If the moon formed from a chunk of earth why is moon rock so unique?

443 viewsOtherPlanetary Science

So as I understand it the moon was formed when a chunk of earth was blown into orbit when a mars like planet collided with earth, if this is the case why is the composition of moon rock not similar to the composition of rock found on earth, when the moon if just a piece of the earth its self?

In: Planetary Science