Geoscience Reference
In-Depth Information
periodicity has not been falsified. We have also seen that in theory,
impact could have caused all extinction. Both ideas are far from
corroborated but at least deserve the status of working hypotheses.
If both are correct, impact cratering ought also to be periodic, at
least in part, and on the same time cycle as the mass extinctions.
In the April 19, 1984, issue of
Nature,
Walter Alvarez and astro-
nomer Richard Muller reported that they had found a periodicity of
28 million years for terrestrial craters,
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the same within its error as
the 26-million-year cycle that Raup and Sepkoski had reported for
mass extinctions. Alvarez and Muller used Grieve's 1982 compila-
tion of terrestrial craters, selecting only those that are older than
5 million years and whose ages are known to better than ± 20 mil-
lion years. Unfortunately, this tight filter produced only 13 imper-
fectly dated craters, too small a sample to allow their statistical con-
clusions to be convincing.
Reporting in the same issue of
Nature,
Rampino and Stothers
applied a different statistical technique to Grieve's database and
found a periodicity of 31 million years for impact craters.
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Stothers
later culled, from Grieve's list, a set of seven Cenozoic craters with
age errors of less than 1 million years.
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0
He compared each of the re-
sulting ages with the ages of seven geologic stage boundaries. These
data are plotted in Figure 26. (Stothers used the Manson, Iowa, crater
FIGURE 26
Cenozoic crater ages and geologic stages. [From data of
Stothers.]
