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VOLCANIC HAZARDS AND VOLCANO MONITORING 177

In some cases, especially in areas of high rainfall and animals, although flying into the distal parts
and hence high surface erosion, the topography of of atmospheric eruption clouds poses a major
a volcano is such that there are obvious paths that threat to aircraft.
lava flows, pyroclastic density currents, lahars and • Pyroclastic density currents and surges are fatal
mudflows are likely to take. In these cases it is to people and animals, and flows exert enough
possible to erect barriers in an attempt to stop force on obstacles to destroy most buildings. The
the flows, or at least divert them from populated speeds of flows and surges are so large that real-
areas. As seen earlier the forces that lavas flows and, istically they cannot be outrun and pre-eruption
especially, pyroclastic density currents can exert evacuation is the only defense.
on obstacles in their paths are very large, and there • Volcanic mudflows and lahars produced by
have been few attempts to erect barriers intended mobilization of pyroclastic deposits by water
to stop them, although a few at least partially suc- have a similar mechanical destructive power to
cessful attempts at lava flow diversions have been pyroclastic density currents.
made on Mount Etna. Barriers against mudflows • All volcanic gases except water vapor are chemi-
and lahars have had more success, for example in cally poisonous and all of them are unbreathable.
the case of lahars from Mount Pinatubo. All except water vapor are heavier than air when
If there is no opportunity to evacuate before- at ambient temperature and so can collect in
hand, then the actions that can be taken to reduce hollows with lethal consequences to animals and
the impact of an eruption are limited. If the erup- people.
tion is explosive and involves the formation of a fall • Hazard assessment for a volcano requires the
deposit, sweeping roofs clean of accumulating ash recognition of the products of previous erup-
will minimize the chances of structural collapse. tions and the analysis of their emplacement to
But if the density of ash in the atmosphere is too obtain an idea of the likely range of possibilities
great, the health hazard will outweigh the econom- for future eruptions.
ic benefit of staying to protect property. In that case • Numerous volcanoes that erupt frequently, or are
the best choice is to walk (assuming no working near inhabited areas, are continuously monitored
vehicles are available) away from the axis of de- by recording seismic activity, measuring the de-
position at right-angles to the direction of the formation of the volcanic edifice, or monitoring
wind, bearing in mind that if the activity evolves the gases being released from shallow magma.
to produce pyroclastic density currents or surges, • Even given detailed monitoring, it is still not easy
high ground may offer a small advantage. to give accurate warnings of impending activity,
and volcano alert systems give qualitative assess-
ments of the probability of a given level of activ-
11.6 Summary ity of a given type within a given time frame.

• The hazards from volcanoes can be direct, in the
form of the immediate effects of lava or ash on 11.7 Further reading
people and buildings, or indirect, for example in
the form of direct destruction of crops or agricul-
Casadevall, T.J. (1992) Volcanic hazards and aviation
tural land causing starvation, or effects on the
safety: lessons of the past decade. FAA Aviat. Saf.
climate (see Chapter 12).
J. 2(3), 1–11.
• Lava flows rarely kill people or animals but Chester, D.K. (2005) Volcanoes, society and culture.
commonly completely destroy property and ruin In Volcanoes and the Environment (Eds J. Marti &
agricultural land. G.J. Ernst), pp. 404–39. Cambridge University Press,
• Pyroclastic fall deposits are mainly a threat to Cambridge.
buildings unless the density of fine ash particles Chester, D.K., Degg, M., Duncan, A.M. & Guest, J.E.
in the air becomes a health hazard to people (2001) The increasing exposure of cities to the