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expected to be bigger in all their dimensions on walked on the surface of the Moon and brought
low-gravity planets. We shall see below that the back many rock samples, and satellites orbiting the
sizes of volcanic features on the other planets Moon had surveyed most of its surface. It became
amply support these conclusions. clear from putting all of the information together
that the craters on the Moon were virtually all due
to impacts, most of which took place in the first 500
13.4 The Moon million years after the Moon formed. Only a hand-
full of crater-like features were produced by vol-
Relative to the other silicate planetary bodies, the canic processes. Furthermore, the very fact that the
Moon has had a very strange history. The rocks Moon’s surface is saturated with well-preserved
brought back from the Moon by the Apollo astro- impact craters and basins from the time just after its
nauts show that the composition of the Moon is formation is proof of the fact, confirmed by all other
similar to that of the Earth apart from the fact that lines of evidence, that the Moon never developed
all volatile compounds and elements have either even a trace of the plate tectonic processes that
partly or completely been lost. This is now inter- play such an important part in removing heat from
preted to mean that, around the time the Earth had the Earth.
nearly finished forming, it collided with another However, this does not mean that the Moon has
planet, probably about the size of Mars. Most of not been volcanically active. In fact, all of the dark
the bulk of these two bodies coalesced to form areas on the Moon’s surface that can easily be seen
the present Earth. Some material was thrown off at from the Earth are regions where basaltic lavas have
high speed, being partly melted in the process and been erupted onto the floors of large impact basins.
so losing all of its volatile compounds, and cooled These lava-filled basins are called maria (singular:
to form a ring of solid particles around the Earth. mare), latin for “seas”, because the earliest tele-
This ring accumulated to form the Moon, doing this scopic observers thought that they were expanses
so rapidly towards the end of the process that the of water. The basins were formed by asteroid
outer few hundred kilometers of the Moon melted impacts during the first few hundred million years
completely again to form a magma ocean. As this of lunar history and they were flooded by large
ocean cooled and crystallized a crust accumulated lava flows (Fig. 13.2), some more than 200 km
from light minerals forming a rock called anorthosite, long, about 500 million years later. The delay was
while denser minerals such as olivine sank to join because it took this long for heat liberated by the

the unmelted interior. All of this took place within decay of radioactive elements to warm up the interi-
a time span of a few tens of millions of years. or of the Moon to the point where extensive melt-
From the Earth it is easy to see that much of the ing took place in the mantle. Large eruptions took
Moon’s surface is covered by craters of a vast range place preferentially inside the impact basins partly
of sizes, from more than 1000 km down to the because the dikes carrying magma from the man-
smallest features visible, only a few hundred meters tle melt sources had a shorter distance to travel to
across. Before rock samples were obtained from reach the floors of the basins than elsewhere. Other
the Moon, some people interpreted these craters, factors may have played a role in concentrating
depressions with raised rims surrounded by blan- large eruptions into basins. These include the fact
kets of material fairly obviously thrown out from that to compensate for the removal of part of the
them, to be explosive volcanoes. These people crust when a large basin forms, the mantle beneath
tacitly assumed that the Moon was at least as vol- the basin rises to some extent, which changes the
canically active as the Earth, perhaps more so given temperature profile and helps to encourage subse-
that there were so many craters. Others argued quent partial melting.
that the craters must be the result of the impacts Not all eruptions took place inside impact basins:
of meteoroids, comets and asteroids, the pieces of a few occur on the surface of the old cratered
interplanetary debris left over from the formation anorthosite crust. Also, there are many linear val-
of the planets. By the mid-1970s astronauts had leys, usually called linear rilles, on the Moon