Nature
5
About 480,000 years ago, after the last caldera-forming eruption, lava flowing from the east covered this area and hardened into erosion-resistant rock. Over time, superheated water rose through the hardened lava, altering it and making it easier to erode. The ancestral Yellowstone River, flowing around other hardened lava flows, reached the soft, hydrothermically altered rock, which eroded faster than the hard rock at the brink of the falls, causing the canyon and falls to form. This change did not happen quickly; the Upper Falls has likely been in its current location for approximately 100,000 years.
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Wyoming · United States
