1. Tectonic Plate Movement: The main force is the divergence of the African Plate. This means that the plate is splitting apart, creating a rift along the East African region. The movement is driven by convection currents within the Earth's mantle, which pull the plates apart.

2. Volcanism: As the plates move apart, magma rises from the Earth's mantle to fill the gap. This magma often erupts, forming volcanoes along the rift valley. These volcanic eruptions further contribute to the widening of the rift and the formation of the valley.

It's important to note that these forces work together to create the Great Rift Valley. Tectonic movement provides the initial opening, while volcanism further shapes the landscape and adds features like volcanic mountains and lakes.