Two major catastrophes combined to produce a mini-Grand Canyon near Mount St. Helens. During the initial blast, more than two-thirds of a cubic mile (2.8 km3) of rock material slid down the side of the volcano, blocking the drainage from Spirit Lake into the Toutle River. It was the largest debris avalanche observed in human history.2 The deposits averaged 150 feet (45 m) thick.
Nearly two years later, on March 19, 1982, the hot volcanic ash from another explosive eruption of Mount St. Helens melted a thick snowpack in the crater, creating a destructive, sheet-like flood of water and mud, which became a mudflow. Reaching the earlier deposits blocking Spirit Lake, the flow cut channels through the debris at a speed of 40 miles per hour (65 km/h). Individual canyons up to 140 feet (43 m) deep appeared in a single day.2,3,4,5 On either side of the canyons were elevated plateaus resembling the North and South Rims of the Grand Canyon.
Side canyons also appeared, resembling the side canyons of Grand Canyon (technically known as gully-headed and amphitheater-headed side canyons). The breach did not cut straight through the obstruction, but took a meandering path, similar to the meandering path of the Grand Canyon through the high plateaus of northern Arizona. Indeed, this “Little Grand Canyon of the Toutle River” is a one-fortieth scale model of the actual Grand Canyon.
This amazing feature was cut through soft debris, but another eruption two months later (on May 19, 1982) melted a snowpack that cut through hard basalt bedrock. The resulting Loowit Canyon was more than 100 feet (30 m) deep. Nearby, the avalanche cut through lava and ash layers to form a third canyon, Step Canyon, up to 600 feet (180 m) deep.
Small creeks now flow through these deep canyons. In other places, geologists assume creeks cut the surrounding canyons very slowly over a very long time period. Yet at Mount St. Helens they know that the canyons formed first and formed extremely rapidly!
Four Lessons from the Mount St. Helens Eruption
Nearly two years later, on March 19, 1982, the hot volcanic ash from another explosive eruption of Mount St. Helens melted a thick snowpack in the crater, creating a destructive, sheet-like flood of water and mud, which became a mudflow. Reaching the earlier deposits blocking Spirit Lake, the flow cut channels through the debris at a speed of 40 miles per hour (65 km/h). Individual canyons up to 140 feet (43 m) deep appeared in a single day.2,3,4,5 On either side of the canyons were elevated plateaus resembling the North and South Rims of the Grand Canyon.
Side canyons also appeared, resembling the side canyons of Grand Canyon (technically known as gully-headed and amphitheater-headed side canyons). The breach did not cut straight through the obstruction, but took a meandering path, similar to the meandering path of the Grand Canyon through the high plateaus of northern Arizona. Indeed, this “Little Grand Canyon of the Toutle River” is a one-fortieth scale model of the actual Grand Canyon.
This amazing feature was cut through soft debris, but another eruption two months later (on May 19, 1982) melted a snowpack that cut through hard basalt bedrock. The resulting Loowit Canyon was more than 100 feet (30 m) deep. Nearby, the avalanche cut through lava and ash layers to form a third canyon, Step Canyon, up to 600 feet (180 m) deep.
Small creeks now flow through these deep canyons. In other places, geologists assume creeks cut the surrounding canyons very slowly over a very long time period. Yet at Mount St. Helens they know that the canyons formed first and formed extremely rapidly!
Four Lessons from the Mount St. Helens Eruption