Some well-exposed, nearly circular areas of intensely deformed
sedimentary rocks, in which a central vent-like feature is surrounded by
a ring-shaped depression, resemble volcanic structures in gross form. As
no clear evidence of volcanic origin could be found in or near these
structures, scientists initially described them as “cryptovolcanic,” a
term now rarely used. Recent studies have shown that not all craters are
of volcanic origin. Impact craters, formed by collisions with the Earth
of large meteorites, asteroids, or comets, share with volcanoes the
imprints of violent origin, as evidenced by severe disruption, and even
local melting, of rock. Fragments of meteorites or chemically detectable
traces of extraterrestrial materials and indications of strong forces
acting from above, rather than from below, distinguish impact from
volcanic features.
Other possible explanations for these nonvolcanic craters include
subsurface salt-dome intrusion (and subsequent dissolution and
collapse); collapse caused by subsurface limestone dissolution and/or
ground-water withdrawal; and collapse related to melting of glacial ice.
An impressive example of an impact structure is Meteor Crater, Ariz.,
which is visited by thousands of tourists each year. This impact crater,
4,000 feet in diameter and 600 feet deep, was formed in the geologic
past (probably 30,000-50,000 years before present) by a meteorite
striking the Earth at a speed of many thousands of miles per hour.
In addition to Meteor Crater, very fresh, morphologically distinct,
impact craters are found at three sites near Odessa, Tex., as well as 10
or 12 other locations in the world. Of the more deeply eroded, less
obvious, postulated impact structures, there are about ten
well-established sites in the United States and perhaps 80 or 90
elsewhere in the world.
[Illustration: Meteor Crater, Arizona.]
[Illustration: Mount St. Helens, about noon, May 18, 1980.]