Unified Interior Regions

Mudline left on tree by May 18, 1980 lahars from Mount St. Helens near the Old Highway 99 bridge across Toutle River, geologist 6 ft tall.

Ash from the May 18, 1980 eruption of Mount St. Helens covering the ground and road at a farm in Connell, Washington, approximately 300 km (180 mi) from the volcano.

Plinian eruption column from May 18, 1980 Mount St. Helens. Aerial view from the Southwest.

Plinian column from May 18, 1980 eruption of Mount St. Helens. Aerial view from southwest. Mount Adams is in the background (right).

For more than nine hours a vigorous plume of ash erupted, eventually reaching 20-25 km (12-15 mi) above sea level. By early May 19, the devastating eruption was over.

On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens. The bulge and surrounding area slid away in a gigantic rockslide and debris avalanche, releasing pressure, and triggering a major pumice and ash eruption of the volcano. Thirteen-hundred feet (400 meters) of the peak collapsed or blew outwards. As a result, 24 square

May 18, 1980 eruption of Mount St. Helens from southwest. Note the pyroclastic density currents spilling over the crater rim.

The May 18, 1980 eruption of Mount St. Helens, Washington, generated lahars that swept down river valleys. The St. Helens bridge on Highway 504 was carried over a quarter-mile (a half-kilometer) downstream and partially buried.  The USGS Cascades Volcano Observatory continues to monitor Mount St. Helens and other very high threat volcanoes. Photo by R.L. Schuster from USGS

Eruptive activity at Mount St. Helens captured the world’s attention on May 18, 1980 when the largest historical landslide on Earth and a powerful explosion reshaped the volcano. A volcanic ash cloud spread across the US in 3 days, and encircled the Earth in 15 days.

This mini-movie compiled from individual satellite images taken in 1980 shows the ash cloud as it

Mount St. Helens on May 17, 1980, one day before the devastating eruption. The view is from Johnston's Ridge, six miles (10 kilometers) northwest of the volcano.

Bulge on the north slope of Mount St. Helens before the May 18, 1980 eruption. Sugar Bowl on left (east) side of bulge and Goat Rocks on right (west) side, viewed from the air.