S10: Earth’s internal dynamics

Earth’s internal dynamics : Impact on gravity field and elliptical figure

Alessandro Forte,
Dept. Geological Sciences, Univ. Florida, USA & IPGP, Paris, France,

Web Lecture on March 17,  2022 at 3:00 PM (Paris Time)

Temporal evolution of Earth’s degree-2 nonhydrostatic geoid from -40 Ma (left map) to the present day (right map), predicted by a tomography-based reconstruction of mantle convection that spans the past 70 million years (Glišović & Forte 2017, 2019).

The observed departures of Earth’s external gravitational field from that expected for a rotating hydrostatic (slightly ellipsoidal) planet provide direct evidence of large-scale heterogeneity deep inside the Earth. The ability to directly map the geometry and amplitude of this internal heterogeneity on a planetary scale is possible thanks to progress in seismic tomographic imaging of 3-D Earth structure using global arrays of seismic stations and earthquakes. I will discuss how these global seismic maps are incorporated into theoretical calculations of the slow creeping movement of hot solid material (i.e. thermal convection) throughout Earth’s rocky mantle, extending down from the crust to the boundary separating the mantle and liquid core. Such tomography-based mantle convection models elucidate how the dynamics and thermal structure of the mantle are directly linked to large-scale variations in Earth’s gravity field and external shape. I will review the recent progress we have made in developing joint seismic-geodynamic models of the thermal convection process in Earths mantle that are consistent with a variety of global surface data (e.g., tectonic plate motions, global gravity anomalies, dynamic surface topography). These models of present-day mantle convection are used as a starting condition for time-reversed simulations of the evolution of the temperature anomalies in the mantle over the past 70 million years. These reconstructions yield direct estimates of the changes experienced by a wide variety of surface geophysical processes. I will in particular focus on the implications for convection-induced temporal variations in Earth’s external nonhydrostatic figure, in particular the time-dependent evolution of dynamic ellipticity over the past 70 million years.