![]() The weight of a cooling volcanic construction depresses the lithosphere into an underlying plastic layer of the mantle called the asthenosphere. The Hawaiian Islands and Iceland are examples of oceanic islands created by vigorous hot spot volcanism.Īfter growth and emergence, large volcanic islands evolve through several stages of decline and submergence. Most seamounts never grow tall enough to become islands, but some very large ocean-floor volcanoes, particularly those above well-established hotspots, emerge above sea level before plate motion removes them from their magmatic sources. Seamounts are volcanic hills or plateaus formed by extrusion of lava onto the seafloor in places where plates of oceanic lithosphere override hot areas in the mantle near divergent plate tectonic boundaries called mid-ocean ridges, and over localized intraplate mantle upwellings called hot spots. The British naturalist, Charles Darwin (1809 –1882), observed atolls in the southwest Pacific Ocean during his nineteenth century travels aboard the Beagle, and he was the first to suggest that an atoll is a crown of coral on a newly-submerged guyot. ![]() In some cases, carbonate reefs fringing an aging volcanic island continue to grow as the island sinks below sea level, leaving a circular island of coral, or an atoll, surrounding a round lagoon where the peak of the extinct volcano once stood. ![]() ![]() Erosion by wave activity during submergence creates the characteristic flat-topped profile of a guyot. A guyot is a flat-topped submarine mountain, or seamount, that once emerged above sea level as a volcanic island, and then resubmerged when volcanic activity ceased.
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