Geologic Processes
Section M.4.
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A Geological History of the Moon

Our understanding of the history of the moon was revolutionized by Apollo and the other missions of the past four decades. Despite this, much is unknown. This document attempts to review current understanding of the geological history of the Moon. The descriptions of rocks are limited to those samples returned from the Apollo and Luna missions. Orbital geochemical data from Apollos 15 and 16 suggest that there is a greater variety of mare basalts, and that the Highlands are probably more diverse. Even so, the undiscovered rocks will probably be similar to those already discovered.


There are a number of competing theories to explain the origin of the moon. Currently, the most favored is the Earth-impact hypothesis. This suggests that Earth was hit by a Mars-sized planetesimal approximately 4.6Ga. The moon condensed out of the material thrown into orbit by the impact.


Initially the moon was molten. As it cooled, two things of note happened. Firstly, an iron-nickel core formed -- similar to Earth's. Initially, this was molten, and would have produced a notable Lunar magnetic field. This is recorded as palaeomagnetism in many of the mare basalts. The field appears to have been strongest approximately 3.8-3.6Ga. Eventually, the core solidified and the "dynamo" which powered the moon's field stopped. The moon's core is currently thought to be approximately 400km in size. This low figure is based on bulk density figures and the minimal interaction with the solar magnetosphere. One of the objectives of the Lunar Prospector Mission, is to confirm the existence of an "extinct" magnetic core.

Also, as the moon cooled, plagioclase crystals formed. These are of low density and floated to form a "scum" floating on the moon's surface. This was in effect the moon's first crust. This crust is thought to have been "tens of kilometers" thick.

Samples of this plagioclase have been found. Rocks which are virtually all plagioclase (like these) are known as anorthosite (anorthite is a kind of plagioclase). Lunar anorthosites are referred to as "Ferroan anorthosite" due to their high Fe content. They are extremely ancient: 4.5-4.6Ga.

The highlands are also made of another dominant rock group: "the Mg-suite". These rocks are also abundant in plagioclase but contain substantial olivine and Ca-poor pyroxene.

The anorthosites and the Mg-suite cannot have come from the same source. This means that there were at least two "parent" magmas.

The final crystallization of the global magma system is thought to be marked by rocks of KREEP composition. KREEP stands for Potassium (K), Rare Earth Element (REE), and Phosphorus (P). These are incompatible elements, elements which do not fit well into the more common minerals. KREEP is a component of many highland soils, breccias, and impact melts, but the trace abundancies are remarkably uniform. Also, the age is a consistent 4.35Ga.


The maria are basalts -- rich in Fe and Mg, and fine-grained. These consist mainly of pyroxene, plagioclase, some olivine, and various accessory minerals. As such, basalts like these are common on Earth, but there are some differences. First: they are devoid of water and hydrated minerals. Second: some of the samples are very rich in Ti. Basalts returned from Mare Tranquillitatus have approximately ten times the Ti content of terrestrial basalts.

Mare basalts are thought to have been produced by radiogenic heating and melting of regions of the moon's mantle. Terrestrial basalts are also produced by partial melting of a silicate mantle, although the mechanism (depressurization, or convective heating) is different. The basalt plains range from 3.1Ga to 3.8Ga in age. Some fragments in highland breccias indicate the existence of mare basalts as long ago as 4.3 Ga. We don't have any samples of the younger mare basalts, but detailed analysis of high resolution photographs, suggests that some flows may embay (hence post-date) young, rayed craters. These may be younger than 1 Ga.

A distinct group of volcanic glasses (different from the impact-generated glass beads) occur across the moon. These have a composition similar to the mare basalts and are thought to be produced by Hawaiian-style lava fountaining.

The maria average a few hundred meters in thickness, and are thinner nearer the rims. The mass of each layer of mare basalt has weighed the crust down. Hence the next layer of lava pooled in the middle. This produced concentric compositional rings in each impact basin. The youngest basalts are on the top and in the middle. Thicknesses in the centers are in the range of 2-4km. The mass can also lead to compression in the centers of the maria, and stretching at the edges. Hence the wrinkle ridges (tectonic compression structures) and grabens ("rift valleys").

The mare basins exhibit strong gravitational anomalies -- so large that they can produce considerable errors in orbital predictions. One theory is that these anomalies are due to the mass of the impacting object which formed the basin. A more popular theory is that the "mascons" are due to mantle-upwelling under the thin and cracked crust resulting from the impact. The basalt flows then added to the gravitational anomaly.

Lower Crustal Composition

On the rims of the large impact basins, fragments of rocks thought to originate in the lower crust, are found. Due to their compositions, they cannot have been formed by melting of any other lunar rocks. These rocks are notably mafic (MgFe rich). Soil/regolith fragments are also missing, and they date to 3.9-3.8 Ga (the age of the last basin impacts).

The Mantle

The mantle constitutes about 90% of the moon. It is thought to consist of a mixture of olivine and pyroxene. The exact composition is thought to vary in a complex manner. For example the Ti-rich basalts of Mare Tranquillitatis are thought to have been derived from a particularly Ti-rich patch of the mantle.

Further References

The New Solar System, J.K. Beatty and A. Chaikin (eds.), Cambridge, Sky Publishing.

Geologic Processes

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