Drowned 14-m.y.-old Galapagos archipelago off the coast of Costa Rica: Implications for tectonic and evolutionary models

Volcanic rocks were dredged from the Cocos and Fisher ridges and seamounts along a 250 km profile parallel to the Pacific coast of Costa Rica, The com position and laser Ar-40/Ar-39 ages of the Cocos Ridge and Seamounts are co nsistent with their formation above the Galapagos hotspot 13.0-14.5 Ma. The reconstructed paleoenvironment and chemistry of the Fisher Ridge are consi stent with it hating originated at a mid-oceanic ridge system. Laser Ar-40/ Ar-39 dating of fresh basalt glass from the Fisher Ridge yielded isochron a ges of 19.2 +/- 0.3 Ma and 30.0 +/- 0.5 Ma. The Fisher Ridge is along a lit hospheric fault that may represent an extensional fracture formed when the oceanic floor rode over the Galapagos hotspot. Even though the younger stru ctures are currently at water depths of >1000 m, volcanological, geochemica l, and geophysical observations indicate that they once formed an emerged a rchipelago very similar in morphology to the Galapagos islands. The diversi ty of the biota on the isolated Galapagos islands, as first described by Ch arles Darwin, has had an important influence on the development of the theo ry of evolution, The existence of a now-drowned Galapagos archipelago 14.5 ha considerably increases speciation times for the Galapagos biota and prov ides a complete solution to a long-standing controversy concerning the dive rgence of the Galapagos marine and land iguanas from a single ancestral spe cies.