What is lake over turn? Why does it happen?

Lake overturn, also referred to as limnic eruption or exploding lake, is a rare type of natural disaster in which carbon dioxide gas suddenly erupts from the deep lake waters, posing the threat of suffocation for those on the shores. CO2 saturation usually takes place due to volcanic activity or decomposition of organic matter. Such an eruption may also cause tsunamis in the lake as the rising CO2 displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption.

An easy way of understanding it is to equate it to a bottle of Coke. It is saturated with CO2 but as long as it is capped nothing happens. Uncapping it and causing it to shake will result in the contents gushing out- that is actually the CO2 escaping.

Lakes succeptible to overturning have the following traits:

- CO2-saturated incoming water
- A cool lake bottom indicating an absence of direct volcanic interaction with lake waters
- An upper and lower thermal layer with differing CO2 saturations
- Proximity to areas with volcanic activity.

Lake overturn to me is an annual phenomenon. To explain, you need to understand that the maximum density of water occurs at about 4°C. In the spring during ice melt, the water temperature is near zero, so it is less dense than the water below, but as it warms, it becomes denser, and eventually becomes more dense than the underlying water. When this occurs, the surface water sinks and mixes with the underlying water. This is called lake overturn and it is a good thing for lakes because it mixes oxygenated water from the surface with less oxygenated water at depth.

A limnic eruption, also referred to as a lake overturn or exploding lake, is a rare type of natural disaster in which suddenly erupts from deep lake water, posing the threat of suffocating wildlife, livestock and humans. Such an eruption may also cause tsunamis in the lake as the rising displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption.

Causes

For a limnic eruption to occur, the lake must be nearly saturated with a gas. In the two known cases, the major component was CO2. This CO2 may come from volcanic gas emitted under the lake, or from decomposition of organic material. Before a lake is saturated, it behaves as 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 when it is at a higher pressure. This is why bubbles in a can of soda only form after the drink is open; 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 temperature can lead to the release of a large amount of CO2.

Once the lake is saturated with CO2, it is at a critical and very unstable point, and 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, an explosion, 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. Then 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 also displaces water to form a tsunami.

There are several reasons this type of eruption is very rare. First of all, there has to be a source of the CO2, and so regions with no volcanic activity are not prone to this kind of eruption. Also, temperate lakes turn over each spring and fall, 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. A lake must be quite deep to have enough pressure to put enough CO2 into solution too. So only deep, stable, tropical, volcanic lakes such as Lake Nyos are prone to limnic eruptions. As for Lake Kivu, scientists are concerned about the concentrations of methane gas along with CO2 and the possibility of a volcanic eruption.