**2. Mantle structure**

The section of crust and upper mantle with a profile of the S wave velocity (Vs, **Figure 2**) recognizes some physical characteristics that help to understand the dynamic of magma in the mantle. Probably, much of the magma that reaches the crust is produced in the asthenosphere, although deeper mantle origin can be envisaged from tomography data (410 and 660 km). Although, it is not clear if some magma can be produced in the outer core and/or in the lower mantle. Helffrich and Wood [15] affirm that seismological images of the Earth's mantle reveal three distinct changes in velocity structure, at depths of 410, 660, and 2700 km. The first two as said are best explained by mineral phase transformations (**Figure 2**), whereas the third D″ layer (2700 km) probably reacts due to change in chemical composition and thermal structure. In addition, tomographic images of cold slabs in the lower mantle, and the occurrence of small-scale heterogeneities in the lower mantle all indicate that subducted material penetrates the deep mantle, implying whole-mantle convection. In contrast, geochemical analyses of the basaltic products of mantle melting are frequently used to infer that mantle convection is layered, with the deeper mantle largely isolated from the upper mantle. The geochemical, seismological, and heat-flow data are all consistent with whole-mantle

#### **Figure 3.**

*(a): Earth slide highlighting the convective cycle on all the mantle (author's collection). (b): Earth slide highlighting the mantle with 2 convective cycles—Upper mantle and lower mantle (author's collection).*

#### **Figure 4.**

*Mantle slice with a characteristic chemical and physical heterogeneity (modified from Helffrich and Wood [15]). The blue blob is residual slabs and/or metasomatized mantle. The pink blobs are identified with the D layer.*

convection provided that the observed heterogeneities are remnants of recycled oceanic and continental, respectively, of mantle volume (**Figure 3**). The convective cells are the engine of the plate tectonic for whole-mantle versus upper-mantle cells (**Figure 3**). **Figure 4** is the best guess for a model of mantle recently published [16, 17].
