Given that the Earth and Moon are often said in the literature to have very similar geochemistry, what is the current theory as to how the Earth's Moon formed?
The current accepted theory is known as the Giant Impact Hypothesis, where according to this NASA webpage "Origin of the Earth and Moon" (Taylor) a Mars sized object collided into the early Earth.
This theory allows explanations of (from the link above):
The chemical makeup:
and in terms of orbits:
However, recently, there has been some revisions to this theory.
According to NASA's page "NASA Lunar Scientists Develop New Theory on Earth and Moon Formation", based on concerns that the Mars-sized would likely to have had a different composition to the Earth (inconsistent with the current similarities in geochemistry).
The new hypothesis is (from the link above):
This is also discussed in "Huge Moon-Forming Collision Theory Gets New Spin" (Wall, 2012), basing the revised hypothesis on the rotation rate, which is theorised to have been very fast, from the article:
Further discussion, based on the geochemical makeup is discussed in "Geochemical Constraints on the Origin of the Earth and Moon" (Jones and Palme) conclude that:
So, there is still some question as to precisely how the Moon was formed.