This contribution explores the consequences of experimentally derived ultramafic plutonic rocks for estimates of continental crust composition. The formation of high-pressure cumulates from primitive to evolved hydrous arc magmas was investigated in the range 0.8 - 1.5 GPa and 1350 - 800 degrees C, to provide constraints on solid assemblages that form the roots of island arc crust. At pressures exceeding 0.8 GPa ( 25km), 40 - 60% of ultramafic cumulates are produced to obtain andesite to dacite compositions that are typical for evolved island- arc volcanic and plutonic rocks. Calculated cumulate densities are comparable or higher than upper mantle values, while seismic velocities ( V-p) vary continuously between 7.3 and 8 km/ s, resulting in a broad crust- mantle transition. The Moho represents a plagioclase saturation boundary. Our results are consistent with seismic imaging from continental margins and island arcs, revealing that beneath an active volcanic front a sharp geophysical Moho signature is sometimes absent.