Geoscience Reference
In-Depth Information
FIGURE 8
The distribution of known terrestrial craters.
[R.
Grieve,
Geological Survey of Canada. Adapted from their web page at
http://
geologic time. Comparison with cratering on other bodies in the solar
system shows that craters less than about 20 km in diameter are
largely missing on the earth, presumably having been removed by
erosion. Thus the observed record is biased toward younger, larger
craters. More craters will be discovered on the earth in the future, but
the smaller the original crater, and the older it is, the greater the like-
lihood that erosion has removed it forever.
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UTBACK
What are the objects that strike the earth from space to form
craters, and how is it that they can have such devastating effect, not
only to dent the solid earth, but, as claimed by the Alvarezes, to play
the starring role in dinosaur extinction? Astronomers have discov-
ered that two types of cosmic objects are in orbits that sometimes
intersect that of the earth: comets and asteroids.
Comets are "dirty snowballs"—mixtures of mineral dust and ices
that evaporate under the heat of the sun to produce the visible tails
that follow behind them for thousands of miles. The great comets of
the 1990s, like Hayukatake and the spectacular Hale-Bopp, have been
