Extinction of the Dinosaurs
Sixty-five million years ago, about 70 percent of all species then living on Earth
disappeared within a very short period in what is termed the Cretaceous-Tertiary
Mass Extinction--commonly known as the K-T Event (K is used to denote
the Cretaceous period rather than C to avoid confusion with other periods such
as the Cambrian). Among the species that disappeared were the last of the dinosaurs.
The cause of this and other sudden species extinctions has long been an important
and controversial topic.
In 1980, physicist Luis Alvarez and coworkers reported finding unusually large
amounts of the element iridium in the sedimentary clay layer laid down
at the time of the K-T extinction. On Earth, iridium is very rare in the crust
because it was concentrated in Earth's core when it was largely molten. However,
chondritic meteorites often still have the primordial solar system abundances
of these elements. This led Alvarez and his collaborators
to suggest that a chondritic asteroid
10 kilometers in diameter struck the Earth in the K-T period. Such an
asteroid could have contained
enough iridium to account for the worldwide clay layer iridium enhancement,
and this meteor impact could also have triggered dramatic climatic changes
that produced the K-T extinction
(animation).