Our Moon originated soon after the formation of the Earth itself, in the wake of a collision between a Mars-sized body and the Earth (see chapter 1). Upon collision, the impacting body disintegrated; most of its core probably sank into the Earth to combine with Earth’s core, while its mantle, along with fragments of the Earth’s mantle, formed a ring of dust encircling the Earth. The Moon condensed out of this ring. Today (or tonight), even with the unaided eye, you can see two types of surface provinces on the Moon: light-colored lunar highlands consist of a type of ultramafic igneous rock called anorthosite, while the dark-colored mare (from the Latin word for "sea") consists of mafic igneous rock (basalt). Most of the anorthosite solidified before 4.35 Ga, while most of the basalt formed later, between 3.9 and 3.1 Ga.

Rocks at the Moon’s surface have not passed through the rock cycle, for plate tectonics does not occur on the Moon, and the Moon does not have an ocean or atmosphere. Thus, we still can see relicts of the Moon’s earliest history. From these relicts, geologists deduce that soon after it formed, the surface of the Moon was a magma ocean. Direct freezing of this ocean produced the anorthosite of the lunar high-lands. The basalt compromising the mare developed later, when huge meteor impacts pulverized much of the surface rock, creating a thick coating of dust and debris and excavating deep craters. Decompression beneath the craters produced mafic (basaltic) magma that rose to fill them. If this explanation for Moon-rock formation is correct, then the present surface of the Moon provides examples of the types of material that would have formed on the surface of Earth in the Hadean eon, had it frozen. But because the rock cycle does operate on Earth, its surface features have changed radically since the Hadean time.

Why is the Moon so dead? Because of its small size, the Moon has cooled more than has the Earth, and the thickness of its lithosphere accounts for almost 60% of the thickness of the whole planet. Such a thick lithosphere simply cannot break into moving plates.

We can get a sense of the frequency of meteor impact during the Hadean eon by observing the intense crating of the Moon’s surface, for the craters have not eroded away. Considering the relative lack of craters in the mare, it seems that most cratering occurred before about 3.9 Ga, by which time, apparently, most debris in the solar system had been incorporated into larger planets. The pummeling of the Earth and Moon might have been worse were it not for the gravitational attraction of giant planets like Jupiter and Saturn, which sucked in much of the debris in the solar system. Notably, particularly large impacts probably changed both the rotation rate and the tilt of the Earth.

Other Feature Articles in this chapter:  1  :  2  :  3

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