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Cooling the inferno.

Prof. Bill McGuire examines ways to mitigate the terrifying effects of volcanic eruptions.

Over the last century, a range of volcanic hazards have claimed over
60,000 lives, and between 1980 and 1990 alone they have affected the
day-to-day lives of over 600,000 people.

Although a battery of measures are available to reduce the impact of
volcanic hazards, rapidly increasing populations near active volcanoes in
the developing world will ensure increasing vulnerability to such hazards,
and new initiatives are required to prevent more disasters.


Unlike other natural hazards, such as floods or earthquakes, volcanoes are
unique in generating a range of destructive phenomena of widely differing
characteristics. These have the potential not only to cause injury and
loss of life on a major scale, but also to seriously disrupt the local or
regional economy for several years, and to make large tracts of land
unusable for centuries. It is estimated that there are currently around
550 active volcanoes with over 10% of the world's population living
sufficiently close to them to be at risk from eruptions.


Lava flows

Due to their low speeds, which normally range from a few metres to several
kilometres per hour, lava flows are rarely life-threatening. Nevertheless,
all but the most fluid lava flows have the strength to demolish buildings
and their volume may be sufficient to inundate hundreds of kilometres of
usable land.

Lava flows are generally categorised into aa flows - which are fed by
tubes or channels and move forward on a relatively narrow front - and
pahoehoe flows, which are generally more fluid and move forward faster on
a wider front.

A number of measures can be taken to try to block or divert aa flows,
including cooling the front with water and erecting physical barriers.

Another method is reducing the lava supply to the front by blowing up the
feeder tubes, thereby forcing the lava to travel over the surface of the
flow, causing thickening rather than lengthening.

As lava flows are driven by gravity, their paths can be predicted and
effective hazard maps constructed relatively easily.

Pyroclastic flows

Pyroclastic flows are among the most life-threatening of volcanogenic
hazards. These gravity-driven mixtures of high temperature gases, pumice,
hot ash and coarser debris are typically generated when a lava dome or
eruption column collapses. They are very quick (up to hundreds of metres
per second) and very hot (300-800 degC). Like lava flows, they can inflict
near-total structural damage and inundate large areas of land.

The only effective way of limiting the effects of pyroclastic flows is by
evacuating threatened areas identified by hazard mapping and up-to-date
monitoring. The flows are usually confined to valleys and low-lying areas,
so the threat to inhabitants can be greatly reduced by adopting a sensible
building policy.

Debris flows and floods

Debris flows and floods - or lahars - are responsible for nearly all the
volcano-related deaths over the last 15 years. For example, the eruption
of Nevado del Ruiz in Colombia in 1985 resulted in the worst volcanic
disaster in the last 95 years. A relatively minor eruption melted part of
the snow and ice field capping the volcano. The resulting flash flood
travelled down a river valley, rapidly turning into a debris flow. At the
valley entrance, over 40km from the summit, the flow engulfed the town of
Armero, killing 25,000.

Mitigating the effects of debris flows is relatively simple because they
are severely constrained by topography. Early warnings sensors on the
slopes can allow time for evacuations, while baffles and sediment dams can
reduce the mass and destructive potential of the flows.

Other hazards

Volcanoes can also produce other hazards, including tephra (volcanic
ejecta, like ash and ballistic projectiles), poisonous gases, volcanogenic
earthquakes and tsunami (see box).

These hazards can have effects far beyond the volcano itself. Heavy ash
falls can cause economic disaster and famine by destroying and
contaminating crops, while rising columns of ash endanger aircraft. More
than 80 civil aircraft have suffered damage from volcanic eruptions in the
last 15 years, although initiatives such as satellite tracking of ash
columns should reduce future risks.

Mitigating the effects of tsunami is a major challenge for scientists, as
the largest of these events have the potential to devastate a vast area.
For example, the collapse of the unstable western half of the Cumbre Vieja
volcano in the Canary Islands could generate a tsunami that would pose a
serious threat to the eastern seaboard of the US. Only an Atlantic tsunami
warning system combined with evacuation plans for threatened areas could
reduce casualties in such cases.


Monitoring is critical in reducing the impact of volcanic hazards. A
comprehensive seismic array and a range of geodetic monitoring methods may
be supplemented by the results of microgravity, magnetic, electrical and
gas geochemistry surveys to provide a picture of the disposition of magma
within a reactivated volcano before it erupts.

Such a monitoring network may allow the timing of the onset of an eruption
to be better estimated, but it is less effective in forecasting the nature
and extent of an impending eruption. These parameters may be estimated,
however, from studies of past eruptions to enable the drafting of hazard

While such maps should never be regarded as a precise guide to future
activity, together with a comprehensive monitoring programme they do offer
the best means of reducing injury and loss of life, mainly through timely


The Nevado del Ruiz tragedy demonstrated the importance of communicating
the seriousness of the threat to the local authorities. At the later
eruption at Pinatubo in the Philippines thousands of lives were saved by
the distribution of a video describing volcanic hazards, which persuaded
over a quarter of a million people to leave the threatened area.

However, even successful communications between scientists and local
authorities have been largely the result of a developing crisis. If the
numbers of volcanic disasters are to be reduced, more attention must be
paid to educating local officials and local populations prior to any
crisis, particularly where volcanoes have not erupted in living memory and
there is no understanding of the threat.


At present little more than 20% of the world's 550 active volcanoes are
monitored to any extent. It is vitally important that greater numbers of
active volcanoes are monitored, even if only at a basic level. This may be
accomplished through increased use of satellites capable of observing
volcanoes. Volcano awareness programmes should be initiated at all active
volcanoes, targeted at making the local population understand the
potential threat. These should be accompanied by pre-emptive and
responsive contingency plans formulated by the civil authorities, on the
advice of informed scientists.

Modern society has yet to face a volcanic eruption on the largest possible
scale. The consequences of a major eruption could be catastrophic on a
global scale, with the potential for a global temperature fall of 3-5

Furthermore, such an event may be beyond our ability to effectively
mitigate, particularly given the short-sighted and short-termist
approaches of most national governments and international political

- Ways to mitigate volcanic hazards Lava flows - damming and/or diversion;
flow-front water cooling

- Pyroclastic flows and surges - judicious siting of settlements;

- Debris flows and floods - judicious siting of settlements; construction
of elevated refuges; construction of sediment dams and baffles; dredging
and levee construction; seismometer and trip-wire warning systems

- Tephra - evacuation of poorly constructed buildings; removal of
accumulating tephra from roofs; availability of face masks/protective
headgear; appropriate medical care for respiratory problems; contingency
plans for power, communication and transport disruption; availability of
uncontaminated water supplies; measures to minimize crop and livestock
damage; warnings to air traffic

- Landslides and debris avalanches - identification of collapse-prone
areas; slope-stability monitoring; pre-evacuation

- Directed blasts and shock waves - pre-evacuation

- Volcanic gases - gas monitoring; resettlement if a persistent problem;
pre-evacuation if episodic and predictable; public safety guidelines;
construction of elevated refuges where appropriate

- Tsunami - identification of unstable slopes adjacent to water; slope
stability monitoring; pre-evacuation; establishment of tsunami warning

Mt Pelee Martinique 1902 29 000 Pyroclastic flows
Kelut Indonesia 1919 5 110 Debris flows
Lamington Papua New Guinea 1951 2 940 Pyroclastic flows
El Chichon Mexico 1982 1 700 Pyroclastic flows
Nevado del Ruiz Colombia 1985 25 000 Debris flows
Lake Nyos Cameroon 1986 1 746 Volcanic gases
Pinatubo Philippines 1991 500 Various
Source: Benfield Greig Hazard Research Centre.
Professor Bill McGuire is head of the Benfield Greig Hazard Research
Centre at University College London.
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