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.
