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Nonetheless, stratigraphy and radiometric dating of Precambrian rocks have clearly demonstrated that the history of the Earth extends billions of years into the past.Radiometric dating has not been applied to just a few selected rocks from the geologic record.The abundance and variety of fossils in Phanerozoic rocks have allowed geologists to decipher in considerable detail the past 600 million years or so of the Earth’s history.In Precambrian rocks, however, fossils are rare; thus, the geologic record of this important part of the Earth’s history has been especially difficult to decipher.Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock.Moreover, many of the oldest dated rocks intrude still older but undatable rocks.A particularly fascinating question about the history of the Earth is “When did the Earth begin?

Thus, the ages of the Earth, the Moon, and meteorites as measured by different methods represent slightly different events, although the differences in these ages are generally slight, and so, for the purposes of this chapter they are here treated as a single event.The rocks in these shields are mostly metamorphic, meaning they have been changed from other rocks into their present form by great heat and pressure beneath the surface; most have been through more than one metamorphism and have had very complex histories.A metamorphic event may change the apparent radiometric age of a rock.Add to this number the age measurements made by from 50 to 100 other laboratories worldwide, and it is easy to see that the number of radiometric ages produced over the past two to three decades and published in the scientific literature must easily exceed 100,000.Taken as a whole, these data clearly prove that the Earth’s history extends backward from the present to at least 3.8 billion years into the past.

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