IMPACT CRATERS
The most direct and obvious evidence for
impacts on the surface of Earth comes
from studying the craters they produce.
The 50,000-year-old Barringer Crater in
Arizona is perhaps the most famous
meteorite crater in the United States.
Known as "Meteor Crater," it is an
extremely well-preserved, bowl-shaped
depression with a pronounced, upraised
rim (Figure 11.6a). The rim of the crater is
overlain by an ejecta blanket (or layer of
debris blown out of the crater upon
impact) which today can be identified as
hummocky terrain surrounding the
crater.
FIGURE 11.6 A. SIMPLE CRATER
The rocks that the asteroid impacted were
shattered and deformed, forming what is known
as brecciated rock, composed of angular broken
pieces of rocks produced during the impact.
Barringer Crater, discovered in the late 19th
century, is a 1.2 km diameter crater with a depth
of 180 meters. It was initially believed to be
formed by an asteroids, but careful study and
evaluation led to its discovery. Impact craters are
distinguish from other geologic processes due to
their high velocity, energy, pressure, and
temperature. The energy of the impact is kinetic
energy, which is transferred to Earth’s surface
through a stock waves. This energy compresses,
heat, melts, and excretes Earth materials,
producing the crater. The shock can
metamorphose rocks in the impact area, with high
pressure modification of minerals like quartz. The
shape of a larger crater may vary, with complex
craters growing to over 100 kilometers in
diameter.
FIGURE 11.7 B COMPLEX CRATER
Geologically, ancient impact craters are difficult to
identify because they have often been either eroded
or filled with sedimentary rocks that are younger
than the impact. For example, subsurface imaging and
drilling below the present Chesapeake Bay have
identified a crater about 85 km (53 miles) in
diameter, now buried by about 1 km (0.62 miles) of
sediment (Figure 11.8). The crater was pro- duced by
impact of a comet or asteroid about 3 to 5 km (1.9 to
3 miles) in diameter about 35 million years ago.
Compaction and faulting above the buried crater may
be in part responsible for the location of the bay.
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A good example of an eroded impact crater is found
near Quebec, Canada. A ring-shaped lake about 70 km
(45 miles) across is eroded along impact-brecciated
(broken) rocks, marking most of the crater, which is
about 100 km (62 miles) in diameter (Figure 11.9).
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Figure 11.9 Canada impact