Probing Earth's Deep Mass Structure for Insights into Mantle Dynamics

Convection in Earth’s mantle is a dominant engine of geological change, driving plate tectonics and affecting human life in the form of earthquakes, tsunamis, and volcanic eruptions. These surface processes are coupled to the slow movements within Earth’s deep layers over a wide range of spatial and temporal scales. Earth’s interior is indeed not homogeneous; its variations of temperature and chemical composition lead to density variations: hotter rocks become lighter, and some minerals can be heavier than others. These density variations create buoyancy forces driving material up and down. Knowledge of these density variations is needed to understand the Earth’s thermal and chemical evolution and how it slowly cools. Yet the deep mass distribution and its links with temperature and composition variations are poorly known. The propagation of seismic waves radiated within the Earth after an earthquake provide an important window on our planet’s interior, but converting seismic velocities into density requires independent information.

Satellite-derived gravity gradients can be used to probe our planet’s deep mass structure, a pre-requisite for deciphering mantle flow patterns. Measurements from the recent satellite missions GOCE and GRACE have made it possible, for the very first time, to build global maps of the subtle variations of Earth’s gravity vector in the different directions of space. These data reveal our planet's gravity as never before. Because they can delimit the edges of the mass anomalies, these gravity gradients are extremely sensitive to the masses' geometry and shed new light on the deep Earth’s mass distribution and dynamics. We identify in the maps large-scale gravity variations following former tectonic plate boundaries on each side of the Pacific Ocean. They likely reflect remnants of ancient tectonic plates that have sunk into the lower mantle. Deep mantle super-plumes, anchored below a depth of 2000 km, also leave a detectable signal in the gravity maps. The consistency of the gravity variations with seismic velocity anomalies in the lower mantle, as revealed by global tomography models, indicates that the two kinds of information can be combined, together with mantle flow models and tectonic plate history reconstructions, in order to unravel the deep Earth evolution and its links with plate movements.