When the Moon was conceived during the formation of the solar system, it was comprised of incredibly hot material. Years’ worth of space rock impacts only exacerbated the Moon’s heat. Fast-forward to today, and all that hot material is finally cooling down. Moreover, the lack of massive space rock impacts means that the Moon isn’t being continuously warmed.
If you remember anything you learned in science class when you were in high school, then you probably understand that warm objects expand and cold objects contract. With that in mind, it shouldn’t come as much of a surprise to anyone that a cooling Moon is also a contracting Moon, and so it’s actually shrinking in size, very slowly over time.
The Moon’s contracting is much too small to observe in person, but stress marks and scarps found all over the world’s surface tell the story quite clearly. Planetary scientists believe that these scarps are created by thrust faults, which occur when the lunar crust presses against itself from the contractions and eventually gives under its own pressure.
Some of the Moon’s scarps tower up to 100 meters in height, but don’t let the numbers fool you, because scientists think that the Moon has only shrunk a grand total of 250 meters since it first formed.