Description of the Volcanic Event: More Details

On May 18, 1980

A. Earthquake at 8:30 a.m. followed by a much larger earthquake at 8:32 a.m. (Richter magnitude 5.1) B. Landslide triggered by the earthquake 1. Volcano lost one-half a cubic mile of rock (almost 10% of total mass) 2. Landslide formed a horseshoe-shaped crater that was 1.2 miles in diameter 3. The mountain lost more than 1,300 ft. in summit elevation 4. Part of the debris avalanche flowed into Spirit Lake and displaced the water causing an 860 foot wave of water. The giant wave caused erosion of the bank, and stripped off all the trees. 5. Debris deposits an average of 150 ft. thick blocked the Toutle River (into which Spirit Lake had drained previously). a. Debris was deposited across the entire width of the valley along 16 miles of the North Fork of the Toutle River. b. The greatest thickness of deposit was 600 ft. (called Langes Crest) C. Pressure released by landslide led to a horizontal steam blast 1. Released energy equivalent to 20 million tons of TNT 2. The blast cleared 150 square miles of forest in six minutes D. The heat of the steam blast melted snow and ice, creating mudflows down six major rivers 1. The mud moved down the mountain slope at speeds of 90 miles per hour 2. The mud picked up and carried trees and debris. 3. On level floodplains, the mud moved less than 10 miles per hour 4. The mud flows caused a significant amount of erosion a. One mudflow through Muddy Creek was 30 feet deep and 1/4 mile wide, and eroded the valley down to bedrock b. Over the bedrock, 15 feet of mud was deposited. E. Vertical eruption column of volcanic ash 1. The volcano erupted violently for nine hours 2. The highest cloud reached at least 60,000 feet.

Reference
Austin, Lumsden, Morris, and Vardiman. Mt. St. Helens Tour Guidebook. Institute for Creation Research, 1997.