According to the giant impact hypothesis

Giant impact hypothesis

The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...

 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

Crust (geology)

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 in the lunar highlands

Geology of the Moon

The geology of the Moon is quite different from that of the Earth...

, as well as the existence of rocks with a high concentration of the geochemical component referred to as KREEP

KREEP

KREEP, an acronym built from the letters K , REE and P , is a geochemical component of some lunar impact melt breccia and basalt rocks...

.

Ages of formation and crystallization of the lunar magma ocean have been constrained by studies of isotopes of hafnium

Hafnium

Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...

, tungsten

Tungsten

Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

, samarium

Samarium

Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...

, and neodymium

Neodymium

Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...

. The magma ocean formed about 70 million years after the history of our solar system

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

began, and most of the ocean had crystallized by about 215 million years after that beginning (Brandon, 2007).

Extended scientific references

• Wood, J. A., Dickey, J. S., Jr., Marvin, U. B., and Powell, B. N. (1970) "Lunar Anorthosites". Science, v. 167, no. 3918, p. 602.
• Wood, J. A. (1972) "Thermal History and Early Magmatism in the Moon". Icarus, v.16(2), p. 229-240.
• Wood, J. A. (1972) "Fragments of Terra Rock in the Apollo 12 Soil Samples and a Structural Model of the Moon". Icarus, v. 16(3), p. 462-501.