of the Earth
Methods 9 - Potassium-Argon Dating
Argon (KAr) Dating
Radiometric Dating Game
Dating and the Geological Time Scale
Methods 8 - Radiocarbon Dating
Series Decay Scheme
Kevin C. Hartzog
Geologic Time Scales
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 carbon-12 is more stable
than carbon-14. So overtime, carbon-14 will decay, losing two neutrons,
and become carbon-12.
The processes that release carbon dioxide will create some carbon dioxide
with C-12, while a certain proportion of the carbon dioxide will have
C-14. 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 C-12:C-14.
But when they die, plants and animals do not consume carbon products.
Over time their C-14 will turn into C-12 by losing 2 neutrons, changing
the C-12:C-14 ratio. The rate that C-14 turns into C-12 is constant, so
by measuring the C-12:C-14 ratio in a fossil, paleontologist can put a
fairly accurate date on when that organism died.
C-14 has a half-life 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 potassium-argon
ratio, which has a half-life of 1,300 million years. Here, potassium-40
decays into argon-40 and calcium-40. Potassium-argon dating is accurate
from 4,600my-100,000 years. Another isotope, uranium (U-235) has several
isotopes that it decays into. Uranium-238 will decay into lead 206 with
a half-like of 4.6 million years.