Study of samples of the main volcanic units of the Afar rift, from margins to late axial series, have yielded the following conclusions: 1) the magmatic liquids are strongly differentiated by a fractional crystallization process in shallow chambers, the evolution of which is limited by injection of new primary liquids as shown in the latest axial series; 2) primary liquids are generated by melting of a homogeneous mantle source, a process whereby successive partial melting in a closed system is stopped by percolation of the generated liquid. The degrees of partial melting are closely related to spreading rates; 3) the magmatic processes involve a discontinuous dynamic evolution of spreading displayed by a pulsatory magmato-tectonic activity which is controlled by the speed of decompression of mantle material.