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