Geoscience Reference
In-Depth Information
The idea that a giant impact could cause mass extinctions,
though consistently rejected by geologists, has a surprisingly long
history, dating back at least to 1742, when Frenchman Pierre-Louis
Moreau de Maupertuis suggested that comets have struck the earth
and caused extinction by changing the atmosphere and the oceans.
2
His countryman, astronomer Pierre-Simon Laplace, wrote in 1813
that a meteorite of great size striking the earth would produce a
cataclysm that would wipe out entire species.
3
In our own century,
the distinguished paleontologist Otto Schindewolf sought an extra-
terrestrial cause for mass extinction. In 1970, Digby McLaren used
his presidential address to the Paleontological Society to present the
idea once again, leading some uniformitarians to assume that he
could only have been speaking tongue-in-cheek.
4
American Harold
Urey, winner of the Nobel Prize in chemistry, proposed in 1973 in
the widely read journal
Nature
that impact was responsible for mass
extinctions and the periods of the geologic time scale on which they
are based.
5
Urey, who had published a variety of important research
papers, had developed enough of a reputation in the earth and plan-
etary sciences to be taken seriously, yet still no one paid any atten-
tion. These suggestions were catastrophist, unorthodox, and without
evidence or predictions; therefore, even when made by distinguished
scientists in important journals, they languished.
By 1980, when the Alvarez theory appeared, conditions had
begun to improve. Iridium at the parts per trillion level was not eas-
ily measured, but it could be done at several laboratories around the
world. The space age was nearly two decades old and the surfaces
of other heavenly bodies were known in great detail—the map of
the moon was more complete and accurate (when the ocean basins
were included) than any map of the earth. It was impossible not to
notice that, whatever its effect on the earth, impact had scarred every
other object in the inner solar system innumerable times.
CRATERING
IN THE SOLAR
SYSTEM
The first person to observe lunar craters was Galileo. In 1609 he
trained his telescope on the moon and saw the seas (the
maria),
the
highlands, and some circular spots. He observed that as the termina-
tor—the sharp line separating the light and dark sides of the moon—
moved across, the far edges of the circles lit up before the centers. This
told him that the rims of the circles were higher than their centers,
which meant that they were depressions, or craters. From the length
of their shadows, Galileo calculated the heights of the crater walls.
