It was formed 600,000 years ago by a giant volcano which emptied a large underground magma chamber.


A thermal basin developed in this lava flow, altering and weakening the rhyolite lava by action of the hot steam and gases. Steam rises from vents in the canyon today and the multi-hued rocks of the canyon walls are also evidence of hydrothermally altered rhyolite.


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.