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    Volcanoes2009 Volcanoes2009 Presentation Transcript

      • Volcano :
      • an opening in the earth’s surface through which lava, hot gases, and rock fragments erupt
      Introduction to volcanoes
        • Magma 50-100 miles below the earth’s surface slowly begins to rise to the surface
        • As the magma rises it melts gaps in the surrounding rock
        • As more magma rises a large reservoir forms as close as 2 miles below the surface (magma chamber)
      How do they form?
        • Pressure from the surrounding rock causes the magma to blast or melt a conduit (channel) to the surface where magma erupts onto the surface through a vent (opening)
      How do they form?
        • The magma, now called lava, builds up at the vent forming a volcano
      How do they form?
        • Often the volcano sides will be higher than the vent forming a depression called a crater
      How do they Form?
      • Crater :
      • Caldera:
      • an unusually large crater or the remains when the cone collapses into its own magma chamber
    • Anatomy of a Volcano
      • Cone :
      • the above ground structure built from lava and/or tephra
      • Conduit :
      • the path that magma takes from the magma chamber to the vent
      • Magma Chamber :
      • the reservoir located under the volcano where magma collects and becomes the supply of magma/lava to build the volcano
      • Lava:
      • molten, liquid rock on the surface of the earth
      • Parasitic Cone:
      • a smaller secondary volcano built on the side of or near the main volcano, but sharing the same conduit to the magma chamber
      • Fumarole:
      • a secondary vent that emits only gases
      • Fissure :
      • a long fissure ( crack ) from which lava flows
      • Vent : opening of the volcano, through which lava, ash and gases flow
    • What comes out of volcanoes?
      • Lava
      • Tephra
      • Gases
      • Lava— 3 kinds:
      • Pahoehoe lava:
      • Hot, thin, fast flowing
      • harden with a relatively smooth surface
      • Often has a ropy or wrinkled appearance
      • Pahoehoe lava:
      • Aa lava:
      • Cooler, thicker , slow moving
      • Hardens with a rough, jagged, sharp edge surface
        • Pillow Lava:
        • Lava suddenly cooled by water
        • shows sack-like segments (stuffed pillows)
    • Tephra
      • Basically, rock fragments
      • Also known as pyroclastic rock fragments.
      • There are many different possible sizes, from very small (volcanic ash or dust to much larger rocks (called volcanic bombs )
    • Ash & dust
    • Volcanic Bombs This dacite breadcrust bomb (about 15 cm in diameter) was erupted from the lava dome at Mount St. Helens, Washington.
      • Lahar (mudflow):
      • mixture of ash, eroded land , and water flowing down river valleys
      • Lahar (mudflow):
    • GASES
      • water vapor, carbon dioxide , nitrogen, sulfur dioxide , hydrogen sulfide, chlorine
    • Where do Volcanoes Occur?
    • Where do Volcanoes Occur? location Examples Divergent Boundaries : where plates move APART Iceland
    • Where do Volcanoes Occur? location Examples convergent Boundaries : where plates come together Cascade range volcanoes, Andes
      • Cascade Volcanoes
    • Where do Volcanoes Occur? location Examples Hot Spots : plates riding over an especially hot place in the mantle Hawaii, Yellowstone, Iceland
    • Hot Spots
        • Two factors determine the type of eruption :
        • Amount of water vapor & other gases in the magma
        • The chemical composition of the magma
      Types of Volcanic Eruptions
        • Trapped gases under high pressure will violently explode when the magma reaches the lower pressure of the surface.
      Explosive Eruptions
        • Has granitic magma is very thick and plugs the vent causing the pressure to build until it blows violently out the vent
      Explosive Eruptions Mt. St. Helens
        • The high water content of the magma produces more water vapor which when mixed in granitic magma produces explosive eruptions
      Explosive Eruptions Mont serrat
        • Low pressure gas
      Quiet Eruptions
        • Has basaltic magma (is more fluid and will flow instead of explode)
      Quiet Eruptions
        • … and has low water content
        • Examples : Hawaii
      Quiet Eruptions
    • Cinder cones
      • Small base, steep-sided, loosely consolidated
      • Up to 1000 feet tall
      • Life span of a few years
      • Commonly built from gravel size lava rock fragments called cinders
      • Violent eruptions, dangerous when close---High pressure gas bubbles causes thick lava to explode into the air, lava begins to cool as it rises and falls becoming very sticky
      • When lava hits the ground it sticks rather than flows
      • This builds a steep cone with a small base
        • Cinder Cones :
      Cinder Cones
    • Shield volcanoes
      • Large base, gentle slope , lava rock layers
      • A few miles high
      • Life span of a million years or more
      • The lava is hot, thin, very fluid often basaltic
      • Example: Hawaiian Islands, Iceland
    • Shield volcano on Mars; Taken from space Take a look at these examples: http://www.volcano.si.edu/world/tpgallery.cfm?category=Shield%20Volcanoes The Mauna Loa volcano in Hawaii—the largest volcano on Earth—has the broad expanse characteristic of shield volcanoes. It spreads across half the island of Hawaii.
      • Shield Volcanoes
      Mauna Kea Shield volcanoes
    • Composite or strato
        • Large mountain volcano often snow capped , a few miles high
        • Life span of million years or more
    • Composite or strato
        • Have alternating eruptions of tephra (air-borne) and lava . The tephra adds height to the volcano and the lava cements the tephra together and adds to the base .
        • Found mostly in subduction zones and have violent eruptions.
    • STrato/Composite Llaima Volcano, Chile.
        • Composite (strato) Volcanoes examples:
      Mt. Rainier Mt. Fuji Mt. Kilimanjaro
    • Composite/strato volcanoes
        • Active (awake):
        • Has erupted within recent time and can erupt again at any time .
        • Pre-eruption activities :
          • Increase in earthquake activity under the cone
          • increase in temperature of cone,
          • melting of ice/snow in the crater
          • swelling of the cone
          • steam eruptions
          • minor ash eruptions
      Volcano Activity Levels (Stages)
    • Mt St. Helens
      • Dormant (sleeping):
      • No eruption within recent times, but there is record of past eruptions
      • Can become active and erupt again after a “wake up” period
      • Example : Mt. Rainier
      • Extinct:
      • No eruption within recorded history
      • Not expected to ever erupt again
      • Example: Mount Mazama (Crater Lake)
    • Crater Lake
      • Mount Rainier
        • The most dangerous volcano in the US
        • The danger is mostly from lahars traveling down river valleys at a speed of 25mph and destroying everything in its path
        • 100,000 people live on the solidified mudflows of previous eruptions
      • Mount Rainier
        • The mountain is dangerously unstable , a tall, steep heap of loose rock held together by the force of gravity and a cubic mile of glacier ice that could be melted or shaken loose
        • Lahar flows average every 500 years and have gone as far as the Puget Sound lowlands
        • Mount Rainier has erupted 4 times in the last 4000 years with the last eruption 200 years ago