neptunism

Neptunism is a historical geological theory proposed by Abraham Gottlob Werner in the late 18th century. It held that the Earth's crust originated through the crystallization of chemical constituents from a primeval global ocean, and that rocks were deposited in a systematic sequence reflecting their position in the ancient seafloor. In Werner's scheme, the oldest rocks—often labeled primitive or crystalline, such as granite and gneiss—formed first from dissolved substances in the sea. Younger materials were laid down as sediments on top of the earlier rocks, producing a vertical order that could be read as a history of crustal development. As the sea receded in Werner's view, land emerged and the geologic sequence became exposed as continents.

Neptunism was challenged by plutonism, advocated by James Hutton and others, which argued that rocks could originate from molten material generated beneath the crust and later intruded or erupted through it. The neptunian–plutonist debate helped establish key geological concepts, including the recognition of rock succession, the stratigraphic scale, and the notion of deep time. Although neptunism was eventually abandoned as a comprehensive account of rock formation, it contributed to the early framework of geology and terminology, and served as a historical stepping-stone toward modern theories of igneous and sedimentary processes.