From global tectonics to global geodynamics

Abstract

Observations suggest that global (plate) tectonics operates on the Earth. The most characteristic features of the global tectonics are ocean floor spreading in mid-ocean ridges and subduction in deep-sea trenches. These processes imply the existence of mantle flow. However, within the framework of the plate tectonics, it is impossible to build a consistent quantitative theory of mantle convection because one cannot answer the question of where the tectonic plate “terminates”. From a mathematical point of view, the difficulty of global tectonics is that there are no boundary and initial conditions that would allow one to consider the evolution of some isolated part of the planet (e. g., the upper mantle). Therefore, to obtain a physically justified answer to the questions about the causes and energy sources of mantle motions, it is necessary to consider an evolution of the planet as a single whole. This formulation of the problem leads to the global geodynamics. Unlike the global tectonics, which in fact ignores the existence of the Earth’s core, for the global geodynamics the liquid outer and solid inner core, as well as the processes at the boundary between them and at the boundary between the core and the mantle, which decisively influence the mantle dynamics, are the main objects of the study. In this review, we confine ourselves to the global heat balance of the Earth. In the coming years, the results of the geoneutrino experiment will make it possible to obtain a reliable estimate of the total rate of radiogenic heat production in the Earth and to estimate the heat flow from the core to the mantle. Even this alone will significantly narrow the choice of models describing processes in the core. An ascertainment of the temperature at the inner/outer core interface and an elucidation of the mixing nature in the outer core will allow one to reduce an uncertainty of the temperature at the base of the mantle and to formulate a boundary condition problem for the mantle flow dynamics. Thus, a bridge from global geodynamics to global tectonics will be thrown and the conceptions of the latter will be put on a firm physical basis.