Geology of Lac 'Assal

The hypersaline Lake Assal, with a water level 509 feet below sea level, is the lowest place in Africa. It is one of the few modern examples of a marine-water fed rift basin that is actively accumulating bedded evaporates (i.e. Halite and gypsum). The lake is completely continental with no surface connection to the ocean. The lake level has remained relatively unchanged over the last few centuries, and with the high evaporation rate, demonstrates that the lake receives a steady flow of seawater infiltrating through the highly fractured 10 km wide volcanic ridge that separates Lake Assal from the seawater source of the Ghoubatt al Kharab. This is an excellent example of a drawndown marine seepage basin fed through a volcanic ridge.

The volcanic ridge formed only within the last one million years, cutting off Lake Assal from the Ghoubatt al Kharab and the rest of the Gulf of Tadjoura. The volcanic ridge is the local source of geothermal activity observed in the Lake Assal area and heats the infiltrating seawater along the way from the sea to Lake Assal. Ardoukoba, on the southeastern shore of Lake Assal, is one of the recent volcanic cones in the area, still active in the area, and subsurface temperatures still very high.

Many actively flowing springs, locally called manda, are dotted along the eastern strandzone of the lake. Their geochemical characteristics are similar to seawater proportions. The northeastern part of the lake is characterized by stacked halite (sea salt) crusts and pans (halite plain) approximately 60 km wide by 20 to 80 meters thick. Gental wave action can also precipitate halite in small pearl-like balls or halite oolites.

In road cuts on the road down to Lake Assal, many dikes such as the one I am sitting on in this picture, can be observed cutting the volcanic country rock.