The Grand Canyon of the Yellowstone River



	THE GRAND CANYON OF THE YELLOWSTONE

	The yellow-hued rocks lining the canyon walls are soft, hydrothermally altered rhyolite lavas. The rocks at the brinks of the falls consist of less altered, more resistant rhyolites. A brief look at the geologic history will help to explain this story: About 600,000 years ago, huge volcanic eruptions occurred, emptying a large underground magma chamber. Volcanic debris spread for thousands of square miles in a matter of minutes. The roof of this chamber collapsed forming a giant smoldering pit - a caldera 30 miles across, 45 miles wide, and several thousand feet deep. Eventually the caldera was filled with lava.
One of these flows was the Canyon Rhyolite flow, approximately 590,000 years ago, which came from the east and ended just west of the present canyon. A thermal basin developed in this lava flow altering and weakening the rhyolite lava by action of the hot steam and gases. Look for steam in the canyon, evidence of the old thermal area. Other lava flows created large lakes that overflowed cutting through the various hard and soft rhyolites creating the canyon. Later, the canyon was blocked three different times by glaciers. Each time these glaciers formed lakes which filled with sand and gravel. Floods from the melting glaciers at the end of each glacial period recarved the canyon, deepened it, and removed most of the sand and gravel. The large rocks in the river upstream from Chittenden Bridge and the Upper Falls were left behind in the last glacial flood. The 308 foot Lower Falls were formed by the leading edge of the Canyon Rhyolite lava flow and the western edge of the old thermal basin. The hard, resistant lava at the brink did not erode, while the altered and weakened lava in the thermal basin eroded easily. The 109 foot Upper Falls was also formed at a contact point of hard and soft rhyolite lavas. In this case, the brink and the massive cliffs are of a dense, resistant rhyolite, while immediately downstream the rhyolite lava contains much volcanic glass which erodes more easily. The present appearance of the canyon dates from about 10,000 years ago, when the last glaciers melted. Very little change has occurred since that time. The canyon today is approximately 800-1200 feet deep and 1500-4000 feet wide, 20 miles long.

THE GRAND CANYON OF THE YELLOWSTONE