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Emergence and Submergence of the Galapagos Islands Josh's Figure
The formation of an oceanic island starts with the thermal expansion of the seafloor. Then as the heat source becomes more active, magma will intrude into the crust and eventually lava will be erupted onto the seafloor. Volcanoes take a long time to appear above sea-level, for every 1 km of elevation growth, 4 km of sinking occurs, forming a big root with in the lithosphere. Volcanoes grow internally, as well as externally, by intruding magma into the base of the volcano. In fact, it is believed that only 20% of the volcanoes molten material is ever emitted as lava (Crisp, 1984). Motion of the lithosphere eventually carries a volcano away from the plume and its magma source, so the volcano then becomes extinct. The volcano and the lithosphere beneath it then begin to cool. As it cools it contracts. As a result of this contraction, the volcano slowly sinks beneath the sea. Also, the oceanic lithosphere will start to sag due to the weight of the volcanic island, submerging the volcano even further.
The youngest Hawaiian volcanoes are islands, but the older ones are now seamounts, the tops of which become progressively deeper to the northwest. Likewise, many of the Galapagos seamounts were once islands. Because the Carnegie and Cocos Ridges disappear into subduction zones, it is uncertain how old the Galapagos mantle plume is. A 1990 oceanographic expedition, however, did locate an 8 million year old seamount on the Carnegie Ridge that was certainly once an island. This volcano, though now 1500 m beneath sealevel, has rounded cobbles on a flat top, which provide clear evidence of wave erosion. Thus there have been islands in the Galapagos for at least 8 million years. The plume, however, is certainly even older. Many scientists believe that the Galapagos mantle plume is responsible for the abundant volcanic rocks of Creteaceous age that occur in the Carribean and on the northwest margin of South America. Thus the Galapagos mantle plume could be as old as 90 million years and there may have been islands in this locality this long.
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