Is a rift valley a convergent plate boundary?
No a rift valley is a divergent boundary
In a rift valley two plates are moving apart. The rift is where the crust of the earth is being pulled apart. The widening of the rift can result in the formation of a body of water, a lake, sea or ocean.
The rift valley is thought to be above a place where the convection currents in the mantle is bring hot magma up to the surface. As the magma reaches the crust it cools and moves in two opposite directions as part of the convection current. As the magma moves in two opposite directions the magma pulls the crust apart creating a rift valley.