The volume of volcanic rock produced by the first Yellowstone caldera eruption was about 600 cubic miles... Yellowstone ash is found in Ventura, California to the west and the Iowa to the east. It is likely the earth has seldom in its long history experienced caldera explosions on the scale of those that created Yellowstone.
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Three gigantic caldera eruptions rocked the Greater Yellowstone Ecosystem. The first and largest, Huckleberry Ridge caldera, blew up about 2.1 million years ago. Its center was in western Yellowstone National Park, but it extended into Island Park, Idaho. Welded tuff from this cycle is called the Huckleberry Ridge Tuff. The yellow rocks along the road in Golden Gate between Mammoth Hot Springs and Swan Lake Flats are Huckleberry Ridge Tuff. So are the tuffs that hold up much of Signal Mountain in Grand Teton National Park, and that crop out along the west side of the Teton Range, in Idaho.
The second great explosion formed the Island Park caldera 1.3 million years ago. This caldera, the smallest of the three, lies just west of Yellowstone in Idaho, within the western part of the Huckleberry Ridge caldera.
The youngest caldera, Lava Creek, erupted the Lava Creek Tuff, 0.65 million years old. It overlaps the Huckleberry Ridge caldera, but its eastern margin is about 10 miles farther east. Because it is the youngest, its tuffs and associated lava flows are best exposed and its history best known. Its eruption may have destroyed the south part of the Washburn Range.
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Geologists don't expect another caldera explosion any time soon, but sometime new lava flows quite likely will once again consume lodgepole forests, and a new generation of geysers will burst forth, perhaps in Hot Springs Basin.
http://www.yellowstonenationalpark.com/calderas.htm