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have pointed out that many flood basalt eruptions are not associated with mass ex-
tinctions and that many extinctions in the geologic record are not associated with
flood basalts. More important, whereas large flood basalt provinces are impossible
to miss, geologists know they have missed most impact craters. Crater counts on
the Moon and observations of asteroids in space show that over the last 250 milli-
on years, at least one impact crater larger than twenty kilometers formed on Earth
every million years. However, geologists have found only about twenty-five such
craters. The other 90 percent are covered, have been eroded away, or are yet to be
discovered. 13
Another rebutting letter argues that the review paper did “not give sufficient
and accurate consideration ofthe volcanic hypothesis,” referring specifically to the
Deccan flood basalts. 14 The third letter, from Keller and colleagues, accused the
authors of using “a selective review of data and interpretations by proponents” of
the Alvarez theory. Again came the claims that the Chicxulub impact predated the
K-T boundary and that Deccan volcanism had likely caused the K-T mass extinc-
tion. 15
The authors of the original review paper responded that Deccan volcanism las-
ted for 400,000 years with no evidence that it destabilized the global biosphere. 16
Most of the Deccan eruptions predate the K-T boundary and did not cause extinc-
tions or leave global environmental traces.
Case Closed
As of 2010, it was clear that the geological community continued to be divided
over the Alvarez theory and the cause of dinosaur extinction. One way to cut
through the debate would be for scientists to measure precisely both the age of the
Chicxulub impact and the age of the K-T boundary. As recognized from the ori-
ginal objections to the Alvarez theory, if the two ages are not the same, then the
Alvarez theory is in doubt, even falsified. If they are the same, then the skeptics
are left with no choice but to appeal to coincidence. In February 2013, a group of
scientists reported those very measurements. 17
Paul Renne of the University of California at Berkeley and nine colleagues used
ultraprecise argon dating on a group of glassy impact melt droplets from the K-T
boundary in Haiti. These analyses date the K-T impact event to 66.038 ± 0.025
million years (an error of 380 parts per million). To date the K-T boundary , the
team collected in the Hell Creek area of Montana, source of the finest T. rex speci-
mens. Samples from nearest to the iridium spike there, and thus nearest in time to
the impact, gave 66.043 ± 0.011 years. Uranium-lead dating of the same layer gave
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