In the Afar depression in northern Ethiopia, where Dallol is located, plate tectonics are slowly splitting the African Continent.

This separation of the African, Somalian and Arabian plates will eventually lead to the formation of a new ocean, but before this happen, the associated geological processes are producing exciting and beautiful geological sites like Dallol or the Erta Ale volcano. These sites are at the triangle between the Red Sea and the the two rift branches that join the East African Rift System close to the center of Ethiopia. The picture below shows a sketch of the distribution of plates in the region and its movement. The relative motion of the Arabian, African and Somalian plates produces a deformation of the crust, that gets thinned by normal faults and ductile deformations of the heated rocks.

The resulting valley, bound by parallel fault escarpments at both sides and with volcanic activity in different places in the valley axis, is called a rift. In the Afar depression, rifting is mainly controlled by the Africa-Arabia divergence, whereas in the East African Rift System extension is controlled by the Africa-Somalia motion.

Within the Afar triangle, The Danakil Depression is such a low area between the Ethiopian Plateau to the west and the Danakil Block to the east. Elevation ranges from around 200m in the north to 120m below sea level in its center, where Dallol is located. The depression is dominated by extensive salt deposits, from the evaporation of waters flooding the depression periodically from the
Red Sea, and by the northwest-southeast Axial Volcanic Range that includes the active Erta Ale and other volcanic shield complexes (Beyene & Abdelsalam, 2005). All these features are related to the active rifting process in the region.

To the north of the region, the Red Sea spreading ridge splits into two branches, the most active one (SSW trend) crossing the Gulf of Zula into the Northern Afar Rift near the Erta Ale range (Barberi & Varet, 1975, Chu & Gordon, 1998, Beyene & Abdelsalam, 2005). Lateral extension of the crust along this trend has thinned the crust to about 16km and lowered the rift elevation below sea level. The magmatic activity observed in the region is also a consequence of this stretching and thinning of the crust (Hayward & Ebinger, 1996). This process started almost 30 Ma ago during the Oligo-Miocene, when long, high angle border faults formed by distension of the rocks. Later, 16-7Ma, volcanoes started forming within the hinge of the rifts. From this time up today, spreading magmatic segments have developed along with dyke injections (Wolfenden et al, 2005). One of this injections, along with the massive presence of saline materials, seems to be responsible for the formation of Dallol.

Figure 1: Schematic view of the plates in the Indic coast of Africa. White arrows indicate plate movements. (Fermín Otálora, imagery from Google maps, TerraMetrics & GISrael)

To know more

• Web site of the Afar Rift Consortium
• “Continental break‐up: The East African perspective” article by Cynthia Ebinger
• Web site of the Institute of Geosciences and Earth Resources of Firence (Italy)

References

• Barberi, F & Varet, J. (1975) Recent volcanic units of Afar and their structural significance. In: A. Pilger and A. Rosler, Editors, Afar depression of Ethiopia, proceedings of an international symposium on the Afar region and rift related problems, Bad Bergzabren, Germany, 1974, vol. 1, E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany (1975), pp. 174–178.
• Beyene, A. & Abdelsalam, Mohamed. (2005). The tectonics of the Afar depression: a review and synthesis. J Afr Earth Sci. Journal of African Earth Sciences. 41. 41-59.
• Chu, D.& Gordon, R.G., (1998). Current plate motions across the Red Sea, Geophys. J. Int., 135, 313–328.
• Hayward, N. J., & C. J. Ebinger (1996). Variations in the long-axis segmentation of the Afar rift system, Tectonics, 15, 244–257.
• Wolfenden, E., Ebinger, C., Yirgu, G., Renne, P. & Kelley, S.P., (2005). Evolution of the southern Red Sea rift: birth of a magmatic margin, Bull.
Geol. Soc. Amer., 117, 846–864.