The Causes of Lake Overturn
The Carbon Dioxide Lakes are all volcanic lakes. Volcanic lakes are caps of water over the vents of active volcanoes. They need the correct balance between volcanic heat flux and atmospheric cooling, and rain and evaporation to exist. Fluid enters the lake from the vents below. The nature of the fluid entering the lake varies depending on the rock and magma below. Only in three lakes in Africa is the composition of the fluid sufficiently dominated by carbon dioxide that it can lead to lake overturn.
This diagram shows how fluids and gases such as carbon dioxide enter a volcanic lake.
Analysis of CO2 in Lake Nyos shows that it originates from magma below the earth's surface. Gas from the magma which is rich in carbon dioxide reaches the earth's surface and is dissolved in ground water. The CO2 rich ground water is discharged into the bottom waters of the lake by springs. Due to the solubility of carbon dioxide in water a lake can dissolve a volume of CO2 more than five times its water volume. The lake is stable despite the ever-increasing amounts of CO2 in its bottom waters due to the high pressures at the bottom of the lake from the weight of the water above, and the fact that surface waters and bottom waters do not mix.
The stability does not last as the solubility of CO2 is temperature dependent leading to different solubilities of CO2 at different depths. Water containing dissolved CO2 is denser than pure water leading to density stratification of the water column. If the layers of water in the lake are disturbed then the density layers will be disturbed triggering a chain reaction in which CO2 comes out of solution and bubbles of gas start to nucleate and rise to the surface, triggering the release of more carbon dioxide as they rise. Once the CO2 gas is released into the air water form on the surface of the gas, making it appear white. This has been termed a "limnic eruption". Limnonology is the study of rivers and lakes. The term limnic eruption is used to account for the similarity between the eruption caused by ascending gas bubbles in a lake and volcanic eruptions.