Summary: We’ve been talking about the dismemberment of Pangaea and its biggest piece, Gondwana, for months now. The process is still going on, and the newest break within the old Gondwana continent is in its largest surviving portion, Africa. Map from Digital Tectonic Activity Map of the Earth (NASA) with annotations by Gibson.The East African Rift System is a present-day break that extends from the Dead Sea in Israel and Jordan, south through the Red Sea, separating Arabia from Africa, and into the African continent through Ethiopia, Kenya and Uganda, eastern Congo and Zambia, and into Mozambique and on offshore. All told, the system is more than 5,000 miles, 8,000 kilometers, long. It’s a big, complex break in the continental crust. As much as we have a pretty good handle on how rifting proceeds, our understanding of how and why such rifts begin is still pretty poor. There are multiple ideas for how the East African Rift started, ranging from some deep-seated mantle plumes, whose upwelling heat broke the crust apart, to crustal thickness variations that allowed magma to flow upward in some locations preferentially to others, initiating the rift process. Differences in crustal density might have the same effect as thickness variations. Whatever started the rift, it has since followed a pretty standard and expectable development. Early in the process, around 30 million years ago, early Oligocene time, the upwelling magma breached the surface and flowed as extensive flood basalts in what are now Ethiopia, Somalia, Yemen, and adjacent areas. This point is called the Afar Triple Junction, because it is the focus for three branching rifts. To the northwest it’s the Red Sea, a young ocean basin where sea-floor spreading has just barely begun, to the northeast is the Gulf of Aden, true oceanic crust, and the mid-ocean ridge there continues into the Indian Ocean as the Carlsberg Ridge, the divide between the Indian tectonic plate and the African Plate. The third branch of the system extends from the Afar Triple Junction into the African Continent. Where the rift is in continental crust, in East Africa, the result is long narrow down-dropped troughs, called grabens. They are bounded by normal faults that have large offsets, many thousands of feet in some cases. The situation is very much like eastern North America must have been back in the Triassic as the Atlantic Ocean began to open. In Africa, it’s not one simple linear zone, but it curves and branches into two major segments on either side of Lake Victoria. The fault-bounded troughs, the grabens, are obviously lower that the uplifted flanks, which tend to be mountainous, and the grabens or basins accumulate thick piles of sediment eroded off the mountains. In East Africa, the long, narrow lakes, such as Abaya in Ethiopia, Turkana in Kenya, Lakes Albert, Edward, and Kivu along the eastern border of Congo, Lake Tanganyika between Congo and Tanzania, Lake Rukwa, and Lake Malawi all lie in the down-faulted basins of the East African Rift. Lake Victoria, the second largest freshwater lake in the world, after Lake Superior, isn’t in a narrow fault basin, but it is related to the tectonic activity. It lies between the two big branches of the rift system, and formed when the uplifts to east or west dammed rivers flowing into the central basin. Victoria is a young lake, only about half a million years old or less, and it has dried up completely at least three times in its history, a reflection of changing climate conditions during the recent ice ages. Victoria is a shallow lake, less than 300 feet deep. In contrast, the deep troughs of the rift system hold some of the deepest lakes in the world. Lake Tanganyika, for example, reaches a maximum depth of more than 4,800 feet, and holds about 18% of all the fresh water on earth. Volcanic activity continues in the region related to the rift process, including Mt. Kenya and Mt. Kilimanjaro, and the active volcanoes of Ethiopia and the
