Most major rocks, including granite and many volcanic rocks, precipitated from a primordial ocean that once covered Earth.

In the late eighteenth century, Abraham Gottlob Werner proposed an ambitious explanation for Earth’s rocks. He argued that a vast primordial ocean had once covered the planet and that, as the water receded, different rocks precipitated from it in a fixed sequence.

This theory became known as Neptunism. It even treated granite—today recognized as an igneous rock—as one of the earliest deposits from the ancient ocean.

The model made sense of many layered rocks around Werner’s mining academy at Freiberg, and his influence helped spread it across Europe. But geologists working elsewhere kept finding rocks that were difficult to explain as ocean deposits.

Basalt appeared in forms that looked like cooled lava flows. Granite sometimes cut through older rocks instead of lying neatly beneath them, and the rocks beside those contacts showed signs of strong heating.

These observations supported the competing idea that heat and molten rock played a major role in geology. Over time, field evidence made a single ocean-precipitation origin impossible to maintain.

Modern geology recognizes three broad rock families. Igneous rocks crystallize from magma or lava. Sedimentary rocks form from deposited sediments or material precipitated from water. Metamorphic rocks form when existing rocks are changed by heat, pressure, and fluids.