The 2960 meter high Ol Doinyo Lengai rises in the north of Tanzania about 2° south of the equator and in the middle of the East African Rift Valley (map). At this continental junction a unique stratovolcano was (and still is) formed, which produces a very low viscous, sodium carbonate lava. Normal silicate melts have temperatures of about 1000°C, but the lava of Ol Doinyo Lengai is only half as hot: during our expedition in October 2000 we measured temperatures of 510°C. Only at night we could see a light red glow; otherwise the carbonate lava is black and reminds in its consistency of mud.
The minerals of the cooled lava react with the humidity of the air. Within a few days they turn white and disintegrate into a fine powder. Especially in strong winds the soda dust is very unpleasant. It penetrates every body opening and irritates lungs and eyes and grinds between the teeth while eating.
Of importance is the high concentration of alkaline earth metals and heavy metals - such as strontium and niobium - in the lava: these elements give hints to the place of origin of the magma. Volcanologists suspect it to be deep in the mantle; they assume that sodium carbonatite is formed from carbonated magma, but do not completely rule out the possibility that it may have formed from the partial melting of crustal rock. A new theory of American researchers is based on basalt as a trunk magma, which is typical for mid-oceanic ridges. It is only in the magma chamber that the sodium carbonate is differentiated from this trunk magma. However, the researchers have definitely excluded an older theory according to which lime sediments are melted during the process. Only about 5% of Ol Doinyo Lengai consists of this special type of lava; the remaining rocks are silicates, and among these the phonolites predominate. The Ol Doinyo Lengai is currently the only volcano in the world that produces this exotic lava species. In the past it was also extracted from volcanoes such as the Kaiserstuhl in the Upper Rhine Graben (also a continental rift system), Canada and China.
LARGE ERUPTIONS OF LENGAI
Large ash eruptions are known for the Ol Doinyo Lengai from 1917, 1926, 1940, 1966/67 and 2008; in 1966 a four kilometer high ash cloud was observed. In 1983 only small ash eruptions occurred, but in 2008 large ash eruptions occurred, with tephra rising up to 15 km high. The ash was deposited on the rift shoulder, on the crater highlands and in the Serengeti. A new cone with a crater diameter of 280 m was created on the crater platform.
Before these eruptions, activity was limited to a comparatively mild lava flow inside the crater. On the crater platform, chimney-like lava towers, the so-called hornitos, were formed; the rising lava collected in pools, reemerged from cracks, and often formed pahoehoe lava flows only a few centimeters thick. Aa lava flows occurred less frequently, but were usually more powerful. Lava spattering was not uncommon: The lava sprayed under pressure in small fountains from openings of the hornitos. The ejection distance of this Spattering was often limited to a few meters. Sodium carbonate lava is extremely rich in gas and foams up at the vent.
After the major eruption in 2008, Ol Doinyo Lengai returned to its "usual" activity. However, the sodium carbonate is now bubbling in the 130 meter deep crater, whose flanks are so steep that it is completely inaccessible. It will take years for the crater to fill to the point where you can reach its bottom.
The surroundings of Ol Doinyo Lengai are also dominated by sodium. Within sight is Lake Natron with its strongly alkaline water and its unique living world.
