Three-dimensional mantle structure: seismic tomography and waveform modeling. Large structures at the base of the mantle: primordial reservoir or plate graveyard? Relationship with surface observations (hot spots, etc.). Introduction of fine structure through seismology.
To a very good first approximation, the Earth's structure is made up of concentric spherical layers, and is described by a "1D" model, where variations in all properties depend only on depth (or radius). As we said earlier, this "static" Earth model does not account for dynamic phenomena, such as mantle convection, which introduce "lateral variations" with respect to the 1D model. Plate tectonics defines a global mantle circulation with upward flows of hot material under oceanic ripples, and downward flows of cold material in subduction zones. However, certain observations do not fit this simple model, in particular the presence of intra-plate volcanism (the famous "hot spots" and continental "trapps" thought to be fed by mantle "plumes"). Their origin gives rise to several fundamental questions:
1) How deep is the source of the mantle plumes (660 km, CMB)?
2) Does this source represent a "primordial" reservoir with a different composition from that of the basalts collected in the mid-oceanic ripples?
3) could imaging these plumes enable us to determine the proportion of heat released by radioactivity in the mantle compared with that corresponding to the cooling of the Earth?
More generally, we reviewed the various possible sources of heterogeneity within the Earth's mantle. This led us to describe methods for determining the Earth's composition, by comparing rock samples with the composition of meteorites, and to draw up the Earth's heat balance.
We then addressed the question of communication between the upper and lower mantle, and the role of the 660 km discontinuity.
Finally, we reviewed the principles of seismic tomography, the different types of seismic data used and the difficulties encountered, in contrast to medical tomography, by the non-uniform distribution of sources and stations.
