GEOMORPHIC EVOLUTION OF A NEAR-RIDGE HOTSPOT AND RELATION TO PHYLOGEOGRAPHY AND BIODIVERSITY: THE GALAPAPAGOS

The Galapápagos Archipelago is situated in a unique tectonic and climatic environment, which gives rise to diverse volcanic landforms and ineffective surficial erosion. The major islands are in 4 geomorphic provinces. The western volcanoes grow from a massive terraced submarine platform at the leading edge of the hotspot. They are the tallest, have enormous calderas, and emerged between 400,000 and 50,000 years ago. Their subaerial sectors are nearly radially symmetric, but sharp rifts delineate their submarine parts. The eastern volcanoes are shorter, lack calderas, and as old as 2.3 Ma. It is possible that they cap western-like edifices (analogous to Hawaiian post-shield phases), but they may have differed from the time of their origin because they erupted nearer to the Galapápagos Spreading Center. The northern islands are the emergent tops of elongate seamounts, where near-ridge lithospheric stresses are illuminated by the unusually hot mantle. The oldest islands are up to 3 Ma and are cut by faults and strongly wave eroded. Otherwise, surface erosion is a nearly insignificant process throughout the archipelago, with the possible exception of during strong ENSO events. Stream-incision is only important where there are thick accumulations of unlithified tuff and on the southwest part of San Cristobal. In those places it is limited to gullies only 10s of meters deep. Sector collapse has occurred on three of the younger islands, but it is nowhere near as important as in Hawaii or the Canary Islands. The Galapápagos have suffered fewer extinctions than any other tropical archipelago and have a high proportion of endemic species. Phylogentic data indicate that organisms inhabited proto-Galapápagos Islands >10 Ma, but natural colonization has taken place since. Because the islands are not in a linear chain, colonization and radiation does not follow the progression rule. Some of the evolutionarily oldest organisms are on the youngest islands and visa versa. Therefore, there is no predictable pattern of radiation, once the islands are colonized. The main control on biodiversity and the proportion of single-island endemics (which may represent speciation rate) is island area. Island age is a second-order, but statistically measurable effect. There is anecdotal evidence that the type of geologic substrate plays a role locally.