Igneous rocks form when molten magma pushes up into the crust or through it. Igneous rock contains various minerals in seemingly randomly arranged, interlocking crystals. There are two types of igneous rock - intrusive and extrusive.

Intrusive rocks cool slowly with big crystals. They cool slowly because they're underground. An example of an intrusive rock is granite. Granite is very hard.

Extrusive rocks cool quickly with small crystals. An example of an extrusive igneous rock is basalt.