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.
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.
[Illustration: 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._)]