KREEP

More than 4.5 billion years ago the surface of the Moon was a liquid magma ocean. It is believed that KREEP represents the last chemical remnant of that magma ocean after the lunar crust formed. KREEP floated to the surface the magma because its component elements are "incompatible" with each other, that is, they do not fit together to form a compact crystal structure. KREEP is used by scientists to trace the volcanic history of the Moon and to record the impacts by meteorites.

Results from Lunar Prospector[?]'s gamma ray spectrometer shows that KREEP-containing rocks are concentrated in the Mare Imbrium rim, the nearside maria and highlands near Imbrium and the Mare Ingenii South Pole-Aitken basin and are distributed at a lower level in the highlands. The distribution seen by Lunar Prospector supports the idea that the impact which formed Mare Imbrium excavated KREEP-rich rocks and ejected them over the Moon and the South Pole-Aitkin basin impact also exposed KREEP-rich material.