Tides induced in the solid Earth by the Sun and Moon have allowed researchers to characterize the density of the deep mantle. The results suggest that two large, low-shear-velocity provinces (LLSVPs) under equatorial Africa and the Pacific are, on average, denser than the surrounding rock. The finding has consequences for our understanding of mantle circulation.

The existence of the two LLSVPs has long been known from seismic tomographic observations. Such studies, which make use of the seismic waves generated naturally by earthquakes, have indicated that the features extend for thousands of kilometres laterally, and for approximately 1000 km upwards from the core–mantle boundary (CMB).

Hot rocks

Low shear-wave velocities typically indicate the presence of hotter material, and the LLSVPs have been interpreted as the source of buoyant mantle plumes that ascend from near the base of the mantle. Sharply changing seismic properties at the edges of the features, however, are not consistent with a purely temperature-based origin, suggesting that compositional differences are involved. How these variations are reflected in the regions’ density distribution has long been an issue for debate.

Now, using probabilistic modelling based on high-precision global positioning system (GPS) measurements, Harriet Lau of Harvard University, and collaborators at Harvard, Columbia and Princeton universities in the US, the University of Science and Technology of China, and the University of Cambridge in the UK, have investigated the response of the Earth’s body tides to different density profiles in the lower mantle. The group’s surprising outcome is that, taken as a whole, the LLSVPs are not more buoyant than the surrounding mantle after all.

Mass concentration

For this conclusion to be consistent with the results of previous studies, the LLSVPs would both have to be heterogeneous features, in which the anomalously dense parts are restricted to within 100 km of the CMB. The spatial resolution of Lau and colleagues’ analysis means that this cannot currently be ruled out, but future work should better define the fine-scale structure.

The research is described in Nature.