Map showing the location of Lake Nyos

The Lake Nyos tragedy was a disaster that occurred on August 21, 1986, when a cloud of carbon dioxide gas suddenly boiled at Lake Nyos, Cameroon, killing over 1700 people within a radius of 20 km. The gas killed humans and animals by suffocation.

Background

Lake Nyos is a deep volcanic crater lake, 5,900 feet (1,800 m) across and 682 feet (208 m) deep, that is thermally stratified, with layers of warm, less dense water near the surface floating on the colder, denser water layers near the lake's bottom. Over long periods, carbon dioxide gas seeping from underground lava dissolve into the cold water at the lake's bottom in great amounts. The amount of CO₂ entering the lake is estimated to be about 90 million kilogrammes annually.

The disaster

Over time, the bottom layers of the lake become supersaturated with CO₂. When this occurs, the lake becomes dangerously unstable, and an event such as an earthquake or landslide can trigger a catastrophic outgassing. This was the situation on August 21 1986.

Most geologists believe that a small landslide occurred on the steep wall of the lake, although some believe that a small volcanic eruption beneath the lake was the trigger. A third possibility is that cool rainwater falling on one side of the lake triggered the overturn. Whatever the cause, the event caused the rapid mixing of the supersaturated deep water with the upper layers of the lake, where the reduced pressure allowed the stored CO₂ to effervesce out of solution.

It is believed that up to a cubic kilometre of gas was released. Because CO₂ is denser than air, the gas flowed down two valleys in a layer tens of metres deep, displacing the air and suffocating all the people and animals. The normally blue waters of the lake turned a deep red after the outgassing, due to iron-rich water from the deep rising to the surface and being oxidised by the air. The level of the lake dropped by about a metre, representing the volume of gas released.

The outgassing probably also caused an overflow of the waters of the lake. Trees near the lake were knocked down.

Degassing the lake

Studies showed that at the rate CO₂ was entering the lake, outgassing events might occur as often as every 10-30 years. To prevent a future disaster in the region, a pipe was installed running from the surface anchored to a raft to allow the deeper areas of the lake to release their CO₂ to the surface in controlled small amounts. It is hoped this will reduce the maximum levels of CO₂ in the future, and prevent any possibility of the lake turning over. Degassing began in 2001 and is continuing steadily.

Other supersaturated lakes

Following the Lake Nyos tragedy, scientists investigated other African lakes to see if a similar phenomenon could happen elsewhere. Lake Kivu in Rwanda, 2000 times larger than Lake Nyos, was found also to be supersaturated, and geologists found evidence for outgassing events around the lake about every thousand years. The eruption of nearby Mount Nyiragongo in 2002 sent lava flowing into the lake, raising fears that a gas eruption could be triggered, but fortunately it was not.

References

• Cotel A (1999), A trigger mechanism for the Lake Nyos disaster, American Physical Society, Division of Fluid Dynamics Meeting, November 21-23, 1999
• Decker, R. and Decker, B. (1997) Volcanoes, 3rd edition, WH Freeman, New York.
• Sano Y., Kusakabe M., Hirabayashi J. et al (1990), Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst, Earth and Planetary Science Letters, v. 99, p. 303-314
• Sano Y., Wakita H., Ohsumi T., Kusakabe M. (1987), Helium isotope evidence for magmatic gases in Lake Nyos, Cameroon, Geophysical Research Letters, v. 14, p. 1039-1041

External links

• www.biology.lsa.umich.edu/~gwk/research/nyos.html
• www.smithsonianmag.si.edu/smithsonian/issues03/sep03/pdf/killer_lakes.pdf