EXPLOSIVE VOLCANISM

Some 600,000 years ago the rumblings of an impending volcanic eruption sounded ominously across the Yellowstone country. Suddenly, in a mighty crescendo of deafening explosions, tremendous quantities of hot volcanic ash and pumice spewed from giant cracks at the earth's surface. Towering dust clouds blackened the sky, and vast sheets of volcanic debris spread out rapidly across the countryside in all directions, covering thousands of square miles in a matter of minutes with a blanket of utter devastation. Abruptly, a great smoldering pit -a caldera 30 miles across, 45 miles long, and several thousand feet deep - appeared in the central Yellowstone region, the ground having fallen into the huge underground cavern that was left by the earth - shaking eruptions. Lava then began oozing from the cracks to fill the still smoking caldera.

Thus, in one brief "moment" of geologic time there was launched an incredible chain of events which led to the creation of many of the natural wonders of Yellowstone National Park. Heat from the enormous reservoir of molten rock which produced the massive eruption still remains deep within the earth beneath Yellowstone, sustaining the spectacular hot water and steam phenomena for which the Park is so justly famous. The formation of the caldera and the eruption of lavas profoundly influenced the shape of the present-day landscape. Once a land covered almost entirely by mountains, the part that collapsed, nearly one third of the total Park area, is now characterized by low rolling plateaus formed from the thick lava flows that filled the caldera. Moreover, the carving of the spectacular Grand Canyon of the Yellowstone and the fashioning of the large interior basin now occupied by beautiful Yellowstone Lake were closely related to this mighty volcanic event.

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