Since its eruption in May 1980, Mount St Helens has become the best know of North America’s Volcanoes. Before 1980, St Helens was just another of the line of snow-clad cones that stretch along the United States’ Pacific coast. It was distinguished by its fine, symmetrical shape, in market contrast with the gnarled peaks of its neighbours Mount Rainer and Mount Adams. In early 1980, St Helens began to pour from fractures near the summit. The snow began to darken as rock fragments became caught up In the violence of the explosions, and a small crater began to form and deepen near the summit.
During April and early May 1980, the northern side of the volcano began to push out, bulging under the pressure of the molten rock beneath. On the morning of May 18, a moderate earthquake proved to be the final straw. The ‘bulge’ slid off the volcano’s side, triggering a massive explosion. Over the course of the next couple of minutes, a blast of hot ash raced across six hundred square kilometers of prime forest, tossing trees aside like matchsticks. For the rest of the day, a column of ash and pumice rose twenty kilometers above the volcano. Ash rained out across the northwestern states. The top four hundred meters of the volcano was replaced by a gaping grey amphitheatre. For the next six years, a small dome of lava pushed its way out into the new crater, until gradually the conduit was plugged and activity stopped. In all, about one cubic kilometer of rock was thrown out during the whole eruption.
Mount St. Helens was a tiny eruption by global standards. Yet it taught us a lot. Until St Helens, nobody realised quite how fragile volcanic mountains are. Since then, numerous examples of volcanoes that have collapsed sideways have been recognized from the telltale carpet of hummocks and hills that stretch away from the remains of ancient amphitheatres. St Helens also gave an inkling of nuisance that even a small eruption can cause to a society that relies on sensitive equipment for transport, communications and day-to-day life.
View south along west arm of Spirit Lake to the decapitated Mount St. Helens in 1983. Part of the gigantic landslide (debris avalanche) that removed the former summit swept entirely through Spirit Lake (hummock in foreground), causing a tsunami to sweep as high as 250 m (820 ft) above the lake.