Geologic Time
Scales
Eras
of the Earth
Law
of Superposition
Radiometric
Dating
References:
Radiometric
Dating
Chronological
Methods 9  PotassiumArgon Dating
Potassium
Argon (K–Ar) Dating
The
Radiometric Dating Game
Radiometric
Dating and the Geological Time Scale
Chronological
Methods 8  Radiocarbon Dating
Uranium
Series Decay Scheme
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Kevin C. Hartzog

Geologic Time Scales
Radiometric Dating
It was not until radiometric dating that paleontologist and geologist
could put a reliable number on the Eons and Periods of Earth's past. Radiometric
dating is based upon a simple chemical property of isotopes. Atoms, such
as carbon, may have more than one form. In the case of carbon, it may
have 12 or 14 neutrons. Both occur naturally, but carbon12 is more stable
than carbon14. So overtime, carbon14 will decay, losing two neutrons,
and become carbon12.
The processes that release carbon dioxide will create some carbon dioxide
with C12, while a certain proportion of the carbon dioxide will have
C14. Through photosynthesis, plants incorporate carbon dioxide into sugar,
which, through the carbon cycle, will consumed by animals. As long as
they live, both plants and animals will maintain a constant ratio of C12:C14.
But when they die, plants and animals do not consume carbon products.
Over time their C14 will turn into C12 by losing 2 neutrons, changing
the C12:C14 ratio. The rate that C14 turns into C12 is constant, so
by measuring the C12:C14 ratio in a fossil, paleontologist can put a
fairly accurate date on when that organism died.
C14 has a halflife of 5,370 years, so carbon dating is useful only
to about 50,00 years. But geologist state that the Earth is 4,600 millions
of years old. For older times, geologist use other isotopes. In determining
the age of the Earth, moon, and meteorites, geologist look at the potassiumargon
ratio, which has a halflife of 1,300 million years. Here, potassium40
decays into argon40 and calcium40. Potassiumargon dating is accurate
from 4,600my100,000 years. Another isotope, uranium (U235) has several
isotopes that it decays into. Uranium238 will decay into lead 206 with
a halflike of 4.6 million years.
