KREEP is a component of lunar basalts. It is an acronym for K (potassium), REE (rare earth elements), and P (phosphorous). These basalts are unusually rich in K, P, and incompatible REE compared to the anorthosites and gabbros of the lunar highlands and the mare basalts. Ages of the KREEP basalts are from 3.8 to 3.6 Ga. When the Mars-size protoplanet Theia hit the Earth and formed the Moon, the large amount of energy liberated formed in the interior of the Moon a magma ocean. As crystallization of this magma ocean proceeded, olivine and pyroxene precipitated and sank to form the lunar mantle. Less dense anorthositic plagioclase floated, forming an anorthositic crust. Residual melt was progressively enriched in incompatible elements (i.e., those that partition preferentially into the liquid phase) forming the “KREEP”-rich layer ultimately trapped at the base of the anorthositic crust. Subsequent melting of this layer produced KREEP-rich basalts.