Deadly Lake Nyos in Cameroon

by Dale H. Easley
Department of Natural and Applied Sciences
University of Dubuque

8/17/2000

When you pop the top on a can of Coke, dissolved CO₂ comes bubbling out. Take another can of Coke scuba diving with you, and the farther down you dive, the less the Coke will fizz when opened. (Drinking it without getting salt water in your mouth is another challenge.) Likewise, if you leave it sitting out, it loses its fizz as most of the CO₂ gradually bubbles off---it equilibrates with atmospheric pressure. Lakes and streams naturally contain small amounts of CO₂, but you don't see them fizz because they are also equilibrated. However, under very rare conditions, such as occur at Lake Nyos in Cameroon¹, a sudden bubbling up of CO₂ can occur. In 1986, a massive release of CO₂ created a 250-foot-high geyser in the lake. The cloud of CO₂ released killed 1,700 people.

Lake Nyos, nearly 700 feet deep, is located in a volcanic crater. Volcanic activity deep below the crater releases CO₂, which rises through fractures to where it encounters ground water. There, much of it dissolves into the ground water. The ground water in turn flows into the bottom of Lake Nyos. In most lakes, there is fairly regular turnover in water, with winds and currents bringing bottom water toward the surface and taking surface water downward. However, in Lake Nyos, the bottom water is stagnant. The CO₂ continues to flow in with the ground water, but stays dissolved because of the pressure in the deep lake. Essentially, a bottom layer forms, rich in CO₂, ready like a Coke can to have its top popped.

It is unclear what set off the CO₂ eruption in 1986. Perhaps wind stirred the lake more than usual. Perhaps a landslide. Regardless, something caused some of the cold, CO₂-saturated water to move upwards. When it did, CO₂ began to bubble out of solution. As the bubbles formed, they rose toward the surface, pulling with them more cold water. As this additional water rose, more CO₂ bubbles formed and rose. And so on. What resulted was a huge, rapid upwelling of bottom water, and a massive amount of CO₂ was released suddenly.

CO₂ is about 50% heavier than our everyday mixture of air. And this CO₂ was cold, too. As it erupted from the lake, it stayed low to the ground, moving as fast as 45 m.p.h., flowing in a cloud for more than 10 miles. It was a suffocating cloud, killing nearly every animal in its path. A day later, it still lurked in low spots, still capable of killing a small girl who descended into a ravine. The area was subsequently evacuated. However, people have returned, attracted by the rich, volcanic soils, and tilapia, thriving fish introduced into the lake.

The buildup of CO₂ in the lake is once again extremely dangerous, according to the US Geological Survey. Pilot studies have shown that the lake can be easily degassed, perhaps for a little as $1 million, very little as these sorts of things go. Sticking several pipes into the depths of the lake, starting an upward movement of the water with a pump, and then letting the resulting bubbling lift the water, has been shown to work in slowly releasing the CO₂. Though politics has slowed the development of a full-scale project, a variety of recent events has helped push the degassing project back on track. Until the degassing is completed, Lake Nyos is prepared to kill again.

Footnotes:

¹ See the websites at http://www.biology.lsa.umich.edu/~gwk/research/nyos.html and http://www.eawag.ch/research_e/apec/aquaticphysics/lakenyos.html.
Also see The Killing Lakes, Scientific American, July, 2000, a major source for this essay.