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period of eruption was in the mid-19th century. This was a sign that Mount Saint Helens was the kind of volcano that erupted only sporadically. That being the case, it was therefore likely to be the type that erupted violently.
As with all volcanoes, the first signs of an eruption were given by an increase in earthquake activity in the vicinity of the mountain. This showed that the pressure from the magma chamber below was causing the brittle rocks of the cone to begin to crack apart. Soon, enough lines of weakness would be created by the earthquakes for the top of the mountain to be dangerously unstable. The earthquakes were particularly numerous below
the north flank of the volcano, indicating that it was the likely location of the eruption.
In fact, the first eruption was from the summit rather than from the flank and took place on the 27th March, just a week after the rapid increase in earthquakes. This small eruption continued in fits and starts thereafter. The material that came from the volcano was mainly gas and ash, and a small cloud of material was sent up over the cone. At this stage it was a Vulcanian type of eruption; but because the size of the eruption was small, the ash fell down to the ground close to the summit, and the main effect was simply to blacken the snowfields.
The cause of these eruptions was probably water from the icefields seeping down to the hot rock below and then flashing into steam. The effect
of this explosive change from water
to steam was to throw out material
from the vent. But this was all old
material, and no new magma was
involved at this stage.
As the days went by, a large bulge, about two kilometres across, appeared to be forming on the northern flank of the mountain, separate from the central vent. It showed that the material blocking the central vent was stuck fast, and that the magma was seeking a way
(Above) Mount Saint Helens is in the Cascade Mountains in the northwest United States.
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Mount Saint Helens North America
(Below) Mount Saint Helens (foreground) and other volcanic peaks in the Cascade Mountain chain before the eruption of 1980.