If you want to gain a good understanding of how the Moon probably formed, you have to first understand the intricacies and limitations of “equations of state”, or EOS for short. An EOS is essentially a mathematical expression that tells you how the pressure of some material changes when other properties of that material also change. Those properties can be the density (how much of the material there is in a certain volume), the temperature (something we are all familiar with), or the internal energy.
The material from which the Moon formed was pretty exotic: it was a mixture of very hot magma (that is, molten rock) surrounded by quite hot rock vapor. Where did that stuff come from? It was going around the Earth right after our planet was smashed by another planet, shortly after the Earth had formed. The collision obliterated the planetary projectile, and gave the Earth a pretty big dent. Most of the Earth’s surface was melted by the blow, and about two-Moons’ worth of hot liquid and vapor was sent into Earth orbit. The Moon would form from that stuff shortly afterwards.
The thing is that the Moon-forming debris was composed of different rock materials, and so describing its thermodynamic behavior (that is, how its pressure changed with temperature or density) is not so easy. But there are several EOS out there that have been used by scientists to study how the planetary debris behaved, mainly through computer simulations. It’s both a fascinating and challenging tale of thermal physics, and I’m in the process of writing it clearly and concisely.
(Video by Canup 2012)