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.
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 Mountains of the Moon in Congo. Over time, the rifting has been sporadic. After the initial pulse of rifting and flood basalt eruption in the Oligocene, the faulting and real rift formation began in early Miocene time, around 22 million years ago or so. There was a period of several million years without too much going on, and the volcanic activity and earthquakes in the region today began in earnest about 5 million years ago or thereabouts.
The present rate of extension, about 6 millimeters per year, is slow, compared to typical oceanic crustal spreading rates, which are more like 20 millimeters a year, two centimeters. The slow rate is probably at least partially related to the strength and thickness of continental crust – it is more brittle, and harder to move than oceanic crust. But, at this rate, we should have a narrow ocean similar to the Red Sea separating the two parts of Africa by about 10 million years from now. The dismemberment of Gondwana continues, at least on this side. But to the north, Gondwana – India, Arabia, and North Africa, are more or less in a state of collision with Eurasia. All these things go on simultaneously.
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Today we have the anniversaries of two significant earthquakes. On December 16, 1920, a quake hit Haiyuan County in Gansu Province, central China. The death toll estimate, 200,000, has been increased by modern estimates to almost 275,000, making it one of the most deadly earthquakes in human history. Its magnitude has been given variously from 7.8 to 8.5, but whatever it was, it shook the earth enough so that seiches – basically, a sloshing of the water in a relatively enclosed body, such as a lake – were recorded in the fjords of Norway. The location was along the Gansu thrust fault, a major fault where one of the continental blocks of North China is being pushed over the rocks to the south. Or maybe it’s better to think of it as the rocks to the south being pushed under the rocks of North China, because this quake is a result of the ongoing collision between India and Eurasia.
The second big earthquake on this day, December 16, was in 1811, at New Madrid, Missouri. It was the first of three quakes over a two-month period there that had magnitudes of about 7.5. They are among the largest historic earthquakes ever in North America. There were few deaths because the region was so sparsely populated, but the sequence of three quakes resulted in the formation of Reelfoot Lake in Tennessee, and the Mississippi River temporarily flowed backwards as a result of the forces. The December quake was strong enough to awaken people in New York City and to damage buildings in Cincinnati, Ohio. There’s a nice book chronicling these quakes, titled When The Mississippi Ran Backwards, by Jay Feldman (2012, Free Press Publishing).
—Richard I. Gibson
East African Rift
Map from Digital Tectonic Activity Map of the Earth (NASA) with annotations by Gibson.