Astronomy
Name ____________________________
THE GEOLOGY OF THE MOON
The Apollo astronauts brought back 381.69kg of rock and soil samples referred to as regolith.
Regolith is the geological term for loose, solid material that covers the bedrock of a planet type
body; in this case the Moon. The original 2,196 samples collected have now been separated into
35,600 samples. The regolith have been dated between 4.6 and 3.1 billion years old. The regolith
were taken only from the topmost few centimeters of the Lunar crust.
All the regolith returned is igneous or
breccias. There are no sedimentary or
metamorphic samples. (Igneous rocks are
those that have formed under extreme
temperatures and pressures, during
volcanic activity or catastrophic impacts;
Sedimentary rocks are those that have
formed by wastage of existing rocks and
deposited on, for example, sea beds;
Metamorphic rocks are those that have
been re-introduced to extreme
temperatures and pressures thereby
altering their characteristics).
Studies have shown that the samples are similar to Earth rocks. Generally Lunar rocks have one
tenth as much sodium and potassium as Earth rocks. Moon basalts contain more titanium than
terrestrial basalts.
Lunar Surface
Can be classified into two distinct types:
 MARIA
Dark, young, smooth areas (16% of surface).
Mare (plural of maria) is Latin for sea. To early observers these areas appeared to be oceans as
they were flat, smooth and dark. The maria have never contained any water but are actually areas
of hardened lava that have formed in basins, created by impact craters. For a reason as yet to be
discovered the maria are more plentiful on the near side.
 TERRAE
Bright, old, highlands which are very cratered and rugged.
Mountainous and heavily cratered areas. Ranges can be 5 km high, bordering maria.
Crust
The Moon's crust averages 68km thick. (107km thick at a maximum on the far side near crater
Korolov). The crust is thinner on the near side.
There is a layer of regolith 1 to 20 meters deep, (on average 5 meters deep in the maria and 10
meters deep in the highlands).
On the highlands, the regolith is comprised of anorthosites with minerals such as pyroxene,
plagioclase and olivine. The mare regolith contain basalt, pyroxene, plagioclase and ilmenite.
See Lunar Rock Types for definitions of these rocks.
Under the regolith sits a 1 km layer of fragmented bedrock. Below which sits a 25 km layer of
solid bedrock. Beneath this there is a 30 km layer of rock, similar to anorthosites of the
highlands. The bottom of this layer is 50 km deep. There is then another 150 km of dense rock.
Then there is a small metallic core.
Core
The core extends to a radius of approximately 340 km (no more than 450 km). That is, the core
radius is less than 25% of the Moon's radius, which is small. (The Earth's core radius is 54% of
it's total radius). The Moon's core is approximately 2% of the its mass. The core probably
consists of mainly Iron and Sulphur in two zones; a molten zone and a solid zone. The Moon's
center of mass is not at its geometric center, being 2 km nearer the Earth. This information
comes from the Lunar Prospector Probe (1998), which used gravity measurements and the
Passive Seismic Experiments on the Apollo missions 11, 12, 13, 14, 15 and 16. Also NASA
scientists have carried out calculations using mass and rate of spin.
Lunar Rocks
 ANORTHOSITIC ROCKS
Plagioclase feldspar (CaAl2Si2O8) is formed in pools
of lava. Contains minor quantities of Kreep Norite
rocks, named after potassium (K), rare earth metals
(REE) and phosphorus (P). Kreep Norite is also present
in Earth rocks. (picture on right)
 BASALT ROCKS
These contain the minerals pyroxene, plagioclase and
ilmenite. (picture on left)

BRECCIA ROCKS
Complex, shattered, crushed igneous rocks collected from
every landing site. (picture on right)
MINERALS
 PYROXENE (Ca-Mg-Fe-Si compound)
This is the most common mineral in Lunar lavas, forming about half of all samples. Forms
yellowish crystals about 1cm in size.
 PLAGIOCLASE (Na or Ca - Al Silicates)
Forms elongated white crystals, can be found in anorthositic rocks. Makes up half the highland
samples.
 OLIVINE (Mg-Fe silicate)
Pale green crystals, can be found in anorthositic rocks.
 ILMENITE (Fe-Ti Oxide)
This is found in basalt rocks.
Lunar Features
 CRATERS
Found more often in terrae regions. Craters can be formed by volcanic activity or by meteorite
impacts. Lunar craters resemble meteorite impact craters as the volume of displaced material is
approximately equal to the estimated volume of the crater walls. (Impacts carried out in
simulated laboratory environments result in craters with similar physical characteristics). They
are also mostly broad and flat and some have central peaks. A high proportion of the near side
and most of the far side are covered with millions of craters. Most that have been named are after
eminent scientists, philosophers and historical figures. On the near side there are about 30,000
craters large enough to be seen with Earth bound telescopes.
 DOMES
Small surface bulges within the maria areas, whose heights are 100 meters or so and diameters of
several kilometers.
 MASCONS
Areas of very high mass concentration that lie beneath regular shaped maria, of debateable
origin.
 RILLES
Trench like features between half and five kilometers wide, not more than 400 meters deep,
usually hundreds of kilometers long. They cross craters and hills indicating that they formed later
than most other Lunar features.
 WRINKLE RIDGES
Irregular elevations 100 meters high and 30 km wide.
 MOONQUAKES
As many as 3,000 moonquakes can happen each (terrestrial) year caused by Earth's gravity
"pulling" on the Lunar surface and exerting forces within the rock. The Apollo missions left
seismometers on the Moon which reveal weak, naturally occurring moonquakes and incidentally
reveal much about the internal mantle rocks.