Absolute Time

Opportunities to come as close to a geological event as that at Mecca are few. Certainly it is generally far beyond the scope of the system of relative dating described above, which is one of the two measures that geologists have when they speak of time. The other measure they use is a clock that records absolute time by measurement of changes in radioactive elements in the rocks. Such elements decay at a constant rate, the works of an atomic clock that ticks steadily but so slowly that its face is marked in thousands and millions of years. This great-great-grandfather of all clocks is capable of timing the span of life from the amoeba to man.

Radioactive decay or nuclear fission, the force that powers the atomic bomb and nuclear power plants, causes transmutation of a radioactive element into a new element. Because this decay takes place at a predictable rate regardless of heat, pressure, and solution, measurement of the amount of the parent radioactive element and the transmuted daughter element in the rock makes it possible to set a quite exact age in years for the rock.

Fission changes uranium into lead and helium, potassium into calcium and the rare gas argon, and rubidium, an element chemically close to sodium, into strontium, the element that puts brilliant reds into fireworks.


In the same way, one radioactive form of carbon turns into a more stable form of the same element.

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