Limnic Eruption - Causes

Causes

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. This CO₂ may come from volcanic gas emitted from under the lake or from decomposition of organic material. Before a lake is saturated, it behaves like an unopened carbonated beverage (soft drink): the CO₂ is dissolved in the water. In both the lake and the soft drink, CO₂ dissolves much more readily at higher pressure. This is why bubbles in a can of soda form only after the can is opened; the pressure is released and the CO₂ comes out of solution. In the case of lakes, the bottom is at a much higher pressure; the deeper it is, the higher the pressure at the bottom. This means that huge amounts of CO₂ can be dissolved in large, deep lakes. Also, CO₂ dissolves more readily in cooler water, such as that at the bottom of a lake. A small rise in water temperature can lead to the release of a large amount of CO₂.

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 rain 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 (buoyancy), 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, temperate 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 CO₂ 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 CO₂. So only in deep, stable, tropical, volcanic lakes such as Lake Nyos are limnic eruptions possible.