Why do some plateaus formed by thermal expansion have little volcanic rock?

Plateaus formed by thermal expansion occur when the lithosphere is heated by upwelling material in the underlying asthenosphere, causing the surface to rise. While this process can be associated with volcanism and rift valleys, these features are not universal.1

Some plateaus, such as the portion of the East African plateau containing Lake Victoria, lack significant volcanic rock because the uplift is driven primarily by the heating and expansion of the uppermost mantle rather than the eruption of magma through the crust. In these instances, the localized heating is sufficient to cause regional uplift without necessarily melting the lithosphere enough to produce widespread volcanic activity. Conversely, in areas where the mantle is particularly hot, the lithosphere may melt, leading to the formation of volcanoes and the deposition of volcanic rocks, as seen on the Ethiopian Plateau.12