Santiago, Pinzón, and Rábida Islands are each single volcanoes that lie along a north-trending line near the center of the Galápagos Archipelago. The core volcano of Santiago Island and its flanking lava fields are composed of basaltic to intermediate lava of alkalic parentage. The smaller islands of Rábida and Pinzón are constructed of tholeiitic lava and tuff, ranging in composition from basalt to siliceous trachyte. On each of these two islands, products of eruptive cycles are preserved as tuff-flow sequences of decreasing degree of differentiation and increasing phenocryst abundance up-section, possibly a consequence of tapping successively deeper levels of compositionally zoned magma chambers.
Island age generally increases to the south away from the Galápagos rift. Volcanic activity on Santiago spanned most of the past 0.7 m.y. K-Ar ages of rocks exposed on Rábida range from about 1.0 to 0.7 m.y.; those on Pinzón are approximately 1.2 to 0.8 m.y. in age.
A part of the tectonic history of the central and southeastern Galápagos Islands is recorded in conspicuous, east-trending alignments of volcanic vents on Santiago and parallel normal faults and some aligned vents on six other islands to the southeast. The central islands among this group are also characterized by uplifted submarine rocks. On the basis of geological and geophysical observations, we hypothesize that this uplift, north-south distention, and associated volcanism may have resulted when an east-trending curtain of mantle upwelled beneath the center of the archipelago during the past 0.5 m.y. The east-trending tectonic pattern appears to be superimposed on the older northwest and northeast trends that have been cited for the entire island group.