Dating rocks by radioactivity
Using fossils as guides, they began to piece together a crude history of Earth, but it was an imperfect history.
After all, the ever-changing Earth rarely left a complete geological record.
For example, isotopes with very long half lives are no good for dating rocks younger than about 100 million years.
Scientists discovered that rocks could be timepieces -- literally.
Many chemical elements in rock exist in a number of slightly different forms, known as isotopes.
For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years.
By measuring the ratio of lead to uranium in a rock sample, its age can be determined.
Using this technique, called radiometric dating, scientists are able to "see" back in time.