The use of such water, which is not being recharged under the current climatic regime, is termed groundwater...
The early studies of the radioactive decay of uranium and thorium into lead caused the British physicist Ernest Rutherford to suggest that this process could be used to determine the age of rocks and consequently of the Earth by observing the amount of helium retained by a rock relative to its uranium and thorium contents.
Scientific knowledge of Earth’s geologic history has advanced significantly since the development of radiometric dating, a method of age determination based on the principle that radioactive atoms in geologic materials decay at constant, known rates to daughter atoms.
Another role of isotopic geochemistry that is of great importance in geology is radiometric age dating. Beginning with studies in the 1950s, a much better chronology and record of Pleistocene climatic events have evolved through analyses of deep-sea sediments, particularly from the oxygen isotope record of the shells of microorganisms that lived in the oceans.
The ability to quantify the geologic time scale—i.e., to date the events of the geologic past in terms of numbers of years—is largely a result of coupling radiometric dating techniques with older, classical methods of establishing relative geologic ages. ...water of the Nubian sandstone aquifer, which extends through several countries in an area that is now the Sahara desert.
Using such techniques, investigators have been able to determine the ages of various rocks and rock formations and thereby quantify the...
..to the extent of about 100 parts per million (ppm) in tektites—far below the value for terrestrial igneous or sedimentary rocks.
Studies of isotopes formed from the decay of radioactive parent elements with short half-lives, in both lunar samples and meteorites, have demonstrated that the formation of the inner planets, including Earth, and the Moon was essentially complete within 50 million years after the interstellar cloud region collapsed.