Drowned islands downstream from the Galapagos hotspot imply extended speciation times

Abstract

THE volcanic islands of the Galapagos archipelago are the most recent products of a long-lived mantle hotspot1,2. Little is known, however, of the submarine Galapagos platform on which the islands are built, or of the Cocos and Carnegie submarine ridges produced by past motion of the Cocos and Nazca plates across the hotspot3,4. In 1990 we surveyed selected areas around the Galapagos platform and as far east as 85°30' W on the Carnegie ridge, where we dredged abundant well-rounded basalt cobbles from a small sea-mount with a terraced summit region. Cobbles were also dredged from several other seamounts. We interpret these features, especially the presence of cobbles, as evidence for erosion near sea level and conclude that these seamounts were volcanic islands before subsiding to their present depths. Radiometric ages for these drowned islands range from 5 to 9Myr, consistent with predicted plate motions. They indicate that the time available for speciation of Galapagos organisms is much longer than the age range of the existing islands.