Geoscience Reference
In-Depth Information
minor volcano. However, research in the mid-1970s
began to indicate that ash found in the region origi-
nated mostly from Mt St Helens. On 20 March 1980, a
swarm of micro-earthquakes around Mt St Helens
signaled renewed activity, which culminated in a small
eruption on 27 March sending clouds of ash 6 km
skyward. Harmonic tremors began on 3 April, signaling
the movement of magma into the magma chamber
below the volcano. By this time, over 70 million m
3
of
tephra had been ejected. Prior to the eruption, the
north side of the volcano bulged more than 150 m at a
rate of 1.5 m per day.
At 8:30 am on 18 May, an earthquake of magnitude
5.1 on the Richter scale started landslides in the bulge
region. Immediately, the volcano erupted and sent out
a lateral explosion, which was felt 425 km away. A dark
cloud of ash, containing 2.7 km
3
of material weighing
520 million tonnes, rose to a height of 23 km over the
next nine hours. The height of the mountain was
reduced by 500 m (Figure 11.12). Over 500 km
2
of
forest were devastated by the resulting base surge that
reached temperatures of 2600°C. The blast overrode
ridges over 10 km away. In some places, debris
deposits accumulated to depths of 150 m. Snow and ice
on the mountain were melted by the blast and formed
lahars that rushed into Spirit Lake, filling the lake to
depths of 60 m. The lahars then moved over 50 km
down the north and south forks of the Toutle River,
reaching Kelso on the Columbia River. Lahars also
flowed down into the Muddy River to the east. In
all, over 300 km of roads and 48 road bridges were
extensively damaged. The lahars and fine ash that
settled in the surrounding area were soon carried into
the Columbia River, where the 180 m wide shipping
canal was reduced from a depth of 12 m to one of
4.3 m. Only 60 people lost their lives, mainly because
the government had ordered evacuations as the inten-
sity of the eruption increased. However, some of this
death toll included people who were permitted back
into the area only days before the eruption. Tephra
fallout became the worst nuisance after the initial
eruption. While most of the dust was blasted out the
side (Figure 11.12), ash fell to the ground in eastern
North America and over the Atlantic. Within 17 days,
ash had encircled the globe at a height of 9-12 km,
at the top of the troposphere. Some ash moved into
the stratosphere, but this amount was minor. Temper-
atures were estimated to have decreased by 0.5°C for
a few weeks directly downwind because of the reduced
The city of St Pierre, Martinique, after the 8 May 1902
eruption of Mt Pelée. The pyroclastic flow, which
destroyed the city, swept over the ridge at the top of
the photograph. The waterfront had been wrecked by a
tsunami caused by a lahar several days previously
(photograph from a book by A. Lacroix, courtesy of the
Geological Museum, London).
Fig. 11.11
stripped from them by the force of the blast. Others
were grotesquely disfigured, either as their body fluids
boiled and burst through their skin, or as their muscles
contracted in spasms as they fought for air. There were
only two survivors, one of whom, Auguste Ciparis, was
a condemned prisoner in the local jail. He suffered
severe burns and later had his sentence commuted.
Subsequent blasts on 19 May and 20 August swept
across most of the northern and western slopes,
causing further destruction. The last explosion killed
2000 people in five mountain villages. Of all historical
volcanic disasters, the residents of the city of St Pierre
and the island of Martinique appear to have suffered
one of the most devastating eruptions known.
Mt S
t Helens (18 May 1980)
(Hays, 1981; Lipman & Mullineaux, 1981; Keller, 1982;
Blong, 1984; Coates, 1985)
The Mt St Helens eruption is notable for several
reasons. Firstly, it was the first large explosive eruption
to occur in the world for several decades. Secondly, it
heralded a period of substantial volcanic activity at the
beginning of the 1980s. Thirdly, it had associated with
it several hazard phenomena. Finally, it was the largest
eruption to occur in the conterminous United States in
recorded history. Mt St Helens is situated in the
Cascade Mountains of Washington State (Figure 11.4).
It had not had a major eruption for 123 years and,
compared to Mt Rainier, was considered a relatively
