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

In the two months preceding May 18, 1980, magma moved high into the cone of Mount St. Helens and shoved the volcano's north side outward by at least 150 m. This dramatic deformation was called the "bulge." Within minutes of a magnitude 5.1 earthquake at 8:32 a.m., a huge landslide completely removed the bulge, the summit, and inner core of Mount St. Helens, and triggered a series of massive explosions.

Magnitude 5.1 Earthquake Strikes Beneath North Flank
	08:32:21 A.M. Northeast flank of Mount St. Helens; the bulge is located in right center of the volcano. Just after the earthquake, an eyewitness in an airplane directly over Mount St. Helens reported seeing material avalanche into its summit crater. Then, "everything north of a line drawn east-west across the northern side of the summit crater began to move as one gigantic mass... the entire mass began to ripple and churn up, without moving laterally. Then the entire north side of the summit began sliding to the north along a deep-seated slide plane."
North Flank Collapses
	08:32:47 A.M. The north side and summit collapsed into two huge landslide blocks. In this photo, the first block has formed a scarp or slip surface about 700 m tall and the second block (the summit area) has slipped downward about 100 m. The time is an estimate of Gary Rosenquist's first image that shows movement of the north flank; the time of Rosenquist's images below are estimates based on this initial time.
Collapse Triggers Explosions
	08:32:53.3 A.M. A series of explosions ripped through the second landslide block. As the landslide moved down the volcano at a velocity of nearly 300 km/hr, the explosions grew in size and speed and a low eruption cloud began to form above the summit area.
Gas and Steam Drive the Explosions
	08:33:03.7 A.M. The explosions were triggered both by the sudden release and expansion of gas contained in the magma that had risen into the volcano and the "flashing" into steam of superheated groundwater around the magma. When the landslide removed the north flank, it depressurized the magma and hot groundwater system and caused the series of sudden, violent explosion of gas, steam, and rocks. These explosions are sometimes referred to as the directed blast or lateral blast.
Directed Blast Overtakes Landslide
	08:33:18.8 A.M. The lateral blast accelerated to as fast as 540 km/hr in the first minute of the eruption and quickly overtook the moving landslide. The initial explosions hurled hot rocks and magma northward above the volcano's flanks, which then fell back to the ground and moved as a pyroclastic surge from the volcano northward as far as 27 km.

Landslide Divides Into Three Parts
Photograph by H. Glicken on September 9, 1980	The landslide slid north from the volcano into the headwaters of the North Fork Toutle River valley and split into 3 parts. One part of the landslide slammed into Spirit Lake (left hand side of photo), which caused waves to splash 225 to 275 meters up surrounding ridges and raised the lake level by 60 m. Another part of the landslide surged up and over a 400-m-tall ridge (Johnston Ridge in foreground), located 8 km due north of the volcano; this part of the landslide then slid down South Coldwater Creek. The final and largest part of the landslide was diverted west by Johnston Ridge and slid 22 km down the North Fork Toutle River.
Landslide Fills North Fork Toutle River Valley
Photograph taken in 1980	Within about 10 minutes, the landslide filled 22 km of the North Fork Toutle River valley with rock debris to an average depth of 45 meters. In places the deposit is 195 m thick! This view is toward the east; Mount St. Helens (note steam plume above new crater) is about 15 km upstream.

More information about 1980 landslide at Mount St. Helens

• A USGS report about this landslide, Rockslide-Debris Avalanche of May 18, 1980, Mount St. Helens, Washington is available online.