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Not to be confused with pluton.

Plutonism (or volcanism) is the geologic theory that the igneous rocks forming the Earth were formed by intrusive magmatic activity, with a continuing gradual process of weathering and erosion wearing away rocks, which were then deposited on the sea bed, re-formed into layers of sedimentary rock by heat and pressure, and raised again. It proposed that basalt was solidified molten magma. It was named after Pluto, the classical ruler of the underworld, or alternatively after Vulcan, the ancient Roman god of fire and volcanoes.

The theory was first proposed before 1750, by Abbé Anton Moro who had studied volcanic islands, and was subsequently developed by James Hutton as part of his Theory of the Earth published in 1788. It contested Abraham Werner's neptunist theory which proposed that the Earth had formed from a mass of water and suspended material which had formed rocks as layers of deposited sediment which became the continents when the water retreated, further layers being deposited by floods and some volcanic activity.

The plutonists strongly disputed the neptunist view that rocks had been formed by processes that no longer operated, instead supporting Hutton's uniformitarianism. A key part of the debate was the neptunist belief that basalt was sedimentary, and some fossils had been found in it. Against this, Hutton's friend John Playfair argued that this rock contained no fossils as it had been formed from molten magma, and it had been found cutting through other rocks in volcanic dykes. The arguments continued into the 19th century, and eventually the plutonist views on the origin of rocks prevailed. However, sedimentary rocks such as limestone are considered to have resulted from processes like those described by the neptunists, and so modern theory can be seen as a synthesis of the two approaches.