Geology of the Moon's Surface Features

"Magnificent Desolation" Includes Craters, Maria, Rilles & Mountains

Mar 2, 2009

What geologic and tectonic features are found on the lunar surface and what are the processes that shaped the surface of the Moon?

Buzz Aldrin walked on the Moon's surface in 1969 and described it as "magnificent desolation". Geologic features on the lunar surface include craters, maria, rilles, and mountains. What are these features? How did they form?

Lunar Impact Craters

Much of the Moon's surface is covered with craters. The crater covered regions are at higher altitudes, so they are called the lunar highlands. Lunar craters range in size from over 100 miles in diameter to microscopic size. The craters are universally circular. Some have a small hill in the center.

On Earth craters form either from meteorite impacts or from volcanic eruptions. Lunar craters more closely resemble impact than volcanic craters. Hence astronomers and lunar geologists think lunar craters are impact craters. They formed when meteorites struck the lunar surface with enough energy to explode and form a circular crater.

Most of the craters formed about 4 billion years ago when the lunar surface had recently solidified and the solar system still contained plenty of debris crashing into the Moon. Craters can continue to form, but at a much slower rate because there is less debris to strike the lunar surface. Hence cratered surfaces in the solar system, including the Moon's, are geologically older than uncratered surfaces. That does not mean the moon or planet formed at an earlier time; rather that the surface has had less geological or tectonic activity to obliterate the craters.

Many lunar craters have rays, which are radial spoke-like patterns centered on the crater. When the impact formed the crater, it blasted different colored rocks from beneath the surface outward to form the ray pattern. Rays are one piece of evidence that lunar craters are impact rather than volcanic craters.

Lunar Maria and the Man in the Moon

The maria (singular=mare) are relatively smooth flat areas on the Moon. They form the dark areas many people call the man in the Moon.. When Galileo first observed them, he thought that they were oceans and named them with the Latin word for seas. The maria are also called the lunar lowlands because they have a low altitude. The side of the Moon facing Earth contains maria, but the far side of the Moon facing away from Earth does not. Astronomers do not know why?

Maria formed a little over three billion years ago, after most of the lunar craters formed. Very large objects struck the moon forming large basins. Lava from beneath the lunar surface oozed up through cracks in the basins, partially filled in the basins, and solidified. The maria are smooth because they were once liquid. They have few craters because they solidified after the major era of cratering was over.

Mountains on the Moon

The lunar surface contains some mountains and hills, but their origin is different than Earth's mountains. Lunar mountain ranges often border the maria. They probably formed from the impacts that formed the maria basins. Energy and explosions from these impacts uplifted the surrounding rock. Some mountains or hills may be huge rocks thrown out from these impacts.

Rilles - Ancient Lava Rivers

The lunar surface also has features known as rilles. The best known is Hadley's Rille. Rilles wind and meander to resemble rivers, however there is no liquid water on the Moon. Rilles were once, however, rivers of molten rock. The lava solidified as the Moon cooled leaving beds were lava rivers once flowed, rilles.

The geology and geologic history of the Moon's surface is much different from Earth's. Hence the Moon has geologic features that are different from those found on Earth.