The formation of a caldera is one of the most violent events in geology. During a “Super-eruption,” so much material is ejected that the volcano effectively loses its foundation. the summit then collapses into the partially emptied magma reservoir below. This creates a circular depression that can be dozens of kilometers wide. Calderas are found at the sites of the world’s most powerful volcanoes, such as Toba, Santorini and Yellowstone. They are often the sites of “resurgent domes,” where new magma begins to push the caldera floor back up.