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1980 Eruption
Debris Avalanche
Lahars
Lateral Blast
Scorch Zone
Ashfall Zone
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Ashfall Zone
After the landslide and blast, a 15-mile (24 km) high column of ash pumped ash into the atmosphere for nine hours.
The massive plume of volcanic ash drifted to the northeast.
The ash drifted northeast turning day into night in Eastern Oregon [Yakima, WA. Noon. May 18, 1980]
Ash was deposited across western states and the ash cloud eventually encircled the globe.
The ashfall blanketed meadows and forests northeast of the volcano.
Forests within 15 miles (24 km) northeast of the volcano were covered with ash.
Ash drifted to the northeast covering forests downwind of the volcano.
The ash coated small trees and shrubs until it was washed off by rainfall.
Fir trees were coated with ash damaging foliage and temporarily retarding growth.
The ash covered forests and meadows… [3 months after eruption]
…and coated small trees and shrubs.
It also coated the canopy of old-growth forests downwind of the volcano.
The ash covered understory trees and shrubs. [1980 photo]
On deeper ashfall sites, the eruption deposited 6 inches (15 cm) of popcorn sized pumice and fine-ash.
Falling ash filtered through small trees and shrubs and covered the forest floor.
The ash buried understory shrubs and small plants.
On deeper ash sites, small trees and shrubs bent over under snow remained stuck under descending ash.
In forests with snow, the ash descended as a “gooey glob” as underlying snow melted away.
On snow covered sites, as the underlying snow melted the ash descended as a single unit crushing and burying plants.
When dry, the surface of the ash formed a hard crust.
Vigorous plants like false hellibore were able to push through even deep ash deposits. [3 months after eruption]
Surprisingly, many plants were able to survive bured for several seasons and eventually sprouted.
In the absence of erosion, few plants sprouted from deep ash sites.
On deep ash sites, small trees were among the only plants that escaped burial.
As time passed, shrubs and other plants sprouted from the ash.
Scientists installed long-term studies to monitor how plants responded to ashfall.
Scientists cleared away the ash from some plots to compare plant responses with and without burial.
Repeated measurement revealed individual plant species responded through time.
Plants adapted to growing up through snow like the glacier lily grew long stems up through the ash.
Plants like trailing blackberry spread rapidly by sending new stem and root along the ash surface.
On shallow ash sites, deposits were only a few inches (cm) thick.
Forest herbs, like twinflower, also benefited from moving their roots from buried soil into th ash deposit.
Some plants, like huckleberry, responded more slowly.
The roots of this huckleberry sruvived nine years under the ash before reaching the surface.
Northeast of the volcano, where most ash falls, beargrass is restricted to steep slopes where ash is quickly eroded.
Ashfall buries small understory plants. [3 months after eruption]
Subalpine forest before eruption. [photo before 1980 eruption]
Trees and shrubs againcover the forest floor. [2 years after eruption]
Surviving trees and shrubs expand. [1 year after eruption]
Studies looked at plants from the smallest herbs to sapling-sized trees.
On areas where ash was shallow understory area plants were able to sprout.
Tree seedlings established in large numbers on the ash surface.
On deep ash sites, depth exceeded six inches (15 cm).
Fallen branches and leaves from overstory trees formed a layer on the ash surface.
Plants like the avalanche lily that are adapted to growing up through late-lying snow were able to sprout from the ash.
Where surface erosion removed the overlying ash, mosses and small plants survived.
Repeated ashfalls from past eruptions have produced deep, well drained soils downwind of the volcano.
The long-term effects of ashfall is illustrated by beargrass. Ash pins its evergreen leaves to the ground killing the plant.
Geologists use tree rings to date past eruptions.