Description of the Volcanic Event (Sidebar 1) Overview
In March 1980, there was an earthquake below the mountain. The summit was observed to be swelling at 3 – 5 feet per day.
On May 16 and 17, it was swelling at a rate of 50 feet per day.
May 18—another earthquake:
1. Landslide from the mountain displaced the water in Spirit Lake and blocked Toutle River 2. Landslide followed by steam blasts—composed of volcanic ash and steam
a. 550o F traveling along the ground at 200 miles per hour
b. Force of 20 million tons of TNT
c. Leveled 150 square miles of forests
3. Steam blast followed by 9 hours of violent eruption (400 million tons of TNT energy) 4. Mud flows through 6 major rivers downstream of volcano. 1/4 mile wide and 30 feet deep, the mud flows deposited up to 15 feet of mud
Summit lost 1,300 feet of elevation; one half a cubic mile of landslide material slid off the mountain on May 18, 1980.
The crater is covered by a lava dome formed by molten material that has oozed to the surface and solidified.
Spirit Lake is covered with 1,000,000 logs which float in mats and drift.
