Many crustal estimates suggest that the continents are approximately andesitic in composition. However, in recent subduction related environments, andesites are thought to originate by intracrustal differentiation from basaltic parental magmas. The net flux from mantle to crust along destructive plate margins is therefore basaltic with low Rb/Sr, and is not readily reconciled with the development of intermediate, high Rb/Sr continental crust. Either the continental crust is considerably more mafic than andesite, or the mechanism of crust formation has changed with time, such that a large proportion of the continental crust was formed by processes unlike those active in recent subduction zones. It is significant that there is evidence from Archean rocks to support the latter hypothesis.