Recent seismological determinations of crustal thickness in Iceland, made mainly through identification of wide-angle PmP and SmS reflections, indicate that the depth to the Moho deepens from 11 km beneath the low-lying Reykjanes Penninsula to 39 km beneath the central highlands. By compiling all available crustal thickness measurements, we show that this trend holds generally for Iceland, with elevation (above m.s.l.) increasing at a rate of 35.54¿6.4 (1&sgr;) meters for each kilometer of Moho depth (below m.s.l.). If one assumes that the topography and Moho are in local isostatic equilibrium with thickness changes in a lower crustal layer, while an upper crust maintains a constant thickness, with the part above sea level having a density of of 2500 kg/m3, then the rate of increase of depth with topographic elevation implies a density jump of 89¿12 kg/m3 across the Moho. Petrological and seismological evidence indicate that the lower-crustal densities probably do not exceed about 3060¿50 kg/m3. Mantle densities must therefore be low, 3150¿60 kg/m3. The combined effects of chemical depletion, thermal expansion and melt entrainment in the mantle can account for some, but not all, of the anomaly. ¿ 1999 American Geophysical Union