Deadly Lake Nyos in Cameroon
by Dale H. Easley
Department of Natural and Applied Sciences
University of Dubuque
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.
1 See the websites
Also see The Killing Lakes, Scientific American, July, 2000, a major
source for this essay.
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On 25 Aug 2000, 10:48.