Ascension Island is the exposed part of a large volcano located about 80 km west of the Mid-Atlantic Ridge axis in the northern South Atlantic Ocean. The volcanic rocks of the island form an alkaline suite with a compositional range of basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite. Trachyte and rhyolite compose approximately 14% of surface exposures. Petrochemical studies by other investigators have shown that the igneous suite was formed by fractional crystallization. The abundance of felsic volcanic rocks and the presence of granite and syenite blocks in pyroclastic deposits suggest that felsic magma chambers have formed and could provide the heat for a convective hydrothermal system. The felsic volcanic rocks form two eruptive centers that may be contemporaneous. The oldest rocks exposed on the surface are rhyolite with K-Ar ages of approximately one million years. Faults mapped on the island have four principal trends and appear to be related to regional structures that are apparent on the bathymetry of the island pedestal.