VOLCANIC HAZARDS
GL3: K1: a)
i)
ii)
iii)
iv)
Volcanic hazards result from:
Blast/explosion
Ash fall, pyroclastic flows (nuees ardentes) and gases
Lava flows
Debris flows and mudflows (lahars)
The nature of the hazard depends on the composition, viscosity
and gas content of the magma.
Case studies: Krakatoa, Mt St Helens, Pinatubo, Montserrat
Compare and contrast the nature of volcanic / earthquake /
mass movement hazards.
DISTRIBUTION
@ 500 Active volcanoes
@ 50 Eruptions/yr
Every continent except Australia
Controlled by plate tectonics
80% at subduction zones - stratovolcanoes (composite cones) - explosive calderas
Fujiyama - Japan, Vesuvius - Italy, Mayon - Philippines, Mount Hood - Oregon
USA
Constructive margins - rift volcanoes - less explosive + more effusive - Iceland
Hot spot volcanoes - Hawaii middle of Pacific plate
Type of eruption largely dependent on magma type - dependent on plate
tectonic setting
5% eruptions lead to deaths - @ 650/yr
Deceptive figure - over 1/2 deaths in 20th century - single event;
1902 Mount Pelee, Martinique,West Indies - 29 000 Saint Pierre - only 2
survivors
Hazard impact depends on population density
Volcano flanks attract high density - fertile soil
Level of economic development and time of day may also be important
PYROCLASTIC FLOWS
Most deaths associated with explosive eruptions involving pyroclastic flows +
tsunami
Explosive eruptions - Nuee Ardente 'Glowing Cloud'
Pompeii, Italy 79AD 16000 deaths
Frothing of molten magma in vent; gas bubbles expand + burst explosively breaks lava Dense cloud of lava fragments ejected in turbulent mixture of hot
gases + pyroclastics (Lava fragments, crystals, ash, pumice, glass shards)
May be ejected vertically many 10's km into atmosphere
Most flows rapidly down flank of volcano - dense
Most hazardous if directed laterally by explosive blast - Pelean type
Over 100km/hr
May travel 30 -40km from source
Hot - up to 1000 'C
70% deaths this century - asphyxiation + burning - damage to buildings + crops
AIR-FALL TEPHRA
Ashfall rarely kills - 1902 Guatemala 20cm ash collapsed roofs - 2000 deaths
1815 Tambora, Indonesia 1 400m of cone blasted off - 12km diameter caldera 12 000 deaths; further
80 000; disease + famine - crops destroyed by ash fall
Fragmental material - Ash - less than 4mm + Lapilli - 4 - 32mm + Bombs - over
32mm
Most less than 1 cubic km material - largest may be several x more
Coarse deposited near vent
Fine may be ejected high into atmosphere + windblown 00's km
Ashfall may contain toxic chemicals - eg fluorine - contaminates soil + water
Heavy ashfall destroys crops
Light ashfall can be beneficial - fertilizer
Heavy falls of coarser material - blanket + destroy crops
eg 1963-5 Irazu volcano, Costa Rica; destroyed coffee crop: ruined farm land:
$150m Flat roofed buildings collapse if thick ashfall
Hot tephra may start fires
LAVA FLOWS
Effusive eruptions - molten lava + ash falls - greater threat to property - Hawaii
Most hazardous to people if erupting rapidly from fissure eruptions
Fluid basaltic magma on steep slopes 50km/hr
Niyiragongo, Zaire 1977 - 5 fissure eruptions on volcano flanks - formed lake of
lava - lake drained in under an hour - 40kmph - 72 deaths + over 400 houses
destroyed
Thick lava blankets sterilise farmland for many years
Iceland 1783 24km fissure eruption 5 months - Lakagigar flow covered 560
square km - Few casualties - over 10 000 (22% population) died in resulting
famine
Unlikely to be repeated in modern world
VOLCANIC GASES
Water vapour, Hydrogen, Carbon Monoxide, Carbon Dioxide, Hydrogen
Sulphide, Sulphur Dioxide, Sulphur Trioxide, Chlorine, Hydrogen Chloride
Rarely direct cause of disaster
Carbon Monoxide
- toxic at very low concentrations - colourless, odourless
1.5x denser than air - accumulates in hollows
- Death in 10-15 minutes 10% concentration
- eg Java, Indonesia 1979 142 deaths - evacuees leaving
volcano
area walked into pool of gas and died immediately
Carbon Dioxide
- Lake Monoun, Cameroon 1984 37 deaths
- Lake Nyos, Cameroon 1986 1746 deaths (less than 1%
survived)
+ 8300 livestock
- Deaths up to 23km away from crater
- Very rare ? Accumulation from previous eruptions
released by disturbance ? landslide
LAHARS
Volcanic mudflows - especially in wet tropics
2nd greatest hazard after pyroclastic surges Mudflows 10% deaths
Kelut, Java, Indonesia 1919 5500 deaths
Occur in association with seismic event + large quantities of water + steep
volcano
Water may originate from - violent electrical rainstorms triggered by eruption
- Collapse of crater lake
- Rapid melting of snow + ice; water mixes with ash
Flows 50 - 80kmph
Common hazard in Andes
Cotopaxi, Ecuador 1877 - snowmelt - lahars 160km long
Nevado del Ruiz, Columbia 1985 50km downstream lahar buried town of
Armero 3-8m 22000 deaths
LANDSLIDES
Commonly associated with dacite magma - high viscosity + high dissolved gas
Mount St Helens, USA 1980
Intrudes into mountain + causes cracking + bulging - earthquake triggered
avalanches + landslides (2.7 cubic km rock) 57 deaths + property damage $1
billion
TSUNAMIS
Krakatoa, Indonesia 1883
Explosions heard 500km away - ash 80km up into atmosphere
Cone collapsed into caldera
Tsunami up to 30m - over 36000 drowned
PREDICTING VOLCANIC ERUPTIONS
GL3: K2: a) i)
attempt
A wide variety of monitoring techniques is used in an
to predict hazardous geological events.
Volcanoes. Indicators of underground movement include:
i)
Ground deformation
ii)
Gravity anomalies
iii)
Thermal anomalies
iv)
Gas emissions
v)
Seismic activity
Case studies:
Mt St Helens, Pinatubo
Hazard maps
Evaluation of degree of success of hazard prediction
Only 10-20 volcanoes monitored well enough to allow scientific prediction
Eg Mount St Helens USA 1980 - 57 deaths
If no restrictions on access after warning ?1000 deaths
Ground deformation
- Swelling of cone + changes in ground slope near
volcano; detected by tiltmeters, lasers, GPS
Hydrothermal phenomena - Increased discharge from hot springs + fumaroles
- Increased temperatures of water/steam/gas
emissions
- Increased temperature of crater lakes
- Melting of snow + ice on volcano
- Withering of vegetation on volcano slopes
- Increased temperature can be detected by satellite
thermal imaging cameras
Chemical changes
- Increase in Sulphur Dioxide, Hydrogen Sulphide
from fumaroles
Seismic activity
- increase in local earthquake activity + audible
rumblings
- increase in size and frequency of long period
events
VOLCANIC HAZARD REDUCTION
GL3: K2: c) The destructive effects of volcanoes can to some extent be
managed and controlled in order to reduce risk.
Volcanoes - Diversion of lava flows, control of lava speed and
direction eg Etna
No defence against pyroclastics - avoid flat roofs 20' slope minimum
Lavas
Bombing
1 High on volcano to spread lava + stop advancing lava flow
2 AA flows to breach levee like walls + spread out lava
3 Walls of cone at vent to spread flow - never tried
eg Etna, Hawaii nb problems with poor visibility
Artificial Barriers - only possible if favourable topography
Only works for thin, fluid flows
Walls 3m thick for each in depth of lava
eg Etna 1983, Hawaii
Water sprays - quite effective in Iceland 1973 Heimay - saved 400 buildings
Hazard potential mapping - quite effective for Mt St Helens
BENEFITS OF VOLCANIC ACTIVITY
GL3: K2: d) Benefits are associated with some natural hazards:
Volcanic activity- geothermal energy, fertile soils
Geothermal power - Iceland, New Zealand, Italy
Fertile soils - Indonesia
Tourism Etna
– Sicily
Fujiyama
– Japan
Bromo
- Java (Indonesia)