Divergent margins typically begin on continental crust but quickly wind up on oceanic crust. There are several distinct stages in these margins. The process of pulling crust apart is called Rifting. The initiation of rifting appears to involve the formation of triple junctions. Mantle plumes strike the underside of the plates, leaving a hole with three cracks emanating from it at 120ΓΈ angles. Two of the cracks on the Earth will become active rift zones and mature divergent margins that connect with other triple junctions, while the third crack will start to rift but eventually fail. The failed crack is called an aulocogen. The best example of a triple junction is at the southern tip of the Arabian crustal plate. The Gulf of Aden and Red Sea represent the two active cracks, and the East African rift system is the aulocogen.
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