The East African Rift System — the Great Rift Valley — is a 6,000-kilometer chain of geological fractures running from the Afar Triangle in northern Ethiopia southward through Kenya, Tanzania, Malawi, and Mozambique to the Zambezi River delta. The African plate is being torn apart here. This is not metaphor.
Formation
The rift began forming approximately 35 million years ago and continues today at approximately 6–7 millimeters per year. Over geologic time this process will eventually separate the horn of Africa from the mainland, creating a new ocean basin where today there is the Afar Depression.
The rift system branches into two arms. The Eastern (Gregory) Rift runs through Ethiopia, Kenya, and northern Tanzania — a chain of alkaline lakes and active volcanoes including Ol Doinyo Lengai in Tanzania, the world's only active carbonatite volcano, which erupts with lava so cool and low in silica it appears almost black.
The Western (Albertine) Rift is deeper, containing Africa's deepest lakes: Tanganyika at 1,470 meters and Malawi. The western rift runs through the Congo-Uganda-Rwanda border region — home to Virunga National Park, the Rwenzori Mountains, and the mountain gorillas of Bwindi Impenetrable Forest.
Where Humanity Began
The dramatic environmental changes caused by rift formation — new highland environments, fragmented forests, expanding grasslands — are believed to have created the selection pressures that drove the evolution of bipedal hominins. The oldest fossils of the human lineage have been found along the rift.
The Laetoli footprints in Tanzania — preserved in volcanic ash 3.7 million years old — are the most affecting physical record of our ancestors anywhere on earth. Two individuals walked side by side across wet volcanic ash. Their footprints hardened. We are reading them now.