Altered rhyolite lava flows at Grand Canyon of the Yellowstone, YNP

This view of the Grand Canyon of the Yellowstone is from below Lower Falls looking downstream. The splendid yellow-brown walls of the canyon consist predominantly of rhyolite tuff and lava (Sulfur Creek Tuff and Canyon flow, respectively), which were erupted nearly 500,000 years ago. Both units have been intensely altered by hydrothermal fluids.