The “crack in Africa” refers to the East African Rift System (EARS), one of Earth’s most significant and active geological features. This zone is where the African continent is slowly splitting apart, a process unfolding over millions of years. Stretching thousands of kilometers, the rift represents a developing divergent tectonic plate boundary, providing scientists a rare view of how continents break up. The system extends from the northern Afar region all the way down to Mozambique, shaping the landscape of East Africa.
Defining the Great Rift Valley
The East African Rift System is a vast, linear-shaped lowland feature formed by a geological rift valley, a long trough where the Earth’s crust stretches and subsides. This trough is typically bounded by high escarpments or mountain ranges. The entire system is commonly known as the Great Rift Valley.
The EARS is not a single continuous crack but a complex network of fault lines and basins divided into two major branches. The Eastern Rift Valley, also known as the Gregory Rift, runs primarily through Ethiopia and Kenya, characterized by extensive, active volcanism and a drier climate. The Western Rift Valley, or Albertine Rift, follows an arc through the Great Lakes region, defined by deep, sediment-filled basins that hold some of the world’s deepest lakes. The Western Branch is more seismically active but exhibits less volcanism compared to its eastern counterpart.
The Geographical Path of the Rift
The East African Rift System begins at the Afar Triple Junction in northern Africa, a unique point where three tectonic plates meet and pull away from each other. From this northern apex, the rift zone follows the Red Sea and the Gulf of Aden before entering the African continent through Eritrea, Djibouti, and Ethiopia. The rift then continues southward, splitting into its two major segments.
The Eastern Branch (Gregory Rift) travels through the highlands of Kenya and continues into Tanzania. This path is marked by a chain of volcanoes and a series of alkaline lakes like Lake Turkana and Lake Magadi.
The Western Branch (Albertine Rift) arcs westward, creating a boundary that includes the borders of Uganda, the Democratic Republic of Congo, Rwanda, and Burundi. This path is defined by the deep trenches that contain the African Great Lakes, such as Lake Albert, Lake Kivu, and Lake Tanganyika. Both branches eventually converge in the south before the system finally fades into the landscape of Mozambique.
The Geological Mechanism of Continental Rifting
The reason the African continent is splitting lies in the process of plate tectonics, specifically at a divergent boundary. The African Plate is separating into two distinct pieces: the Somalian Plate to the east and the Nubian Plate (which makes up the majority of the continent) to the west. These plates are pulling away from each other at a very slow, measurable rate of approximately 6 to 7 millimeters per year, about the speed of a fingernail growing.
This continental breakup is driven by forces originating deep within the Earth’s mantle. Upwelling plumes of hot rock, known as mantle plumes, are rising beneath the region from the asthenosphere. This heat causes the overlying lithosphere, the rigid outer layer of the Earth, to bulge upward, creating large domes like the Ethiopian and Kenyan plateaus. The combination of this upward doming and the resulting lateral tension causes the brittle continental crust to stretch, fracture, and thin, a process called lithospheric thinning. As the crust stretches, it breaks along normal faults, creating the characteristic elongated, down-dropped blocks, or grabens, that form the rift valley floor.
Associated Features and Long-Term Predictions
The ongoing rifting process has created a landscape rich with geological features. The upwelling heat and fracturing of the crust have led to significant volcanic activity along the rift zone. The most notable volcanic massifs created by this process include Mount Kilimanjaro and Mount Kenya, though many other active and dormant volcanoes dot the landscape, particularly in the Eastern Branch. The fractured crust also results in frequent seismic activity, with numerous shallow earthquakes occurring as the plates continue to move apart.
The rifting also resulted in the formation of the Great Rift Valley Lakes, which fill the deep basins created by the subsiding valley floor. These lakes, such as Lake Tanganyika and Lake Malawi, are known for their exceptional depth and unique biodiversity.
The continued separation of the Nubian and Somalian plates will eventually lead to the complete rupture of the continent. Geologists predict that over the next 1 million to 5 million years, the rift valley floor will sink low enough to be flooded by ocean water, initially from the Red Sea and Gulf of Aden. This will ultimately result in the formation of a new, narrow ocean basin, separating the Horn of Africa (including Somalia and parts of Ethiopia, Kenya, and Tanzania) from the rest of the African continent.