Yellowstone caldera eruption: preliminary scoping of mortality
1. Pre-eruption ( mortality 0.01 - 1%)
Scenario needs definition for whether there is warning, e.g., an eruption which begins
in a small way and then builds up over days or weeks. Volcanologists may provide a
single day’s warning or the mere mention of the possibility of a caldera eruption will
have lead to mass evacuation for hundreds of miles without any certainty that there
will be such an eruption. Many people will not move until the evidence grows, but
then it will be too late to get far enough away. However, with one day warning or less
as the volcano ramps up, those remaining will try to get out in a hurry and so road
accidents and violence, looting by those staying behind, will cause some deaths and
injuries.
2. Eruption.
2.1 Pyroclastic flows (mortality 90%)
Assume these run radially out from volcano in all directions. Only survivors would
be people inside houses in the peripheral areas, or inside houses well sheltered by
topography closer in. Everyone outside at the time of the eruption (within a few hours)
and within the PF radius will be killed. 90% mortality within radius of pyroclastic
flows.
3. Post-eruption 1-7 days
3.1. Ashfall in > 50 cm ash deposit area (mortality 1-10%)
Not clear how long (how many days) eruption will go on for. Complete, impenetrable
darkness will descend over the area for up to 2 days, probably, and down to one day,
hours or minutes as far away as < 1cm isopach line.
 No movement, even walking, will be possible during about 2 days. Anyone
caught in car or on foot when ash fall starts will be marooned, especially those
people who delayed their evacuation until the last moment: this could be many
thousands of people. They may have no food or water in the car. Small
children, elderly and sick will be in trouble. When ash fall clears the cars will
not be able to move in the ash, or not far before filters clog, etc. Roads will be
invisible. Walking through deep ash near impossible unless consolidated by
rain and people can walk on top – the only means of escape.
 People outdoors or no shelter. They will not be able to see where they are
going once the ash fall starts, and in the > 50 cm area this could be life
threatening. They will get covered in ash and the effect will be like being in a
dusty mine inhaling large quantities of fine dust, or sandstorm. People die in
sandstorms in the open. Indeed, the eruption could generate unusual weather
condition for many miles radius, and strong winds blowing over dry ash would
create very hazardous dust conditions. Many fit people could suffocate within
a few hours of such conditions in the open.
 All agriculture and horticulture will be destroyed or badly damaged in this
area. All livestock will perish from lack of water and grass, etc.within days.
All ponds, troughs, etc. will be destroyed by ash.
 Total breakdown of all normal life. Nothing works at all or gradually grinds
(literally) to a halt within a few days. Mortality dependent on rescue from
outside. Rising mortality amongst vulnerable people (old, chronic sick, very
young) from beginning and increasing on daily basis, then fit people, if no
access to water or food. People will break into shops, etc., when visibility
returns, but many people will be marooned far away from shops and unable to
drive anywhere. Some houses will have private wells but many will have
become filled up with ash. Medicines will run out and cannot be replenished,
hospitals will cease functioning - no power, water or communications
 All transport will cease – including rescue vehicles, rail and air transport.
 Communications – all landlines will be brought down by ash and questionable
whether mobile phone microwave transmitters will work. Power will fail, so
all phones will fail eventually.
 Power - power lines are mostly overhead and moist ash will short the
insulators. There will be outages until repaired. Generators will have trouble
functioning, fuel oil will run out.
 Water – dependent on pumping: no water if power fails, except from rivers,
streams, etc. Pumping stations at reservoirs, etc., can get clogged with ash.
Ash will affect turbidity, water courses, etc., but water drinkable if filtered.
Lack of chlorination will lead to gastro-intestinal infections (diarrheoa and
vomiting).
 Food – will run out quickly. People will have to revert to hunting, after
looting shops, but wild animals die from destruction of habitats. Domestic
animals, e.g., cattle, die in absence of adequate food and water. Harvesting –
not possible until normal life resumes, all crops/fruit lost in > 30 cm area due
to destruction by ash and even > 15 cm area without adequate mechanisation.
 Roof collapse – people sheltering in houses, buildings at high risk of roof
collapse and death from injury and asphyxiation in >50 cm area, especially if
it rains and wet ash is double its weight on roof.
3.2. Ash fall in the 15-50 area (mortality 1-5%)
 As above, but half as many people seriously affected.
3.3. Ash fall 6-14 cm.
 Normally, not very serious effects, but area surrounded by ash for hundreds of
miles. Continuous high levels of fine ash in the air, even after rain. (mortality
1%).
3.4. Ash fall 1-5 cm.
 Vast area, with regular dusty conditions preventing life returning to normal,
even after rain, but conditions begin to improve after 5-7 days. Negligible
mortality.
4. Week 2.
4.1. Conditions in > 15 cm become very bleak. No food, water contaminated, waiting
for rescue and food drops, etc. A further 1-10 % die.
5. Week 3.
5.1. Conditions begin to improve in > 15 cm area, with roads cleared and supplies
getting through, but major clearance and keeping roads clear of ash will depend upon
movement of vehicular traffic and availability of petrol. More unusual infectious
diseases start to spread. No antibiotics or routine drugs left in many areas. A further
1-10 % die in > 15 cm and especially > 50 cm areas.
6. Week 4.
Anticipate gradual recovery, with supplies getting through and ash being cleared.
6. Urban-rural differences.
Cities will be paralysed in > 15 cm areas for days until streets cleared and failed
infrastructure repaired. Gradual run down of power (lack of fuel, hydroelectric
stations put out by ash), water and food supplies in the > 50 cm areas.
Problem in rural areas is isolation and inability to move with all transport halted for
days or even weeks in > 15 cm areas. Communication failure, with problems with
radio and TV with huge quantities of ash resuspended in the air and windblown
clouds of ash.
7. Recent analogue eruptions
7.1. Mount St Helens, USA, 1980.
Ash fall depth maximum 5 cm in central Washington. No rain for 5 days, all transport
stopped. Problems with power outages, water supplies, communications, and waste
water. Rapid resumption of normal living, ash cleared effectively. Increase in asthma
and bronchitis and people with pre-existing lung disease moderately affected. No
infectious disease epidemics or increase in mortality. This is a small eruption by
comparison ( Yellowstone 1000 times greater fallout of ash).
7.2. Mount Pinatubo, Philippines, 1992.
Second largest eruption of the 20th Century. 30,000 to 40,000 people evacuated from
around the volcano (30 km radius) to < 15 cm ash fall area. Over 300 deaths from
collapsing roofs (rain accompanied ash fall). No asthma or infectious disease
outbreaks. Most ash went into S. China Sea, so small eruption compared to
Yellowstone (at least 300 times bigger fallout).
7.3. Tambora, 1815. The largest historic eruption was Tambora, Indonesia, 1815.
(Yellowstone at least 20 times larger fallout). Darkness up to 2 days. Around 70,000
people may have died in that part of Indonesia from the direct and indirect effects of
the eruption (depth of ash fall 25-50 cm), but we have no details. Famine and disease
are mentioned by some authors as a significant cause of death.
Conclusion: We have no modern day experience of caldera eruption- sized ash falls
and how they would impact on modern society. The above figures are very
preliminary estimates only.
Factors to consider:
 Could it be worse? Yes. It depends upon whether the eruption occurs in the
summer or winter, and the effect the eruption could have on the
climate/weather. The layer of ash would reflect the sunlight in an abnormal
way and could alter rainfall. The above figures assume it is summer.
Mortality will be higher in the winter in the freezing cold and without power.
Snow will add to the visibility and transport problems, but would dampen
down the ash and enable it to be cleared away more easily (when it melted!). I
have not factored in deaths from floods and lahars in > 15 cm areas subjected
to heavy rainfall. The worst health scenario would be the absence of rain for
days or weeks out in the Mid-west and western USA, or a change in the
weather pattern leading to arid, windy conditions.
 Could it be better? Yes. With all its wealth and technology the USA could
prepare for this eruption and have the hardened facilities to enable them to dig
themselves out and respond more effectively. It would be planning on a
massive scale, but possible, I think. There is no planning feasible around the
volcano within the radius of the pyroclastic flows – that area will need
evacuating, no matter what. The key questions would be how to maintain the
US defence capability, whether the enemy be Cuba or El Quaeda, and the
effects on Wall St. A world economic collapse could be triggered and the
mortality in the USA and even world-wide following this eruption, due to the
knock-on effects, would escalate.
 A lesson from history? Modern technological societies have freed themselves
from the Four Horsemen of the Apocalypse scenario of poor harvests leading
to famine and pestilence (epidemics), war and sudden death – which you still
see in Stone-age, throw-back societies, such as Ethiopia. This would have
been the scenario in the part of Indonesia affected by the Tambora 1815
eruption. The devastation to agriculture in the Yellowstone eruption could be
overcome by pre-planning, stock-piling, agreements with the EC, etc., but a
breakdown in society due to economic collapse could see the Horsemen riding
again, with world economic collapse. The Steinbeck novel Grapes of Wrath
comes to mind…on a mini-scale by comparison.
Peter J Baxter
13/2/04