Three moderate earthquakes south of the caldera and one beneath the caldera on May 25-26, 1980, marked the beginning of unrest that continues into the 1990’s. Swarms of earthquakes beneath the caldera, changes in several hot springs, and the formation of new springs have occurred since 1980. Precise surveys have also shown that the central part of the caldera has risen by more than 50 centimeters since 1975. This unrest is probably related to the stretching (east-west extension) of the Earth’s crust that is known to be occurring in the region around the caldera, and it probably also involves the rise of magma beneath the caldera. Scientists do not know if this unrest will lead to volcanic activity, but the geologically recent eruptions along the Mono-Inyo Craters Volcanic Chain suggest that future eruptions are possible. [Illustration: South Inyo Crater, Long Valley Caldera, California. Explosive eruptions formed the crater about 500 years ago. (_Photograph by Steven R. Brantley._)] Yellowstone Caldera, Yellowstone National Park, Wyoming. Yellowstone Caldera is one of the largest and most active calderas in the world. The spectacular geysers, boiling hot springs, and mud pots that have made Yellowstone famous—and even the strikingly beautiful Grand Canyon of Yellowstone through which the Yellowstone River plunges—owe their existence to the tremendous volcanic forces that have affected the region during the past 2 million years. Cataclysmic eruptions 2.0, 1.3, and 0.6 million years ago ejected huge volumes of rhyolite magma; each eruption formed a caldera and extensive layers of thick pyroclastic-flow deposits. The youngest caldera is an elliptical depression, nearly 80 kilometers long and 50 kilometers wide, that occupies much of Yellowstone National Park. The caldera is buried by several extensive rhyolite lava flows erupted between 75,000 and 150,000 years ago. [Illustration: Map of most recent Yellowstone Caldera and its main thermal features. After the caldera formed, many vents erupted thick rhyolite lava flows, and the central part of the caldera was pushed upward to form resurgent domes. The star marks the magnitude 7.5 Hebger Lake earthquake.] IDAHO _Hebgen Lake_ West Yellowstone MONTANA Silver Gate Gardiner WYOMING Yellowstone National Park Mammoth M 7.5 Aug. 18 1959 Norris Geyser Basin Mud Pots Old Faithful West Thumb Geyser Basin _Yellowstone Lake_ CALDERA RIM COLORADO Resurgent dome