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Volcanoes

D. Types of Volcanic Eruption

  1. Quiet
    • Low silica (basalt)
    • Low viscosity; gas escapes easily.
    • Lava flows, fountains, lakes
      Types of flows:
      pahoehoe - smooth, ropy lava
      aa (cooler, and therefore higher viscosity than pahoehoe): rough, jagged

  2. Explosive
    • Intermediate to high silica (andesite-rhyolite)
    • Medium-high viscosity: gas is trapped, building up pressure.
    • Produces pyroclastic material ("fire-broken"). General term is tephra.


    Ash
    Cinders
    Bombs (plastic) & Blocks (solid)
    1/8- 2 mm
    2-64 mm
    > 64 mm

    size
    ----------------------------------------------------->

    • Pyroclastic flows (nueé ardentes) - hot, glowing clouds of ash and gas

  3. Stages of a Volcano
    Volcanoes have extremely variable "lifetimes" from less that a decade to millions of years.
    Active - erupting now or signs that it will in near future
    Dormant ("sleeping") - may be thousands of years between eruptions
    Extinct ("dead") - not likely to erupt again

E. Forecasting Volcanoes

  1. Monitor for changes in the following (may signal movement of magma):
    • Elevation and tilt (using tiltmeter, lasers)
    • Water levels in crater lakes
    • Temperature of groundwater
    • Gas composition
    • Frequency of small earthquakes

  2. Study the volcano's history to understand how it may behave in future

    Success story: 1991 eruption of Pinatubo, evacuation saved lives.

F. Volcanic Hazards Lava flows: Cause fires; destroy roads and houses.

Ash: Aviation hazard (engine failure), respiratory problems, collapse of buildings, damage to crops, electronics, and machinery

Gas: May cause acid rain (sulfuric acid form when SO2 reacts with water), global cooling due to aerosols (tiny droplets of sulfuric acid). Toxic carbon dioxide is denser than air and accumulates in low areas (Example: 1986, Lake Nyos, Cameroon, 1700 people killed) .

Lahars: Mudflows formed when water from rain or melting snow and ice mixes with volcanic debris. Lahars rush down sides of volcano, moving at speeds of 20-40 mph. May bury everything in their path. Lahars are one of the most dangerous volcanic hazards.

Pyroclastic flows: high-speed avalanches of gas and debris; may be up to 1,500 °F and move at speeds of 100 to 150 miles per hour. Example: eruption of Mt. Peleé on Martinique (1902) killed 29,000 people.

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updated January 28, 2002

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