Island Park and Yellowstone Calderas formed as the result of enormous rhyolite ignimbrite eruptions, with single eruptions producing up to 600 cubic miles (2,500 km3) of ash. Henry's Fork Caldera, measuring 18 miles (29 km) by 23 miles (37 km), may be the largest symmetrical caldera in the world. The caldera formed when a dome of magma built up and then drained away.