Although very unlikely to occur, a caldera forming eruption would produce massive pyroclastic flows.
The Yellowstone region has produced three exceedingly large volcanic eruptions in the past 2.1 million years. The pyroclastic flows from these eruptions created in rock formations called "tuffs."
The Huckleberry Ridge Tuff was produced during a caldera-forming eruption thought to be one of the five largest individual volcanic eruptions known anywhere on earth.
A similar, smaller but still huge eruption occurred 1.3 million years ago. This eruption formed the Henrys Fork Caldera, located in the area of Island Park, west of Yellowstone National Park, and produced another widespread volcanic deposit called the Mesa Falls Tuff.
The region’s most recent caldera-forming eruption 640,000 years ago created the 35-mile-wide, 50-mile-long (55 by 80 km) Yellowstone Caldera. Pyroclastic flows from this eruption left thick volcanic deposits known as the Lava Creek Tuff, which can be seen in the south-facing cliffs east of Madison, where they form the north wall of the caldera.
Contrary to what is sometimes portrayed, Yellowstone is not overdue for a caldera-forming eruption. There is no requirement that Yellowstone will have a fourth caldera-forming eruption. If it does, it will not necessarily follow any particular schedule. The actual average of the two intervals between the three eruptions is about 730,000 years, significantly greater than the time elapsed since the last super-eruption. Nevertheless, we cannot discount the possibility of another such eruption occurring some time in the future, given Yellowstone's volcanic history and the continued presence of magma beneath the Yellowstone caldera.