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Appendix B
The K-T Event

        Sixty-five million years ago about 70% of all species
then living on Earth disappeared within a very short period. The
disappearances included the last of the great dinosaurs.
Paleontologists speculated and theorized for many years about what
could have caused this "mass extinction", known as the K-T event
(Cretaceous-Tertiary Mass Extinction event). Then in 1980 Alvarez,
Alvarez, Asaro, and Michel reported their discovery that the
peculiar sedimentary clay layer that was laid down at the time of
the extinction showed an enormous amount of the rare element
iridium. First seen in the layer near Gubbio, Italy, the same
enhancement was soon discovered to be worldwide in that one
particular 1-cm (0.4-in.) layer, both on land and at sea. The
Alvarez team suggested that the enhancement was the product of a
huge asteroid impact.

On Earth most of the iridium and a number of other rare elements
such as platinum, osmium, ruthenium, rhodium, and palladium are
believed to have been carried down into Earth's core, along with
much of the iron, when Earth was largely molten. Primitive
"chondritic" meteorites (and presumably their asteroidal parents)
still have the primordial solar system abundances of these
elements. A chondritic asteroid 10 km (6 mi.) in diameter would
contain enough iridium to account for the worldwide clay layer
enhancement. This enhancement appears to hold for the other
elements mentioned as well.

Since the original discovery many other pieces of evidence have
come to light that strongly support the impact theory. The high
temperatures generated by the impact would have caused enormous
fires, and indeed soot is found in the boundary clays. A
physically altered form of the mineral quartz that can only be
formed by the very high pressures associated with impacts has been
found in the K-T layer.

Geologists who preferred other explanations for the K-T event
said, "show us the crater." In 1990 a cosmochemist named Alan
Hildebrand became aware of geophysical data taken 10 years earlier
by geophysicists looking for oil in the Yucatan region of Mexico.
There a 180-km (112-mi.) diameter ring structure called
"Chicxulub" seemed to fit what would be expected from a 65-
million-year-old impact, and further studies have largely served
to confirm its impact origin. The Chicxulub crater has been age
dated (by the 40Ar/39Ar method) at 65 million years! Such an
impact would cause enormous tidal waves, and evidence of just such
waves at about that time has been found all around the Gulf of
Mexico. Similarly, glassy debris of appropriate age called
tektites (and their decomposition products), which are produced by
large impacts, have been found all around the Gulf.

One can never prove that an asteroid impact "killed the
dinosaurs." Many species of dinosaurs (and smaller flora and
fauna) had in fact died out over the millions of years preceding
the K-T event. The impact of a 10-km asteroid would most certainly
have been an enormous insult to life on Earth. Locally there would
have been enormous shock wave heating and fires, a tremendous
earthquake, hurricane winds, and trillions of tons of debris
thrown everywhere. It would have created months of darkness and
cooler temperatures globally. There would have been concentrated
nitric acid rains worldwide. Sulfuric acid aerosols may have
cooled Earth for years. Life certainly could not have been easy
for those species which did survive. Fortunately such impacts
occur only about once every hundred million years.