The Grand Canyon of the Yellowstone River cuts through an ancient hydrothermal basin that developed in rhyolitic lava and ash flowing into the Yellowstone Caldera about 500,000 years ago. The river carved the canyon through hardened lava, or rhyolitic rock, and welded ash, or tuff. Hot areas and steam vents still remain in the canyon, releasing heat, gases, and hot water from the underground Yellowstone volcano. On cool days, steam rises from vents near the canyon bottom, and cinnamon-colored rust spots appear on the walls where iron in the rhyolite is exposed to moisture and oxygen. The canyon is about 20 miles long, with parts nearly 1,200 feet deep, and it continues to grow longer, deeper, and wider through water erosion rather than glaciation. Hydrothermal features occur along faults and fractures in the canyon wall.