The Grand Canyon Of The Yellowstone


What Is There Today:YNPGrandCanyon.jpg

Length: 20 miles (32 km)
Depth: 800-1,200 feet (240-360 m)
Width: 1,500-4,000 feet (450-1,200 m)
Height of Upper Falls 109 feet 133 m)
Height of Lower Falls: 308 feet 193 m)
Primary rock type: Rhyolite/altered rhyolite


The Grand Canyon of the Yellowstone is part of the Canyon Rhyolite lava flow. Even though the canyon is cut mostly through the igneous rock, rhyolite, it is obvious that not all the rhyolite weathered at an even rate. If all the rock was of the same type one would expect a more even flow through that stretch of river. As it turns out, the rhyolite at the base of each of the major falls (upper and lower) has been chemically altered by hydrothermal activity. Evidence of thermal activity can still be seen today as steam released at the canyon bottom. The chemically altered rhyolite is weaker and more suseptable to weathering while the denser, more resisitant rhyolite is found above the falls.





Theoretical Formation:

A brief overview of its geologic history introduces a complex story, the details of which are the focus of ongoing study and debate.


About 600,000 years ago, huge volcanic eruptions occurred in Yellowstone, emptying a large underground magma chamber. Volcanic debris spread for thousands of square miles (kilometers) in a matter of minutes. The roof of this chamber collapsed, forming a giant smoldering pit-a caldera 30 miles (45 km} across, 45 miles (75 km) 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. The multi-hued rocks of the canyon walls are also evidence of hydrothermally altered rhyolite.

Other lava flows created large lakes that overflowed and cut 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 (93 meter) Lower Falls was 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 (33 m) 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. Since that time, erosional forces (water, wind, earthquakes and other natural forces) have continued to sculpt the canyon.


Sources:
1. http://gorp.away.com/gorp/resource/us_national_park/wy/see_yel1.htm
2. Yellowstone, A Visitors Companion
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