1980 Eruption of Mount St. Helens - North Face Slides Away

North Face Slides Away

As May 18 dawned, Mount St. Helens' activity did not show any change from the pattern of the preceding month. The rate of bulge movement, sulfur dioxide emission, and ground temperature readings did not reveal any changes indicating a catastrophic eruption. USGS volcanologist David A. Johnston was on duty at an observation post approximately 6 miles (9.7 km) north of the volcano: as of 7 a.m., Johnston's measurements did not indicate any unusual activity.

Suddenly, at 8:32 a.m., a magnitude 5.1 earthquake centered directly below the north slope triggered that part of the volcano to slide, approximately 7–20 seconds (about 10 seconds seems most reasonable) after the shock. The landslide, one of the largest in recorded history, travelled at 110 to 155 miles per hour (180 to 249 km/h) and moved across Spirit Lake's west arm. Part of it hit a 1,150-foot (350 m) high ridge about 6 miles (9.7 km) north. Some of the slide spilled over the ridge, but most of it moved 13 miles (21 km) down the North Fork Toutle River, filling its valley up to 600 feet (180 m) deep with avalanche debris. An area of about 24 square miles (62 km2) was covered, and the total volume of the deposit was about 0.7 cubic miles (2.9 km3).

Scientists were able to reconstruct the landslide due to a series of rapid photographs by Gary Rosenquist, who was camping 11 miles (18 km) away from the blast. Rosenquist, his party, and his photographs survived because the blast was deflected by local topography 1 mile (1.6 km) short of his location.

Most of St. Helens' former north side became a rubble deposit 17 miles (27 km) long, averaging 150 feet (46 m) thick; the slide was thickest at 1 mile (1.6 km) below Spirit Lake and thinnest at its western margin. Thousands of trees were torn from the surrounding hillside after the lake was sloshed 800 feet (240 m) up the hillside. All the water in Spirit Lake was temporarily displaced by the landslide, sending 600-foot (180 m) high waves crashing into a ridge north of the lake, adding 295 feet (90 m) of new avalanche debris above the old lakebed, and raising its surface level by about 200 feet (61 m). As the water moved back into its basin, it pulled with it thousands of trees felled by a super-heated wall of volcanic gas and searing ash and rock that overtook the landslide seconds before.