Dinosaur Killer - Four Revolutions in the Earth Sciences - page 180

The first and hardest-working opponents of the Alvarez theory were two scientists
fromDartmouth College, Charles Officer andCharles Drake. Both hadbeen Buch-
er's students at Columbia. They began with two papers in Science , one in 1983 and
the other in 1985. 7 Their intent was to show that
• The K-T event took place at different times around the world and therefore could not have
resulted from a simultaneous global catastrophe.
• The dinosaurs did not go extinct all at once, as in the Alvarez model. They and other species
were already dying out well before the K-T boundary. The impact, in the unlikely event one
had occurred, did no more than administer the coup de grâce.
• The evidence that geologists had found at the K-T boundary, which turned out to include not
only an iridium spike but also quartz and zircon that had suffered high-pressure shock, as well
as glassy spherules, was far from diagnostic of impact and often not even consistent with it.
• Volcanism explains the K-T evidence as well or better than the Alvarez theory and does not
rely on a nonuniformitarian bolt from the blue.
Though the Alvarez team was no doubt disappointed that geologists did not rush
to accept their proposal, in the end the skeptics did them and science a favor by
forcing the theory to confront and pass critical tests. The result was that scientists
determined that the K-T boundary is the same age everywhere: it is a true global
event. The dinosaurs and other species disappeared suddenly and had not started to
decline earlier. Though high iridium, shocked minerals, and glassy spherules can
each have other causes, taken together they are markers of extraterrestrial impact.
A major volcanic eruption, the Deccan Traps in India, did occur near the end of
the Cretaceous, but the detailed chronology of the eruption and the geochemistry
of the boundary clay showed that the clay cannot be attributed to volcanism.
Of course, or so one would think, the clincher would be to find the “Crater of
Doom,” as Walter Alvarez named the K-T impact site in his fine topic. 8 By 1990,
tenyearsafterthepublicationoftheoriginalpaper,scientistsstillhadnotfoundthe
crater—or had they? Calculations showed that it should be about 150 kilometers
in diameter, yet that did not mean it would be easy to spot. Any arriving meteorite
has a two-thirds chance of landing in the ocean, where the crater could have dis-
appeared down a subduction zone or lie hidden beneath younger sediments. Had
the meteorite landed on a continent, 65 million years of erosion might have made
the crater hard to recognize. Had it landed near one of the poles, thousands of feet
of ice might now bury the crater. It was easier to think of reasons why the crater,
if it ever existed, should not be found than reasons why it should. Clever detective
work, and even more importantly, good luck, would surely be required.
Geologists had noted that the K-T boundary clay is thicker, and the particles that
comprise it larger, in North America than in other places, suggesting that ground