Lake Nyos disaster
The Lake Nyos disaster occurred on 21 August 1986, when a limnic eruption at Lake Nyos, in northwestern Cameroon, produced a large cloud of carbon dioxide (CO2), which descended onto nearby villages, killing 1,700 people and 3,500 livestock.
The eruption triggered the sudden release of about 100,000–300,000 tons (some sources state as much as 1.6 million tons) of CO2. This gas cloud rose at nearly 100 kilometres per hour (62 mph) and spilled over the northern lip of the lake. It then rushed down two valleys, branching off to the north, displacing all the air and suffocating people and livestock within 25 kilometres (16 mi) of the lake.
A degassing system has since been installed at the lake, with the aim of reducing the concentration of CO2 in deep waters and therefore the risk of further eruptions.
It is not known what triggered the catastrophic outgassing. Most geologists suspect a landslide, but some believe that a small volcanic eruption may have occurred on the bed of the lake. A third possibility is that cool rainwater falling on one side of the lake triggered the overturn. Others still believe there was a small earthquake, but as witnesses did not report feeling any tremors on the morning of the disaster, this hypothesis is unlikely. Whatever the cause, the event resulted in the rapid mixing of the supersaturated deep water with the upper layers of the lake, where the reduced pressure allowed the stored CO2 to effervesce out of solution.
It is believed that about 1.2 cubic kilometres (0.29 cu mi) of gas was released. The normally blue waters of the lake turned a deep red after the outgassing, due to iron-rich water from the deep rising to the surface and being oxidised by the air. The level of the lake dropped by about a metre and trees near the lake were knocked down.
Scientists concluded from evidence that a 100 m (330 ft) column of water and foam formed at the surface of the lake, spawning a wave of at least 25 metres (82 ft) that swept the shore on one side.
Carbon dioxide, being about 1.5 times denser than air, caused the cloud to "hug" the ground and move down the valleys, where various villages were located. The mass was about 50 metres (160 ft) thick, and travelled downward at a speed of 20–50 kilometres per hour (12–31 mph). For roughly 23 kilometres (14 mi), the gas cloud was concentrated enough to suffocate many people in their sleep in the villages of Nyos, Kam, Cha, and Subum. About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gas cloud.
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