The theme of this book is the study of basaltic volcanism as a stage in planetary evolution: to use the eruption of lava from the interior of a planet as evidence of the thermal and chemical processes of the planet. All lavas extruded on a planet’s surface can provide information about events in the interior. From a petrologic point of view, however, it is the more primitive lavas, which rise to the surface of a planet with minimal changes, that can provide the most information about the interior. In general, these more primitive lavas are basalts and so in Chapter 1 our sampling has concentrated on basaltic rocks.