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ranges. Mt St Helens also highlighted the fact that the
previous 60 years worldwide were generally quiescent
volcanically.
CONCLUDING COMMENTS
There is no doubt that the Earth is experiencing one
of the most intense periods of volcanism in the last
10 000 years. This period began at the beginning of
the seventeenth century, concomitant with global
cooling that peaked in the Little Ice Age. The volcanic
events of the latter half of the twentieth century
must be viewed in this context, rather than as freak
eruptions of supposedly dormant volcanoes. The
eruptions of El Chichon, Mt St. Helens, and Mt
Pinatubo are the most dramatic examples of this
reawakening. While Mt Pinatubo stands out as a
dominant event, there is little realization that it
was but one of three major volcanic eruptions in
1991-1992. The other two were Mt Hudson in Chile
and Mt Spurr in Alaska. Since then, volcanic activity
has become quiescent. Whether or not this represents
a resumption of inactivity similar to that between the
eruption of Caribbean volcanoes in 1902 and Mt
Agung in 1961 is academic. However, since the
eruption of Mt St Helens in 1980, renewed volcanic
activity and associated phenomena have taken more
than 24 000 lives. In our present era, volcanic
eruptions are pervasive, unpredictable, and deadly.
The north side of Mt St Helens after the 18 May 1980
eruption. The crater has not formed by collapse, but by a
lateral blast. The material in front represents the
pyroclastic debris from that blast (photograph courtesy of
James Ruhle and Associates, Fullerton, California).
Fig. 11.12
incoming solar radiation, but the worldwide effect on
climate was virtually irrelevant. The greatest effect
occurred within 700 km of the eruption. Ash accumu-
lated to depths of 10-15 mm, 150 km downwind of the
eruption. It also made driving conditions in and around
the area treacherous for five days because of low
visibility and slippery roads. Speed limits had to be
reduced to 8-10 km hr
-1
to prevent accidents. The
deposition of ash proved a major clean-up problem in
urban centers. The small town of Yakima, Washington,
with a population of 50 000, took ten weeks to
remove half-a-million tonnes of ash, a task that cost
$US2 million. The fine dust clogged air filters on cars,
got into brakes and wrecked motors. Dust fallout on
electrical equipment caused equipment failure, while
crop yields noticeably decreased because the ash
coated leaves and lowered the efficiency of photosyn-
thesis.
Given the magnitude of the event, the media hype,
and the reaction by the public and government, the
true economic effects of the disaster tended to be over-
estimated. Washington State estimated the damage at
$US2700 million, and Federal Congress appropriated
just under $US1000 million for disaster relief. In
effect, the eruption caused $US844 million damage.
Clean-up cost $US270 million; agricultural losses
amounted to $US39 million; property damage, mainly
to roads and bridges, cost $US85 million; and com-
mercial timber losses amounted to $US450 million.
The eruption sparked renewed research interest into
other volcanoes along the Cascade and Sierra Nevada
REFERENCES AND FURTHER
READING
Blong, R.J. 1984.
Volcanic Hazards: A Sourcebook on the Effects of
Eruptions
. Academic Press, Sydney.
Bolt, B.A., Horn, W.L., MacDonald, G.A., and Scott, R.F. 1975.
Geological Hazards
. Springer-Verlag, Berlin.
Branney, M and Zalasiewicz, J. 1999. Burning clouds.
New Scientist
17 July: 36-41.
Cita, M.B., Camerlenghi, A., and Rimoldi, B. 1996. Deep-sea
tsunami deposits in the eastern Mediterranean: new evidence
and depositional models.
Sedimentary Geology
104: 155-173.
Coates, D.R. 1985.
Geology and Society
. Chapman and Hall, New
York.
Fielder, G. and Wilson, I. 1975.
Volcanoes of the Earth, Moon and
Mar
s. Elek, London.
Halldórsson, M.M. and Brandsdóttir, B. 1998.
Subglacial volcanic
eruption in Gjálp, Vatnajökull, 1996: The jökulhlaup
. <http://
www.hi.is/~mmh/gos/vat-update.html>
Hays, W.W. 1981. Facing geologic and hydrologic hazards:
Earth-science considerations.
United States Geological Survey
Professional Paper
1240-B: 86-109.
