Volcano Hazards: | Types and Effects | Location | | Gas | Lahars | Landslides | Lava Flows | Pyroclastic Flows | Tephra |

The famous May 18th eruption of Mount St. Helens followed two months of intense restless activity. Within 15 to 20 seconds of a magnitude 5.1 earthquake at 8:32 a.m., the volcano's north flank and summit slid away in a huge landslide -- the largest in recorded history. The landslide depressurized the volcano's magma system, triggering powerful explosions that ripped through the sliding debris. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north. The directed blast of hot material, called a pyroclastic surge by scientists, accelerated to nearly 500 km per hour (300 miles per hour), then slowed as rocks and ash fell to the ground and spread away from the volcano; several people escaping the pyroclastic surge on its western edge were able to keep ahead of the advancing surge by driving 100 to 160 km per hour (65 to 100 miles per hour)!

Pre-eruption View of Spirit Lake

Photograph by K. Jacobsen (U.S. Forest Service) in 1979

This view north from the summit of Mount St. Helens shows the pristine forest that surrounded Spirit Lake (lower right) at the base of the volcano before the 1980 eruption. The snow-covered area above Spirit Lake is the Mt. Margaret back country; Mount Rainier looms in the distance. Spirit Lake formed during earlier eruptions of Mount St. Helens as volcanic debris from landslides, lahars, and pyroclastic flows was deposited at the base of the volcano. The deposits blocked two small river valleys, which subsequently filled with water (note the "fan-shaped" area of the lake's south shore and the two arms of the lake).

Post-eruption View of Spirit Lake

Photograph by L. Topinka on October 4, 1980

View north from above the crater of Mount St. Helens after the 18 May 1980 eruption at about the same elevation as the former summit. The gray, ash-covered area surrounding Spirit Lake is the former forest that was destroyed by the eruption's enormous pyroclastic surge, commonly known as the directed blast or lateral blast. Note the increased surface area of Spirit Lake compared to the pre-eruption photograph. The lake's elevation was raised by about 60 m (197 ft.) to 1038 m when part of the eruption's landslide swept into the lake (the landslide began about 20 seconds before the pyroclastic surge). Most of the lake's surface is covered with tree trunks swept into the lake by the surge.

Spirit Lake -- Middle of Devastation Zone
Photograph by R. Everts on August 20, 1982	This view south from the Mt. Margaret back country shows the new crater of Mount St. Helens, Spirit Lake (lower-left corner), and de-forested ridges in the middle of the area devastated by the 18 May 1980 pyroclastic surge. Note the logs floating on the surface of Spirit Lake.

Pyroclastic Surge Shatters and Blows Down Trees

Photograph by R.B Waitt, August 9, 1980

As the pyroclastic surge moved away from the volcano, the local topography partly deflected the direction in which it traveled. Can you determine the direction the surge was traveling as it swept across this small basin? Note the pattern of the tree trunks on the ground and look for the "rootballs" attached to the base of individual trunks. In this image, the surge was moving generally from bottom to top away from the volcano, but when it encountered the tall ridge at the top of the photograph, part of the surge was deflected to the right.

Pyroclastic Surge Snaps Massive Tree Trunks

Photograph by S.R. Brantley in July 1982

In the middle of the devastated area above Spirit Lake and 12 km NNE of Mount St. Helens, this massive tree trunk was knocked over by the fast moving pyroclastic surge. Hot rock, ash, and gas traveling within the surge, which reached speeds of least 480 km/hour (300 mph), easily blew down or snapped trees with trunks as large as 2 m in diameter. An estimated 4 billion board feet of timber was destroyed by the surge.

Rocks and Woody Debris Carried by Pyroclastic Surge
Photograph by C.D. Miller in 1980	Located atop a ridge about 10 km NNE of Mount St. Helens, this tractor was struck by rock and wood debris carried by the pyroclastic surge. The surge traveled across the ridge from right to left. Top photograph: the tractor's large steel grate served as a "net" to capture material carried by the surge between 2 and 3 m above the ground. Bottom photograph: close view of the tractor's steel grate reveals rock and woody debris that was carried by the surge. Note the tree trunk as well as the shattered pieces of wood.
Photograph by C.D. Miller in 1980

Pyroclastic Surge Burns Trees

Photograph by L. Topinka on May 4, 1981

The pyroclastic surge devastated an area of about 600 km2 north of Mount St. Helens and reached as far as 27 km (17 mi) from the volcano. As the speed of the surge decreased toward the outer edge of the devastated area, trees were left standing but their needles were burned or seared by the intense heat of the surge. This "seared zone" is visible behind the blown-down trees; note that the transition from the seared zone to the standing green trees is very abrupt. Scientists estimate temperatures inside the surge was between 100° and 300°C within the devastated area and between about 50° and 250°C along the outer edge of the devastated area.

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