Geoscience Reference
In-Depth Information
Soon after the lateral blast, a vertical column of ash and steam began to rise. In less than 10
minutes it was 20 kilometres high and began to expand into a typical mushroom cloud. The
swirling ash particles generated static electricity, and lightning started many forest fires.
Wind soon spread the ash to the east, and space satellites were able to track it right around
the Earth. Between 1 and 10 centimetres of ash fell over most of northwest USA. During
the nine hours of vigorous eruption, about 540 million tonnes of ash fell over 57,000 square
kilometres.
Yet another hazard of such eruptions are so-called pyroclastic flows. These are made up
of particles of rock or magma shattered by explosions and swept along at several hundred
kilometres per hour in a mass of hot gas. Their speed and temperature make them particu-
larly deadly. When Mount Pelée in Martinique in the West Indies erupted in 1902, a pyro-
clastic flow swept down on the city of St Pierre, killing almost all the 30,000 inhabitants.
Ironically, one of the two survivors lived because he was in solitary confinement in a thick-
walled, poorly ventilated cell in the prison. Pyroclastic flows on Mount St Helens reached
no further than the avalanche debris, though in some places where the material came to rest
in old lakes, the heat was still sufficient to flash the water into steam and cause what looked
like secondary eruptions. It may have been a pyroclastic flow that destroyed Pompeii in 79
AD
.
Mount St Helens itself was left 400 metres lower than before the eruption, with a large new
crater in the middle. There were several more explosive eruptions during 1988 and one in
1992, but none was as spectacular as the first. Today the mountain is bristling with scientif-
ic instruments that could pick up signs of further activity.
Blasts from the past
The eruption of Mount St Helens may seem tremendous, but it is small in comparison with
others in the historical and prehistoric past. It threw 1.4 cubic kilometres of material into
the air. The eruption of Tambora in Indonesia in 1815 by comparison ejected about 30 cu-
bic kilometres, and that of Mount Mazama in Oregon in 5000
BC
produced an estimated
40 cubic kilometres of ash. In 1883, the island of Krakatau (which is west of Java, not east
as in the film title) blew up, leaving a 290-metre deep crater in the sea floor. Most of the
36,000 casualties were drowned in the resulting tsunami, in which a 40-metre wave stran-
ded a steam ship deep in the jungle. Around 1627
BC
it was the turn of the Aegean island
of Santorini, or Thera, to blow. This happened at the height of the Minoan Bronze Age and
may have contributed to the downfall of that civilization and the legend of the lost land of
Atlantis. On the geological timescale, those are just the latest in a series of violent erup-
tions.
