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On March 20, 1980 a magnitude 4.2 earthquake signaled the reawakening of the volcano after 123 years. During the Spring of 1980, rising magma pushed the volcano's north flank outward 5 feet per day. On the morning of May 18, 1980, the largest terrestrial landslide in recorded history reduced the summit by 1,300 feet and triggered a lateral blast.  Within 3 minutes, the lateral blast, traveling at more than 300 miles per hour, blew down and scorched 230 square miles of forest. Within 15 minutes, a vertical plume of volcanic ash rose over 80,000 feet. By afternoon of May 18, 1980, the dense ash cloud turned daylight into darkness in eastern Washington, causing streetlights to turn on in Yakima and Ritzville. The volcanic ash cloud drifted east across the United States in 3 days and encircled Earth in 15 days. (Source: USGS)

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