What is the Moon made of?
view of the lunar surface from the Galileo spacecraft
to popular belief the Moon is not made of cheese! It is made up of many
different types of rocks, many of which have been broken up and formed
back together again and again because of the numerous impacts
have occurred over the Moon’s 4.6 billion year lifetime.
types of rocks are known as breccias.
the different rock types are related to the colours of the rocks that
make up the highlands and the maria. The most common type of Moon rock
is the light coloured anorthosite, which contains a lot of the mineral
feldspar, and makes up the rugged lunar highlands. It is also the most
common type of rock on the Earth. The sheer amount of anorthosite on
the Moon tells geologists that the Moon was entirely molten at some
point in its history because the feldspar crystals that began to
crystallise out of the melt were less dense than the melt itself, and
so floated to the top of this molten ‘magma ocean’,
eventually solidifying into an almost entirely feldspar crust.
type of highlands rock is similar to anorthosite but contains some
other minerals, like olivine, which is rich in magnesium. This rock
group is called the Mg-suite. KREEP makes up the rest of the highlands
rock and stands for potassium (chemical symbol K), rare earth elements
(REE) and phosphorous (P). These chemical elements do not mix with the
other rock forming minerals of plagioclase feldspar, pyroxene and
olive, so they get left behind in the melt while everything else
crystallises. Rocks formed from this left-over melt has unusual mineral
compositions, which helps geologists learn about the final stages of
the Moon’s geologic evolution.
is the main
component of the lunar maria. Basalt also makes up a large component of
Earth and Moon rocks, forming from solidified lava, and is made up of
the minerals pyroxene and plagioclase feldspar, sometimes with a bit of
olivine. The basalts were erupted into giant impact basins several
hundred of thousands of years after the impact event.