Lake Assal is a small East African lake in central/east Djibouti. It lies within the Great Rift Valley. When the nineteenth century geographer John Walter Gregory first used the phrase Great Rift Valley he used it to describe a geological trench that extends from the Dead Sea in Israel through the Red Sea into East Africa and as far as the Indian Ocean on the coast of Mozambique. At one time the Great Rift was once thought to be caused by the movement of the African landmass away from that of Asia. Current thinking considers the situation to be more complex. In East Africa Lake Assad lies close to the boundary of three tectonic plates through which geologists believe the Ethiopian lowlands, Djiboiti and Somalia are tearing themselves from the African mainland and drifting, in the slowness of time, away from the Asian landmass.
Lake Assal lies at the heat of this geological activity. It lies at the heart of the Afar depression and is the lowest point on the African continent and the third lowest point on Earth after the Dead Sea the Sea of Galilee. The water level is 509 feet below sea level. Within the depression the Earth’s crust is thought to be particularly thin extending to a depth of just 15 kilometres compared to the usual 120 kilometres. The lake is surrounded by hills make of volcanic basalt. The nearby Ardoukoba Volcano last erupted in 1978 following an earthquake having been dormant for 3,000 years.
Lake Assal is though to be a crater lake lying within a volcanic hotspot. It is also one of the saltiest places on Earth. The lake is oval in shape and can be broadly divided into two sections. In the north west the dried up lake bed reveals extensive salt deposits. In the south east the lake is the second saltiest on Earth. The surface water has a salt concentration of 276.5 grams per litre which is 3% higher than that of the Dead Sea. A higher concentration only occurs in Don Juan Pond in the Antarctic.
Lake Assad has a high salt concentration because there is a limited influx of fresh water combined with a very high evaporation rate. The local climate is extremely hot with summer temperatures reaching 52 Celsius and winter temperatures averaging 34 Celsius. Drying winds cross the lake in summer and the average surface water temperature is 34 Celsuis. Although the lake is supplied by seasonal runoff from the hills the winter rains are intermittent and there is no year round freshwater supply. The lake is instead supplied by hot salt water springs. The spring water has the same composition as that of the Indian Ocean and ther is thought to be a conduit between the lake and the nearby Ghoubbet El Kharab bay.
Although Lake Assal has such a high salt content that it is practically lifeless the lake bed shows three distinct layers. A geological layer lying between the basalt bed and the salt crust contains fossils from a freshwater and saltwater period. The fossil record can be used to support one of two alterative theories of lake formation. The lake may have forward when a lava flow isolated it from Ghoubbet El Kharab bay which is itself forming in the process of East Africa seperatng from the mainland, or, it may have always been a distinct lake which became fed by saltwater which found its way through the aquifer when sea levels rose.