Mount St. Helens

Mount St. Helens erupted on May 18, 1980, at 8:32 a.m. Pacific Daylight Time (Larger image)

An earthquake, measured at 5.1 on the Richter scale, marked the start of the 1980 eruption of Mount St. Helens. The earthquake triggered a massive collapse of the north face of the mountain, which slid downslope, entraining ice, rock and explosive materials, and giving rise to a massive pyroclastic flow (a volcanically-induced avalanche) which flattened vegetation and buildings in an area of over 150 square miles (approximately 389 km²).

For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20-25 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (95 kilometers/hour), with ash reaching Idaho by noon. By early May 19, the devastating eruption was over.

The pyroclastic flow was one of the largest witnessed in modern memory. Moments after its formation, the flow crossed Spirit Lake and passed right over a ridge 1,300 feet (395 m) high. The flow then continued for 14 miles (22 km) down the Toutle River.

Noticeable ash fell in eleven states. The total volume of ash (before its compaction by rainfall) was approximately 0.26 cubic mile (1.01 km³), or enough ash to cover a football field to a depth of 150 miles (240 km).

Harry R. Truman, an 84-year old innkeeper who had lived near the mountain for over 50 yearsa became nationally famous when he decided not to evacuate before the impending eruption, despite repeated pleas by local authorities. His body was never found. In total, 57 people were killed or never found.

In 1982, the President Ronald Reagan and Congress designated a 110,000-acre area around MSH the Mount St. Helens National Volcanic Monument. The Monument resides inside Gifford Pinchot National Forest. The area is being left to gradually return to its natural state following the devastation.

Mount St. Helens on May 17, 1980 (Larger image)	3000-foot steam plume on May 19, 1982 (Larger image)