Geoscience Reference
In-Depth Information
Mount St Helens. By firing laser beams at reflectors placed around it, geophysicists were
able to measure its growth. It was pushing northwards at an incredible 1.5 metres per day.
By 12 May, certain parts of the bulge were more than 138 metres higher than before the
magma intrusion began. The volcano was literally being wedged apart into a highly un-
stable and dangerous condition.
Early on the morning of Sunday 18 May, Keith and Dorothy Stoffel were in a small plane
above the mountain when they noticed rock and snow sliding inwards into the crater. With-
in seconds, the whole north side of the summit crater began to move. The bulge had col-
lapsed in a great avalanche. It was like taking out the cork from a champagne bottle. The
magma inside it was exposed. The explosion was almost instantaneous. The Stoffels put
their plane into a steep dive to gain speed and escape. David Johnston of the US Geological
survey was not so lucky. An hour and a half before he had radioed in the latest laser beam
measurements from his observation post 10 kilometres north of the volcano. As the north
flank fell away, the blast headed straight for him. He was one of 57 people to die in the
eruption.
18. The eruption of Mount St Helens in Washington State in 1980 was one of the most
spectacular and best documented in recent times. The column of ash and smoke rose
nearly 20 kilometres into the atmosphere.
Though it began several seconds later, the blast quickly overtook the avalanche. It fanned
out at more than 1,000 kilometres per hour. Over a radius of about 12 kilometres, trees were
not only flattened but swept away. Nothing living or man-made was left. As far as 30 kilo-
metres away, trees were toppled, though isolated pockets survived in hollows. Even further
afield, the leaves were seared by heat and branches were snapped.
