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of 500 million years for each—far older than the greatest age of the earth it-
self as calculated by Kelvin and others. Soon other wotkers began analyzing
rocks that contained radioactive elements. By 1907 a slightly improved
method yielded a series of ages all much greater than the 100 million years
proposed by Kelvin as the maximal age of the earth. The greatest of these
was 2.2 billion years. The discovery of radioactivity—and the natural clock
with which it keeps time—made Kelvin's simple conductive calculation ob-
solete. Kelvin's mistake was to assume that no new heat had been created in
the earth since its origin. Radioactive decay produces such "new" heat.
The new method of age dating, based on radioactive decay, required ac-
curate measurement of the isotopic ratios of various elements. The isotopes of
an element are atoms of that element that have the same number of protons but
different numbers of neutrons. Perhaps the best-known example is carbon-14,
which is found in very small quantities compared to its far more common sis-
ter isotope, carbon-12 (the numbers refer to the atomic weight of the atom).
The transformation of one isotope into another that we call radioactive decay
takes place at a known and constant rate. There are 339 isotopes of 84 ele-
ments known in nature; 269 of these are stable (they do not change), and 70
are radioactive. Of the 70 radioactive isotopes 18 are too "long-lived" to be
useful; they have too long a half-life (the amount of time required for exactly
half of the parent isotope to be transformed into the daughter isotopes).
The early geochronologists examined only the breakdown of uranium
into lead for age dating. Today other isotopic pairs are examined, the most
useful being potassium/argon and argon/argon. The potassium/argon (K/Ar)
method measures the telative amounts of potassium-40 and successor argon-
40. This method is the most widely used of all radiometric dating method-
ologies
because
it is
cheap, reliable, and the method least likely to he com-
promised by contamination. Because argon is an inert gas, it never combines
chemically with any other material in the rock, and it escapes as a gas when
a rock is heated. Potassium is one of the most abundant elements in rocks
and is constantly breaking down into argon, which then escapes in most
cases. However, under special circumstances the potassium/argon "clock" is
