Igneous rocks are formed by the cooling of lava/magma. Igneous rocks are divided into two categories- intrusive and extrusive. Intrusive is when the magma cools slowly beneath the Earth's surface, which causes large crystals to form. When lava cools above the Earth's surface, it is called extrusive. Extrusive rocks have smaller crystals compared to intrusive, the difference always being noticeable.
