Degassing Lake Nyos

In the middle of an August night in 1986 in the west-African country of Cameroon a misty cloud of carbon dioxide bubbled out of a lake and swept silently down the surrounding valleys - thousands of animals and 1700 people died, many in their sleep. 

A chain of volcanoes extends in a straight northeasterly line from Annobon Island in the Atlantic Ocean into the western part of Cameroon.  It is thought that this feature is related to a rift which was first activated during the initial formation of the Atlantic Ocean (although the existence of a mantle plume has not been ruled out).  The volcano at Lake Nyos is now extinct, although the Mt. Cameroon volcano, near to the coast, 400 km to the southwest, is still active.

Lake Nyos is a few square kilometres in area, and is around 200 m deep.  It is situated in the crater formed from the collapse of the pipe feeding a now extinct volcano. The lake is

 

compositionally stratified, with fresh water in the upper 50 m and heavier sodium and carbon dioxide rich water below that.  The water below 180 m is particularly rich in sodium and carbon dioxide.  Most of the sodium and carbon dioxide come from numerous sodium-bicarbonate bearing springs - derived from an underlying magma chamber - feeding into the bottom of the lake. 

In August of 1986 some event – perhaps a mudslide, heavy rain or wind blowing across the lake – caused the water column to be disturbed.  Some of the deep carbon dioxide rich water moved towards surface where it was subjected to lower pressure.  The dissolved carbon dioxide quickly converted to carbon dioxide gas and rushed to the surface starting a chain reaction of degassing the deeper water.  A huge cloud of carbon dioxide spilled over the lake’s outlet and down into the surrounding valleys.

 

Lake Nyos (and nearby Lake Monoun where a similar but less disastrous event occurred in 1984) are now being monitored closely.  Carbon dioxide levels are continuing to build and it is estimated that there is now at least as much carbon dioxide as there was prior to the 1986 limnic eruption.


In 1995 an international team of scientists and engineers (mostly from France, the UK and the US) tested a procedure to degas the lower parts of Lakes Nyos and Monoun in a controlled way, and a team is now in the region to begin the project in earnest.  The procedure involves lowering a strong polyethylene pipe to the lake bottom.  Some water is pumped out at the top, and as the deep water rises through the pipe the carbon dioxide starts to bubble out. The gas and water then become buoyant and suck more water in at the bottom in a self-sustaining process.

 

Test of the de-gassing operation in 1995

Some scientists are worried that the procedure could get out of control and cause a repeat of the 1986 disaster, however it will be carefully monitored, and the process can be shut down quickly.  Another potential problem is that volcanic rock forming the natural dam which holds water in the lake is weak. If the dam should fail the upper 40 metres of water would spill out and this would lead to an immediate limnic eruption and a major flood which could extend all the way into Nigeria.  The area around the lake was evacuated after 1986, but people are starting to move back to take advantage of the good grazing land.

Reference

Clarke, T., Taming Africa’s killer lake, Nature, V. 409, p. 554-555, February 2001.

Also refer to: http://perso.wanadoo.fr/mhalb/nyos/index.htm

http://www.biology.lsa.umich.edu/~gwk/research/nyos.html

Steven Earle, 2000. Return to Earth Science News