What happened at Lake Nyos
on the night of the 21st of August 1986

 

The CO2 limnic eruption theory

Volcanoes are well known as producers of poisonous or asphyxiating gases and, in some instances, these gases kill people caught in the volcanic plumes : such was the case on Dieng Plateau in Central Java, Indonesia, where 149 people died in 1979, in the wake of a comparatively minor phreatic eruption - an eruption driven by the vaporisation of groundwater, without any ejection of magmatic material. The possibility that a phreatic eruption occurred through the Cameroonian lakes cannot be completely ruled out. However, in these cases, the culprit could well have been the lake itself : indeed, the possibility that the Monoun gas bursts originated from huge amounts of carbon dioxide dissolved in the deep layers of the lake was first investigated by H. Sigurdsson and his team, who concluded that a disturbance of unknown origin had upset the density stratification of the water column, triggering an overturn of the lake and the subsequent release of carbon dioxide. Being denser than air, CO2 flows over the ground surface, asphyxiating people unfortunately present in the gas cloud. 

Most probably, the same explanation holds for lake Nyos but, owing to the much larger size of the latter lake and the topography of the surroundings (the perched at lake, c. 1,000 m , is drained by deep and long valleys, sheltering several villages), the death toll was much higher, including the last victim stricken some 27 km downstream from the lake. 

The carbon dioxide which erupted from the lake water is undoubtedly of volcanic origin, continuously seeping through the sediments of the lake bottom, most probably as a cluster of warm springs of CO2-bearing water. Due to the high solubility of carbon dioxide in water, a lake can dissolve a volume of CO2 more than five times its water volume. The stability of such a "time bomb" stems from the fact that CO2-rich water (e.g., soda water) is denser than pure water, as long as gas bubbles do not nucleate. The horizontal layering of the water column is due to the differential diffusion of CO2 and heat but, contrary to salt (which stabilises the thermohaline stratification of the oceans), carbon dioxide has a solubility that is limited by temperature, making the stratification intrinsically unstable. Thus, there is even no need of an external trigger (landslide, earthquake or heavy rain) to upset the stratification of the lake.

 

It is probable that the depths of the lake are fed by hydrothermal springs saturated in carbonic gas. Because of the high density stratification these waters do not mix with upper levels of water and accumulate under the level of the high density pressure gradient (cf. figure). The concentration in dissolved gas increases and reaches the limit of solubility  (oversaturation of the water).
Once CO2 bubbles nucleate within a saturated layer of the lake water, they rise and grow, attracting in their wake deeper water available for ex-solution, feeding the chain reaction process : the entire lake overturns through an ascending column of rising and expanding bubbles. 

The term "limnic eruption" was first coined by J.C. Sabroux at the UNESCO Conference on the lake Nyos disaster, in March 1987 at Yaounde, in order to take into account the analogy with volcanic eruption, also powered by gas bubbles ascending and expanding in a liquid (the magma). The limnic eruption, the mechanism of which was further refined by K. Tietze, explains satisfactorily the sequence of events and consequences of the Monoun and Nyos disasters, and suggests a possible mitigation of this new geological hazard.