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,