Re: How are rocks formed on the moon?

Your question is a good one. As you will see, the answer shows how much like the Earth the Moon (and other planets) can be, yet at the same time, how different they are.

It turns out that the processes that drive geological activity, like volcanos or formation of rocks, are pretty much the same on all rocky planets (like the Earth and Mars) and moons. But on different planets, we find that different processes are more or less important.

For example, let's look at how rocks form on the Earth. Here, we have three basic types of rocks: igneous, sedimentary and metamorphic. Igneous rocks are "fire-formed", that is, are results of lava or magma cooling into solid rocks. Sedimentary rocks are formed as layers of material are laid down on the bottoms of bodies of water, then pressed into rock (sandstone and limestone are examples). Metamorphic rocks are rocks that have changed their types through heat pressure. For example, granite, an igneous rock, can be turned into marble.

On the Moon, there is no liquid water, as I'm sure you know. So we can't get sedimentary rocks at all. However, billions of years ago, when the Moon was young and its inside was hotter than it is now, there was molten rock inside. This lead to the formation of igneous rocks on the surface. You can see them for yourself when you look up at the full moon. You will quickly notice dark areas on the face of the Moon. We call the maria (Latin for "seas," because some early astronomers thought that they might be seas). We now know that these dark, smooth areas are in fact made of basalt. Basalt is another igneous rock we see commonly on the Earth. So we know that sometime in the Moon's distant past, there were volcanic eruptions that created these maria.

I don't know if there are any metamorphic rocks on the Moon. I've never heard of anyone finding any, but there could be some somewhere. The problem with metamorphic rocks on the Moon is simply that the Moon cooled down pretty fast, so that there was not a lot of time for these kinds of rocks to form. Today, the Moon is geologically dead, making no new rocks at all. It has probably been this way for billions of years, too.

There is one other sort of process on the Moon that doesn't create rocks, but does reshape them. Here on Earth, we have erosion to wear down rocks and turn them into gravel over time. On the Moon, there is no wind or water to do this, but the Moon does get hit by meteors. Meteors hit hard enough to break rocks up and send them flying across the Moon. You can see this, too, when you look at craters on the Moon through binoculars or a telescope. The same thing can happen on the Earth, but luckily, our atmosphere helps protect us from meteors.

If you want to learn more about the Moon and the Moon's geology, I suggest you go to The Nine Planets website. Not do they have good information on their site, they have links to lots of other good sites.

Hope this helps!