Divergent boundary in an area of continental crust
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Tectonic activity splits continental crust much in the same way it does along mid-ocean ridges.
The East African Rift Valley is an example of a divergent boundary in an area of continental crust. It is one the geologic wonders of the world, a place where the earth’s tectonic forces are presently trying to create new plates by splitting apart old ones.
Watch this simple video on the formation of a rift valley.
The East African Rift Valley formed around 25 to 30 million years ago where the Somalian and Nubian plates are pulling away from the Arabian Plate. The eastern branch of the rift passes through Ethiopia and Kenya, and the western branch forms a giant arc from Uganda to Malawi. The valley, which actually consists of two broadly parallel rifts, extends for 4000 km from Mozambique to the Red Sea.
When a divergent boundary occurs beneath a thick continental plate, the pull-apart is not strong enough to create a clean, single break through the thick plate material. Here the thick continental plate breaks into a rift-shaped structure. As the two plates pull apart, normal faults (breaks) develop on both sides of the rift (valley) and the central floor of the valley sinks lower.
Early in the rift-forming process, streams and rivers will flow into the sinking rift valley to form lakes. In East Africa, the rift valley is still above sea level but occupied by lakes at several locations.
The area experiences volcanic activity, suggesting that the crust has been weakened and thinned by tension, resulting in rising magma escaping onto the surface at volcanoes such as Mt Kilimanjaro.
As the rift grows deeper it might drop below sea level allowing ocean waters to flow in. This will produce a narrow, shallow sea within the rift. This rift can then grow deeper and wider.
If rifting continues a new ocean basin could be produced. The Red Sea is an example of a more completely developed rift. There the plates have fully separated and the central rift valley has dropped below sea level.