Volcanic Systems and Thermal Regimes

Volcanism, and more broadly -- where melting is not involved -- thermal regimes, are a fundamental aspect of planets. It is the internal generation of heat by radioactive decay, or in the special case of Io by tidal friction, that drives planetary differentiation. Upward migration of melts occurs when heat generation exceeds what can be dissipated by conduction, and results in the development of crust and atmosphere, and in the cases of Earth and ancient Mars, the hydrosphere. A feature that may be unique to Earth is the bimodal character of its crust. Beneath the oceans lies thin, young, continuously rejuvenated basaltic crust. The continents are thicker; substantially granitic, a lithology virtually absent in ocean crust; and they are in general much older. It is ironic that we now understand much more about the origin of oceans than about the origin of the continents on which we live. This is fundamentally because we do not understand why some volcanic systems develop silicic plutons that can resist subduction and add to continental crust, whereas others do not.

 (by John Eichelberger & Kozo Uto, 2005)