Selous Game Reserve is located on one of the oldest geological regions in the world: the Tanzania Craton.
This three billion year old rocky region was laid down when the Earth’s continents first began to form, and has remained stable while the rest of the African continent has grown around it.
The craton forms the highest part of the East African Plateau, a huge flat-topped highland area that covers most of Tanzania and its neighbouring countries; the east and west boundaries of the plateau are lined by branches of the East African Rift Valley.
The plateau is thought to have formed around 30 million years ago, as the result of a hot plume of molten rock (known as magma) under the Earth’s surface (or crust); the heat caused the crust to expand and rise, like a cake baking in an oven.
As the crust rose, the ancient Tanzania Craton rose too, forming the ‘lid’ of the plateau.
Elsewhere in the East African Rift, the magma plumes have pushed through the surface of the Earth’s crust at cracks and weak points, creating volcanoes such as Mount Kilimanjaro.
The western edge of the East African Plateau runs along the border of Tanzania and is marked by a string of vast lakes, including Lake Tanganyika and Lake Malawi; these lakes formed in the deepest valleys of the East African Rift, fed by rivers draining water off the western edge of the plateau.
The Rufiji River provides one of the largest drainage basins for the East African Plateau; the river and its tributaries drain the eastern portion of the plateau, carrying the water on a broad meandering path through Selous Game Reserve.
The Rufiji is the largest river in Tanzania and is 600km long and more than 1.5km wide in places; its drainage basin covers most of the south-eastern portion of the East African Plateau.