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All the major continents contain a core of very old rocks fringed by younger rocks.These cores, called Precambrian shields, are all that remain of the Earth’s oldest crust.Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets.The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites.This method is thought to represent the time when lead isotopes were last homogeneously distributed throughout the Solar System and, thus, the time that the planetary bodies were segregated into discrete chemical systems.

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.Three basic approaches are used to determine the age of the Earth.The first is to search for and date the oldest rocks exposed on the surface of the Earth.Most commonly, the event causes partial or total loss of the radiogenic daughter isotope, resulting in a reduced age.Not all metamorphisms completely erase the radiometric record of a rock’s age, although many do.

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