How big was the eruption of Mount St Helens?
Notable Statistic: The May 18, 1980 blast devastated 596 square kilometers (229 square miles) and destroyed timber valued at several million dollars. This was the deadliest and most economically destructive volcanic event in the history of the United States.
What was on the northern flank of Mount St Helens?
The landslide removed Mount St. Helens‘ northern flank, including part of the cryptodome that had grown inside the volcano. The cryptodome was a very hot and highly pressurized body of magma.
When did magma start to build up on Mt St Helens?
Magma began intruding into the Mount St. Helens edifice in the late winter and early spring of 1980. By May 18, the cryptodome (bulge) on the north flank had likely reached the point of instability, and was creeping more rapidly toward failure.
How did the removal of the cryptodome cause Mount St Helens to erupt?
Removal of the cryptodome and flank exposed the conduit of Mount St. Helens, resulting in a release of pressure on the top of the volcano’s plumbing system. This caused a depressurization wave to propagate down the conduit to the volcano’s magma storage region, allowing the pent-up magma to expand upward toward the vent opening.
What was the most active stage of Mount St Helens?
The Cougar Stage was probably the most active eruptive stage in Mount St. Helens‘ history before the Spirit Lake Stage. During this time the volcano produced explosive eruptionsthat ejected large volumes of ash, lava domes, lava flows, pyroclastic flows, a debris avalanche, and lahars.
Where did the tephra fall from Mount St Helens?
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.
Where did the debris from Mount St Helens go?
The second lobe of the debris avalanche surged over a 1,300-foot ridge and spilled into the South Coldwater Creek drainage. The third and largest lobe traveled 14 miles down the Toutle River valley, filling the valley to an average depth of 150 feet and leaving mounds of sediment in a bumpy pattern of hummocks.
How tall was the ash plume from Mt St Helens?
The towering column of ash rose for more than 9 hours and reached a height of about 80,000 feet. Wind carried ash mostly to the northeast where it darkened skies and covered the ground with gray, volcanic ash. Some ash remained aloft, and this part of the plume circled the Earth in 15 days.