OREGON STATE UNIVERSITY

What is known about the volcanoes of Cameroon?

Category: 
Africa

 

Volcanoes in Cameroon are part of the Cameroon line, a chain of volcanoes extending from Annobon Island in the Atlantic Ocean northeastward through Cameroon. The oldest rocks have been dated at 70 million years old. Nine volcanoes along the line are active. A fissure eruption occurred at Mt. Cameroon in 1982.

Volcanism along the Cameroon line is related to rifting – where a continent breaks into two pieces. About 110 million years ago a giant rift broke apart what became Africa and South America and the South Atlantic Ocean began to form. A smaller rift formed within the African continent. This older rift, called the Benue Trough, is north of and parallel to the Cameroon line. About 80 million years ago, during a reorganization of plate boundaries, the African plate rotated counter-clockwise. Then a new rift formed that failed to split Africa but apparently did form conduits that allowed magma to ultimately reach the surface and form the volcanoes of the Cameroon line.

In 1986, a cloud of gas burst out of Lake Nyos because a landslide disturbed the stratification with the lake. The stratification (layering) is caused by different amounts of CO2 dissolved in the water. The layers are stable (they don’t mix or changes position). The alternative mechanism, that the burst was caused by a phreatic eruption (involving lava into the lake), is not generally accepted.

Dr. Niels Oskarsson proposed reducing the amount of CO2 in the lake by continuously draining the bottom waters from the lake. Water from rainfall would displace an equal volume of water from the bottom of the lake. Over time this would establish low CO2 levels in the lake.

The 1987 conference recommended continuous telemetered monitoring of the deep water temperature, pH, conductivity, and alkalinity of the lake. Unfortunately, no funding or plans for implementation were developed (in 1987).  

There is a pretty good consensus as to what happened at Lake Nyos (in 1986, by the way). A few folks disagree on the details, but the main event was the sudden release of a large cloud of carbon dioxide gas from the lake. Lake Nyos is a water-filled throat of an old volcano and it is deep and funnel-shaped. Although no longer erupting, there is still gas being released by the old plumbing system under the lake. Carbon dioxide gas was released directly into the deepest waters of the lake, where it could remain in solution (the way that carbon dioxide stays in solution in an un-opened soda or beer). In this situation the lake could build up a large amount of carbon dioxide dissolved in the deeper water. This was a stable situation. The carbon-dioxide charged water was slightly denser than the normal water in the upper levels of the lake, and the weight of the overlying water kept the carbon dioxide in solution in the deeper parts of the lake.

However, nature decided to unbalance the situation. This is where the disagreement among volcanologists comes in. It is agreed that somehow some of that carbon dioxide-rich water was displaced upward into shallower depths to the point where the overlying water pressure was lower and carbon dioxide bubbles could start to form (like when you lower the pressure on a soda by opening the bottle and suddenly bubbles start to form). At Lake Nyos, once \these bubbles started to form they wanted to rise to the top, this brought up more carbon dioxide-rich water which then also started to develop bubbles, and pretty soon there was a big rush of carbon dioxide bubbles to the surface. What people don’t agree on is what the trigger for this unbalancing event was. Most people, I think, feel that there was some sort of landslide into the lake that stirred up the water. There are a few volcanologists who think there was some type of eruption in the deeper part of the lake, but they are in the minority.

Once all this carbon dioxide reached the surface, it splashed some lake water out of the lake, like a big bubble bursting. Carbon dioxide is denser than air, so it hugged the ground and flowed down the stream valley that leads away from the lake. Unfortunately many homes and at least one town are also along this valley and the inhabitants were caught by this cloud of ground-hugging gas. Carbon dioxide usually kills people by displacing the air that they need to breathe, but in high-enough concentrations it is poisonous as well.

So you see that although volcanologists might disagree on the tectonic details underlying Cameroon and might even disagree on the triggering mechanism for the 1986 disaster, but the danger is pretty well understood. Obviously one way to minimize the chances of this happening again is to prevent the deep lake waters from becoming gas-charged. A program was started to have a pump running that brought up the deep water (in a small controlled way), where it was pumped into the air like a fountain. This allowed smaller amounts of deep water to lose their carbon dioxide gradually rather than having the potential of a big bubble occurring again. Another worry is that the lake walls themselves are not very strong (they are constructed of tuff, partially solidified ash). The problem is that there is this lake at an elevation higher than the main towns nearby, and if for any reason the walls of the lake were breached there would be a flood of water that could be just as dangerous as a flood of carbon dioxide. I’m not sure but I think there were plans to pump water out of the lake to try and keep the level and pressure down.