[Illustration: 91. A sample of vesicular basalt, produced by lunar
volcanism 3.7 billion years ago, in the Lunar Receiving Laboratory.
Devices record size and orientation of the rock. The cavities in
this sample were formed by gases escaping from the still-molten
rock. This sample is 13.5 centimeters (5.5 inches) long. A fragment
of this lunar rock is on display in the “Apollo to the Moon”
gallery.]
During the six Apollo program moon landings, astronauts collected and
returned to Earth samples of the lunar surface. The samples were
collected both from the flat maria regions—great basins created by
ancient meteoric impacts and later filled with lava from the moon’s
interior—and from the highland regions.
Subsequent analysis of the samples has indicated that the moon’s surface
is largely composed of three kinds of rock.
Basalt, the rock of the maria regions, was formed as lavas from the
interior of the Moon welled to the surface, filled the great meteoric
impact basins, and then cooled.
Anorthosite, the highland rock, is believed by many scientists to have
formed when the original crust of the Moon cooled and solidified.
According to this theory, a light mineral, plagioclase, floated to the
surface of the Moon and formed the anorthosite.
Breccia, the shocked rock, is composed of large and small fragments of
rocks which were shattered and redistributed on the lunar surface by
meteoric impacts. Subsequently, the fragments were recombined into new
rocks by heat and pressure.
Lunar soils are largely composed of fragments of the three types of
rocks and their minerals, and glass produced by meteoric impacts and
volcanic eruptions.
Lunar rock samples are on loan from the National Aeronautics and Space
Administration.
[Illustration: 92. Astronaut Schmitt collects samples with the lunar
rake, a hand tool used to collect rocks and rock chips ranging in
size from 1.3 centimeter (½ inch) to 2.5 centimeters (1 inch).]