Africa's Deadly Lakes

Let us start with a definition: a limnic eruption is a rare event in which dissolved carbon dioxide (CO2) suddenly erupts from deep lake waters, forming a cloud of exsolved gas that can suffocate wildlife, livestock and humans. Scientists believe earthquakes or volcanic activity can trigger for such phenomenon.

To date, this phenomenon has been observed only twice. The first was in 1984 at Lake Monoun (Cameroon), causing the asphyxiation and death of 37 people living nearby[1]. The event was associated with a landslide from the eastern crater rim, which slumped into deep water. A second, decidedly deadlier eruption happened in 1986 at neighbouring Lake Nyos, a deep lake high on the flank of an inactive volcano. This time the eruption released over 80 million cubic meters of CO2 and killing around 1,746 people and 3,500 livestock, again by asphyxiation[2].

While data indicate a volcanic source of the carbon dioxide, the gas is probably gradually build-up and not able to rise to the surface because of thermal layers in the water. The carbon dioxide is hovering near saturation and an earthquake or volcanic activity may suddenly alter the status quo. The thermal layers of cold and warmer waters are disturbed and the dissolved carbon dioxide is released.

Lake Monoun and Lake Nyos are relatively small lakes but other much larger density-stratified equatorial lakes might potentially harbour much more danger. Lake Kivu in east Africa, has a methane and carbon dioxide gas content that is higher by two to four orders of magnitude than that of the Cameroon lakes.

 

A gas burst from Lake Kivu in Rwanda - with two million people living nearby - would form a carbon dioxide and methane cloud up to 340 cubic kilometers in volume and expansion of the exsolving gas from deep water to atmospheric pressure would correspond to an energy release equivalent to 8 megatons of explosive[3]. Which is quite a large explosion.

[1] Sigurdsson et al: Origin of the lethal gas burst from Lake Monoun, Cameroun in Journal of Volcanology and Geothermal Research – 1987
[2] Kling et al: The 1986 Lake Nyos Gas Disaster in Cameroon, West Africa in Science – 1987
[3] Sigurdsson: Gas bursts from Cameroon crater lakes: a new natural hazard in Disasters - 1988