In geology, an intrusion's a body of igneous rock that's crystallized from molten magma below the surface of the Earth. Bodies of magma that solidify underground before they reach the surface of the earth are called plutons, named for Pluto, the Roman god of the underworld. Correspondingly, rocks of this kind are also referred to as igneous plutonic rocks or igneous intrusive rocks. This's to be contrasted with extrusive rocks. The rock surrounding a pluton's called country rock. Varieties In c… (More on Intrusion)