Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified by igneous differentiation to produce a range of other volcanic rock types, for example, rhyolite in Iceland, and phonolite and trachyte at the intraplate volcano Fernando de Noronha. Unlike mid-ocean ridge basalts (MORBs), which erupt at spreading centers (divergent plate boundaries), and volcanic arc lavas, which erupt at subduction zones (convergent plate boundaries), ocean island basalts are the result of intraplate volcanism. However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of a mid-ocean ridge, and Samoa, which is located near a subduction zone.
Figure 1. Age-progression of volcanic islands and seamounts at the Hawaii hotspot
Ocean island basalt formations at Rochester Falls in Mauritius
Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.
An illustration of competing models of crustal recycling and the fate of subducted slabs. The plume hypothesis invokes deep subduction (right), while the plate hypothesis focuses on shallow subduction (left).
