Nature 355, 246 - 248 (16 January 1992); doi:10.1038/355246a0

Drowned islands downstream from the Galapagos hotspot imply extended speciation times

D. M. Christie^*, R. A. Duncan^*, A. R. McBirney^†, M. A. Richards^‡, W. M. White^§, K. S. Harpp^§ & C. G. Fox^¶

^* Oregon State University, College of Oceanography, Corvallis, Oregon 97331-5503, USA
^† University of Oregon, Department of Geology, Eugene, Oregon 97403, USA
^‡ University of California at Berkeley, Department of Geology and Geophysics, Berkeley, California 94720, USA
^§ Cornell University, Department of Geology, Ithaca, New York 14853, USA
^¶ NOAA, Hatfield Marine Science Center, Newport, Oregon 97365, USA

THE volcanic islands of the Galapagos archipelago are the most recent products of a long-lived mantle hotspot^1,2. 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 hotspot^3,4. 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.

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