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 0.5 meter in diameter. (Photograph by J.C. Reed, Jr.)

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.

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Maintained by John Watson
Updated 06.24.97