The big one Here's what happened when Mount St. Helens exploded 25 years ago today: At 8:32 a.m., a 5. 1 earthquake rumbled 1 mile beneath...
At 8:32 a.m., a 5.1 earthquake rumbled 1 mile beneath the volcano.
One second later, the mountain’s north flank collapsed, creating the largest landslide in recorded history. In less than 10 minutes, debris filled nearly 25 square miles of the North Fork Toutle River valley to an average depth of 150 feet.
Rock and hot gases exploded sideways at 220 to 670 mph. The blast felled trees up to 12 miles away in a 180-degree arc north of the volcano. Some 57 people are killed, most by suffocation.
A slurry of melted snow and ice, boulders and sediments swept down mountain streams around the volcano. The largest mudflow, or lahar, raced down the North Fork Toutle River valley at up to 27 mph. It deposited more than 45 million cubic yards of sediment in the Columbia River, blocking oceangoing shipping for 13 days.
A vertical column of ash and stream rose 15 miles above the mountain. The cloud drifted to the northeast, depositing more than 3 feet of ash near the volcano. By 11:45 a.m., ash fell in Spokane. The ash cloud eventually circled the globe.
Hours after the avalanche and blast, super-heated pumice and gas flowed from the crater and into the valley north of the mountain. Temperatures of these pyroclastic flows reached 1,300 degrees Fahrenheit.
The eruption subsided about 5:30 p.m. after ejecting 540 million tons of ash.
Sources: U.S. Geological Survey, Seattle Times archives