The catastrophic Mount Saint Helens volcanic eruption that took place on May 18, 1980 was the result of several factors that began with an accumulation of magma within the mountain's edifice earlier that year. A resulting bulge on the mountain's north flank was further weakened when an earthquake triggered a debris avalanche that relieved the pressure that had been acting as a counter-force to the magma buildup. Relieving the pressure resulted in the water that was contained within the system turning into steam, which then forced its way out explosively through a landslide scar.
The steam and magma released by the explosive lateral blast reached speeds estimated to be as high as 300 mph. A 230 square-mile area was devastated as the fast-moving and incendiary-temperature debris covered it. Buildings and dense forest areas were flattened during the initial pyroclastic flow.
Within a few minutes of the lateral blast, an eruption plume burst through the summit crater that soon reached a height of about 15 miles. By the time the more extreme events brought on by the eruption subsided, 57 people were killed and more than 200 homes were destroyed or severely damaged.