Plutonism

Plutonism (or vulcanism) is the geologic theory that the rocks forming the Earth were formed in fire by volcanic 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.