According to the giant impact hypothesis a large amount of energy was liberated in the formation of the Moon and it is predicted that as a result a large portion of the Moon was once completely molten, forming a lunar magma ocean. Evidence for the magma ocean hypothesis comes from the highly anorthositic compositions of the crust in the lunar highlands, as well as the existence of rocks with a high concentration of the geochemical component referred to as KREEP.
Ages of formation and crystallization of the lunar magma ocean have been constrained by studies of isotopes of hafnium, tungsten, samarium, and neodymium. The magma ocean formed about 70 million years after the history of the Solar System began, and most of the ocean had crystallized by about 215 million years after that beginning (Brandon, 2007).