An analysis of the geological history of the eastern Greater Antilles suggests a model for the tectonic evolution of this area which may be applicable to island arcs in general. The island arc is found to consist of a continuously emerging geanticlinal platform developing over a thickening root of oceanic crustal material. The axis of island-arc deformation is localized along a boundary between plates of oceanic crust with contrasting physical properties in response to horizontal stresses in the mantle. Volcanic magmas originate through the partial melting of partially hydrated upper mantle material along the axis of deformation. The geological history of the eastern Greater Antilles shows that successively younger deposits are characterized by increasing facies diversification which in turn reflects increasing movement along old high-angle normal and reverse faults. Island arcs may be considered to be zones of deformation analogous with classical continental-margin orogens, but distinct from these in that the immense quantities of terrigenous sediment which are present in normal mountain systems are absent here. Hence, the observed structural geology directly reflects primary orogenic forces without the modifying effect of a very thick sedimentary section.