Folllowing a lull in its recent frequent earthquake and minor volcanic activity, the summit of Mt. St. Helens in Washington disappears in a massive landslide, releasing a powerful (300mph) lateral explosion that flattens nearby forest land ahead of a devastating release of debris and snowmelt mud known as a lahar. Within an hour, with the summit crater exposed, the remaining magma stored under Mt. St. Helens surges upward, resulting in a massive eruption from the summit, lasting nine hours and wiping out hundreds of square miles of forest and killing dozens of people, including geologists who had been on station to monitor the volcano’s activity. Following the eruption, Mt. St. Helens is over 1,000 feet shorter, its peak replaced by a mile-wide crater. This is the first significant volcanic eruption on the American mainland since 1915.
