Most volcanic eruptions occur in stages, each of which may cause different degrees of destruction. Two months prior to the massive 1980 eruption of Mount St. Helens, a series of small earthquakes precipitated steam explosions that blasted a crater through the volcano's ice cap. By May 17, more than 10,000 earthquakes had shaken the volcano, and a large bulge — visual evidence that molten rock had risen high into the volcano — appeared on its northern flank. A day later, a stronger earthquake shook loose the bulge, resulting in the largest known landslide in history. The landslide released pressure inside the volcano, triggering a lateral blast northward that deposited blocks and smaller rock fragments over a wide area and splintered mature trees like matchsticks.
Almost immediately following the lateral blast, a vertical column of steam, water, and volcanic debris erupted from the newly formed crater. Within 10 minutes, the column had risen into the atmosphere more than 19 kilometers (12 miles) above sea level. Hundreds of thousands of tons of ash completely darkened the sky to the east of the volcano for more than 200 kilometers (125 miles).
Parts of the eruption cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano. The hot rocks and gas quickly melted some of the snow and ice capping the volcano. The resulting surge of water mixed with loose rock debris to form lahars, volcanic mudflows with the consistency of wet concrete. Flowing at speeds up to 130 kilometers per hour (80 miles per hour), the lahars uprooted trees, destroyed roads and bridges, and buried everything in their paths.
Beginning just after noon and continuing over the next five hours, multiple avalanches of hot ash, pumice, and gas poured out of the crater. During explosive eruptions, fiery, pyroclastic flows travel downslope from a volcano. The Mount St. Helens pyroclastic flow spread as far as five miles north of the crater. Based on measurements of the plume height as well as the volume of expelled material, the Mount St. Helens eruption was extraordinary in size — a so-called Plinian eruption that occurs once every hundred years or more.
The destruction caused by the eruption was widespread. Nearly all exposed life forms were decimated over a 240-square-kilometer (150-square-mile) area. Remarkably, beginning just a few months after the initial blast, vegetation returned to Mount St. Helens. First to appear were small trees and plants that had been protected by snowpack. Gradually, new seeds arrived — carried by the wind and by animals that moved in from adjacent areas. Twenty years later, significant populations of mammals, birds, insects, and fish had been re-established and, as images from space-based monitoring satellites show, most of the affected area has again been covered by vegetation. It will take 200 years or more, however, to restore the old-growth forest conditions that preceded the blast.