Geology & History
Yellowstone's world-famous natural history is marked by such colossal volcanic events that their reflections in today's landscape are difficult to grasp and impossible to take in at just a glance, even for those familiar with the signs of past volcanism.
The features of Yellowstone National Park result from great explosive eruptions and profound collapse of the ground, enormously thick lava flows, uplift and extensive faulting, and the erosive power of flowing water and ice. For more than a century, geologists have discovered and analyzed evidence of the dramatic events that have shaped the land here. When combined with growing knowledge about how volcanoes work and the never-ending motion of Earth's surface, the evidence tells a remarkable story of the Yellowstone landscape.
The volcanism most directly identified with the Yellowstone region has, during about the past 2 million years, built an immense volcanic plateau that straddles a high mountain divide—the Yellowstone Plateau volcanic field. This volcanic region has evolved through 3 cycles of voluminous outpourings of rhyolite lava and volcanic ash, each of them climaxing with one of Earth's greatest pyroclastic-flow eruptions and the resulting collapse of a central area to form a large caldera. Other eruptions have poured out basalt lava flows around the margins of the volcanic field. Large earthquakes occur just off the plateau along the nearby Teton and Hebgen Lake faults, the latter of which ruptured in 1959 (Ms = 7.5), causing considerable damage to the region.
Yellowstone's volcanism is only the most recent in a 17 million-year history of volcanic activity that has occurred progressively from southwestern Idaho to Yellowstone National Park. At least six other large volcanic centers along this path generated caldera-forming eruptions; the calderas are no longer visible because they are buried beneath younger basaltic lava flows and sediments that blanket the Snake River Plain.