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Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.When we look at sand in an hourglass, we can estimate how much time has passed based on the amount of sand that has fallen to the bottom.Radioactive rocks offer a similar “clock.” Radioactive atoms, such as uranium (the parent isotopes), decay into stable atoms, such as lead (the daughter isotopes), at a measurable rate.The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.

James Joly calculated that the Earth’s age was 89 million years on the basis of the time required for salt to accumulate in the oceans.The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).he question of the ages of the Earth and its rock formations and features has fascinated philosophers, theologians, and scientists for centuries, primarily because the answers put our lives in temporal perspective.Until the 18th century, this question was principally in the hands of theologians, who based their calculations on biblical chronology.