Limnic Eruptions

The term “limnic” refers to bodies of freshwater, such as lakes and rivers. Limnic eruptions are gaseous expulsions that bubble up from the bottom of lakes. To date, there have been two limnic eruptions that we have knowledge about.

The first killed 37 people traveling the road adjacent to the shore of Lake Monoun on August 15, 1984; the second killed 1800 people sleeping in their villages near to Lake Nyos, and the 3500 animals they owned, on August 21, 1986. Both lakes are located in the nation of Cameroon in central Africa. How frequent such events may occur and what situations may trigger them has yet to be determined.

Unlike volcanic eruptions, limnic eruptions leave little evidence behind them and none that lasts for any significant period of time. Prior to in-depth investigation, it was thought the deaths were due to biological or chemical warfare; central Africa being regularly troubled by internecine warfare since independence from European colonial powers. Even the concept of limnic eruptions was unknown prior to these two natural disasters.

The word “limnic” comes from the Greek “limne” meaning lake. “Lake turnover” is the other term used to describe this natural event. Both Monoun and Nyos are crater lakes located in the Oku volcanic field of central Africa. Testing has determined that one other crater lake in this field, Lake Kivu, which lies between the Democratic Republic of the Congo and Rwanda, has the potential to produce a limnic eruption. Kivu is considerably larger than Monoun and Nyos, with concentrated populations near its shores. A limnic eruption from Lake Kivu could threaten up to two million people living nearby.

All three of these central African lakes are volcanic crater lakes, and investigation since these tragedies has found other volcanic crater lakes around the world with the potential to produce limnic eruptions. The Albano and Nemi crater lakes in the Alban Hills near Rome in Italy have the potential but are considered low risk, although monitoring will be continued. Lake Quilotoa in Ecuador is currently considered to present no risk of a dangerous limnic eruption, but has the characteristics necessary to develop the potential, so will also continue to be monitored.

All of the above lakes have formed in craters created by volcanic eruptions; whether lakes formed by other means might develop the potential for limnic eruptions is unclear at this time. How frequently limnic eruptions happen, for example in comparison to the frequency of volcanic eruptions, is also difficult to determine because they leave insufficient evidence for scientists to determine historic occurrences.

A limnic eruption occurs when concentrated amounts of dissolved carbon dioxide and/or methane gases at the bottom of deep lakes are disturbed, causing them to rise to the surface. The pressure exerted by water decreases the closer you get to the surface of the lake, enabling these substances to reconstitute as gases separate from the water they were previously dissolved in. Gases occupy a greater volume than liquids, generating a bubbling effect that can produce a large wave on the surface of the lake. The Lake Monoun eruption produced a wave estimated at five meters high that flattened vegetation up to 100 meters from the shoreline.

These gases enter the bottom of the lake through vents in the lake bed; the structure of volcanic fields makes this more likely, driving speculation that limnic eruptions are only feasible in deep crater lakes. They need to be deep to provide the pressure to dissolve the gases and keep them in the depths, otherwise they would simply leak to the surface and dissipate into the air in a harmless manner.

When the concentration of these gases nears saturation point and a traumatic geophysical event such as an earthquake or landslide causes disturbance, a limnic eruption may occur. Although we have only become aware of this type of natural disaster since the 1980s, there is no doubt that they have been occurring since the youth of our planet Earth.

If the gas involved is carbon dioxide, you may get a loud bang as the bulk breaches the surface of the lake and a large wave flooding the neighboring shoreline. Because carbon dioxide has a molecular mass heavier than air, the gas cloud will stay close to the surface of the lake and neighboring land while slowly dissipating. Any wind will blow this lethal, asphyxiating cloud in its prevailing direction, smothering any people or animals in its path.

When methane is involved, the tragedy could be even worse. Neither the Monoun nor Nyos events involved methane, although a limnic eruption from Lake Kivu could. Methane is an inflammable gas. If a cloud of methane gas from a limnic eruption met open flame in a human habitation, it could generate a massive explosion, igniting all inflammable materials over an extensive area.

The good news about this type of natural disaster is that now we know about them, we can stop them happening. Unlike volcanic eruptions, earthquakes and tidal waves, we have the technology to allow us to determine if a deep lake is susceptible, the ability to monitor circumstances if they do and the engineering capability to siphon of the gases when that proves necessary. Installing a pipe connecting the depths of a prone lake to the surface can release the build up of gases, mitigating the situation.