How are rocks formed?

Rocks are made up of collections of minerals and can be formed in a variety of ways. There are three main types of rocks:

Sedimentary rocks are formed by much smaller pieces of other rocks and minerals. These other bits of rock are the result of erosion by wind, water, and other natural processes and are often carried quite far from their original location. The pieces come to rest in a low area such as a valley floor or dry river basin and solidify into rock with time. Sandstone is a good example of a sedimentary rock. When you scrape it, you may notice that it will start breaking down into the tiny grains of sand from which it was formed.

Metamorphic rocks are the result of other rocks that become buried deep enough to be exposed to high heat and pressure. Over time, the different layers are compressed and flattened into metamorphic rock. This is why you can often see multicolored stripes or lines in this type of rock.

Igneous rocks start out as magma, but can be formed in one of two ways following this state. Intrusive igneous rocks cool very slowly deep beneath Earth's surface. This slow cooling process allows for the formation of large, obvious crystals that are visible to the naked eye, like those seen in granite. Extrusive igneous rocks are those that cool rapidly at or very close to the surface. Volcanic glass, such as obsidian, is an example of this. Contrary to intrusive igneous rocks, extrusive igneous rocks contain very tiny crystals that must be magnified to be viewed.

Keep in mind that these are all part of the Rock Cycle - the constant process of breaking down and reforming rocks that has been going on since Earth first formed.