The geology of the Bruneau-Jarbidge and Owyhee Canyons is spectacular, especially from the vantage point of a white-water craft. You will find yourself looking up at the caves, columns,
and spire-like hoodoos towering hundreds of feet above. These natural monuments are carved by the river as it exploits weak layers of the volcanic rock. As interesting as these features
are, the most unique aspects of southwestern Idaho’s geology are so large they are nearly impossible to see. For example, the volcanism in the Bruneau-Jarbidge area formed an oval basin 60 miles long by 30 miles wide.
The geology of the canyons is shaped by an astounding combination of volcanism, glacial melt, and regional drainage patterns. The Bruneau-Jarbidge and Owyhee areas were the sites of two massive volcanic eruptive centers, fueled by what is dubbed the Yellowstone Hotspot. When tectonic plates slowly move over molten hotspots, large eruptions are inevitable. The Owyhee-Humboldt eruptive center was active about 13.8 to 12 million years ago. Although it is over 1 million years older than the Bruneau-Jarbidge eruptive center, the size of the explosions and the rock types of the two areas are very similar. In each area, there were multiple events of volcanic activity related to the hotspot.
Both the Owyhee-Humboldt and Bruneau-Jarbidge eruptive centers began with very explosive events that led to gradual collapse, creating basins. The next series of events was the eruption of massive rhyolite flows that filled the basins, the largest flow was over 200 cubic kilometers. Basaltic eruptions followed. Then, about two million years ago, glacial rivers slowly
began to carve out the extraordinary canyons that we see today, brilliantly showcasing the events of the last 13 million years.