A limnic eruption, also known as a lake overturn, is a very rare type of natural disaster in which dissolved carbon dioxide () suddenly erupts from deep lake waters, forming a gas cloud capable of asphyxiating wildlife, livestock, and humans. A limnic eruption may also cause tsunamis or seiches as the rising displaces water. Scientists believe earthquakes, volcanic activity, and other explosive events can serve as triggers for limnic eruptions. Lakes in which such activity occurs are referred to as limnically active lakes or exploding lakes. Some features of limnically active lakes include: -saturated incoming water A cool lake bottom indicating an absence of direct volcanic heat with lake waters An upper and lower thermal layer with differing saturations Proximity to areas with volcanic activity
Investigations of the Lake Monoun and Lake Nyos casualties led scientists to classify limnic eruptions as a distinct type of disaster event, even though they can be indirectly linked to volcanic eruptions.
Historical occurrences
Due to the largely invisible nature of the underlying cause ( gas) behind limnic eruptions, it is difficult to determine to what extent eruptions have occurred in the past. The Roman historian Plutarch reports that in 406BC, Lake Albano surged over the surrounding hills, despite there being no rain nor tributaries flowing into the lake to account for the rise in water level. The ensuing flood destroyed fields and vineyards before eventually pouring into the sea. This event is thought to have been caused by volcanic gases, trapped in sediment at the bottom of the lake and gradually building up until suddenly releasing, causing the water to overflow.
In recent history, this phenomenon has been observed twice. The first recorded limnic eruption occurred in Cameroon at Lake Monoun in 1984, causing asphyxiation and death of 37 people living nearby. A second, deadlier eruption happened at neighboring Lake Nyos in 1986, releasing over 80 million m3 of , killing around 1,700 people and 3,000 livestock, again by asphyxiation.
A third lake, the much larger Lake Kivu, rests on the border between the Democratic Republic of the Congo and Rwanda, and contains massive amounts of dissolved . Sediment samples taken from the lake showed an event caused living creatures in the lake to go extinct around every 1,000 years, and caused nearby vegetation to be swept back into the lake. Limnic eruptions can be detected and quantified on a concentration scale by taking air samples of the affected region.The Messel pit fossil deposits of Messel, Germany, show evidence of a limnic eruption there in the early Eocene. Among the victims are perfectly preserved insects, frogs, turtles, crocodiles, birds, anteaters, insectivores, early primates, and paleotheres.
Causes
For a lake to undergo a limnic eruption, the water must be nearly saturated with gas. was the primary component in the two observed cases (Lake Nyos and Lake Monoun). In Lake Kivu, scientists including lake physicist Alfred Johny Wüest are also concerned about the concentrations of methane gas. may originate 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 (e.g., a soft drink): the is dissolved in the water. In both the lake and the soft drink, dissolves much more readily at higher pressure (Henry's law). This is why bubbles in a can of soda form only after the can is opened; when the pressure is released, the 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 is at the bottom. Therefore, huge amounts of can be dissolved in large, deep lakes. also dissolves more readily in cooler water, such as that found at a lake bottom. A small rise in water temperature can lead to the release of a large amount of .