Igneous rocks form when magma or lava cools and solidifies. Magma is molten rock located below Earth's surface, while lava is magma that has reached the surface. Intrusive igneous rocks form when magma cools and crystallizes slowly below Earth's surface, resulting in large crystal sizes. Extrusive igneous rocks form when lava cools and crystallizes quickly on Earth's surface, resulting in small crystal sizes. Common intrusive and extrusive rock types are listed.