Table 141

Some Isotopes Used in Radiometric Dating

Isotope

Half-life (years)

Carbon-14

5,730

Uranium-235

704,000,000

Potassium-40

1,250,000,000

Uranium-238

4,500,000,000

When an organism dies, its uptake of carbon stops, and decay of the existing carbon-14 continues. Thus, over time, the amount of carbon-14 declines with respect to the original amount of the stable carbon-12. After 5,730 years, half of the carbon-14 in a sample will have decayed. After another 5,730 years, half of the remaining carbon-14 in the sample likewise will have decayed. Use of carbon-14 dating is limited to organic remains less than about 60,000 years old, such as the leather quiver and wooden bow and arrows shown in Figure 14-5. Isotopes with longer half-lives are used to date older rocks.

Radioactive isotopes occur naturally in all matter. Some of the isotopes commonly used in radiometric dating procedures appear in Table 14-1. Radiometric dating is accurate within certain limits. The techniques depend on making careful measurements and obtaining samples that are uncontaminated with more recent material. Scientists compare several types of independent measurements to determine a range of plausible dates.

Scientists have estimated Earth's age by using a dating method that is based on the decay of uranium and thorium isotopes in rock crystals. Collisions between Earth and large pieces of space debris probably caused the surface of Earth to melt many times as the planet was formed. Therefore, the age of the oldest unmelted surface rock should tell us when these collisions stopped and the cooling of Earth's surface began. The oldest known rocks and crystals are about 4 billion years old. So, scientists infer that organic molecules began to accumulate about 4 billion years ago.

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