Many hitherto puzzling features of the isotope and trace-element geochemistry of the Earth’s mantle and crust can be explained if Earth history is punctuated by episodes of enhanced exchange between the lower and upper mantle. Such episodes would replenish the upper mantle with trace elements, and also cause rapid growth of continental crust. This picture is consistent with recent geophysical models in which two-layer convection alternates with episodes of penetrative or whole-mantle convection.