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Why do some rift valleys have volcanoes along their axes?

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tectonic basins and rift valleys Escarpments of the Great Rift Valley rising above the plain north of Samburu Game Preserve, central Kenya. Beisa oryxes graze in the foreground.

Volcanoes occur along the axes of some rift valleys because of the processes associated with the separation of lithospheric plates. As these plates diverge, the crust thins, forcing the underlying mantle to diverge as well. This movement allows hot material from the asthenosphere to rise to shallow depths.1

In many cases, this upwelling of hot material creates large domes, which push the overlying lithosphere upward and cause it to stretch, manifesting as a rift valley. This stretching and thinning of the crust creates fractures that are invaded by ascending magma, which then erupts to form volcanoes along the rift axis.12

While volcanism is common in rift valleys formed over these large domes, such as parts of the East African Rift System, it is rare or absent in rift valleys created by distant tectonic forces where such domes are not present, such as the Baikal Rift.1