Continental volume and freeboard through geological time

Abstract

Approximate constancy of continental freeboard since the Archaean^1–3 has been used to argue that no growth of the continental crust occurred during the Proterozoic and Phanerozoic^4,5 and that there was limited vertical movement of old cratons.⁶ However, it has been shown^7,8 that constancy of freeboard implies net growth of the continental crust, because the ocean basins deepen as the heat flow from the mantle decreases with time (crustal thickness d_c, the volume of the oceans V₀ and the Earth's surface area A_e being constant). A net growth of about 25% or 1 km³ yr⁻¹ is necessary to maintain freeboard at a constant value⁷. We further explore here the consequences of approximate constant freeboard for continental growth using a model that relates the volumes of isostatically compensated continents and oceans to the secular decline in terrestrial heat flow. We find that a post-Archaean increase in freeboard by 200 m requires continental growth of only 10%, while a decrease in freeboard by 200 m during this same period necessitates a crustal growth of 40%. Shrinkage of the continental crust since the end of the Archaean can be ruled out. Changes of >10% in post-Archaean crustal thickness are highly unlikely.