saturation and degassing limit.
at work at Lake Kivu.
is the phenomenon of the formation of gas bubbles in a liquid which has become
saturated. The saturation limit of a gas in a liquid depends on two factors: the
total quantity of dissolved gas and the pressure of the liquid (that is to say
the depth, in the case of a lake).
The formation of bubbles can take place
either by increasing the quantity of dissolved gas or by diminishing the pressure
(that is to say, depth). If one of these conditions is fulfilled, and if the level
of saturation is reached, tiny bubbles of gas form in the water which then rise
to the surface. These little bubbles carry with them liquid already saturated
in gas. New bubbles appear in this liquid. On their way up, the size of the bubbles
increases. A chain reaction is set in motion. The ex-solution, once begun, will
rapidly develop in an 'avalanche' process.
If the phenomenon is not controlled,
a veritable explosion of a mixture of water and gas will occur at the surface.
This is precisely the phenomenon which can be observed on opening a bottle of
champagne or of lemonade : by lowering the pressure inside the bottle, bubbles
of dissolved gas form suddenly and, taking liquid with them, rise to the surface.
a huge scale, this is also the phenomenon which produced the catastrophe of Lake
Nyos (Cameroon) on the night of the 21st August, 1986. The saturation limit was
surpassed in the deep waters of the lake, the ex-solution process was triggered
off and developed in all the lake waters in a chain reaction. A gas explosion
threw up a column of water over 80 m high. There was a violent overturning of
all the lake water. Following this, the enormous quantity of carbonic gas thus
liberated, being denser than air, 'flowed' through the neighbouring valleys, suffocating
all forms of life as far as 30 kms from the lake. I800 people thus perished because
of this catastrophe.