A solid rock body that formed by cooling and crystallization of molten rock (magma) within the Earth. Most plutons, or plutonic bodies, are regarded as the product of crystallization of magma intruded into surrounding “country rocks” within the Earth (principally within the crust). During the middle part of the twentieth century some geologists believed that many igneous or plutonic rock bodies were the result of ultrametamorphism (granitization). However, that model has largely been abandoned.