What Makes a 'Killer' Lake Explode?

Rwanda's Lake Kivu is considered a "killer" lake because of the huge amounts of biogas trapped beneath a top layer of heavy water. Wikipedia/USED UNDER CREATIVE COMMONS CC BY-ND 3.0

Floods and wildfires make the news with depressing regularity. But today we're going to look at a different kind of natural disaster; one that hasn't been documented in more than three decades. Rare as it may be, this phenomenon shouldn't taken lightly as it can wipe out entire communities with little to no warning.

We're talking about a limnic eruption. A limnic eruption is what happens when deadly gasses (like CO2) explode out of volcanic lakes. Sometimes the carnage unfolds on multiple fronts. Just as lethal clouds suffocate humans and animals, the abrupt displacement of water is liable to create tsunamis. That exact combination of events killed more than 1,700 people on a grim summer day in 1986 in the West African country of Cameroon. And now scientists wonder if an even bigger limnic eruption is in the making.

A Recipe for Trouble

Water pressure increases with depth; that's why scuba divers can't venture too far below the surface without the right equipment. "Hydrostatic pressure" is the name given to the force that is exerted upon a submerged object by the weight of all the liquid above it. Normally, this pressure intensifies by 14.5 pounds per square inch (or 99.9 kilopascals) for every 34 feet (.6 meters) of water.

Gases dissolve more easily in cold, high-pressure water. Therein lies the key to limnic eruptions. Such outbursts can only occur in deep bodies of water with a lot of hydrostatic pressure at the bottom. There must also be a significant difference in both pressure and temperature between the surface water and the lower depths. (The latter will be much chillier.)

Stratification will act like a barrier, keeping that dissolved gas confined to the lake bottom where it can't depressurize and then escape into the atmosphere. Because it's trapped, the dissolved gas accumulates in massive and potentially deadly quantities. Explosions are impossible in lakes whose lower and upper water levels intermingle on the regular.

And while we're on the subject, the water needs a continuous supply of some highly soluble gas like carbon dioxide (CO2) or methane. That's where volcanism comes in. At localities with active volcanoes, buried magma is liable to send methane, CO2, and other gasses seeping through thin sections of Earth's crust. If a lake is overhead, the gas may pass right into the water, traveling by volcanic vents and other routes.

Up From the Depths

That brings us to Lake Nyos and Lake Monoun. They are both located in a volcanic field in Cameroon. Both lake bottoms are oversaturated with CO2, which underlying magma sends their way. On Aug. 15, 1984, some of the deep water in Monoun that had been loaded up with the dissolved gas ascended to the surface. No one knows why this happened; it's possible that heavy rainfall, and an earthquake or a landslide displaced some of the lake-bottom water. Regardless, as the water rose, the dissolved CO2 lurking inside it became depressurized and formed bubbles. Those bubbles drove even more of the water to the top of the lake, resulting in a massive, foul-smelling cloud of carbon dioxide gas.

Under the wrong set of circumstances, this gas is extremely dangerous to people. Large quantities of CO2 cling to the ground and displace oxygen, which can lead to death by suffocation. On that awful day in 1984, no fewer than 37 humans perished as a direct result of all the CO2 Monoun suddenly let loose.

Just two years later, on Aug. 21, 1986, Lake Nyos experienced a limnic eruption of its own. Once again there was a sudden, mysterious upheaval of CO2-laden water from its frigid, high-pressure depths. But this time, the body count was much higher: Carbon dioxide from the Lake Nyos disaster killed approximately 1,746 people and more than 3,500 domestic animals. Somewhere in the ballpark of 330,000 to 1.7 million tons (300,000 to 1.6 million metric tonnes) of CO2 gas burst out of the water with enough force to set off a 65.6-foot (20-meter) tsunami.

The extraction barge of the KivuWatt powerplant in Rwanda draws CO2-rich water from the depths of the lake, then siphons off the methane and sends it to a power plant to generate electricity for the area.

Coming to a Lake Near You?

If you're worried about a killer limnic eruption breaking out in Lake Superior or Loch Ness, University of Michigan geoscience professor Youxue Zhang says you shouldn't be. The two most recent limnic eruptions were the Lake Nyos and Lake Monoun cataclysms that we've just described. Both bodies of water are located just above the equator, where it tends to be warm all year round.

There's just no way for a limnic eruption to happen in a temperate body of water. In places where seasonal temperatures vary wildly (like the Great Lakes), lake surfaces often cool down, causing the water at that level to sink and swap places with the layers of water beneath it. "Temperate lakes experience turnovers yearly, [so] it is not expected that any gas would be able to accumulate in lake bottom water," Zhang says via email. "Without [dissolved] gas accumulation, there would be no lake eruptions."

However, Zhang and many of his colleague have taken a healthy interest in Lake Kivu, a 1,042-square mile (2,700-square kilometer), up-and-coming vacation destination on the border of Rwanda and the Democratic Republic of the Congo. Why? Because it seems to have all the necessary criteria for a truly colossal limnic eruption.

There are about 10.5 billion cubic feet (300 million cubic meters) of dissolved CO2 and 2.1 billion cubic feet (60 million cubic meters) of methane lurking near the bottom. Were those gases to explode from the lake's surface, the 2 million people who live around Kivu might find themselves in jeopardy.

One possible solution: Harvest those very gases as a possible energy source by an extraction barge. KivuWatt is a one-of-a-kind $200 million facility that uses an offshore barge to draw up water from the lake. It then siphons off the methane and sends it to a power plant generating electricity for the area. When life gives you lemons, turn it into electricity.