Contemporaneous alkalic and tholeiitic basalts from Cerro Azul volcano, in the Galápagos archipelago, are related by fractional crystallization in the lower crust and upper mantle. The higher pressures enhance augite crystallization, which depletes the residual liquids in silica while enriching them in K and Na, just as predicted by phase-equilibria experiments. Such occurrences have also been reported from other ocean islands. The common aspect of these alkalic suites is that they occur at volcanoes that have relatively low magma-supply rates, which leads to the development of deep magma bodies and the production of alkali-olivine basalts by fractional crystallization of a tholeiitic parent. During more robust phases of magmatism, tholeiitic magmas ascend into the upper crust and produce a low-pressure tholeiitic differentiation trend.