摘要
Recent space geodetic and gravimetric studies have given indications that the Earth’s radius is increasing at 0.1-0.4 mm yr-1 at present. Seismic studies have also shown that earthquakes alone could be causing the radius to increase at 0.011-0.06 mm yr-1. Deep mantle plumes provide a geophysical context within which such radial expansion, if confirmed, could possibly be explained. Both theory and observation suggest that these rising plumes more readily penetrate the 670 km barrier than do subducting slabs moving in the opposite direction towards the core-mantle boundary. If so, there would be a net flow of mass from the deep lower mantle into the upper mantle. Due to the lower pressures in the upper mantle,the excess mass of plume materials reaching there would transform to minerals with lower densities than they had at the mantle base. An increase in the mantle volume and the Earth’s radius would therefore be implied. Using previously published data for the African superplume. it is estimated that this mechanism could cause the Earth’s radius to increase at rates of 0.02-0.3 mm yr-1, similar to the rates possibly indicated in the present studies. This mechanism could also explain the very large range in current estimates of mantle plume heat and volume fluxes. A possible energy source for this plumedriven mode of expansion is discussed.
Recent space geodetic and gravimetric studies have given indications that the Earth’s radius is increasing at 0.1-0.4 mm yr-1 at present. Seismic studies have also shown that earthquakes alone could be causing the radius to increase at 0.011-0.06 mm yr-1. Deep mantle plumes provide a geophysical context within which such radial expansion, if confirmed, could possibly be explained. Both theory and observation suggest that these rising plumes more readily penetrate the 670 km barrier than do subducting slabs moving in the opposite direction towards the core-mantle boundary. If so, there would be a net flow of mass from the deep lower mantle into the upper mantle. Due to the lower pressures in the upper mantle,the excess mass of plume materials reaching there would transform to minerals with lower densities than they had at the mantle base. An increase in the mantle volume and the Earth’s radius would therefore be implied. Using previously published data for the African superplume. it is estimated that this mechanism could cause the Earth’s radius to increase at rates of 0.02-0.3 mm yr-1, similar to the rates possibly indicated in the present studies. This mechanism could also explain the very large range in current estimates of mantle plume heat and volume fluxes. A possible energy source for this plumedriven mode of expansion is discussed.
