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Limnic eruption

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From Wikipedia, the free encyclopedia

Type of natural disaster

Lake Nyos, the site of a limnic eruption in 1986

A limnic eruption , also known as a lake overturn , is an extremely rare type of natural hazard in which dissolved carbon dioxide (CO2) suddenly erupts from deep lake waters, forming a gas cloud capable of asphyxiating wildlife, livestock, and humans. Scientists believe earthquakes, volcanic activity, and other explosive events can serve as triggers for limnic eruptions as the rising CO2 ejects water from the lake. Lakes in which such activity occurs are referred to as limnically active lakes or exploding lakes . Some features of limnically active lakes include:

CO2-saturated incoming water

A cool lake bottom indicating an absence of direct volcanic heat in the lake waters.

An upper and lower thermal layer with differing CO2 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 hazard event, even though they can be indirectly linked to volcanic eruptions.[1]

Historical occurrences<br>[edit]

Lake Monoun

Lake Nyos

Locations of the two recorded limnic eruptions in modern history, Cameroon

Due to the largely invisible nature of the underlying cause (CO2 gas) behind limnic eruptions, it is difficult to determine to what extent, and when, eruptions have occurred in the past. The Roman historian Plutarch reports that in 406 BC, 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.[2] 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.[3]

In recent history, this phenomenon has been observed twice.[4] The first recorded limnic eruption occurred in Cameroon at Lake Monoun in 1984, causing asphyxiation and death of 37 people living nearby.[5] A second, deadlier eruption happened at neighboring Lake Nyos in 1986, releasing over 80 million m3 of CO2, killing around 1,700 people and 3,000 livestock, again by asphyxiation.[6]

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 CO2. 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 CO2 concentration scale by taking air samples of the affected region.[7]<br>Lake Monoun situated in the West Region of CameroonThe 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<br>[edit]

Diagram describing the occurrence of limnic eruption<br>For a lake to undergo a limnic eruption, the water must be nearly saturated with gas. CO2 was the primary component in the two observed cases, Lake Nyos and Lake Monoun. In Lake Kivu's case, scientists, including lake physicist Alfred Johny Wüest, were also concerned about the concentrations of methane.[8][9] CO2 may originate from volcanic gas emitted from under the lake, or from decomposition of organic material.

Before a lake becomes saturated, it behaves like an unopened carbonated soft drink: the CO2 is dissolved in the water. In both lakes and soft drinks, CO2 dissolves much more readily at higher pressure due to Henry's law. When the pressure is released, the CO2 comes out of solution as bubbles of gas, which rise to the surface. CO2 also dissolves more readily in cooler water, so very deep lakes can dissolve very large amounts of CO2 since pressure increases, and temperature decreases, with depth. A small increase in water temperature can lead to the release of a large amount of CO2.

Once a lake is saturated, it is very unstable but a trigger is needed to actually set off an eruption.[10] In the case of the 1986 Lake Nyos eruption, landslides were the suspected triggers, but a volcanic eruption, an earthquake, or even wind and rain storms can be potential triggers. Limnic eruptions can also be caused by gradual gas saturation at...