Plutonic

(Greek mythology, Roman mythology) Synonym of Plutonian (“of or relating to Pluto, the Greek and Roman god of the underworld; demonic, infernal”)

(by extension) Synonym of Plutonian (“of, relating to, or having characteristics associated with the underworld; dark, gloomy; mournful”)

(by extension, geology, mineralogy) Of or pertaining to rocks formed deep in the Earth's crust, rather than by volcanoes at the surface of the Earth.

(by extension, geology, historical) Of, pertaining to, or supporting plutonism (“the theory that the rocks of the Earth were formed in fire by volcanic activity, with a continuing gradual process of weathering and erosion, then deposited on the sea bed, re-formed into layers of sedimentary rock by heat and pressure, and raised again”).