Igneous rocks form by the cooling and crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive (or volcanic) igneous rocks. If this happens deep underground, they are intrusive (or plutonic) igneous rocks.

Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.