For a limnic eruption to occur, the lake must be nearly saturated with gas.
In the two known cases, the major component was CO2, however, in Lake Kivu, scientists are concerned about the concentrations of methane gas as well.
The gas may come from
CO2 may come from volcanic gas emitted from under the lake or from decomposition of organic material.
Once the lake is saturated with CO2
It is very unstable. A trigger is all that is needed to set off an eruption. In the case of the 1986 eruption at Lake Nyos, landslides were the suspected triggers, but an actual volcanic eruption, an earthquake, or even wind and storms are other possible triggers. In any case, the trigger pushes some of the saturated water higher in the lake, where the pressure is insufficient to keep the CO2 in solution. Bubbles start forming and the water is lifted even higher in the lake, where even more of the CO2 comes out of solution. This process forms a column of gas. At this point the water at the bottom of this column is pulled up by suction, and it too loses its CO2 in a runaway process. This eruption pours CO2 into the air and can also displace water to form a tsunami.
There are several reasons this type of eruption is very rare.
First, there must be a source of the CO2, so only regions with volcanic activity are at risk.
Second, temperature lakes, such as North America's Great Lakes, turn over each spring and fall as a result of seasonal air temperature changes, mixing water from the bottom and top of the lake, so CO 2 that builds up at the bottom of the lake is brought to the top where the pressure is too low for it to stay in solution and it escapes into the atmosphere.
Finally, a lake must be quite deep to have enough pressure to dissolve large volumes of CO 2. So only in deep, stable, tropical, volcanic lakes such as Lake Nyos are limnic eruptions possible.