Environmental Engineering Reference
In-Depth Information
SUPER VOLCANOES
new developments
Satellite imagery reveals large volcanic caldera structures, previously undetected by
ground-based geological surveys, just as it shows us ancient impact craters. They raise
questions about the size and frequency of what the media like to call super-volcanoes,
interested in the probability and likely impact of such eruptions in the near future.
Radiometric dating and geochemical assessment of volcanic ashes allow us to reconstruct
the age, frequency, size and impact area of previous eruptions. Their specific gas and
particulate emissions also permit estimates of their effect on atmospheric radiation
balance and hence climate. Although the
dust veil index
of ash in the stratosphere points
to reduced sunlight at Earth's surface, the effect of H
2
SO
4
(sulphuric acid) aerosol mist -
produced when sulphur emissions mix with water vapour - is more critical to climate by
raising atmospheric albedo. Direct, geophysical hazards apart (see box, p. 236), short- or
long-term climate changes threaten serious economic and social disrupton through
atmospheric pollution, famine and disease.
A
volcanic explosivity index
(VEI) is used to measure key eruption parameters such as
blast, duration and eruptive rate and the volume, height and material content of the
eruptive column. The latter influences the extent of tropospheric-stratospheric
penetration, downwind dispersal and atmospheric residence time (persistence). The index
operates on a logarithmic scale from 0 (gently effusive) through 3-4 (explosive) to 7-8
(cataclysmic) eruptions. Corresponding emission volumes and column heights are,
respectively, under 0·0001 km
3
and 0·1 km; 0·01-0·1 km
3
and 3-25 km; 10-100 km
3
and
over 25 km. They may, in turn, stimulate global temperature reductions of up to 0·1° C,
0·2-0·3° C and 2°-4° C for periods of months or years. Recent VEI scores are as follows:
Etna
(
2001
) 1-2,
Soufrière Hills
(
1995
) 3,
El Chicón
(
1982
) 4,
Mount St Helens
(
1980
) 5,
Krakatau
(
1883
) and
Pinatubo
(
1991
) 6, and
Tambora
(
1815
) 7.
Of over 6000 eruptions during the
Holocene
temperate stage of the last 10,000 years,
one in fifty has equalled or exceeded the VEI 5 of Mount St Helens with recurrence
intervals of 50-100 years. VEI 7 events have recurrence intervals of less than 1000 years.
With much longer intervals of 50,000 years, the risk of a VEI 8 eruption seems remote -
except that it is 73,000 years since Toba erupted in Indonesia. Spewing more than a
billion tons of sulphurous gases into the atmosphere, together with over 2500 km
3
of ash
and tephra, Toba reduced surface radiation. This lowered global temperatures by 3-4° C
at a time when the
Devensian
cold stage entered its coldest phase. Where is a statistically
overdue repeat performance
