National Volcanic Monument - The Climatic Eruption of May 18, 1980
After 123 years of silence, Mount St. Helens resumed volcanic activity in March 1980. On March 27, seven days after the first earthquakes rumbled under the southwest Washington mountain, a steam and ash vent opened at the summit.
At 8:32 a.m. on May 18, 1980, a magnitude 5.1 earthquake triggered the largest landslide in recorded history. Reaching velocities up to 150 mph in only 10 minutes, the avalanche raced 15 miles and covered 25 square miles of the North Fork Toutle River Valley in debrits that measured an averange depth of 150 feet. Within minutes a lateral blast, equal to an explosion of 10 million tons of TNT, ripped through this sliding avalanche of debrits. The blast accelerated to 670 mph and to temperatures of 570 degrees F. (298.8 degrees C.) impacting 230 square miles. Ash clouds rose to 70 000 feet and on the first day of the exsplosion almost 50% of Washington State was covered in ash.