From the park website...
The specifics of the geology of the canyon are not well understood, except that it is an erosional feature rather than the result of glaciation. After the caldera eruption of about 600,000 years ago, the area was covered by a series of lava flows. The area was also faulted by the doming action of the caldera before the eruption. The site of the present canyon, as well as any previous canyons, was probably the result of this faulting, which allowed erosion to proceed at an accelerated rate. The area was also covered by the glaciers that followed the volcanic activity. Glacial deposits probably filled the canyon at one time, but have since been eroded away, leaving little or no evidence of their presence. The present canyon is no more than 10,000 to 14,000 years old, although there has probably been a canyon in this location for a much longer period. The canyon is 800 to 1,200 feet deep and 1,500 to 4,000 feet wide.
~ CK
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