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A. Russell, “The Mass Extinctions of the Late Mesozoic,” Scientific American ,
246(1):58-65 (January 1982); Steven M. Stanley, “Mass Extinctions in the Ocean,”
Scientific American , 250(6):64-72 (June 1984); and Rick Gore, “Extinctions,”
National Geographic , 175(6):662-699 (June 1989).
In their 1980 Science article (Alvarez et al. 1980 ), Alvarez and his colleagues, a
team of a physicist, a geologist, and two nuclear chemists, proposed an impact
theory to explain what happened in the Cretaceous and Tertiary extinction. In
contrast to the widely held view at the time, especially by paleontologists, the impact
theory suggests that the extinction happened within a much shorter period of time
and that it was caused by an asteroid or a comet.
In the 1970s, Walter Alvarez found a layer of iridium sediment in rocks at the
Cretaceous-Tertiary (K-T) boundary at Gubbio, Italy. Similar discoveries were made
subsequently in Denmark and elsewhere, both in rocks on land and in core samples
drilled from ocean floors. Iridium normally is a rare substance in rocks of the Earth's
crust (about 0.3 parts per billion). At Gubbio, the iridium concentration was found
more than 20 times greater than the normal level (6.3 parts per billion), and it was
even greater at other sites.
There are only two places one can find such high concentration of iridium: one
is in the earth's mantle. The other is in extra-terrestrial record. Iridium can be found
in the earth's mantle and in extra-terrestrial objects such as meteors and comets.
Scientists could not find other layers of iridium like this above or below the KT
boundary. This layer of iridium provided the crucial evidence for the impact theory.
However, the impact theory has triggered some of the most intense debates between
gradualism and catastrophism. The high iridium concentration did not necessarily
rule out the source could not be from the Earth.
Gradualists believed that mass extinctions occurred gradually instead of catas-
trophically. The volcanism camp is the leading representative of gradualism. The
volcanism camp had a different explanation of where the iridium layer in the KT
boundary came from. They argued that this iridium layer may be the result of a
massive volcanic eruption. The Deccan Traps in India was dated 65 million years
ago, which coincided with the KT extinction; the Siberia Traps was dated 248
million years ago, which coincided with another mass extinction - the Permian-
Triassic mass extinction, in which as many as 95 % of species on Earth were wiped
out. The huge amount of lava produced by such volcanic eruptions would cause
intense climatic and oceanic change worldwide.
Another line of research has been focusing on the periodicity of mass extinctions
based on an observation that in the past there was a major extinction about every 26