Henry's Fork Caldera

The Henry's Fork Caldera in eastern Idaho is a caldera located in an area known as Island Park, west of Yellowstone National Park. The caldera was formed by an eruption 1.3 million years ago of the Yellowstone hotspot, that had a volume of 280 km3 and is the source of the Mesa Falls Tuff.

The Henry's Fork Caldera is nested inside of the Island Park Caldera, and the calderas share a rim on the western side. The earlier Island Park Caldera is much larger and more oval and extends well into Yellowstone Park. Although much smaller than the Island Park Caldera, the Henry's Fork Caldera is still sizeable at 18 miles long and 23 miles wide and its curved rim is plainly visible from many locations in the Island Park area. Of the many calderas formed by the Yellowstone hotspot, including the later Yellowstone Caldera, the Henry's Fork Caldera is the only one that is currently clearly visible. It is one of a succession of supervolcanoes and calderas that formed the Snake River Plain.

Wikipedia