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Ocean island basalts (OIB) are basaltic rocks found on many volcanic islands away from tectonic plate boundaries, typically associated with hot spots.
Ocean island basalts are found on islands and volcanoes located on oceanic crust. The chemical composition of these basalts can vary from tholeiite to alkali basalt within the same island group, but it 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.
The theory of plate tectonics provides excellent explanations for both the existence of volcanic eruptions at plate boundaries such as ocean ridges and subduction zones and why geological processes produces basalts of different geochemical properties at these plate boundaries. The existence of basaltic eruptions in the interior of tectonic plates, however, are not directly related to plate tectonism and models involving magma upwelling from deep mantle sources, such as mantle plumes and hotspots have been developed to explain them. These models explain why OIBs contain more enriched basalt than mid-ocean ridge basalts (MORBs) that originate much closer to the Earth's surface or in the astenosphere. The existence of mantle plumes and hotspots has, however, been hotly debated since they were first proposed in the 1960s and 1970s.