Lava Creek Tuff is a tuff

Tuff

Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...

 formation, in Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

, created when the Yellowstone Caldera

Yellowstone Caldera

The Yellowstone Caldera is the volcanic caldera located in Yellowstone National Park in the United States, sometimes referred to as the Yellowstone Supervolcano. The caldera is located in the northwest corner of Wyoming, in which the vast majority of the park is contained. The major features of...

 erupted about 640,000 years ago.

The Lava Creek Tuff distributed in a radial pattern around the caldera and is formed of 1000 cubic kilometres (239.9 cu mi) of ash in pyroclastic flow

Pyroclastic flow

A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

s.

The tuff has been exposed by erosion at Tuff Cliff along the Gibbon River

Gibbon River

The Gibbon River is a river in Yellowstone National Park, in Wyoming, the United States. It rises in the center of the park at Grebe Lake. It flows for a short distance into Wolf Lake. Below Wolf Lake, the river flows through Virginia Cascades into the Norris valley. It flows near the Norris...

.

Lava Creek Tuff ranges in color from light-gray to pale red in some locales. Density of the tuff ranges from fine-grained to aphanitic

Aphanite

Aphanite, or aphanitic as an adjective, is a name given to certain igneous rocks which are so fine-grained that their component mineral crystals are not detectable by the unaided eye...

and is densely welded. Thickness of the tuff layer ranges from 180–200 m (590.6–656.2 ft).