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.
It was named after Neptune, the ancient Roman name for the ancient Greek god of the sea, Poseidon. There was considerable debate between its proponents and those favouring a rival theory known as Plutonism which gave a significant role to volcanic origins, and in modified form replaced Neptunism in the early 19th century as the principle of uniformitarianism was shown to fit better with the geological facts as they became better known. In modern geology, many different forms of rock formation are acknowledged, and the formation of sedimentary rock occurs through processes very similar to those described by Neptunism.
In the mid-eighteenth century as the investigation of geology found evidence such as fossils, naturalists developed new ideas which diverged from the Genesis creation myth. Georges de Buffon proposed that the Earth was over 75,000 years old, possibly much older, and showed signs of historical development in a series of distinct epochs.
Abraham Gottlob Werner was the inspector of mines and professor of mining and mineralogy at the Mining Academy in Freiberg (Saxony) which became dominant in late eighteenth century geology. His Short Classification and Description of Rocks of 1787 and his lectures set out classification of rocks on the basis of their age based on the sequence of layers of differing material, rather than by the types of minerals as had been previous practice.
He based his historical sequence of rock formation on the theory that the Earth had originally consisted of water containing material which settled out of suspension in a process of sedimentation to form the core of the planet and the continents as a series of layers, the oldest and hardest being granite while newer layers showed an increasing number of fossils. Noah's flood repeated the process, adding new rock layers. Volcanos had a minor effect, modifying the continents and adding more sediment as well as some volcanic rocks, and successive lesser floods added more layers, so that most rocks were a result of precipitates settling out of water.
The Neptunist-Plutonist/Vulcanist Controversy
A rival theory known as Plutonism (or Vulcanism) held that rocks were formed in fire. This was originally proposed by Abbé Anton Moro (1687-1750) with reference to his studies of volcanic islands, and was taken up by James Hutton who put forward a uniformitarian theory extending over infinite time in which rocks were worn away by weathering and erosion, then were re-formed and uplifted by heat and pressure.
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