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Limnic Eruptions

We're all familiar with the lava fountains and massive explosions that come from volcanoes, but did you know that there's another, much wierder kind of eruption? It's what's known as a Limnic Eruption. It's so bizarre and unusual that we've only seen it happen twice: once at two volcanoes which are right next to each other.

A limnic eruption occurs when a large bubble of gas is trapped at the bottom of a lake and then get's released. It reaches the top, explodes outward, and then travels down the slopes, suffocating everything in it's path. No smoke, no fire, no ash, no lava...no heat! It's just a whole bunch of gas getting released all at once. So how much gas are we talking about? The Lake Nyos eruption released 80 million cubic meters of CO2.

The two recorded limnic eruptions occurred at Lake Monoun and Lake Nyos in 1984 and 1986 respectively. The two are located in Cameroon in the Oku Volcanic Field. The Lake Monoun eruption occurred on August 15 and resulted in the death of 37 people in the vicinity. That day people reported hearing an explosion and there were reports of an earthquake at a town 6 km north. Vegetation was flattened for 100 meters around the eastern end of the lake, indicating that a tsunami around 5 meters high (~31 feet) pushed out from the lake at the time of the eruption. People at the edge of the gas cloud described the smell as bitter and acidic while the victims within were both asphyxiated and burned, indicating that there may have been a dissolved acid (such as HCl) mixed with the CO2. The cloud took about 4 hours to dissipate, hanging 0-3 meters above the ground.

The Lake Nyos eruption was much more catastrophic, occurring on August 12, 1986; almost exactly 2 years after the Lake Monoun eruption. The event claimed over 1700 lives with uncounted loss of cattle and wildlife. The cloud descended from the lake into valleys and basins where it rested over a local village, killing nearly everyone.

But how the heck do these bizarre eruptions-these explosions of lake gas-occur?? Well, Lake Nyos and Lake Monoun are both crater lakes and there is carbon-dioxide gas being released into the bottom of these lakes constantly. As you'll recall, volcanoes produce a great deal of gas which is released from magma when it's flowing. The gasses seep up through the crater and get dissolved into the lake as HCO3. You can compare a lot of stuff in volcanology with soda and I'm going to do it again. What you have is the soda bottle effect. You have all this gas dissolved down there, building up and building up. The more gas you have dissolved, the more unstable the water gets. It starts approaching it's saturation point-the point at which it can no longer hold the gas in it. Then, something catastrophic happens-there's a landslide or an earthquake-something shakes up the lake and the disturbance causes the gas to get released and explode upward. As the gas explodes, it disturbs the saturated water even more which releases more gas, etc...that's how you get 80 million cubic meters of gas released at once. Think of it like this. You have a bottle of soda (pick your favorite carbonated flavor). The saturation point of a liquid depends on the amount of pressure it's under, and so your soda, before you open it, is under quite a bit of pressure. When you unscrew the bottle, there's a loud hissing sound as the pressurized gas escapes and you see a cloud of bubbles suddenly appear and raise to the surface. Well, that's a pretty docile way of looking at it. If you really want to simulate what happens during a limnic eruption, screw the cap back on and start the throwing the bottle against a wall. Now open the bottle. Go ahead and try. What happened? You probably had the soda explode all over you. This happened because the shockwaves from the wall disturbed the soda enough that it couldn't handle it and released all the gas at once (it had to wait till you opened it because of the pressure, however). It's virtually an identical situation for the limnic eruptions, substituting a landslide or earthquake for the wall.

So now we know how the limnic eruption works, but the interesting story about Lake Nyos doesn't end there, however. Gas is still building up in the lake...in fact, there's more gas dissolved in there now (2001) then there was before the eruption (300 million cubic meters)! That means the question isn't "if the lake erupts?" but "when will it erupt?" and "how many people will it kill?" While people aren't allowed to live nearby now, people still tend herds of livestock in the area because the land is very fertile. After all, if you were poor, uneducated, and hadn't seen an eruption in over 15 years, you'd be going to the better land too. The job of volcanologists is to save lives, so naturally there were a lot of people wondering just how they could prevent another eruption. You have to either stop the gas from entering the lake or figure out a way to take it out. The first option is impossible so a french team devised a brilliantly simple method of degassing Lake Nyos. They took a long pipe, stuck it into the lake so that it came near the bottom, and started pumping water out. They only needed to pump water out for a minute though, before they got a nonstop fountain of water. As the water rises in the pipe, the pressure decreases and the gas gets released causing it to shoot out very fast (like first unscrewing your soda bottle). The higher it gets, the more gas that get's released, until it comes shooting out the top, so fast, that it reaches a height of over 40m (130 ft). The gas:water ratio for the fountain is 10:1; that means that for every liter of water you have ten liters of carbon-dioxide gas released over the platform. In fact, the soda bottle effect is so dramatic here that when scientists first started bringing up water from the bottom of the lake, the sampling containors would explode when opened! Talk about a surprise to grab your attention. Anyway, the water's ascent to the surface creates a suction at the base and so the water just keeps on shooting out indefinitely. Plans are in place to install more of these pipes, although questions still remain over whether this will accidentally trigger a new eruption.

The lake is fairly small: 1400 m long, 900 m wide, and 208 m deep. The top 50 m of water are normal-there's no dissolved gas there because it's far enough from the crater vents and also because there's not much pressure. The small size of the lake means it will be easy to set up several other pipe platforms to help degass the lake. Hopefully, we can prevent another limnic eruption from occurring here and, in the process, save lives.