Concentrations of Zr, Y, Nb and rare earth elements (REE) are reported for basaltic lavas from the Tertiary to Recent flood basalt succession of eastern Iceland. The samples are drawn from a series of overlapping vertical profiles through the westward dipping succession. The trace element results show that more primitive (low Zr, Y, Nb and Ree) lavas are more abundant in the topographically higher parts of the lava pile. This part of the pile consists of the attenuated edges of overlapping lava units, which thicken as they are traced westward down-dip. The results are thus interpreted in terms of the spatial variation in the chemistry of individual lava units. Lavas erupted from near the spread-center for each eruptive unit are commonly more evolved. Variation of this type is observed at the present day in the neovolcanic zone of Iceland and also controlled the pattern of volcanism in at least two other extensional igneous provinces, Skye and Troodos. In assessing temporal variation in chemistry of the erupted lavas, allowances must be made for the primary spatial variation noted above the secondary chemical effects of the varying degrees of zeolitization. When these allowances are made, there appears to have been no systematic change in the chemistry of the flows with time.