Two greenstone age minima at 2.5--2.2 Ga and 1.65--1.35 Ga may correlate with ages of supercontinents when a greater proportion of greenstones were recycled into the mantle by subduction. From the age distributions of greenstones and juvenile continental crust, earth history can be divided into three stages: (I) >2.8 Ga, when greenstones and microcontinents formed and collided continuously, although probably not forming a supercontinent until about 3 Ga; (ii) 2.8--1.3 Ga, where a clear episodisity is apparent in greenstone eruption and collision ages, and where two greenstone age minima coincide in age with supercontinents; and (iii) <1.3 Ga, when greenstones and continents appear to have formed and collided continuously, perhaps in response to significant overlap in the times of dispersal and assembly of supercontinents. Greenstone age peaks 2.7, 1.9, and 1.3 Ga may reflect catastrophic overturn of the mantle initiated by episodes of sinking of cold lithospheric slabs through the 660-km discontinuity to the D″ layer initiating an episode of major mantle plume activity. Continuous production and preservation of greenstones in the last 1.3 Gy is consistent with seismic tomography data, which suggest a steady sinking of slabs into the lower mantle.