The initial glancing impact of a large differentiated planetestimal,
perhaps one/half the proto-Earth diameter or larger is depected by Hartmann
on the cover of Natural History magazine. Click here for the offical Origin of the Moon site created by Dr. Hartmann & Gregg Herres.
The impactor vaporizes and the proto-Earth is almost destroyed. Most of the volitiles in the minerals of the impactor are lost at this time, leaving "bone-dry" minerals in the ejected material.
The nickle-iron cores merge, but the impactor mantle material is mostly ejected into Earth orbit.
The impactor mantle material forms a disk of volitile depleted minerals around the Earth. There is very little or no nickel-iron core material in the disk for the accreting proto-Moon to incorporate.
The heat of accretion allows volcanic activity on the very early Moon forming the vast lava plains called maria.
The present Moon has been geologically "dead" since just after the formation of the maria. Its surface features the result of eons of external bombardment from meteoroids and asteroids.
The Earth and Moon: a family portrait.