Gregor's Denudation of the Continents

Abstract

IN his article, “Denudation of the Continents”, Gregor¹ estimated the pre-human global denudation rate at about 90–100×10¹⁴ g/yr, and obtained an average rate of about 110×10¹⁴ g/yr for the time interval from Carboniferous to Recent. His method, however, produced a value of 400×10¹⁴ g/yr for pre-Carboniferous Phanerozoic time—a value he questioned seriously himself. We replotted Gregor's data on semilogarithmic paper, converting them from volumes/yr to g/yr using his density factor of 2.6, and fitted them as simple exponential functions by drawing two straight lines; one for Carboniferous to the present, the other for late Pre-Cambrian to Devonian (Fig. 1). The dashed line, which extrapolates to a value of about 300×10¹⁴ g/yr, shows the reason for Gregor's 400×10¹⁴ g/yr as the depositional rate for these Palaeozoic and late Pre-Cambrian rocks. Such a rate would have to prevail if the rocks of Devonian and older ages have been decaying with a 120–150 m.y. half-life since their time of deposition. It seems much more likely that the Devonian and older sequence represents an interrupted cycle of deposition and destruction, for which the half-life increased markedly after the beginning of a new cycle initiated during the Carboniferous.