Apologies as this should have been publishes yesterday.
Lake Nyos, Cameroon 1986
Volcanism takes many shapes and forms, not all volcanoes eruption violently and explosively like Mount St Helens in 1980 neither do they all have a constant flow basaltic lava flowing from their vents such as Kilauea. Degassing can be constant at active volcanoes, poisonous gasses such as carbon dioxide and sulfur dioxide can seep out through the rocks even when all seems quiet. This can have devastating consequences on local population and wild life, but very few times has it been quiet so fatal as the summer on 1986 in Cameroon.
Lake Nyos is a crater lake which lays in the Oku Volcanic Field at the northern end of the Cameroon Volcanic Line. This area is known to degas large quantities of varying substances most of which dissipates in to the atmosphere. However, water is able to dissolve and store quite high quantities on CO2, especially the cold bottom waters of a water body such as Lake Nyos.
On 21st of August 1986 at approximately 9.30pm local time, a cloud rich in CO2 swept the surrounding area from the lake. Over 1700 people were killed as well as thousands of cattle and other wildlife. Air has to be just 15% CO2 for people to loose consciousness and have difficulty breathing; the cloud is believed to have had a much higher concentration then that. People simply fell asleep never to wake up, dying of asphyxiation.
There are several theories to the exact cause of the gas cloud. Firstly was the idea of a ‘carbon dioxide eruption’ from the volcanic system; it literally burped out great amounts of the gas in one go, so much so that it couldn’t be held in the water and seeped in to the atmosphere and down to the surrounding villages. The second theory was based on an event two years previous in the August of 1984 at Lake Monoun. A landslide stirred the water of the lake causing ascent of the carbon rich bottom waters. No longer under the pressure of the depths the CO2 began to exsolve (bubble out of solution) and was release in mass in to the air. Unlike at Nyos this was a smaller incident only claiming 34 lives but it gave scientific insight to what occurred two years later. There were no signs of any landslide on the banks of Nyos in August on 86′ however there had been heavy rainfall. Rain waters would have been much colder, and therefore denser, than the lake water causing a convective overturn. Again as bottom waters rose the dissolved gases would have been released with catastrophic effect.
What ever the cause, a painful lesson was learned and in 2001 a large polyethylene pipe was placed in to Lake Nyos, with a second being put in place in 2011, to siphon water continuously from the lower layers to the surface so there is never an excessive build up of CO2.
1. http://science.howstuffworks.com/environmental/earth/geophysics/lake-nyos1.htm accessed 20/08/14
2. http://www.emigennis.com/2014/04/06/lake-nyos-disaster-reference-photos/ accessed 20/08/14