New geologic, petrologic, and Sr isotope data from Seguam Island, Alaska, reveal substantial variation in the composition of magmas of the central Aleutian arc. The data indicate that variable magmatic processes occur along the strike of the arc. Seguam is situated on extended arc crust and has erupted voluminous basalt and dacite. In contrast to basalt from adjacent volcanic centers atop unextended crust, which evolved by polybaric open-system crystallization, Seguam lavas crystallized at shallow levels under closed-system conditions. Seguam basalt therefore better reflects the character of its source materials than basalt erupted through unextended arc crust. We conclude that along-arc differences in the nature of crystallization were facilitated by changes in the thermal and density structure of the subarc litho-sphere caused by infra-arc extension.