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
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.
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.
|Test of the de-gassing operation
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.
T., Taming Africa’s killer lake, Nature, V. 409, p. 554-555,
refer to: http://perso.wanadoo.fr/mhalb/nyos/index.htm
Steven Earle, 2000. Return to Earth Science News