Geologic Time Scales

Eras of the Earth
Law of Superposition
Radiometric Dating


References:

Radiometric Dating
Chronological Methods 9 - Potassium-Argon 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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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 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.