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In addition, they have had to develop special techniques with which to dissolve these highly refractory minerals without contaminating the small amount (about one-billionth of a gram) of contained lead and uranium on which the age must be calculated.

Since parent uranium atoms change into daughter atoms with time at a known rate, their relative abundance leads directly to the absolute age of the host mineral.

Similarly, in geologic studies, vast quantities of information from widely spaced outcrops have to be integrated.

Some method of correlating rock units must be found.

Just as the use of the fossil record has allowed a precise definition of geologic processes in approximately the past 600 million years, absolute ages allow correlations back to Earth’s oldest known rocks formed more than 4 billion years ago.

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Great uplift, accompanied by rapid erosion, is taking place and large sediment fans are being deposited in the Indian Ocean to the south.

Using this established record, geologists have been able to piece together events over the past 635 million years, or about one-eighth of Earth history, during which time useful fossils have been abundant.

The need to correlate over the rest of geologic time, to correlate nonfossiliferous units, and to calibrate the fossil time scale has led to the development of a specialized field that makes use of natural radioactive isotopes in order to calculate absolute isotopes has been improved to the point that for rocks 3 billion years old geologically meaningful errors of less than ±1 million years can be obtained.

It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced.

In addition, because sediment deposition is not continuous and much rock material has been removed by erosion, the fossil record from many localities has to be integrated before a complete picture of the evolution of life on Earth can be assembled.

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