Limnic Eruptions: The Rarest Natural Disaster

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Podcast Transcript

Early in the morning of  August 21, 1986, in a valley near Lake Nyos, Cameroon, something horrible happened. 

1,746 people were killed, as were over 3,500 cattle and almost every other animal that breathed air. 

There were no signs of violence or destruction. Everyone and everything seemed to have simply died.  

They were the victims of one of Earth’s rarest and most frightening disasters. 

Learn more about limnic eruptions, what they are, and how they can be prevented on this episode of Everything Everywhere Daily.

Every natural disaster you can think of all has one thing in common. You know when it’s happening. 

You can’t miss the high winds of a hurricane, the surging waters of a tsunami, the shaking ground of an earthquake, or the explosions and pyroclastic flows of a volcano. 

While all of those disasters are horrible, at least you know what is happening to you. 

What makes a limnic eruption so horrifying is that you can’t see, hear, or feel what is happening. 

So what is a limnic eruption?

As the name would suggest, it has to do with lakes. 

Limnic eruptions are extremely rare, and only a few have been recorded in history. 

The earliest known case of a limnic eruption was documented by the Roman historian Plutarch. He noted that in the year 406 BC, the waters of Lake Albano, outside of Rome, surged over its banks and across the surrounding hills.

The problem is that Laka Albano is in a volcanic crater. There are no tributaries that flow into the lake, and there was no rainfall. There was nothing that would cause the lake to flood. The water in the lake just spontaneously rose up and spilled out over the rim of the crater, destroying farm fields and vineyards.

Plutarch wrote about this disaster almost 500 years after it occurred, so he wasn’t a witness to the event, nor was he able to speak to eyewitnesses. It was something that was passed along for generations. 

It was a rather odd story to have survived for 500 years. A lake just spontaneously rises up and spills over its banks. 

Fast forward almost 2000 years from Plutarch. This time instead of Italy, the place is the African country of Cameroon. The lake in question is Lake Monoun, located in the Oku Volcanic Field and also situated in a volcanic crater. 

On August 14, 1984, at 10:30 pm, something happened at Lake Monoun. 

A gas cloud reportedly arose from the lake. Sometime between 2:30 am and dawn on the 15th, 37 people on the eastern side of the lake were killed. 

The victims were found with skin damage, and all the plants around the village on the shore of the lake were found with to have been crushed as if they were hit by a large wave.

The deaths were a total mystery, and certain things didn’t add up.

Some victims were found in a truck. The truck stopped working, and two people in the truck got out and were killed. Two people sitting on the top of the truck somehow survived. 

The working theory for the deaths at Lake Monoun was terrorism. There had been fighting and civil strife in Cameroon in the past, so the explanation was plausible, and quite frankly, nothing else made sense. 

There was no means or motive, but what else could it be?

Just two years later, something similar and much more devastating happened. 

Again, this took place in Cameroon. This time the event took place in Lake Nyos, located only 100 kilometers away from Lake Monoun. 

Lake Nyos was very similar to Lake Monoun. It is located in the same volcanic field, and the lake was located in a volcanic crater. 

However, Lake Nyos was five times larger than Lake Monoun and more than twice as deep.

In the early hours of August 21, 1986, an event similar to that which took place at Lake Monoun occurred. 

This time the results were far worse. There were several villages located below the lake in a valley on the side of the crater. Witnesses who came upon the scene described it as if it had been hit by a neutron bomb. A bomb that kills people but doesn’t damage any property. 

1,746 people were killed, along with 3,500 cattle. Most of the dead died in their sleep. Some victims were found as far as 16 miles away.

The few survivors reported a rapid loss of consciousness and the scent of sulfur. Some of the victims had skin damage, as in Lake Monoun, but there was no evidence of heat or chemical burns. 

This was too large of an event to have been terrorism. With more victims and more survivors, enough data was gathered to get an idea of what happened. A limnic Eruption. 

A limnic eruption happens when you have dissolved gasses at the bottom of a lake. In this case, carbon dioxide was trapped at the bottom of both lakes Nyos and Monoun. 

The carbon dioxide got there from venting from a volcano that sat below the lake. Over time, the dissolved CO2 built up at the bottom of the lake.

CO2’s ability to be dissolved in water depends on several factors, including pressure and temperature. The higher the pressure, the more CO2 can be held in the water. 

Likewise, the colder the water, the most CO2 can be held. 

In the lakes in question, they were both deep, with much cooler water at the bottom than at the top. 

In very deep lakes like these, water at the bottom doesn’t mix with water at the top. There is a layer known as a thermocline that separates cooler water from warmer water. 

The thermocline prevents the cooler water from mixing with the warmer water. Such lakes with layers that do not mix are known as meromictic lakes. Most lakes are not meromictic lakes, and the waters will mix at least once per year. Only one lake in a thousand is a meromictic lake.

The result is that the cool water, which is under high pressure, just sits at the bottom of the lake, building up an enormous amount of dissolved CO2, which is never released because the water isn’t disturbed. The dissolved CO2 also makes the water heavier, which further prevents the water from mixing with the lighter, warmer water above.

However, when the water is disturbed, the results can be dramatic. If you want a relatable example of what can happen, just open a can of soda. The sudden release of pressure causes the dissolved carbon dioxide to turn into bubbles and come out of solution. 

Something happened at both lakes Nyos and Monoun, which disturbed the lower layer of water where all the CO2 was being held. The best guess is that it was a landslide on the banks of the lake or some small volcanic event at the bottom of the lake. 

When all of this CO2-rich water was disturbed, it changed the pressure and temperature, which caused a massive release of CO2 gas, just like opening a bottle of soda. 

The release of gas further disturbs the water, causing even more turbulence, releasing even more gas in a cascade.

This giant gas bubble is how the flood that Plutarch spoke of in Lake Albano could have occurred. 

Once this gas reaches the surface of the lake, it doesn’t just float away. CO2 is 1.5 times the density of air. Because it is heavier than air, the cloud will sit there until it dissipates, or it will flow downhill. 

This is why the truck at Lake Monoun didn’t run. The engine was literally deprived of oxygen. It is also why the men who got out of the truck on the ground died, but the men on the top of the truck survived. 

It is also why the villages below the lake crater in Nyos were afflicted because CO2 is heavier than air. 

The victims never knew what hit them. CO2 is odorless, colorless, and tasteless. They just would have asphyxiated, no different than if they had a plastic bag over their heads. 

Despite the fact that two of these events occurred within two years of each other, this is an extremely rare phenomenon. 

Only certain lakes are candidates for a limnic eruption; even then, it might take centuries for enough CO2 to build up to dangerous levels. 

Once limnologists and geologists had figured out what happened, the next question was identifying other lakes which were candidates and how to figure out ways to prevent it from happening. 

There actually is a lake that is a candidate for a limnic eruption, and if it were to happen, it would be far worse than any other limnic eruption which has previously occurred. Lake Kivu is on the border of the Democratic Republic of Congo and Rwanda. 

Lake Kivu is the eighth-largest lake in Africa. Its volume of water is 3,300 times greater than Lake Nyos. 

More importantly, there were only 14,000 people who lived in the vicinity of Lake Nyos. However, there are 2,000,000 people who live around lake Kivu. 

In the case of Lake Kivu, the gas trapped at the bottom is CO2…..and methane.

Were there to be a major event at Lake Kivu, there would be an additional risk of the methane igniting when it reached the surface.

This isn’t a theoretical concern. Researchers believe that a limnic eruption may have occurred in Lake Kivu about 5,000 years ago. 

Over the last 50 years, CO2 levels at the bottom of the lake have increased by over 10%, and methane levels have increased by 20%.

There is a methane extraction industry on the lake, and the hope is that this is causing enough low-level disturbance to vent the methane as well as some CO2.

Likewise, a ventilation pipe has been installed at Lake Nyos to remove some of the dissolved CO2 at the bottom of the lake. 

I will close by answering a question that some of you might be thinking. 

If the conditions necessary for a limnic eruption are a deep lake in a volcanic crater in a region with volcanism, there is a very prominent lake that fits the bill. 

Crater Lake in Oregon, a United States National Park. Crater Lake is the deepest volcanic crater lake in the world. It is part of the Cascade volcanic arc, which includes active volcanoes such as Mount Saint Helens. 

So, could a limnic eruption happen at Crater Lake?

The answer is no. 

The reason has to do with temperature. All of the lakes I’ve mentioned so far lie in tropical areas, or at least in places where it seldom snows or freezes.

Crater Lake gets a tremendous amount of snow. When the cold, freezing snow lands in the lake, it sinks because of its cold temperature. This causes gradual but constant convection in the lake, which slowly circulates the water. 

It isn’t much, but it is enough to prevent a dramatic CO2 build-up. 

Limnic eruptions are certainly very dangerous, but they are not something to worry about on your next trip to a lake. 

Conditions have to be perfect, there needs to be some sort of event to trigger the eruption, and it takes centuries for the gas to build up to dangerous levels.

Moreover, once a potential lake has been identified, steps can be taken to prevent a disaster from occurring. 

Hopefully, now that we know how limnic eruptions work and can identify lakes that are potential candidates, we can avoid tragedies like Lake Nonoun and Nyos from ever happening again.

The Executive Producer of Everything Everywhere Daily is Charles Daniel.

The associate producers are Thor Thomsen and Peter Bennett.

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