GROWTH CURVE OF THE CONTINENTAL CRUST AND ITS IMPLICATION TO THE THERMAL EVOLUTION OF THE EARTH

Growth curve of continental crust has long been one of major concerns among Earth scientists. There are many different ideas proposed by different methods. For example, Fyfe (1978) proposed that rapid growth of continents in Archean up to 120% against present volume, whereas Hurley and Rand (1969) proposed nearly absent continental crust in the Archean and growth in the Proterozoic and Phanerozoic up to 100%.

To estimate the real growth curve of continents on the Earth, our research group employed the method by river mouth zircon from the major 30 rivers on the continents. (Rino (2004), Rino et al (2008)). The full result is shown in this paper.

Rino (2004) demonstrated that the analysis of river mouth zircon does really reflect age population of granitic materials of whole drainage regions. Recycle of continental crust can be evaluated by the comparison between zircon age population and geologic map where sedimentary rock and granitic basement can be identified. We have covered almost all area on the Earth excepting Antarctica. Continental platform which occupied 10% of all continent is estimated by the extension of geologic boundaries from continental margin.

The full result indicates 1) growth curve of continental crust showed only 20% growth from 4.6-2.5Ga, 2) major growth occurred from 2.6-0.6Ga. And 3) Recycling of continental crust dominated since 1.0Ga up to the present. Specifically recycling in the Phanerozoic is twice than before. In addition, the Phanerozoic may be the turning point to decrease continental crust.

The obtained growth curve of continental crust is remarkably different from the model derived from calculated thermal history of the Earth (Korenaga, 2006). It means extensive tectonic erosion occurred in the past, most probably in the Hadean and Archean. The lost continental crust must be present in the mantle transition zone which is about 10 times of the total volume of the present continents on the surface which we call as Second Continents. The primordial anorthositic continents on the Hadean Earth must be somewhere in the CMB or partly middle depth of lower mantle. We call them as the Third Continent. Both second and third continents must have controlled mantle convection through time because of highly concentrated radiogenic elements such as U, Th, and K.