Geologically, Yellowstone is composed of a large caldera, and some of the surrounding areas. This is one of the largest calderas in the world. It lacks the traditional cinder cone, because the volcano which formed it has been dormant for centuries, dormant, but not dead. In truth, there is a considerable amount of seismic activity in the area, along with the thermal areas which have helped to make the park so famous. Some of the park reangers delight in informing the tourists that the volcano which formed Yellowstone, erupts every 500, 000 years. When asked the inevitable question, they inform us that the last eruption was 600, 000 years ago.
    All fun aside, the dormant volcano which formed Yellowstone, is what geologists ominously refer to as a super volcano. No volcano of such power has erupted during historical times, and it is thought by most, that no such volcano has erupted since mankind has lived on earth. It may be that such an eruption would bring an end to historical times.