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