The May 18, 1980 eruption of Mount St. Helens is one that will live forever in modern history. At 8:32am on that day, a magnitude 5.1 occurred and was accompanied by a rapid series of events. At the same time as the earthquake, the volcano's northern bulge and summit slid away as a huge landslide—the largest debris avalanche on Earth in recorded history. A small, dark, ash-rich eruption plume rose directly from the base of the debris avalanche scarp, and another from the summit crater rose to about 200 m (650 ft) high. The debris avalanche swept around and up ridges to the north, but most of it turned westward as far as 23 km (14 mi) down the valley of the North Fork Toutle River and formed a hummocky deposit. The total avalanche volume was about 2.5 km3 (3.3 billion cubic yards), equivalent to 1 million Olympic swimming pools.
You can learn more about the history of eruptions at Mount St. Helens at http://on.doi.gov/
This photo was taken on July 15, 1980 which shows the after-effects of this major event. Here we see mudflow almost to the top of the mailbox posts near Cowlitz River in Cowlitz County, Washington. Photo credit: Lyn Topinka.
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