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Gibbon Falls over the caldera wall
At the heart of Yellowstone's past, present and future, lies volcanism. About two million years ago, then 1.3 million years ago, and again 640,000 years ago, huge volcanic eruptions occurred here. The park's present central portion collapsed, forming a 30- by 40-mile caldera, or basin. Gibbon Falls spills 84 feet over the side of the ancient caldera wall.