From Belinda Smith at Cosmos Magazine:
Around 4.5 billion years ago, an object slammed into Earth vaporising most of the planet into a scorching cloud from which the moon was born.
Geochemists in the US – Kun Wang from Washington University in St Louis and Stein Jacobsen at Harvard – examined minuscule amounts of potassium in moon and Earth rocks and found minute differences – possible only if their raw materials were thoroughly mixed in a superheated fog before they coalesced.
The work, published in Nature, pokes a hole in the theory that the moon was born from a low-impact collision. More.
In truth, we do not know very much about how the moon was formed, and theories rise and fall.
See also: How the Moon Formed: 5 Wild Lunar Theories (Mike Wall at Space.com, 2014)
Our moon formed in collision with embryo planet?
and
Origin of the moon still shrouded in mystery
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