Six hundred and thirty thousand years ago, molten rock filled two large cavities below Yellowstone National Park's Central Plateau. Lava, forced to the surface, formed a bulge that resulted in an incredible explosion estimated to have been 1,000 times larger than the 1980 Mount St. Helens eruption; the future Yellowstone National Park literally blew up. The two volcanic chambers that spewed their contents collapsed, creating an enormous caldera (a collapsed volcano) 30, by 45 miles wide. Eventually, rhyolite lava filled much of the depression constituting the Yellowstone plateau. Later, during the ice age, a 3,500-foot deep alpine glacier ice cap covered the area. As the massive glacier moved, it most likely ground out a segment of the southeast rim of the caldera allowing Yellowstone Lake to take form.