DESCRIPTION:
Blowdown

MSH80_blowdown_smith_creek_09-24-80.jpg
The slopes of Smith Creek valley, east of Mount St. Helens, show trees blown down by the May 18, 1980 lateral blast. Two U.S. Geological Survey scientists (lower right) give scale. The direction of the blast, shown here from left to right, is apparent in the alignment of the downed trees. Over four billion board feet of usable timber, enough to build 150,000 homes, was damaged or destroyed.
USGS Photograph taken on September 24, 1980, by Lyn Topinka.
[medium size] ... [large size]

The May 18, 1980 eruption of Mount St. Helens generated a horizontally directed series of explosions that formed a lateral blast. This blast destroyed an area of 230 square miles. Trees 6 feet in diameter were mowed down like blades of grass as far as 15 miles from the volcano.

The near-supersonic lateral blast -- (May 18, 1980 at Mount St. Helens) -- , loaded with volcanic debris, caused widespread devastation as far as 19 miles from the volcano. The area affected by the blast can be subdivided into three roughly concentric zones:

1. Direct blast zone, the innermost zone, averaged about 8 miles in radius, an area in which virtually everything, natural or manmade, was obliterated or carried away. For this reason, this zone also has been called the "tree-removal zone." The flow of the material carried by the blast was not deflected by topographic features in this zone.

2. Channelized blast zone, an intermediate zone, extended out to distances as far as 19 miles from the volcano, an area in which the flow flattened everything in its path and was channeled to some extent by topography. In this zone, the force and direction of the blast are strikingly demonstrated by the parallel alignment of toppled large trees, broke off at the base of the trunk as if they were blades of grass mown by a scythe. This zone was also known as the "tree-down zone."

3. Seared zone, also called the "standing dead" zone, the outermost fringe of the impacted area, a zone in which trees remained standing but were singed brown by the hot gases of the blast.

Trees amounting to more than 4 billion board feet of salable timber were damaged or destroyed, primarily by the lateral blast. At least 25 percent of the destroyed timber was salvaged after September 1980. Hundreds of loggers were involved in the timber-salvage operations, and, during peak summer months, more than 600 truckloads of salvaged timber were retrieved each day.

[Graphic,21K,InlineGIF]
Graphic of Mount St. Helens area showing May 18, 1980 devastation.

Mount St. Helens May 18, 1980 Eruption Menu

Return to:
[Blowdown Menu] ...