**4. Partial melting into the asthenosphere**

The rise of the S wave velocity through the lower mantle suggests that little is the melting of the mantle, anyhow, little compared to the asthenosphere. Also, because of the variation of temperature, the pressure and water content are little, and however, not as happens in the asthenosphere. On the contrary, the S wave velocity decrease in the asthenosphere from 70 to 250 km. The velocity still keeps lower numbers until 410 km, as is seen in **Figure 3**. Hence, the magma can be produced, as well in the upper mesosphere, till 410 km. In this part of the mantle (from 70 to 410 km), the rocks are partly melted (**Figure 8**). So, partial melting is the most important process acting in the upper mantle for the production of magma [14]. In other words, the lower mantle could melt when it participates in the convective cycles. Partial melting can be advocated each time there is granular compaction that will cause the production of a magmatic liquid that buoyantly rises through the crust till erupting (**Figure 8**). The rise of magma can have a basalt or broadly muchevolved composition (rhyolitic) depending on how much interaction with the crust will have. However, basalts are compositions that help to model the geochemical

#### **Figure 8.**

*Partial melting scheme. Magma is formed by the compaction of grains. The liquid is present between grains and is formed squeezing such material and so the magma aggregate and buoyantly rise through channels to the surface (author's collection).*

**Figure 10.**

*Diverse magma for the diverse tectonic environment—A simple petrographic outline (modified from Bosellini 17).*

processes in the rock origins, so are very important. On the other hand, rocks with an evolved composition help to decipher the processes that occur in the crust partial melting versus crystal fractionation, assimilation, and other minor processes, such as mixing and/or mingling (**Figure 9**) [25–28].
