Plutonism is the geologic theory

Theory

The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...

 that the rocks forming the Earth were formed in fire by volcanic activity

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

, 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

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 by heat and pressure, and raised again. It proposed that basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 was solidified molten magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

. It was named after Pluto

Pluto (mythology)

In ancient Greek religion and myth, Pluto was a name for the ruler of the underworld; the god was also known as Hades, a name for the underworld itself...

, the classical

Classical mythology

Classical mythology or Greco-Roman mythology is the cultural reception of myths from the ancient Greeks and Romans. Along with philosophy and political thought, mythology represents one of the major survivals of classical antiquity throughout later Western culture.Classical mythology has provided...

 ruler of the underworld

Greek underworld

The Greek underworld was made up of various realms believed to lie beneath the earth or at its farthest reaches.This includes:* The great pit of Tartarus, originally the exclusive prison of the old Titan gods, it later came to be the dungeon home of damned souls.* The land of the dead ruled by the...

, or alternatively after Vulcan

Vulcan (mythology)

Vulcan , aka Mulciber, is the god of beneficial and hindering fire, including the fire of volcanoes in ancient Roman religion and Roman Neopaganism. Vulcan is usually depicted with a thunderbolt. He is known as Sethlans in Etruscan mythology...

, the ancient Roman god

Religion in ancient Rome

Religion in ancient Rome encompassed the religious beliefs and cult practices regarded by the Romans as indigenous and central to their identity as a people, as well as the various and many cults imported from other peoples brought under Roman rule. Romans thus offered cult to innumerable deities...

 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

James Hutton

James Hutton was a Scottish physician, geologist, naturalist, chemical manufacturer and experimental agriculturalist. He is considered the father of modern geology...

 as part of his Theory of the Earth published in 1788. It contested Abraham Werner's

Abraham Gottlob Werner

Abraham Gottlob Werner , was a German geologist who set out an early theory about the stratification of the Earth's crust and coined the word Neptunism...

 Neptunist theory

Neptunism

Neptunism is a discredited and obsolete scientific theory of geology proposed by Abraham Gottlob Werner in the late 18th century that proposed rocks formed from the crystallisation of minerals in the early Earth's oceans....

 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 flood

Flood

A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...

s 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

Uniformitarianism

In the philosophy of naturalism, the uniformitarianism assumption is that the same natural laws and processes that operate in the universe now, have always operated in the universe in the past and apply everywhere in the universe. It has included the gradualistic concept that "the present is the...

. 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

John Playfair

John Playfair FRSE, FRS was a Scottish scientist and mathematician, and a professor of natural philosophy at the University of Edinburgh. He is perhaps best known for his book Illustrations of the Huttonian Theory of the Earth , which summarized the work of James Hutton...

 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 dyke

Dike (geology)

A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

s. The arguments continued into the 19th century, and eventually the Plutonist views on the origin of rocks prevailed.