On the Early History of the Earth
According to current astrophysical models of the sun, solar luminosity has increased with time at a rate depending on the nature of the model and the “zero-age” of the sun. The determination of the earliest occurrence of running water on the earth9s surface may establish a calibration point on the luminosity curve. Our investigation of the nature and age of the oldest rocks suggests that erosion by running water occurred at least 4 billion years (b.y.) ago. Calculation of the evolution of terrestrial surface temperatures based on the most favorable solar model and current estimates of the composition of the early atmosphere indicate that either some revision of the model is necessary or that the earth and sun must be older than 4.5 and 5 b.y., respectively. The rate of weathering of bedrock and resulting erosion vary directly with temperature, atmospheric moisture content, and organic acids from vegetation. Since these all increased with geologic time, it is suggested that the rate of bedrock erosion, which would have been slow initially, must also have accelerated. If the presumption of an interaction between sedimentary load and geologic activity is correct, the duration of geosynclinal cycles should have decreased with time. The oldest rocks dated so far (about 3.5 b.y.) are pegmatites, granites, and granite gneiss complexes, all of which represent terminal processes in respective geologic cycles, the beginnings of which are inferred at about 4 b.y. ago. The presence of a substantial sialic crust at this time is indicated by the compositions of both igneous rocks and meta-sedimentary rocks. The origin of the continents is examined in the light of these geologic data and inferences, and on the assumption of an accumulation of the earth by accretion from cold, solid objects, probably in the form of whiskerlike crystals. Theories of origin of continents by segregation from the mantle cannot be reconciled with the time scale provided by calculations from the thermal history of the earth and the currently accepted age of 4.5 b.y. An alternative hypothesis according to which the continents originated by impact of extra-terrestrial material with subsequent modification by geological processes and mantle admixture is reconsidered.