Plutonism is a theory proposed by the Scottish geologist James Hutton in the second half of the eighteenth century, according to which, in the processes generating rocks, should be taken into account also the magmatic ones, that is attributable to the “subterranean heat”. The name derives from Pluto, the ancient deity of Roman mythology, lord of Hades, the underground world of the Underworld.

Plutonism was opposed to Neptunism, the theory proposed by German geologist Abraham Gottlob Werner, according to which all rocks had a marine origin (Neptune, in Roman mythology, was the god of water and sea). According to Neptunism, the center of the Earth would have been cold and solid and its core composed of hard stone. Rocks and mountains would have originated from marine sedimentation processes and the material erupted from volcanoes would not have originated from the depths of the lithosphere but would have been fed by a process of “cooking” of carbon layers. According to the neptunism, in fact, all materials present on the earth’s crust would have been deposited following the retreat of a great sea that originally covered all the earth.

Hutton, the theorist of Plutonism, hypothesized instead the escape of the Earth’s heat by means of periodic volcanic eruptions which would have determined an elevation of the ground. The subsequent erosive processes would have contributed to reduce the height of the reliefs and conveyed the debris into the sea. For the internal heat of the Earth, in the sea the sediments would have melted and pushed again upward starting again the cycle of lifting, erosion and melting.

The diatribe between Plutonists and Neptunists played a very important role in the development of geology. For decades, between the end of the eighteenth century and the first decades of the nineteenth, it was a dogged, if not violent, search for evidence in favor of one or the other theory. In any case the debate, and the research that followed, led to the birth of modern geology. Towards the twenties of the nineteenth century was now peacefully accepted that basalt or granite had a volcanic origin, and not sedimentary, as the neptunists had theorized. On the other hand, even the theories of neptunists, although refuted in the long run, were fruitful for the progress of geology. It is of Neptunist origin, for example, the theory that sediments consolidate by compaction and cementation of granules, as well as the hypothesis of the stratigraphy of rocks and their correlation.

Plutonism and Neptunism were related to the opposition of solidism and fluidism, which enlivened speculations about the quantitative and qualitative nature of the Earth’s center.

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