A compilation of geochemical data from >50 active volcanoes where seismically defined Moho depth measurements are available, representing most volcanic arcs around the world, indicates a relation exists between crustal thickness (Moho depth) and the trace element composition of basalts. The best correlation exists for maximum light/heavy rare earth element (REE) ratios, and we use maximum Ce/Y (R2 = 0.90) to show the exponential function with increasing depth, to 50 km. Application to New Zealand, part of the eastern Gondwanan margin through much of the Phanerozoic, shows that higher Ce/Y ratios in arc basalts correspond to two periods of recognized crustal thickening, plus an additional event at ca. 240 Ma, which links New Zealand to the Gondwanide Orogeny. Conversely, low Ce/Y ratios in basalts correlate with periods of recognized extension in eastern Gondwana. Very high Ce/Y ratios in ca. 120 Ma rocks imply an unusually thickened (∼50 km) arc crust, consistent with independent geological estimates of crustal thickness at that time. The correlation shows that crustal thickness variations during orogenesis can be conservatively estimated to within ±3 km.