Home – Home – Jornals – Russian Geology and Geophysics 2011 number 12

Continental crust is very important for the evolution of life because most bioessential elements are supplied from continents to oceans. In addition, the distribution of continents affects climate because they have much higher albedo than oceans, which is equivalent to that of clouds. Conventional views suggest that continental crust is gradually growing through the geologic time and that most continental crust was formed in the Phanerozoic and Late Proterozoic. However, the thermal evolution of the Earth implies that most of continental crust should be formed in the early Earth. This is "Continental crust paradox".
Continental crust comprises granitoid, accretionary complex, and sedimentary and metamorphic rocks. The latter three components originate from erosion of continental crust because the accretionary and metamorphic complexes consist mainly of clastic materials. Granitoid has two components: a juvenile component through slab-melting and a recycling component by remelting of continental materials. Namely, only the juvenile component contributes to net continental growth. The remains originate from recycling of continental crust. Continental recycling has three components: intracrustal recycling, crustal reworking, and crust-mantle recycling, respectively. The estimate of continental growth is highly varied. Thermal history implied the rapid growth in the early Earth, whereas the present age distribution of continental crusts suggests a slow growth. The former estimate based on the thermal history regards continental recycling as important, whereas the latter estimate based on the present age distribution of continental crusts regards it insignificant, suggesting that the variation of estimate for the continental growth is due to involvement of continental recycling.
We evaluated the erosion rate of continental crust and calculated secular changes of continental formation and destruction to fit four conditions: present distribution of continental crust (no continental recycling), geochronology of zircons (intracontinental recycling), Hf isotope ratios of zircons (crustal reworking), and secular change of mantle temperature. The calculation suggests some important insights. The distribution of continental crust at 2.7 Ga is equivalent to the modern amounts. The distribution of continental crust from 2.7 to 1.6 Ga was much larger than at present, and the sizes of the total continental crust at 2.4, 1.7, and 0.8 Ga became maximum. The distribution of continental crust has been decreasing since then. More amounts of continental crust were formed at higher mantle temperatures at 2.7, 1.9, and 0.9 Ga, and more amounts were destructed after then. As a result, the mantle overturns led to both the abrupt continental formation and destruction, and extinguished older continental crust. The timing of the large distribution of continental crust apparently corresponds to the timing of "icehouse" periods in Precambrian.