Ocean island basalt

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Piton de la Fournaise erupting OIB in 2004

Ocean island basalts or OIBs are basaltic rocks found on many volcanic islands away from tectonic plate boundaries typically associated with hot spots. Islands hosting ocean island basalts always lie above the oceanic crust and are not limited to volcanic islands but occur also on volcanoes of any size under the sea. The chemical compostition of these basalts can vary from tholeiite to alkali basalt within the same island group, but is never calc-alkaline. During the shield volcano stage of many hotspot islands, tholeiitic OIBs build up most of the volcano's structure. Following the post-erosional stage this is usually accompanied by violent eruptions of alkali basalt and other more evolved volcanic rocks with high alkali content.