August 21, 1986 Eruption of Oku Volcanic Field

Lake Nyos. Deaths caused by release of large volume of CO2 from lake. More than 300 survivors hospitalized. (reference #9675)

With its reported 1700 fatalities, the 21 August 1986 limnic eruption of Lake Nyos is, at the time of writing, the most deadly case of this type of natural event. The lake is located on the flank of an inactive volcano in the Oku volcanic plain, northwest region of Cameroon, and has a maximum length of 2.0 km and a maximum depth of 207 m. Its estimated volume of water is 0.15 km3. The event was triggered by a sudden release of about 100 000–300 000 m3 carbon dioxide (Socolow, 2005). The carbon-dioxide-rich cloud was expelled rapidly from the floor at the southern end of Lake Nyos with an estimated speed of about 100 km per hour. Survivors from the valley recalled hearing a rumble that lasted about 20 seconds and seeing a fountain of water and foam that rose about 90m above the lake’s surface. A wave at least 24m tall scoured the rocky lake shore (Brown, 2000). A large amount of gas escaped from the deep waters which resulted in the lake surface level dropping by 1 m. The cloud quickly enveloped houses within the crater that were 120m from the shoreline of the lake. Due to the density of carbon dioxide being greater than that of air, the gaseous mass hugged the ground surface and descended down valleys along the north side of the crater. The deadly cloud was about 50m thick and it advanced downslope at a rate of 20–50 km per hour. This deadly mist persisted in a concentrated form over a distance of 23 km, bringing sudden death to the villages of Nyos, Kam, Cha, and Subum. Today the lake also poses another type of threat, because the volcanic dyke along one side is weakening. If this natural dam breaks it will allow water to rush into downstream villages all the way into Nigeria and allowing much carbon dioxide to escape. (reference #13041)

In August 1986 the waters of Lake Nyos were saturated with CO, of volcanic origin. Late in the evening of 21 August a pulse of volcanic gas -- mainly CO, but containing some H2S --was released above a volcanic vent in the north-east corner of the lake. The stream of bubbles rising to the surface brought up bottom waters highly charged with CO, which exolved, increasing the gas flow and hence the flow of water to the surface. At the surface the release of gas transformed the accompanying water into a fine mist and sent a wave of water crashing across the lake. The aerosol of water and CO, mixed with a trace of H,S swept down the valleys to the north of the lake through Nyos and on to Subum, Cha and Fang, leaving a terrible toll of injury and death in its wake. We estimate that about 200,000 tonnes of water were lost from the lake. If this was mainly bottom water saturated with CO2, then it would have released about 6,000 tonnes of gas, which at atmospheric temperatures and pressures would be a volume of about three million cubic metres. (reference #13036)