The measurement or estimation of temperatures at which geologic processes take place. Methods used can be divided into two groups, nonisotopic and isotopic. Nonisotopic methods involve measurements of earth temperatures either directly by surface and near-surface features or indirectly from various properties of minerals and fossils. The isotopic methods involve the determination of distribution of isotopes of the lighter elements between pairs of compounds in equilibrium at various temperatures, and application of these data to problems of the temperature at which these compounds (commonly minerals) form in nature.