Volcanoes

Volcanoes

Distribution
1. predominantly clustered around plate margins, but not those of conservative boundaries
2. Pacific Ring of Fire
3. Japan and Iceland have a high number
4. Some not near plate margin - hot spots
  1. e.g Hawaii
Cause
1. 2 contrasting types
  1. Lava dominated: formed by diverging plate boundaries , basalt rises freely to surface
    1. e.g Iceland volcanoes
  2. Ash dominated: along subduction zones where large amounts of pyroclastic material ejected
    1. e.g Japan - Fuji Yama
2. Or Hot Spots/Magma Plumes
  1. Hot spot in core - where radioactive decay in core is concentrated
    1. heats the lower mantle, creates localised thermal currents where magma plumes vertically rise
      1. when magma plume rises within centre of tectonic plate it 'burns' through the lithosphere to create volcanic activity on the surface
        Hot spot remains stationary as overlying plate moves, creating a chain of active and extinct volcanoes as the plate moves away from hot spot
        e.g Hawaii
Magnitude
1. VEI: Volcanic Explosivity Index - relative measure of the explosiveness of eruptions
  1. dependent on how much volcanic material thrown out, to what height, how long it lasts
  2. VEI 2 and up - logarithmic scale
Hazards
1. Pyroclastic flow
  1. dense clouds of hot volcanic gases, ash and volcanic bombs
  2. speeds over 100 miles an hour
2. nuees ardentes
  1. means glowing cloud
  2. a type of pyroclastic flow
  3. hot block and ash flows
  4. a pyroclastic avalanche largely hidden from view by an ash cloud
  5. deposits collect from a collapsed lava dome
3. Tephra and ash
  1. ash is anything below 2mm, tephra is anything above
  2. formed when magama is fragmented by eruptions
  3. can stay in the atmosphere causing global variations in weather patterns
  4. breathing difficulties
4. Lava flows
  1. streams of molten rock that seep from a volcanic vent
  2. more dangerous when quick
    1. basaltic, low viscosity,
  3. effusive eruption
  4. 700 to 1200 degrees Celsius
  5. damage to property
    1. large but localised economic losses
5. Gases
  1. water vapour, carbon dioxide, sulphur dioxide, carbon monoxide
    1. carcinogens - carbon monoxide
      1. respiratory illness
    2. sulphur reacts atmosphere to form aerosols
  2. accounts for nearly 1% of world total co2
  3. directly responsible for 3% of human volcano-related deaths between 1900 and 1986
  4. sulphur reacts with water to form acid rain - corrosive
6. Mudflows (lahars)
  1. ash and debris mixes with water
    1. on steep slopes can reach 22 metres per second
Prediction
1. Seismometers and seismographs
  1. measure seismic activity. Microquakes indiate rising magma fracturing overlying rocks
2. Tiltometers and laser - based distance measurement
  1. measure ground deformation caused by rising magma.
3. Magnetometers
  1. measure changing magnetism within volcano which indicates rising magma.
4. Hydrological instrumentation
  1. Measure rising groundwater temperature and gas content due to rising magma - e.g sulphur corrupts water, increases acidity
5. Remote sensing equipment e.g camera surveillance, gas sampling, thermal imaging
  1. record signs like small eruptions and gas eruptions e.g chlorine
Types of eruption
1. Icelandic
  1. e.g Mid Atlantic Ridge
  2. constructive margin
  3. basaltic magma
  4. low silica - 45 - 52%
  5. quiescent
  6. low airborne ejection
  7. frequent
2. Hawaiian
  1. subduction zones, hot spots, rift zones
  2. basaltic - more viscous than icelandic
  3. VEI 2 - 3
  4. eruptions tens - few hundreds of metres
  5. sporadic violence
  6. gas slugs
  7. fairly regular
3. Vulcanian
  1. almost all types magma, usually 55% or more silica
    1. basaltic andesite
  2. explosive
  3. 10^2 to 10^6 tonnes ejected rock
  4. tephra disperses over wide area
  5. ends w/ flow of lava
  6. cannon like explosions
  7. may be precursor to larger eruptions
4. Vesuvian
  1. volatile rhyolitic lava
  2. frothy mobile lava due to gases exsolved in magma like pumice
  3. pumice airfall is heavy - affects area up to 50km^3
  4. eruption columns up to 45km
  5. sustained eruption columns
  6. occur after long dormancy
  7. felsic volcanoes
5. Peleean
  1. acidic lava
  2. rhyolitic/andesitic
  3. glowing avalanche pyroclastic flow
  4. lower tephra volume than p;inian / vulcanian
  5. eruption cycle may last for decades
6. Plinian
  1. rhyolitic
    1. highly explosive / violent
      1. magma de-pressurises as it emerges, allowing gas expansion- propels material up to 45km
        tall eruption plume
        VEI 4,5,6
        continuous gas blast eruptions
        ash plume and magma conduit
        violent, less frequent
Lava
1. basaltic
  1. low silica - 45 - 52%
  2. hot (1,200 degrees C)
  3. flows far
  4. low viscosity
  5. extensive, gently sloping landforms
  6. less explosive
2. Andesitic
  1. 52 - 63% silica
  2. higher viscosity than basaltic
  3. block flow
  4. lava domes /stratovolcanoes
3. rhyolitic
  1. high viscosity
  2. silica greater than 68%
  3. 800 degrees C
  4. slow
  5. more localised features
Types of volcano
1. Fissure eruption
  1. vent linear, few m wide
    1. Iceland, Laki
  2. lava plateaus
  3. gentle flow of lava
  4. constructive boundary
  5. fed by dikes
  6. basaltic lava
2. Basic shield volcano
  1. if active, experiences near continuous eruptive activty over long time period
  2. gentle slope 2 - 3 degrees, steepens to about 10
  3. warrior shield shape - steady accumulation of broad sheets of lava
  4. height is 1/20 width
  5. low viscosity mafic lava
  6. effusive
    1. volatiles escape easily
  7. calmest volcanic event
  8. fire curtains, lava fountains
  9. subaerial
  10. excluding flood basalts, largest volcanic features on earth - Mauna Loa, 80,000 km^3 basalt
3. Acid dome volcano
  1. Puy de Dome, France
  2. steep, layers of lava, piles around vent, hemispherical dome shape
  3. heights several hundred metres
  4. viscous lava, rhyolitic
    1. explosive - pressure build up
    2. shallow seismicity from excess fluid reserves
  5. cryptodomes
  6. effusive
4. Stratovolcano
  1. composite cone, layered
  2. conical shape, steep proifle
  3. one of most common
    1. vesuvius
      1. columns up to 50km^3
  4. felsic magma
  5. layers of lava, tephra, pumice
  6. common at subduction zones
    1. magma rises when water released into asthenosphere, mantle rock lowers melting point of overlying mantle, which rises and pools at base of lithosphere, magma pools in chamber
  7. low pressure in chamber akkiws volatiles to escape - cone overcomne - explosive bombs
    1. greatest hazards to society in history
5. Caldera
  1. Yellowstone caldera, 80km long chamber, Lava Creek eruption - 240 cubic miles rock, ash
  2. cauldron shaped depression, follwoing evacuation of magma chamber. Type of sinkhole, from 1 to dozens of km in diameter
  3. large volumes magma erupt over short time period. structural support for crust above magma chamber lost. collapses into empty chamber.
    1. subsidence rather than eruption
      1. any type of magma, usually rhyolitic, and explosive,
  4. rich ore deposits
    1. mineralised caldera Sturgeon Lake Caldera, Canada

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	Created by Lauren Bradford about 7 years ago