The collision of meteorites with Earth produces
impact craters, which are generally circular bowlshaped
depressions. There are more than 200 known impact
structures on Earth, although processes of weathering, erosion,
volcanism, and tectonics have undoubtedly erased many
thousands more. The Moon and other planets show much
greater densities of impact craters, and since the Earth has a
greater gravitational pull than the Moon, it should have been
hit by many more impacts.
Meteorite impact craters have a variety of forms but are
of two basic types. Simple craters are circular bowl-shaped
craters with overturned rocks around their edges and are generally
less than 3 miles (5 km) in diameter. They are thought to
have been produced by impact with objects less than 100 feet
(30 m) in diameter. Examples of simple craters include the Bar
ringer Meteor Crater in Arizona and Roter Kamm in Namibia.
Complex craters are larger, generally greater than 2 miles (3
km) in diameter. They have an uplifted peak in the center of
the crater and have a series of concentric rings around the
excavated core of the crater. Examples of complex craters
include Manicougan, Clearwater Lakes, and Sudbury in Canada,
Chicxulub in Mexico, and Gosses Bluff in Australia.
The style of impact crater depends on the size of the
impacting meteorite, the speed it has as it strikes the surface,
and to a lesser extent the underlying geology and the angle at
which the meteor strikes the Earth. Most meteorites hit the
Earth with a velocity between 2.5 and 25 miles per second
(4–40 km/s), releasing tremendous energy when they hit.
Meteor Crater in Arizona was produced about 50,000 years
ago by a meteorite 100 feet (30 m) in diameter that released
the equivalent of four megatons of TNT. The meteorite body
and a large section of the ground at the site were suddenly
melted by shock waves from the impact, which released
about twice as much energy as the eruption of Mount Saint
Helens. Most impacts generate so much heat and shock pressure
that the entire meteorite and a large amount of the rock
it hits are melted and vaporized. Temperatures may exceed
thousands of degrees in a fraction of a second as pressures
increase a million times atmospheric pressure during passage
of the shock wave. These conditions cause the rock at the site
of the impact to accelerate downward and outward, and then
the ground rebounds and tons of material are shot outward
and upward into the atmosphere.
Impact cratering is a complex process. When the meteorite
strikes it explodes, vaporizes, and sends shock waves
through the underlying rock, compressing the rock, crushing
it into breccia, and ejecting material (conveniently known as
ejecta) back up into the atmosphere, from where it falls out
as an ejecta blanket around the impact crater. Large impact
events may melt the underlying rock forming an impact melt
and may form distinctive minerals that only form at exceedingly
high pressures.
After the initial stages of the impact crater forming process,
the rocks surrounding the excavated crater slide and fall
into the deep hole, enlarging the diameter of the crater, typically
making it much wider than it is deep. Many of the rocks
that slide into the crater are brecciated or otherwise affected
by the passage of the shock wave and may preserve these
effects as brecciated rocks, high-pressure mineral phases,
shatter cones, or other deformation features.
Impact cratering was probably a much more important
process in the early history of the Earth than it is at present.
The flux of meteorites from most parts of the solar system
was much greater in early times, and it is likely that impacts
totally disrupted the surface in the early Precambrian. At
present the meteorite flux is about 100 tons per day (somewhere
between 107 and 109 kg/yr), but most of this material
burns up as it enters the atmosphere. Meteorites that are
about a tenth of an inch to several feet (.25–60 cm) in diameter
make a flash of light (a shooting star) as they burn up in
the atmosphere, and the remains fall to Earth as a tiny glassy
sphere of rock. Smaller particles, known as cosmic dust,
escape the effects of friction and slowly fall to Earth as a slow
rain of extraterrestrial dust.
Meteorites must be greater than 3.2 ft (1 m) in diameter
to make it through the atmosphere without burning up from
friction. The Earth’s surface is currently hit by about one
small meteorite per year. Larger impact events occur much less
frequently, with meteorites 328 feet (100 m) in diameter hitting
once every 10,000 years, 3,280 feet (1,000 m) in diameter
hitting the Earth once every million years, and 6.2 miles (10
km) in diameter hitting every 100 million years. Meteorites of
only hundreds of meters in diameter could create craters
about 0.6–1.2 miles (1–2 km) in diameter, or if they hit in the
ocean, they would generate tsunami more than 16.5 feet (5 m)
tall over wide regions. The statistics of meteorite impact show
that the larger events are the least frequent.
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