A three-stage model for crustal origin is proposed in which Stage I (4.5-4.0 Ga) is characterized by the production of a transient ultramafic crust rapidly displaced by a widespread mafic crust formed at divergent plate boundaries, consumed at sinks, and recycled through the mantle. During Stage II (4.0–3.5 Ha) a falling geothermal gradient results in the production of isolated andesitic arc systems from partial melting of descending mafic crust. Further decreases in temperature during this stage result in decreased melting and the production of tonalitic magmas which rise, overplating the andesitic crust which is partially melted to produce high-K granites. Sialic arcs grow by tonalitic plutonism and arc collisions into two supercontinents during Stage III (3.5-2.7 Ga). During this stage, greenstone belts form in continental-rift and marginal-basin environments.