At 8:32 a.m. a 5.1 magnitude quake struck one mile below the mountain. While there had been literally hundreds of earthquakes at the mountain since March 20th, the unstable north face could not sustain another. Within moments the largest landslide in recorded history removed more than 1,300 feet from the summit and swept away almost the entire north side of the mountain. The elevation of the mountain dropped from 9,677 feet to its present day 8,363 feet. What was once the 9th highest peak in Washington State was suddenly reduced to the 30th highest peak? The intense high pressure/high temperature steam that escaped instantly turned more than 70% of the snow and glacial ice on the mountain to water. This massive movement of rock, ash, water and downed trees swept into Spirit Lake and down the north fork of the Toutle River Valley at speeds in excess of 175 miles per hour.
As the north face slid away it let loose the trapped gases like a cork removed from a well shaken bottle of champagne. Rosenquist a well known photographer and the rest of his party survived the eruption even though the lateral blast was rushing straight in their direction at speeds nearing 650 miles per hour. Luckily, after rolling over ridge after ridge, the blast suddenly turned. In only three minutes the blast flattened 230 square miles of old growth forest in a fan shape north of the mountain.
As the north face of the mountain collapsed, expanding gases and steam from the molten rock hurtled rock and ash out across the land at speeds up to 670 miles per hour uprooting trees as far as 6 miles from the mountain. It was impossible to outrun and the searing heat too intense. Reid Blackburn, a photographer with The Columbian newspaper was one of the first victims of the eruption. Blackburn had tried to take shelter in his car but the lateral blast blew in the windows, letting in the hot, choking ash. He was found in his car 7 miles north of the mountain.
Along with the loss of human life, it seemed the plants and animals in the area were almost completely wiped out as well. It is estimated that 5,000 black-tailed deer, 1,500 Roosevelt elk, 200 black bears, and 15 mountain goats fell victim. Millions of small game, fish, birds and insects were also in the path of the eruption.
The areas nearest the mountain were completely stripped of topsoil by the lateral blast. Nothing remains but bare rock. For weeks after the eruption, pyroclastic flows of escaping steam and ash rolled down the remains of the north face, sterilizing the soil with its intense 1200 degree heat and depositing pumice and ash up to 60 feet thick in the valley below. Two weeks after the eruption when temperature measurements could safely be made, the pyroclastic deposits still averaged 650 degrees. Today, the pumice plan, a fan shaped area just below the crater, has been heavily eroded by wind and rain.
Directly in the path of the huge landslide was Spirit Lake. Normally a chilly 42 degrees, the landslide instantly raised the temperature to near 100 degrees. As the mud and rock hit the lake, the intense temperatures and choking ash killed all of the life in the water. Like dropping a large rock into a bucket of water, the lake sloshed up onto the surrounding hillsides, pulling trees and other debris into the lake. With the addition of heavy metals from the mountain and all the organic material washed down from the hillsides, huge bubbles of methane gas escaped from the lake for months after the eruption. Today the bottom of Spirit Lake is 100 feet above the original surface. The lake now has two and a half times more surface area than it did before the eruption.
