Geoscience Reference
In-Depth Information
was suitable, Luis invented what he thought was a novel one for measuring the
rate of accumulation of sedimentary rocks. The method depended on measuring
the amount of the rare element iridium in the boundary clay. 6
Samples of the limestones above and below the K-T boundary held about 300
parts per trillion of iridium, about the amount expected. But to the Alvarezes' sur-
prise, the boundary clay had more than 30 times that much. To check that the result
was not an unusual characteristic of clays from this part of the geologic column,
they analyzed clay layers in the limestones above and below the boundary but
found that neither had detectable iridium. The next obvious question was wheth-
er the iridium spike was unique to the K-T boundary clay and Gubbio. They ana-
lyzed a K-T boundary clay from Denmark and found that it contained even more
iridium, as did another from New Zealand. The team measured the abundances of
other chemical elements in the boundary clay and found them also to be high in
meteoritic elements. Evidently the boundary clay worldwide contains an anomal-
ous amount of extraterrestrial material. But how had it gotten there? And what did
it have to do with the K-T extinction?
Luis had once read a topic published in 1888 by the Royal Society on the 1883
explosion of the Indonesian volcano Krakatoa. The topic described how the erup-
tion had ejected eighteen cubic kilometers of volcanic material, about one-quarter
of which had reached the stratosphere. The volcanic dust circled the Earth, remain-
ing aloft for more than two years and causing some of the most remarkable sun-
sets ever witnessed. The example of Krakatoa led Luis to propose by analogy that
65 million years ago, a large meteorite struck the Earth, throwing up a cloud of
cosmic and terrestrial debris so thick that it blocked the Sun. Global temperatures
plummeted, photosynthesis halted, the food chain collapsed, and the dinosaurs and
70 percent of all species became extinct.
LuisphonedWalter,theninItaly,totellhimofhisexcitingdeductionandtopro-
pose that Walter present the finding at an upcoming meeting on the K-T boundary
in Copenhagen, which both Alvarezes planned to attend. Writes Luis, “We thought
the paleontologists would be delighted to learn what caused the extinctions. Walt
knew better and correctly urged us to stay home” (257). Walter was right: pale-
ontologists were not delighted. A knockdown, drag-out battle ensued between the
twocampsthatturnedtheAlvareztheoryintooneofthemostvituperativescientif-
ic controversies in history.
Crater of Doom
Search WWH ::




Custom Search