Abstract
Volcanism became emergent on San Cristobal Island 2.3 m.y. B.P. in the eruption of floods of alkali basalt. Since then eruption continued until 0.6 m.y. B.P. in a central shield volcano. From 0.9 m.y. B.P. to almost historical time, activity moved to the NE and was characterized by a fissure-fed eruption. Three distinct magma series occur on San Cristobal: an alkaline series, a series of MORB-like tholeiites and a series of incompatible-element-enriched tholeiites typical of oceanic islands. Each series is represented by very primitive basalts and more-evolved lavas resulting from a simple fractional crystallization process. The isotope and trace-element data suggest that the different series are generated by different degrees of partial melting and mixing of both a depleted and an enriched source.