Rapid erosion of sediments along the north fork of Toutle River, flowing
out of Spirit Lake on Mount St. Helens, carved a canyon like a
miniature Grand Canyon, showing that the Grand Canyon could form suddenly.
The sediments on Mount St. Helens were unconsolidated volcanic ash, which
is easily eroded. The Grand Canyon was carved into harder materials,
including well-consolidated sandstone and limestone, hard metamorphosed
sediments (the Vishnu schist), plus a touch of relatively recent
basalt.
The walls of the Mount St. Helens canyon slope 45 degrees. The walls of
the Grand Canyon are vertical in places.
The canyon was not entirely formed suddenly. The canyon along Toutle
River has a river continuously contributing to its formation. Another
canyon also cited as evidence of catastrophic erosion is Engineer's
Canyon, which was formed via water pumped out of Spirit Lake over
several days by the U.S. Army Corps of Engineers.
The streams flowing down Mount St. Helens flow at a steeper grade than
the Colorado River does, allowing greater erosion.
The Grand Canyon (and canyons further up and down the Colorado River)
is more than 100,000 times larger than the canyon on Mount St. Helens.
The two are not really comparable.
