A Pluton is a mass of molten igneous rock which is less dense than the surrounding rocks and rises through the crust. It is sometimes called a diapir or montgolfier ( as its appeareance resembles a balloon). Plutons are commonly made of granitoid rocks with high silica contents which gives them a low density such as granite and granodiorite.
As plutons cool slowly crstal sizes can be large and contain even larger crystals called phenocryts. As the pluton rises pieces of country rock can be incorporated into the melt- xenoliths.Convection currents within the pluton circulate a crystal mush and these evidence of these flow can be seen where the pluton is exposed at the surface. They are mostly found in subduction zone and collision zone settings.