Comparing relative dating and radiometric dating

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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.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.The results from these methods indicate that the Earth, meteorites, the Moon, and, by inference, the entire Solar System are 4.5 to 4.6 billion years old.Present evidence indicates, however, that these intervals were rather short (100-200 million years) in comparison with the length of time that has elapsed since the Solar System formed some 4 to 5 billion years ago.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.These oldest rocks are metamorphic rocks with earlier but now erased histories, so the ages obtained in this way are minimum ages for the Earth.Because the Earth formed as part of the Solar System, a second approach is to date extraterrestrial objects, i.e., meteorites and samples from the Moon.

Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1.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.A particularly fascinating question about the history of the Earth is “When did the Earth begin?” The answer to this question was provided by radiometric dating and is now known to within a few percent.The last modification to the geologic time scale of Figure 1 was in the 1930s, before radiometric dating was fully developed, when the Oligocene Epoch was inserted between the Eocene and the Miocene.

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