The Earth’s crust beneath Yellowstone National Park is still restless. Precise surveys have detected an area in the center of the caldera that rose by as much as 86 centimeters between 1923 and 1984 and then subsided slightly between 1985 and 1989. Scientists do not know the cause of these ups and downs but hypothesize that they are related to the addition or withdrawal of magma beneath the caldera, or to the changing pressure of the hot ground water system above Yellowstone’s large magma reservoir. Also, Yellowstone National Park and the area immediately west of the Park are historically among the most seismically active areas in the Rocky Mountains. Small-magnitude earthquakes are common beneath the entire caldera, but most are located along the Hebgen Lake fault zone that extends into the northwest part of the caldera. A magnitude 7.5 earthquake occurred along this zone in 1959. [Illustration: Castle Geyser erupting a column of hot water, Yellowstone National Park. (_Photograph by Steven R. Brantley._)] Active Volcanoes: Windows Into the Past Molten rock has erupted onto the surface of the Earth throughout its 4.5-billion-year history. Although many of these ancient rocks were removed by erosion, volcanic deposits can be found beneath younger rocks in many parts of the United States. To a geologist, such long-lasting volcanic rocks look like those formed by today’s active volcanoes. Many ancient volcanic rocks, however, change somewhat with time, as they become firmly consolidated, buried by younger deposits, and sometimes folded and faulted by the continuous shifting of the Earth’s crust. Even minerals of volcanic rocks may change, if after burial they encounter high pressures and temperatures. [Illustration: Columnar jointing in an ancient lava flow in the Blue Ridge Mountains, Shenandoah National Park, Virginia. The flow that contains the columns is one of an extensive series of lava flows, each averaging about 200 feet thick, that poured over the land more than 570 million years ago. Columns form as cooling or shrinkage joints when a hot lava flow cools quickly; the columns form perpendicular to the cooling surface. These columns are about .5 meter in diameter. (_Photograph by J.C. Reed, Jr._)] Most active volcanoes are built on older volcanic deposits erupted from ancient volcanoes, and visitors to the present-day volcanoes walk or drive across these products of past volcanism. For example, anyone driving across the Cascade Range, sunbathing at Waikiki, or fishing on the Alaska Peninsula is there because old volcanic rocks form the landscape.