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out through the northern flank.
Although geologists could not know this at the
time, the earthquakes had weakened the whole north flank more than any other part of the cone. The rock was simply held together by friction. As the bulge grew, the rock was being pushed apart and replaced by liquid magma, so that the cone was liable to fail at any moment.
The actual moment of failure was recorded in the form of a much larger earthquake on the 18th May just after 8 a.m. At this time the entire north flank of the volcano dropped down in a massive landslide of two cubic kilometres of rock. It was as though the cap had been taken off a shaken soda bottle (see page 36). Now, with nothing to hold it in place, the gas-rich magma was able to escape in a gigantic explosion.
In fact the landslide was shattered into tiny fragments by the explosion, so that a mixture of rock fragment and gas was formed. This part of the eruption
(Below) The destruction was to the north of the volcano because the new eruption was on the northern flank
of the cone. Nonetheless, the whole top of the cone was blown away as well as a large part of the northern flank. The destructive power of the volcano is seen in the satellite image by the contrast between the forested (green) southern flank and the large area of barren land (brown) to the north of the volcano. The feature in the centre of the cone is a lava dome that slowly built up in the years after the explosive eruption. It is made of a very acid form of lava.
Blast
N
Spirit lake
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