Interior and Geological Activity

Before the Apollo missions we knew almost nothing about the interior of the
Moon. The Apollo missions left seismometers on the lunar surface that have
allowed us to deduce the general features of the Lunar interior by studying the
seismic waves generated by "moonquakes" and occasional meteor impacts.
The Structure of the Interior
Our present picture of the Moon's interior is that it has a crust about 65 km
thick, a mantle about 1000 km thick, and a core that is about 500 km in radius.
A limited amount of seismic data suggests that the outer core may be molten.
There does appear to be some amount of differentiation, but not on the scale of
that of the Earth. It has no magnetic field to speak of, but magnetization of
Lunar rocks suggests that it may have had a larger one earlier in its history.
Although there is a small amount of geological activity on the Moon, it is
largely dead geologically (the energy associated with the Earth's seismic
activity is about 10^14 times larger than that of the Moon). Most Lunar
seismic activity appears to be triggered by tidal forces induced in the Moon by
the Earth.
Geological History of the Moon
The weight of the evidence is that the Moon was active geologically in its
early history, but
the general evidence suggests that the Moon has been essentially dead
geologically
for more than 3 billion years. Based on that evidence, we believe the
chronology of Lunar geology was as follows:
The Moon was formed about 4.6 billion years ago; maybe hot or maybe cold.
The surface was subjected continuously to
an intense meteor bombardment associated with debris left over from the
formation of the Solar System.
By about 4.4 billion years ago the top 100 km was molten, from original
heat of formation and from heat generated by the meteor bombardment.
By 4.2 billion years ago the surface was solid again.
As the intense meteor bombardment associated with debris left over from the
formation of the Solar System continued, most of the craters that we now see
on the surface of the Moon were formed by meteor impact.
The fracturing and heating of the surface and subsurface by the meteor
bombardment led to a period of intense
volcanic activity in the period 3.8-3.1
billion years ago. Meanwhile, the meteor bombardment had tapered off because by
this time much of the debris of the early Solar System
had already been captured by the planets.
The lava flows associated with the volcanism filled the low areas and many
craters. These flows solidified to become the flat and dark maria,
which have little cratering because most of the original craters were covered
by lava flows and only a few meteors of significant size have struck the
surface since the period of volcanic activity.
The regions that were
not covered by the lava flows are the present Highlands; thus, they
are heavily cratered, and formed from different rocks than the seas.
The volcanism stopped about 3.1 billion years ago: the Moon has been
largely dead geologically since then except for the occasional meteor impact
or small moonquake, and micro-meteorite erosion of the surface.
Thus, Lunar surface features, particularly in the Highlands,
tend to be older than those of the Earth, which
remains to this day a geologically active body.