  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. 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. One step further back in time from today’s active volcanoes are people who picnic in the White Mountains of New Hampshire, enjoy the autumn colors in the Blue Ridge of Shenandoah National Park, and hike in the rugged Big Bend National Park of Texas. Many of the rocks in these areas were formed by eruptions or by intrusion of magma into the Earth’s crust many millions of years ago. Because volcanic activity has been so important in shaping the Earth, watching active volcanoes today provides a window through which we can glimpse and reconstruct the early volcanic history of our planet. As we increase our knowledge about volcanic processes, by studying volcanoes erupting today as well as those that have lain dormant for hundreds to thousands of years, we increase our ability to predict when and how volcanoes will erupt. Accurate predictions, presented in terms that are meaningful to public officials, will minimize the number of lives lost and the social and economic upheaval that an eruption can cause. Thick layers of volcanic rocks form the Superstition Mountains, located about 60 kilometers east of Phoenix, Arizona. The consolidated deposits of pyroclastic flows, lava flows and domes, and lahars in the Superstition Mountains and adjacent areas testify to a period of intense volcanism about 17 to 25 million years ago in central Arizona. (Photograph by D.W. Peterson.) |
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