Close view of the eruption column of Mount St. Helens on May 18, 1980; the volcano is visible in the bottom of the photograph. The explosive release of volcanic gases into the atmosphere fragmented the erupting magma into tephra, including frothy pumice and gritty volcanic ash. At the beginning of the eruption, the hot column of tephra, gases, and entrained air rose to about 22 km above sea level in less than 10 minutes.

The eruption continued for the next nine hours, reaching a climax in the late afternoon. The prevailing wind blew the ash and gases east across Washington at an average speed of 100 km/hr for the first 1,000 km. About 3 1/4 hours after the eruption began, volcanic ash began to fall on Spokane, Washington, about 400 km downwind from Mount St. Helens. At this distance, between 0.5 and 2 cm of volcanic ash fell to the ground. The eruption cloud encircled the Earth in about 2 weeks.