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St. Helens

St. Helens

This photo, one of a series of dramatic photos taken from Bear Meadow NE of Mount St. Helens, shows the initial seconds of the May 18, 1980 eruption. A vertical eruption plume rises from the summit crater, while immediately below it the onset of the lateral blast can be seen. Both explosions were triggered by collapse of the north flank in a giant landslide, whose crest appears at the lower right as a gigantic wave mantled by fragments of white glacial ice.

St. Helens

St. Helens

The powerful lateral blast from Mount St. Helens on May 18, 1980 swept 30 km away from the volcano, blowing down giant trees like matchsticks. The blast, traveling at velocities up to 1100 km per hour, devastated 600 sq km over a broad area nearly 180 degrees wide north of the volcano.

St. Helens

St. Helens

The powerful lateral blast of May 18, 1980 at Mount St. Helens totally removed large standing trees near the volcano, leaving jagged stumps with splinters facing away from the volcano parallel to the direction of movement of the blast cloud. Warbonnet stumps, such as this one (about 2 m high) on Harry's Ridge 9 km north of the crater, are a common feature near the volcano; farther away trees were felled and left in place.

St. Helens

St. Helens

Incandescence is visible in fractures in a growing lava dome in the crater of Mount St. Helens on October 18, 1980. Several earlier lava domes formed after May 18, 1980 had been removed by explosive eruptions. This photo shows the beginning stages of the lava dome that grew incrementally until the end of the eruption in October 1986.

St. Helens

St. Helens

The gray layer behind the ruler was produced by the May 18, 1980 lateral blast of Mount St. Helens. The deposit is about 50 cm thick at this location 13 km NE of the volcano. The blast deposit is overlain by airfall pumice produced later on May 18, and underlain by a pumice deposit from an eruption in 1482 AD.

St. Helens

St. Helens

This vehicle was parked near Meta Lake, 13 km NE of Mount St. Helens, within the area affected by the devastating lateral blast of May 18, 1980. Tree blowdown occurred to distances of about 30 km from the volcano. Most of the 57 fatalities produced by the eruption resulted from the lateral blast.

St. Helens

St. Helens

The twisted girders of a highway bridge lie entombed in mudflow deposits from Mount St. Helens. This May 18, 1980 mudflow was produced by dewatering of the debris-avalanche deposit in the North Fork of the Toutle River and traveled as far as the Columbia River, decreasing the depth of the navigational channel from 11 meters to 4 meters.

St. Helens

St. Helens

The hill in the background is one of many hummocks forming the surface of the massive debris-avalanche deposit produced by collapse of the summit of Mount St. Helens on May 18, 1980. The different-colored rocks represent portions of the former volcano that were transported relatively intact far from the volcano. The avalanche traveled 25 km, filling the upper North Fork Toutle River to a maximum depth of nearly 200 m. The lighter-colored rocks in the foreground are pyroclastic-flow deposits.

St. Helens

St. Helens

Pumice fragments from the May 18, 1980 eruption form a broad plain below Mount St. Helens in this May 23 photo. Pumiceous pyroclastic flows on May 18 traveled 8 km from the crater of Mount St. Helens to as far as Spirit Lake. A geologist can be seen holding a large, light-weight block of pumice. In addition to May 18, pumiceous pyroclastic flows were erupted on May 25, June 12, July 22, August 7, and October 16-18, 1980.

St. Helens

St. Helens

The shear headwall of the landslide scarp created by collapse of Mount St. Helens on May 18, 1980, towers 550 m above the crater floor. The white areas on the crater rim are glaciers that were truncated by the collapse. Steam rises at the right from the new crater of St. Helens in this August 1980 view. Mount Hood is visible in the distance to the south across the Columbia River.

St. Helens

St. Helens

Pryroclastic surges originating from secondary phreatic explosions at Mount St. Helens in 1980 produced these cross-bedded layers. They were deposited from successive, rapidly moving horizontal clouds of gas, ash, and rock fragments that resulted from the interaction of still-hot pyroclastic-flow deposits from the May 18 eruption with groundwater and fragments of Mount St. Helens glaciers carried down by the collapse of the summit.

St. Helens

St. Helens

The horseshoe-shaped crater at Mount St. Helens is typical of scarps formed by massive landslides. On May 18, 1980 the upper 400 m of the summit was removed, leaving a 2 x 3.5 km horseshoe-shaped crater open to the north. Explosions played only an incidental role in formation of the crater, which resulted primarily from a massive landslide, the world's largest during historical time. The volume of the missing portion of the volcano closely matches the volume of the debris-avalanche deposit filling the North Fork Toutle River below the volcano.

St. Helens

St. Helens

U.S. Geological Survey scientists make precision leveling measurements in the crater of Mount St. Helens in February 1982 with the steaming lava dome in the background. Repeated measurement of deformation in the crater was one of several methods used by scientists to successfully predict future eruptions of Mount St. Helens.

St. Helens

St. Helens

Lava domes are formed by the extrusion of viscous, silica-rich lava that accumulates above the volcanic vent. This steaming lava dome partially fills the crater of Mount St. Helens in April 1983. Dome extrusion often follows explosive eruptions, which decrease the gas content of the remaining magma. Dome growth, however, is commonly accompanied by explosive activity and pyroclastic flows. Lava domes can form within the summit craters of volcanoes or on their flanks. They may build solitary, domical masses, or a complex of overlapping domes.


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