Credit: SPRINGER

Oldoinyo Lengai, Masai for 'Mountain of God', in the East African Rift Valley is a volcano with a difference. It is the only active volcano on Earth that expels molten carbonate rich in sodium and calcium instead of the more usual silica-rich lavas, and it also spews highly alkaline ash. Past explosive eruptions have been reported to have caused health problems to the local Masai people and their livestock as well as wildlife through contamination of the water sources in the area.

A phase of effusive eruption of Oldoinyo Lengai in 2006, spanning late March and early April, was the longest active period recorded so far. During this time, almost a million cubic metres of natrocarbonatite lava extruded from the volcano, allowing Matthieu Kervyn and colleagues to investigate the shallow magmatic system of this unique volcano (Bull. Volcanol. doi:10.1007/s00445-007-0190-x; 2008). They reconstructed the sequence of eruptive events by using satellite data and eyewitness accounts, and by characterizing the morphology of the various lava features in detail.

The main crater area of the volcano is marked by the presence of a number of rather curious-looking volcanic constructs called hornitos. This group of sharply conical features formed after 2001 as a result of localized, mildly explosive eruptions that piled up partially solidified lava fragments. Kervyn and colleagues suggest that during the 2006 eruption, lava first effused from the flanks of hornitos.

Withdrawal of lava from beneath the hornitos rendered them unstable and some collapsed. There were numerous phases of collapse at Oldoinyo Lengai, suggesting that lava was derived from shallow interconnected reservoirs rather than a single reservoir. Some of the lava filled up the southern part of the crater, whereas much of the rest flowed down the western flank of the volcano through a gully. Upon reaching gentler slopes, the lava flow had grown to a thickness of over 10 metres and a length of over a kilometre.

Analysis of the chemical composition of the lava flows revealed less than 4% of silica (by weight), a much lower proportion than the tens of percent found in lavas from most terrestrial volcanoes. Nevertheless, the morphology of the lava flows, as well as features within the crater such as the hornitos, are very similar to that of more typical volcanoes.

The region around the volcano is sparsely populated and the risk to the local population appears to be limited. However, the volcano and its vicinity are a tourist attraction. Effusive events and instability resulting from the collapse of hornitos can be dangerous to visitors. During the initial stages of the 2006 eruption, dust plumes due to collapse were incorrectly reported in the international media as ash plumes attributed to an explosive eruption. Such reports may scare off tourists that the region needs badly. Accurate information on the health risks at Oldoinyo Lengai is therefore all the more important.