Mount St. Helens

Scientists were on the ash-covered ground within days after the eruption and found a complex mosaic of disturbance zones. The eruption included many types of physical forces, such as heat, burial, scour, and so forth, and the intensity of these forces varied substantially over the blast area (for example, thin versus thick deposits, warm versus searing hot temperatures).

Generally, these physical forces were most intense in areas closest to the volcano’s north side and less severe farther away, but the mountainous terrain shielded some spots from heat and funneled mudflows into stream valleys. Also, multiple forces affected many places. So, although the whole landscape looked gray and ashen, scientists found complicated patterns of disturbance and tremendous variation, or heterogeneity, in the effects on the ecosystems.

Rock that used to be the north side of the volcano covered about 23 square miles, primarily in the North Fork Toutle River valley, leaving hummocks (mounds) and basins. The former forest was obliterated and buried under sand and rock from 33 to 640 feet thick.

The pyroclastic flow spread over 6 square miles immediately north of the volcano, which was part of the area already buried by the debris avalanche deposit. Gravel and cobble-size pumice spread out in a fan-shaped flow up to 131 feet thick, creating a barren plain of pumice. No remnants of the former forest remained.

Glacier ice and snow meltwater carrying boulders, stones, and grit scoured the stream channels. Where the mudflows slowed down and eventually stopped, they buried streams and their flood plains. Large mudflows killed most vegetation in their paths, although plants survived along the flow margins. Small, shallow mudflows on the mountain slopes were less destructive and left many plant survivors.

Two to three feet of blast material stripped away all trees and covered the ground. A few small patches of understory vegetation survived in places shielded by ridges or other natural features or protected by late-lying snow. The lateral blast also knocked down trees on about 143 square miles. Fragmented rock and ash blanketed the ground in a layer 4 to 78 inches thick.

Hot volcanic gases killed the trees but left them standing in a 42-square-mile scorch zone that extended along the fringes of the blowdown zone. From 4 to 16 inches of fragmented rock and ash covered the ground.

Beyond the most heavily disturbed zones near the volcano, the wind dropped cool pumice and ash (tephra) over an area of several thousand square miles. Heavier tephra dropped first, and tephra deposits were deepest near the volcano, gradually diminishing farther away. At 25 miles northeast of the volcano, tephra piled up about 8 inches deep and buried tree seedlings, small shrubs, herbs, and mosses. Areas several hundred miles away received only a dusting of ash.