地球における海洋と大陸の形成

抄録

The presence of oceans and continents is one of the Earth's unique features among the planets in our Solar System. Here I discuss the formation and origin of Earth's oceans and continents by combining the results of geological, geochemical and geophysical studies. The geological and geochemical data provide evidence for the presence of Earth's oceans by ca. 3.8 Ga and probably ca. 4.3 Ga. Yet, oceans could be formed even earlier, possibly soon after the solidification of the magma ocean, especially if the Earth had gained water before the Moon-forming Giant Impact. The hydrogen isotope data for meteorites and mantle-derived terrestrial samples suggest that proto-planets and planetesimals, that accreted to form the Earth, could acquire water from solar nebular and water-rich materials delivered from outer Solar System, respectively. Furthermore, the hafnium isotope systematics of Hadean zircon constrain the timing of magma ocean solidification to as early as 4.5 Ga. The infant solid Earth would be characterized by the stagnant-lid convection regime. Meanwhile, radiogenic heat generation induced vigorous mantle plumes and stagnant plates had been cooled and hydrated. Eventually, the hydrated proto-plates were subducted beneath buoyant oceanic plateaus generated by the mantle plumes, resulting in the onset of plate tectonics. Although there is evidence that granitic rocks were locally formed on the Hadean Earth, the geochemical compositions of sedimentary rocks indicate that the emergence and growth of continents occurred during Middle and Late Archean times. This implies that the time interval from the magma ocean solidification to the onset of global plate tectonics is ~1 Gyr.