Geo-Joint: The Lake Nyos Limnic Eruption

The serenity of gazing out over calm waters has drawn people to live near oceans, lakes and rivers even long after it was necessary to be there for the proximity of the drinking supply. Of course, to this day, towns and cities are situated by water to take advantage of fishing, transportation possibilities , sports, and recreation. It’s just a nice place to be. But certain lakes have proven to be the source of ultimate demise for some nearby dwellers, and not because they drowned or drank polluted water.

Africa, stradling the equator, has an abundance of rainfall and a number of large and productive lakes. The basins of some of these lakes are volcanic in origin, with their bodies of water perched in bowls that sit higher than the surrounding countryside. One such volcanic lake is Lake Nyos in northwestern Cameroon, West Africa. It is not that old a body of water, having formed in a crater about 400 years ago. As with a few other such lakes, its volcanic beginnings influence more than its shape and location. Because it sits atop an ancient vent into the earth’s interior, gases from the magma can migrate up the conduit and dissolve in waters that form underground springs which feed the bottom of the lake. This gas transfer may go on for decades or centuries. The processes of an average lake usually involve a mixing of water levels due to winds or thermal differentials, but deep Lake Nyos was extremely stable and unmoving for a long period. This allowed vast quantities of dissolved carbon dioxide to saturate the cold lowest layers of the lake, called the hypolimnion. It is estimated that there were five gallons of CO2 in every gallon of Lake Nyos water.

All that gas just sat there at depth, under the pressure of the warmer overlying water, for many decades or longer. It is doubtful that anyone knew or was worried about it. But on August 21st, 1986, an event of unknown origin caused the waters to be disturbed. It may have been a small earthquake, a landslide or a rapid temperature shift resulting from cold precipitation. In any case, it tipped the balance, and like an explosion, the gas pressure was released. Water shot up 300 feet as 1.2 cubic kilometers of CO2 burst out in less than half a minute. It caused a tsunami of 5 to 20m, which destroyed some shoreline vegetation, but the most destructive aspect of this event, which is called a limnic eruption, was the gas. Heavier than air, it settled to the ground after the burst and suffocated everything it came down on. People and livestock died where they stood, unable to breathe. The blanket of CO2 flowed down the valleys that ran down the exterior of the volcano, killing almost everyone and every breathing thing in its way. The effects of the gas were felt as much as 15 miles away from Lake Nyos. As the cloud got thinner at a distance, those sleeping on the ground were silently suffocated while others who were upright still had air to breathe. Some were only knocked unconscious, but for many hours, and awoke to a ghastly scene. It must have seemed like a horrible spell had been cast over the countryside, with people and animals quietly dying by the hundreds, their lives snuffed out like all the cooking fires in each village. 1,746 people lost their lives, as did over 3,500 livestock. Only a few people managed to survive and tell the story.

Once the cause of the disaster was understood, a rather simple solution was devised in order to avoid a repetition of the event. Vertical pipes extending to the lake’s bottom were installed to allow the gaseous water a path to the surface, where it sprays up in a fountain. Releasing the pressure a little at a time is hoped to be sufficient to defuse the bomb. Lake Nyos is not a large lake—it is roughly 3/4 of a mile by a mile. The only other lake known to have experienced a limnic eruption is Lake Monoun, an even smaller lake in the same region. What happened at these two small lakes could be repeated at the only other known location where all the “right” conditions exist—and where the consequences would be far greater. Along the seismically and volcanically active Great Rift Valley in East Africa, Lake Kivu has similar gas-supplying vents at its bottom. Situated atop the border where western Rwanda meets eastern Democratic Republic of Congo, it is an asymmetrical shape very roughly 20 by 50 miles, making it around 1,000 times the surface area of Lake Nyos, and 3,000 times its water volume. Those waters are highly charged with gases, as much as 350 times the gas volume of the site of Cameroon’s nightmare. Lake Kivu’s waters contain both CO2 and methane, which results from biologic interaction with the CO2. Fossil studies of Lake Kivu’s bottom layers indicate that there have been huge extinction events about every 1,000 years. This suggests that limnic eruptions may occur cyclicly. The environs of Lake Kivu are both seismically active, and subject to incoming lava flow from volcanoes at higher elevation, so the triggers for gas disturbance are readily available.

Trying to get ahead of the volatile situation and to access valuable fuels, some methane removal pipes have been emplaced in Lake Kivu. Whether they can reduce the gas volume significantly remains to be seen, and frighteningly, the operations could themselves possibly set off a reaction. For the two million people who inhabit the area, which includes the city of Goma, the next occurence of a massive release of CO2 by whatever method, could be a disaster of enormous proportions.

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