Guadalupe Island consists of two partly overlapping shield volcanoes that are overlain by a thick series of flank and fissure eruptions. The northern shield volcano is the younger of the two. Together, the shields and fissural volcanism form a complete alkali olivine basalt to trachyte series. This series is mineralogically, chemically, and isotopically similar to other such volcanic series that are characteristic of central volcanoes located on the flanks of active mid-ocean ridges. Guadalupe, however, is located on the axis of a fossil ridge crest. Its origin must therefore be considered in the context of the petrologic consequences of ridge-crest jumps and the subsequent history of abandoned ridge-crest segments. The unique tectonic setting of Guadalupe allows some constraints to be placed on viable geometric models of chemical and isotopic heterogeneities in the suboceanic upper mantle.