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Volcanoes
D. Types of Volcanic Eruption
- 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
- 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
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Bombs (plastic) & Blocks (solid)
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1/8- 2 mm
|
2-64 mm
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> 64 mm
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size
----------------------------------------------------->
- Pyroclastic flows
(nueé ardentes) - hot, glowing clouds of ash and gas
- 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
- 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
- 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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