"pertaining to or involving intense heat deep in the earth's crust," 1796, coined by Irish scientist Richard Kirwin (1733-1812) from combining form of Pluto (as god of the underworld) + -ic. Especially in reference to early 19c. geological theory (championed by Hutton) that attributed most of the present features of the earth's crust to action of internal heat, a theory which triumphed over its rival, neptunism, which attributed them to water. Related: Plutonism; Plutonist.
Plutonic rocks are such igneous rocks as have been formed under conditions of depth and pressure, and have cooled slowly, so as to have acquired in general a distinctly crystalline structure ; the term Plutonic is opposed to volcanic, the former designating rocks formed at some depth beneath the surface, the latter rocks of igneous origin but of superficial formation. As used by Lyell, the word is nearly the equivalent of metamorphic. [Century Dictionary, 1895]