A solidified, underground, igneous mass, varying in size from batholiths to sills and dykes. See McNulty et al. (2000) GSA Bull. 112, 1 on the Mount Givens pluton, Calif., and Stevenson et al. (2007) J. Geol. Soc. 164, 1 on the Eastern Mourne pluton, Ulster. A plutonic rock is an igneous rock which has cooled and crystallized slowly, at great depth.