Lakes are generally thought of as a peaceful location to relax next to and admire the fish
or other wildlife.
Yet, in a select part of the world, a very small number of lakes can be quite dangerous,
via producing rare but absolutely catastrophic disasters.
One such disaster occurred during a peaceful summer evening in the rainforest of Cameroon
in Africa, as it was 9:30 pm local time and the date was August 21st, 1986.
All of a sudden, loud rumbling was heard, causing confused people to awake and venture
outside.
Less than a minute later, these people who went to investigate fell to the ground, lifeless.
As a 50 meter or 164 foot tall cloud of lethal colorless gas advanced across the landscape,
it killed all humans and animals in its path.
This killed almost everyone in a 10 kilometer or 6.2 mile radius with fatalities among people
and livestock occurring up to 25 kilometers or 15.5 miles distant of Lake Nyos, totaling
more than 1,700 people.
What had just occurred was not an act of war or even human caused, but rather a fairly
rare type of natural disaster known as a limnic eruption, with the colorless gas being massive
amounts of carbon dioxide.
Since the gas cloud contained carbon dioxide concentrations well beyond 30% in the air
it flowed into, it caused unconsciousness and death which was immediate.
This limnic eruption caused the catastrophic release of 1.6 million metric tons of carbon
dioxide gas in a short timespan from Lake Nyos, following a similar disaster about 2
years earlier at Lake Monoun which released 200,000 metric tons of carbon dioxide gas.
To the current understanding of the science of limnic eruptions, four factors are needed
for a lake to be theoretically capable of producing a limnic eruption.
The first factor is that the area which the lake resides in must have the air temperature
stay about the same year round.
This means the disaster is most likely to occur in consistently hot or humid climates
such as rainforests that are fairly close to the equator, with all 7 documented limnic
eruptions occurring within a range of latitudes between 8 north and 8 south.
With this being said, it is not unreasonable to assume that limnic eruptions could theoretically
occur in a larger range of latitudes.
The second factor is that a limnic eruption can only occur at a lake which is meromictic,
meaning that it has two or more layers which do not intermix.
Third, although limnic eruptions could originate at shallower lakes, all known lakes which
have produced limnic eruptions are quite deep, being at least 91 meters or 300 feet deep
at their deepest point.
The most important and fourth factor for a limnic eruption to occur is that a persistent
carbon dioxide source is needed which emits large volumes of gas into the body of water
which dissolves, and so far this has only been known to occur at lakes above or adjacent
to specific volcanoes or volcanically active regions such as northern Lake Kivu in Africa.
Lake Nyos and Lake Monoun are both volcanic crater lakes formed through explosive eruptions
in the currently dormant Oku volcanic field.
This volcanic field encompasses a wide stretch of the nation of Cameroon, and is one of a
number of volcanoes in the so-called Cameroon Line which appears to be a broad area of crustal
weakness where magma can easily intrude into and either remain for long periods of time
or erupt.
In the case of Lake Nyos, magma has existed at a depth of approximately 4 miles or 6 kilometers
for at least 12,300 years as that marks the date of an explosive eruption which formed
the overlying crater.
Since then, approximately 5,000 metric tons of carbon dioxide gas released from the underlying
magma chamber has been input into Lake Nyos every year.
This gas largely remained at depth in the lower section of the lake, not interacting
with higher carbon dioxide poor layers.
Eventually, since a sufficient amount of dissolved carbon dioxide was located in the lake, a
limnic eruption became possible and all it needed was for a trigger to occur.
Such a trigger could be a volcanic eruption, an earthquake, or another possible reason
such as a landslide.
In the case of Lake Nyos the trigger was probably a landslide.
This caused the carbon dioxide dissolved in the bottom of the lake to rapidly rise to
the surface much like rapidly shaking a carbonated beverage, causing a runaway reaction where
much of the carbon dioxide in the lake escaped in a short period of time.
This created a large wave in the lake due to the displaced water, and since the carbon
dioxide is denser than air it soonafter advanced downslope, killing all animals and people
the invisible cloud of gas encountered.
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