Volcanic basalt formation. 18th-century illustration of visitors viewing spherical formations (one at centre) embedded in basalt columns in the volcanic mountains of the Ardennes, near Pradelle, France. These structures (extrusive igneous rocks) form when molten rock (magma) from below the Earth's surface rises to the surface during volcanic activity. The geometry of the formations is due to contraction on cooling. Artwork from 'Mineralogie des volcans' (1784) by French geologist Barthelemy Faujas de Saint-Fond (1741-1819).