Melting and magmatism

The German geographer Varenius (1622–1650) was one of the first to suggest that volcanic activity is ultimately caused by the escape of hot melted rock from the interior of our planet. Written at a time when most geologists believed that the Earth’s interior is filled with molten rock, the source of the melt was not problematic: Any break or fracture would allow molten rock to leak out to the surface, just as puncturing the skin of an animal allows blood to flow out. However, with the study of solid earth tides and the advent of seismology at the end of the nineteenth century, it became plain that the bulk of the Earth is solid and the origin of magma became less obvious.

At the present time we believe that melted rock is a secondary manifestation of the thermal regime of our planet and that heat transport by magma is of slight importance compared to thermal conduction and lithospheric recycling, at least on the Earth. Volcanism and its subsurface accompaniment, igneous intrusion, is, nevertheless, an important process affecting the surface of the terrestrial planets, to the extent that almost no planetary surface seems to have escaped its effects.