Geoscience Reference
In-Depth Information
TABLE 5
Frequency of Formation of Craters of Different
Diameters
Crater
Mean waiting time
diameter (km)
between impacts (yrs)
> 10
110,000
> 20
400,000
> 30
1,200,000
> 50
6,200,000
> 60
12,500,000
50,000,000
> 100
> 150
100,000,000
lower curves show how far in each direction the actual curve might
lie.) If we knew which craters had associated extinction events, their
positions could be plotted on the impact-kill curve. As noted in the
discussion of Table 4, however, the only crater about which we are
certain is Chicxulub. Michael Rampino and Bruce Haggerty
2
1
make
an educated guess that three other craters can be added to the list as
shown in Table 6 and Figure 24.
Is the impact-kill curve credible on its face? It predicts that the
largest extinctions are associated with craters at least 140 km in
diameter. This is plausible because the largest extinctions and the
largest craters each occur about every 100 million years. There have
been five major extinctions since the Cambrian began, and, Shoe-
maker's estimates tell us, about the same number of giant impacts.
Note again that these two calculations are completely independent.
Sepkoski's best estimate of species killed at the K-T boundary is
about 70 percent, corresponding to a crater of about 150 km. (If
Chicxulub is actually 300 km in diameter, as some argue on the
basis of its buried topography and gravity structure, then the actual
impact-kill curve would have to be closer to the lower dashed line.)
None of the points other than Chicxulub is precisely located but
they do fall within the upper and lower boundaries of the curve. On
the other hand, as far as we know, some sizable craters have failed to
produce mass extinctions. Neither the 24-km Ries Crater in Ger-
many, nor the 45-km Montagnais Crater, located in the seafloor off
Nova Scotia, is connected to an extinction. If the reasoning behind
the impact-kill curve is correct, and if Ries and Montagnais have no
associated extinction, then the curve must hug 0 on the Y axis until
