Volcanism became emergent on San Cristobal Island 2.3 m.y. B.P. in the eruption of floods of alkaline basalt. Volcanism then coalesced in a central shield volcano, which erupted continually until 0.6 m.y. B.P. At 0.9 m.y. B.P., activity began to jump to the northeast, and the style of volcanism changed to fissure-fed eruptions that have continued almost to historical time.
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 that are typical of oceanic islands. Each series is represented by very primitive basalts that have been little modified since segregation from their sources. Each series also has more-evolved lavas, which range from low-Mg basalts to hawaiites and icelandites. These evolved lavas are the result of simple fractional crystallization of magmas which produces major- and compatible-element compositions similar to those of observed primitive lavas.
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. The first source is thought to be the depleted asthenosphere which generates the MORB's of the Galapagos Spreading Center, and the enriched source is the mantle plume associated with the Galapagos hotspot.