English: Grand Canyon and Yellowstone Falls at Yellowstone National Park. The canyon is up to 900 feet deep (275 m) and a half mile (0.8 km) in width. The canyon below the Lower Yellowstone Falls was at one time the site of a geyser basin that was the result of rhyolite lava flows, extensive faulting, and heat beneath the surface (related to the hot spot). The rhyolite in the canyon contains a variety of different iron compounds. Exposure to the elements caused the rocks to change colors. The rocks are, in effect, oxidizing; the canyon is rusting. The colors indicate the presence or absence of water in the individual iron compounds. Most of the yellows in the canyon are the result of iron present in the rock rather than sulfur.