The Grand Canyon of the Yellowstone River slices through an ancient hydrothermal basin. The basin developed in rhyolitic lava and ash that flowed into the Yellowstone Caldera about 500,000 years ago. The river carved this spectacular canyon through the hardened lava (rhyolitic rock) and welded ash (tuff). Hot areas and steam vents still exist in the canyon. They allow heat, gases, and hot water to escape from the underground Yellowstone volcano.Rhyolite RustOn cool days, puffs of steam waft from vents near the bottom of the canyon. You might also see cinnamon-colored splotches on the canyon walls. The rust spots form when iron in the rhyolite is exposed to moisture and oxygen.* The canyon is about 20 miles (32 km) long.* Parts of the canyon are nearly 1,200 feet (366 m) deep.* The canyon is forever growing longer, deeper, and wider.* Water erosion, rather than glaciation, sculpted the canyon.* Hydrothermal features are found along faults and fractures in the canyon wall.
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