A Caldera is a large basin shaped volcanic depression formed by the explosion or collapse of a volcano. At Yellowstone the caldera, 30 miles (45 km) across, 45 miles (75 km) wide and several thousand feet deep, was filled by later lava flows to make a high, nearly flat plateau. Most volcanic rock exposed within the caldera was formed from viscous rhyolite lava during the last 600,000 years.
