From physicist John Baez, a history of the major disasters that happened to the Earth: the Big Splat, the Late Heavy Bombardment, the Oxygen Catastrophe, and the Snowball Earth. The Big Splat is believed to have formed the Moon:
In 2004, the astrophysicist Robin Canup, at the Southwest Research Institute in Texas, published some remarkable computer simulations of the Big Splat. To get a moon like ours to form -- instead of one too rich in iron, or too small, or wrong in other respects -- she had to choose the right initial conditions. She found it best to assume Theia is slightly more massive than Mars: between 10% and 15% of the Earth's mass. It should also start out moving slowly towards the Earth, and strike the Earth at a glancing angle.
The result is a very bad day. Theia hits the Earth and shears off a large chunk, forming a trail of shattered, molten or vaporized rock that arcs off into space. Within an hour, half the Earth's surface is red-hot, and the trail of debris stretches almost 4 Earth radii into space. After 3 to 5 hours, the iron core of Theia and most of the the debris comes crashing back down. The Earth's entire crust and outer mantle melts. At this point, a quarter of Theia has actually vaporized!
After a day, the material that has not fallen back down has formed a ring of debris orbiting the Earth. But such a ring would not be stable: within a century, it would collect to form the Moon we know and love. Meanwhile, Theia's iron core would sink down to the center of the Earth.
