Earth's Isostatic and Dynamic Topography—A Critical Perspective

Abstract

AbstractEarth's topography arises from the linear superposition of isostatic and dynamic contributions. The isostatic contribution reflects the distribution of thickness and density of the crust overlying a static, non‐convecting mantle. We argue that isostatic topography should be limited to the crust, thereby delimiting all sources for dynamic topography below the Moho. Dynamic topography is the component of the topography produced by normal stresses acting on the Moho that deflect the isostatic topography away from crustal isostatic equilibrium largely as a consequence of mantle flow dynamics. These normal stresses arise from pressure variations and vertical gradients of the radial flow in the convecting mantle. The best estimate of dynamic topography is from the residual topography, which is the difference between observed topography and crustal isostatic topography. Dynamic and residual topography are the same. It is clear that thermal anomalies horizontally advected by plate motions would not exist if the mantle were not convecting, therefore their contribution to topography is inherently dynamic in origin. The global integral of dynamic topography that encompasses all non‐crustal buoyancy sources is demonstrated to be equal to zero. It follows that mantle convection cannot change the mean radius or mean elevation of the Earth. Since changes in ocean basin volume driven by changes in mean depth of the oceans are inherently part of dynamic topography, thereby requiring that continental elevations must also change, such that the global integral of these perturbations must also be equal to zero. This constraint has important implications for global long‐term sea level and the stratigraphic record, among other features of the Earth system impacted by changes in Earth's dynamic topography.