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Once the organism dies, however, it ceases to absorb carbon-14, so that the amount of the radiocarbon in its tissues steadily decreases.

Geologist Ralph Harvey and historian Mott Greene explain the principles of radiometric dating and its application in determining the age of Earth.

This rate of decay is constant for a given isotope, and the time it takes for one-half of a particular isotope to decay is its radioactive half-life.

For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years.

But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.

Before then, the Bible had provided the only estimate for the age of the world: about 6,000 years, with Genesis as the history book.

Radioactive elements were incorporated into the Earth when the Solar System formed.