TUKUL 2: Geology and Volcanology

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Ethiopian Rift

The Ethiopian rift extends for 500 km in a North/North-East direction. The large Plio-Pleistocene calderas formed in the rift and the large volume of the eruption products, played an important structural role and determined an asymmetry such that the age and the style of the tectonic activity are different on the two sides of the rift. From North to South six main zones can be identified: the South-West of the Afar, the Northern zone, the central basin (Galla lakes), the Bilati basin, the bifurcation of the Amaro and the transition of the lower Ebâr.

The Awash Park
The Ethiopian rift is separated from the Kenyan one by a fault of about a hundred kilometers, interrupted by three main basins: the rift of the lower Narok, the symmetric tectonic graben of the lower Ebâr (Chew Bahir) and the Canjuli branch of the Ethiopian rift. An axial strip of faults and Upper Quaternary volcanoes, known as the Wonji strip (2 to 6 km wide) runs along almost the whole length of the Ethiopian rift. This active strip (earthquakes of 1966 and 1975) is the homologue of the axial tectonic graben of the Kenyan rift.

The Blue Nile waterfalls

Almost all the inactive volcanic centers and those that recently became  extinct in the Quaternary fall within the Wonji strip, where they tend to be placed on steps.
One of the most important is the giant complex of the Awasa caldera, a rhyolitic volcano with a diameter of  35 km, active during the Middle Pleistocene; some basaltic cones isolated within the caldera are related to the Wonji strip which runs through it.
The Ethiopian highlands are crossed by several fault systems whose chronological relations to the uplift and rift episodes are still not fully understood.

In Southern Ethiopia, 70-80 km West of the Ethiopian rift, a zone with faulted and dipping blocks, 60 to 100 km wide and with a North-East orientation, cuts the Ethiopian highland and continues in a North/North-East direction along the upper Omo Valley at least up to the Ghibbie basin. At a latitude of 7°N, the zone with East-West faults of the Gojeb is active. Another East-West tectonic line is that of the Addis Ababa-Ambo faults; it crosses the Ethiopian plateau at a latitude of 9°N and separates the basalts of the Abbay basin plateau from the series of the dipping blocks of the Ghibbie basin to the South. On the western edge of the rift, the Guraghe escarpment is reached by the tectonic graben of Butajira, where, during the Holocene, there were active faults and fissural basaltic effusions. This marginal tectonic graben corresponds to the Wonji strip, adjacent to the opposite eastern edge of the Ethiopian rift. To the East, it is separated from the widest part of the basement of the rift by a narrow horst where there are exceptional outcrops of rocks similar to those of the plateau. The tectonic gulf of Addis Ababa rises gently in a North-West direction, starting from  1,500 m at the basement of the rift up to 2,300 m at Addis Ababa. Some acid volcanic centers of the Plio-Pleistocene, some funnel-shaped cavities of volcanic origin (maare), and fissural basaltic flows cross the gulf in a North/North-East direction.

The downward movement of the rift started in the Miocene. The main tectonic activity in the Northern sector started about 15 Myr. The activity of the faults of the rift started again during the Upper Miocene creating a deeper and more symmetric graben.
Since then the graben has become smoother due to important volcanic fillings during the Pliocene and the Quaternary with frequent eruptions of the calderas covering the edges of the adjoining plateaus with thick  rhyolitic tuffs and ash fallout.
Later volcanic and sedimentary deposits now hide the calderas, but it is probable that the diameters of the largest ones were close to the width of the rift.

The caldera of Garibaldi Pass
The huge volume of acid rocks produced by these eruptions very likely contributed to the concomitant subsiding of the basement of the rift.

The Wonji Lake


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