The currently-accepted determined ages of the emergent Galapagos Islands are not old enough to allow for the speciation of the terrestrial life present upon them, including marine and terrestrial iguanas. The discovery of drowned and subducted seamounts off the coast of Costa Rica and magnetic signature data from these seamounts suggest that they were once emergent portions of the Galapagos Island chain.  If terrestrial biota existed upon the seamounts while they were emergent and before the present Galapagos Islands emerged, the amount of time possible for speciation is increased to an amount within the margin of error for the genetic distance between the two iguana species.

        The emergent Galapagos Islands are around three million years old, which is ample time for the speciation of the thirteen species of finch found there by Darwin.  This is not enough time, however, to explain the divergence of the Galapagos marine and land iguanas from a single South American species, based on the calculated genetic distances between the two. The discovery of seamounts off the coast of Costa Rica, thought to be representative of drowned islands, could be a possible explanation for the speciation of the iguanas.  These seamounts have been dated between 5 and 14.5 million years old, which would be ample time for this speciation, assuming that the iguanas did originate from a single South American species.  Geologic data show that the drowned archipelago remained over the Galapagos hotspot longer than the present emergent Galapagos Islands, and therefore existed as an island chain for a longer amount of time, allowing longer time for speciation of life on the islands.  (Werner et al., 1999; Christie et al., 1992)
       The Galapagos Islands (or the Colon Archipelago) are located about 1050 km off the western coast of South America near the equator. The chain consists of fifteen large and several hundred small islands.  The islands are all of volcanic origin and many are mountainous and contain volcanic craters, some of which are still active; of the 21 emergent volcanoes, 13 have erupted in the Holocene (White et al., 1993).  These are basaltic shield volcanoes rising from the submarine Galapagos Platform (Nordlie, 1973).  The origin of Galapagos volcanism is the Galapagos hotspot, a 90 million-year-old mantle plume of Mid-Cretaceous origin (Hauff et al., 1997).
        The Galapagos Archipelago is situated upon the Nazca plate, south of the Galapagos spreading center (Fig. 1).  The Galapagos spreading center accretes crust asymmetrically to both the Cocos and Nazca plates (Hey et al., 1977).  The Galapagos hotspot, along with the Galapagos spreading center, has created two hotspot tracks, the Cocos and Carnegie Ridges.  The Cocos Ridge lies on the Cocos plate while the Carnegie Ridge lies on the Nazca plate.  These ridges extend from the Galapagos Islands to the trenches along Central and South America, where they are being subducted (Werner et al., 1999).

        Figure 1.  Map of the Galapagos Islands and Galapagos spreading center showing the Cocos and Carnegie Ridges. From Feighner and Richards, 1994.

        Many techniques have been used to date the present emergent Galapagos islands.  For many years, it was believed that the islands were very old, in order for the biota present to have become so differentiated and speciated.  Bailey’s (1976) potassium-argon ages date the islands at a maximum of 3 million years old.  Hall’s (1983) dates from Espanola Island are more than 3 million years old, more than twice as long as previously calculated.  Based on stratigraphy and paleontological evidence, Hickman and Lipps (1985) concluded an age of at least 3 million years  for the islands.
        Drowned seamounts have been discovered off the coast of Costa Rica that are believed to have once been emergent parts of the Galapagos Island chain (Fig. 2).  Rocks dredged from these seamounts show rounding and other evidence of sea-level erosion and weathering.  This implies that the seamounts were once emergent islands, and therefore capable of sustaining terrestrial life.   (Christie et al., 1992; Werner et al., 1999)
        These seamounts are located near the convergent boundary between the Cocos and Caribbean plates just off the coast of Costa Rica (Fig. 2).  The subduction of the Cocos plate beneath the Caribbean plate has uplifted the mountain ranges in southern Costa Rica and has led to the extinction of volcanism in the area during the last 5 million years (Barckhausen, et al., 1998).  Questions were posed by many investigators (von Huene et al., 1995; Hinz and Block, 1992) as to whether other seamounts have been subducted beneath Costa Rica along with the Cocos plate and whether they remain intact beneath the overriding Caribbean plate.  Magnetic signature data has shown that the seamounts have subducted, causing furrows in the continental slope, and that the seamounts retain their magnetization, indicating that they remain physically intact for some time after subduction (Barckhausen et al., 1998).

        Figure 2.  Map of seamounts off the coast of Costa Rica. Points 1 through 3 indicate seamounts upon the oceanic crust.  Points 4 through 8 indicate areas where seamount subduction is supposed. Modified from Barckhausen, et al., 1998.

        The drowned seamounts have ages ranging from 5 million years old (Christie et al., 1992)  to 14.5 million years old (Werner et al., 1999).  A small 9 million-year-old seamount at 85 degrees 40’ W 2 degrees S was found to be composed mostly of glassy basalt and to have several wave-cut terraces, indicating its extrusive, subaerial origins and that it was once emergent (Christie et al., 1992).  The discovery of a 14.5-million-year-old seamount at the oldest preserved portion of a continuous hotspot track suggests that there have been emergent islands present above the Galapagos hotspot for at least 14.5 million years (Werner et al., 1999).
        Based on genetic distances (calculated values based on DNA analysis indicating the amount of time that would be needed for one species to evolve into another related species) this date of 14.5 million years is within the margin of error range (15-20 million years) for the land and marine iguanas to have diverged from a common mainland ancestral species (Wyles and Sarich, 1983).  Their discovery suggests that the iguanas’ species divergence may have occurred on these drowned islands and subducted seamounts while they were emergent (Carson, 1992).  The emergence of the seamounts would have significantly increased the time possible for speciation of terrestrial life such as the iguanas.  It is thought that this evidence will inspire biologists to configure new speciation models based on the emergence and disappearance of island chains forming over hotspots (Carson, 1992).

[Word count: 973]

Bailey, K., 1976, Potassium-argon ages from the Galapagos Islands: Science, v.192, p. 465-467.

Barckhausen, U., Roeser, H. A., and von Huene, R., 1998, Magnetic signature of upper plate structures and subducting seamounts at the convergent margin off Costa Rica: Journal of Geophysical Research, v. 103, p. 7079-7093.

Carson, H. L., 1992, The Galapagos that were: Nature, v. 355, p. 202-203.

Christie, D. M., Duncan, R. A., et al., 1992, Drowned islands downstream from the Galapagos hotspot imply extended speciation times: Nature, v. 355, p. 246-248.

Feighner, M. A. and Richards, M. A., 1994, Lithospheric structure and compensation mechanisms of the Galapagos Archipelago: American Geophysical Union paper 93JB03360.

Hall, M. L., 1983, Origin of Espanola Island and the age of terrestrial life on the Galapagos Islands: Science, v. 221, p. 545-547.

Hauff, F., Hoernle, K., et al., 1997, A mid-Cretaceous origin for the Galapagos hotspot: Volcanological, petrological, and geochemical evidence from Costa Rican oceanic crustal segments: Geologische Rundschau, v. 86, p. 141-155.

Hey, R., Johnson, G. L., and Lowrie, A., 1977, Recent plate motions in the Galapagos area: Geological Society of America Bulletin, v. 88, p. 1385-1403.

Hickman, C. S., and Lipps, J. H., 1985, Geologic youth of Galapagos Islands confirmed by marine stratigraphy and paleontology: Science, v. 227, p. 1578-1580.

Hinz, K., and Block, M., 1992, Preliminary scientific results of Sonne cruise 81, in Hinz, K., ed., 1992, Geoscientific investigations off Costa Rica: Bundesanst fur Geowissenschaften und Rohstoffe, Hanover, Germany, p. 55-90.

Nordlie, R. E., 1973, Morphology and structure of the western Galapagos volcanoes and a model for their origin: Geological Society of America Bulletin, v. 84, p. 2931-2956.

von Huene, R., et al., 1995, Morphotectonics of the Pacific convergent margin off Costa Rica, in Mann, P., ed., 1995, Geologic and tectonic development of the Caribbean plate boundary in southern Central America: Spec. Pap. Geol. Soc. Am., p. 291-307.

Werner, R., Hoernle, K., et al., 1999, Drowned 14-m.y.-old Galapagos archipelago off the coast of Costa Rica: Implications for tectonic and evolutionary models: Geology, v. 27, p. 499-502.

White, W. M., McBirney, A. R., and Duncan, R. A., 1993, Petrology and geochemistry of the Galapagos Islands: Portrait of a pathological mantle plume: Journal of Geophysical Research, v. 98, p. 19533-19563.

Wyles, J. S., and Sarich, V. M., 1983, Are the Galapagos iguanas older than the Galapagos?, in Bowman, R. I., Berson, M., and Leviton, A. E., eds., 1983,  Patterns of evolution in Galapagos organisms: San Francisco, Pacific Division, California Academy of Science, p. 177-186.