Geoscience Reference
In-Depth Information
Volcano futures
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 similar eruptions in future. Radiometric dating
and geochemical assessment of volcanic ashes allow us to reconstruct the age, frequency, size and impact area
of previous eruptions (see
Chapter 23).
Specific gas and particulate emissions also permit estimates of their effect
on atmospheric radiation balance and hence climate. Although the stratospheric ash dust veil index 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 volcanic hazards apart (see box,
p. 209), short- or long-term climate changes threaten serious economic and social disruption through atmospheric
pollution, famine and disease.
Volcanic explosivity index (VEI) measures 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 0·1C, 0·2-0·3C
and 2
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, Krakatoa (1883) and Pinatubo (1991) 6, and Tambora (1815) 7.
Of over 6,000 eruptions during the Holocene temperate stage during the past 11.5 ka, 2 per cent have 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 1,000 years. The risk of a VEI 8 eruption seems remote, with much longer intervals of 50 ka -
except that it is 73 ka since Toba erupted in Indonesia. Spewing 1 Gt of sulphurous gases into the atmosphere,
together with over 2,500 km
3
of ash and tephra, Toba attenuated incoming radiation. This lowered global temperatures
by 3-4
-4
C at a time when the Devensian cold stage was entering its coldest phase.
Where might a statistically overdue repeat performance occur? Volcano-seismic activity almost anywhere around the
Pacific Rim and in the eastern Mediterranean region offers several potential sites. One site particularly worth watching
is in Yellowstone National Park in the western United States. Although best known for its hot springs and geysers,
the United States Geological Survey (USGS) co-sponsors the Yellowstone Volcano Observatory, monitoring increased
ground swelling (diapirism) and earthquake activity. Like several others worldwide, the Yellowstone calderas cover
10
3-4
km
2
, and were produced by cataclysmic eruptions 2 Ma, 1.3 Ma and 600 ka ago. A repeat would threaten the
economic and political pre-eminence of the United States and exchange rapid global cooling for the current warming.
high pore pressures and dehydration through surface
evaporation precipitates dissolved minerals, especially
carbonates, silicates and iron compounds. These cement
the grains and give
cohesive strength
to the developing
rock mass.
driven by the same thermal convection responsible for
the mid-ocean ridges and is known as
hydro-thermal
circulation
. It alters the condition of sea-floor rocks
and is capable of generating new minerals. Oceans
are also a major route for recycling crustal lithosphere
by reprocessing terrigenous sediments via subduction
or accretion, albeit on far longer time scales. Together
these processes form a major part of the rock cycle
and influence the quality (geochemistry and turbidity),
performance and biochemical processes of
ENVIRONMENT
It is reckoned that the entire global ocean is cycled through
oceanic crust every few million years. This process is
ocean
water.
