Rates of generation and growth of the continental crust

Models for when and how the continental crust was formed are constrained by estimates in the rates of crustal growth. The record of events preserved in the continental crust is heterogeneous in time with distinctive peaks and troughs of ages for igneous crystallisation, metamorphism, continental margins and mineralisation. For the most part these are global signatures, and the peaks of ages tend to be associated with periods of increased reworking of pre-existing crust, reflected in the Hf isotope ratios of zircons and their elevated oxygen isotope ratios. Increased crustal reworking is attributed to periods of crustal thickening associated with compressional tectonics and the development of supercontinents. Magma types similar to those from recent within-plate and subduction related settings appear to have been generated in different areas at broadly similar times before ∼3.0 Ga. It can be difficult to put the results of such detailed case studies into a more global context, but one approach is to consider when plate tectonics became the dominant mechanism involved in the generation of juvenile continental crust. The development of crustal growth models for the continental crust are discussed, and a number of models based on different data sets indicate that 65%–70% of the present volume of the continental crust was generated by 3 Ga. Such estimates may represent minimum values, but since ∼3 Ga there has been a reduction in the rates of growth of the continental crust. This reduction is linked to an increase in the rates at which continental crust is recycled back into the mantle, and not to a reduction in the rates at which continental crust was generated. Plate tectonics results in both the generation of new crust and its destruction along destructive plate margins. Thus, the reduction in the rate of continental crustal growth at ∼3 Ga is taken to reflect the period in which plate tectonics became the dominant mechanism by which new continental crust was generated.