a dome. Beneath th is dome the gaseous water in the magma createsextreme pressures because of expansion. When the pressure becomes toogreat the dome and magma are sent in to the Earth's atmosphere in atremendous explosion. On the island of Krakatau , a caldera type volcanoexploded in 1883 ejecting 75 cubic ki lometers of material in the air and left adepression in the ground some 7 kilometers in diameter.

    A potentially very destructive caldera covering an area of about 2000 squarekilometers exists under Yellowstone National Park in the United States (Figure10n-7). Investigations have discovered that over the last 2 mill ion years thisvolcano has exploded on a regular interval of about 700,000 years. The lasteruption occurred 630,000 years ago and the next could take placeanytime.When the Yellowstone caldera last erupted, it blasted 1,000 cubic

    kilometers of volcanic ash and rock into the atmosphere. The ash ejected intothe atmosphere created climatic havoc on a global scale. The ash would haveblocked sunlight from being received at the ground surface for a few years. Areduction in the reception of solar radiation would have caused the globleclimate to cool significantly. Over time this ash settled back to the Earth'ssurface covering more than half of North America.