Flexi Says: Igneous rock is the most common type of rock. Extrusive igneous rocks form when lava erupts and cools at the surface. There is no set maximum size for an extrusive igneous rock because magma can pour out over very wide areas. Approximately 80% of Earth's surface and landforms above and below sea level are of volcanic origin. The entire seafloor is made up of extrusive igneous rock. Magma erupts continually from the mid-ocean ridges and cools to form the seafloor.