The Moon

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

Vital Statistics

Significant Features

Theories for Origin of Moon

Geology of the Moon

Geologic Map of the Moon

Geologic Evolution of the Moon

Histories of Earth and Moon Compared

The diagram above compares the geologic histories of the Earth and Moon. The central bar is the terrestrial geologic time scale. The bottom three large divisions, each a billion or so years long, are called eons. The time since 540 million years ago is called the Phanerozoic eon, from Greek roots meaning "visible life." The divisions of the Phanerozoic are called eras. They are in turn subdivided into periods (not shown). On Earth, everything before the Phanerozoic is often called Precambrian (the Cambrian is the earliest period of the Phanerozoic).

Just as with human history, we subdivide planetary history at major break points. The Moon has its own history which doesn't necessarily correspond to ours. For example, the end of the Mesozoic era is defined by a large meteor impact, but that event had no major effect on the Moon at all.

The short Nectarian eon (red) is the time when most of the lunar mare basins formed. Everything before that is Pre-Nectarian. The Imbrian is the time when the last very large basins formed and were filled by lava flows. The Eratosthenian generally corresponds to the interval after the main mare formation when older craters formed. The Copernican loosely corresponds to the interval of young craters with bright rays.

The two outer bars show the preservation of information on the Earth and Moon. On Earth, rocks have a "half life" of a couple of hundred million years, that is about half of the rocks of any given age will have eroded away in a couple of hundred million years. Do not confuse this with radioactive decay. So the farther back in time we go, as in human history, the sketchier the record gets. We have only a very partial understanding of the Precambrian. On the Moon, though, almost everything since the end of the Imbrian is preserved. So the Moon fills a gap in geologic time where evidence on the Earth is almost completely gone.

Surfaces of Earth and Moon

On the Earth, weathering and erosion are constantly breaking down old rocks, moving the fragments to new locations and cementing them together again. Much of the Earth is covered with a few kilometers or less of Sedimentary Rocks

On the Moon, the major process which breaks down and transports rock is meteor impact. Most of the impacts occurred early in the history of the Moon, but small impacts are still occurring at a much lower rate. The surface of the Moon is covered with perhaps a kilometer or so of mixed-up and broken rocks and fine fragments called Regolith or Breccia. Cosmic ray and micrometeorite bombardment degrade regolith material. These processes are called space weathering.


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Created 20 May 1997, Last Update 9 April 1999