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but I have substituted Chicxulub.) This chart tacitly assumes cause
and effect, which may be incorrect. It really asks this question: Does
the age of each of the well-dated Cenozoic impact craters match that
of a geologic stage boundary? Stothers concluded that the answer is
yes, and at a confidence level of 98 percent to 99 percent.
A fruitful line of research to confirm his conclusion would be to
select a significant number of craters whose ages appear to lie close
to the 28-million- to 32-million-year periodicity, but where the age
measurement errors are too large for certainty, and to launch an
intensive dating program so as to determine the ages of those craters
more precisely. Grieve estimates that in order to conduct a fair test,
the age uncertainties would have to come down to no more than
10 percent of the age itself. In other words, for a 30-million-year-old
crater, the uncertainty would have to be no worse than ± 3 million
years, well within the reach of today's technology. 2 1
O THER
C YCLES
Rampino and Stothers have gone on to argue for a 32 ± 3 million-
year periodicity not only in mass extinctions and impact cratering,
but in a variety of other major geologic processes: flood basalt erup-
tions, magnetic reversals, appearance of oxygen-poor oceans, large
changes in sea level, and episodes of seafloor spreading. Later, Ram-
pino and Bruce Haggerty went on to develop what they call their
Shiva (Siva) hypothesis. 2 2 If they are correct, a single cause is likely
to drive most or all of the earth's large-scale processes. Dare I say it?
If Rampino and colleagues are right, as shown in Figure 27, they are
on the trail of a grand unified theory of earth systems!
They imagine the cycle starting with the impact of an asteroid or
comet, say the one that forms the 50-million-year crater, which has a
diameter of 100 km. The asteroid that produced it would penetrate at
least 20 km into the earth. The nearly instantaneous evacuation of a
large section of the crust would relieve the pressure on the underlying
mantle, causing it to melt and giving rise to floods of basalt, which
would then erupt at the surface, possibly hiding the parent crater. The
shock of impact would upset the magnetic dynamo in the earth's
core, causing it to reverse. The ocean floor would rift and spread; sea
level would fall. The poisonous effects of the gases emitted during
flood basalt eruptions, added to those of impact, would cause a mass
extinction and thus neatly tie the whole package together.
This takes us far out on the slenderest branch yet. One can prob-
ably count on one's digits the number of geologists who believe
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