An intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of a planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust. Intrusions vary widely, from mountain range sized batholiths to thin vein-like fracture fillings of aplite. The central cores of major mountain ranges consist of intrusions. When exposed by erosion, these cores called batholiths may occupy huge areas of Earth's sur… (More on Intrusive)