Alfred Wegener's original work on continental drift concentrated on the breakup of
Gondwanaland, showing how South America, Antarctica, India, Australia, and finally
Arabia broke off, leaving only Africa -- which was in the process of breaking up as well,
along the East African Rift Valley system. A microplate underyling the Sinai Peninsula
was also accepted by a few geologists.
This model fit the existing data for decades, and so modern models of plate tectonics
contain an African Plate with Africa at its center. The African Plate has been opening up
along the Rift Valley the way your thumb moves away from the rest of your hand. The plate is remarkable in that there is tectonic spreading around nearly all of its perimeter,
except the inactive boundary with the European Plate to the north.
Up until the late 1990's all plate boundaries had been deduced from earthquakes. South of
a point in Mozambique, there are no earthquakes associated with the Rift Valley, and so there
was no reason to suspect that the rift continued further south.
In the 1990's, Rice University geologist Richard Gordon, working with Dezhi Chu (now at
Exxon), and later James Lemaux II (Rice) and Jean-Yves Royer (IUE, now at BP) studied the changes in magnetic anomalies in the Indian Ocean
off South Africa and found evidence that the rift valley is just the northern, and most
active part of a boundary between two separate plates - the Nubian Plate to the west, and
the Somali Plate to the East.
The plates are grinding together like two immense gears, albeit very slowly. South of a
pivot point off the coast of South Africa, they are coming together at the rate of 2
millimeters per year, much slower than even the most sluggish spreading center.
Further research has revealed that the triple junction between the Nubian, Somalian, and
Antarctic Plates lies in the Andrew Bain Fracture Zone Complex southeast of South Africa,
part of the Southwest Indian Ridge. And so we must relegate the African Plate to the
status of a "former" tectonic plate, along with the Baltic, Farallon, Aluk, and Kula Plates.
"Geologists Find Motion Across Dispaaearing Plate Boundary", Rice University press
relelase, March 3, 1999
"Geologists Locate Boundary Between African Plates", Rice University press release, as
reported by the Australian Institute of Geoscientists