Fish Canyon Tuff is a vast volcanic ash-flow deposit in southwest Colorado formed by an enormous eruption at the La Garita Caldera in the San Juan volcanic field about 28 million years ago, during the Oligocene Southern Rocky Mountain ignimbrite flare-up. With an estimated volume of about 5,000 cubic kilometers, it represents one of the largest known explosive eruptions on Earth, and the caldera collapsed during the eruption. Geologists identify it as the product of a single eruption because of its unusual chemical and mineral uniformity. Its sanidine crystals are also a key standard in 40Ar-39Ar dating, giving the formation lasting importance for measuring geologic time.