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1980 Eruption
Debris Avalanche
Lahars
Lateral Blast
Scorch Zone
Ashfall Zone
Pyroclastic Flows
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Lakes
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Lakes
Prior to the eruption, Spirit Lake was a crystal clear mountain lake.
Before the 1980 eruption, Spirit Lake was surrounded by old-growth forests.
The eruption filled the lake and removed the forest.
The landslide displaced the water from the lake and lifted the lake bed 200 feet in the air.
Large chunks of the volcano were deposited in the lake.
Pre-eruption view showing old-growth forests that cover the valley north of the volcano and surround Spirit Lake.
Same view shows the extensive destruction by the massive landslide and 300 mph lateral blast.
Logs felled by the lateral blast were washed back into Spirit Lake as lake water displaced by the landslide receded.
The lake was littered with blasted trees and transformed into a nutrient-rich bacterial stew. [4 months after the eruption]
A helicopter moves boat used for aquatic sampling.
Biologists navigate the log mat to sample the lake. [1 year after eruption]
Dissolved minerals and organics were much higher (left beaker) in thermal ponds than blast zone lakes.
Bacterial action and dillution by incoming fresh water rapidly cleared the lake water.
Elevated bacterial populations are shown on culture plates. Four heavily impacted lakes at right with two undisturbed at left.
The lake bubbled with carbon dioxide and methane as a result of microbial activity.
Biologists lower a marked disk into the water to determine water clarity.
Disk is barely visible through the organic rich lake water.
The eruption completely alterred the nutrients and productivity of the lake. [20 years after eruption]
Aerial view of blown down forest surrounding Obscurity Lake north of Mount St. Helens
Biologists from the US Forest Service and State of Washington periodically sample lakes to study how fish populations are responding to the eruption.