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Though few people in the United States may actually experience an erupting
volcano, the evidence for earlier volcanism is preserved in many rocks of North
America. Features seen in volcanic rocks only hours old are also present in
ancient volcanic rocks, both at the surface and buried beneath younger deposits.
A thick ash deposit sandwiched between layers of sandstone in Nebraska,
the massive granite peaks of the Sierra Nevada mountain range, and a
variety of volcanic layers found in eastern Maine are but a few of the
striking clues of past volcanism. With this perspective, an erupting volcano is
not only an exciting and awesome spectacle in its own right but a window into a
natural process that has happened over and over again throughout Earth's history.
-- From: Brantley, 1994
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