The study of the abundance ratios of isotopes (both stable and radioactive) of major and trace elements in rocks (e.g. Rb/Sr, Pb/U, etc.), to elucidate a number of geologic problems and processes. These include the age relationships of rocks, and the age of the Earth itself (see geochronology; isotopic dating; radiometric dating); palaeotemperatures and geothermometry (see oxygen-isotope analysis; geothermometer); and the provenance of natural waters, ore-forming fluids, and magmas (see isotope fractionation; smow; d/h ratio; oxygen-isotope ratio; stable-isotope studies).