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Limnic Eruption

January 22, 2022

Earth Science, EarthSphere Blog, Environment, Hydrosphere, Repost, Science

Dangerous Carbon Dioxide Clouds

(Published in The EarthSphere Blog. Cover Image: Bubbling Gases from Below by WM House & CF Lovelace; ArcheanArt)

A story about strange whitish gas clouds rising from the sea and suffocating villages and towns on the shore might be found in the science fiction section of Barnes and Noble. Sadly, it is also found in the news, and it is reality, not science fiction. There is even a scientific term describing such an event; a limnic eruption. Special conditions are required for these eruptions, so there are limited locations where the threat is imminent. But one of these locations is Lake Kivu in Africa.

Lake Kivu nestles between Rwanda and the Democratic Republic of the Congo. The lake is about 90 kilometers long and 50 kilometers wide. It is one of a series of lakes formed along the East African Rift System. This rift system is the result of diverging tectonic plates. About 22 million years ago, Africa started splitting in two. Rift lakes formed in the north-south trending valleys created as the Somali Plate and Nubian Plate moved apart at a rate of about 7 millimeters a year.

The particulars of how these rift valleys form make the lakes long and deep. They are all freshwater lakes positioned well above sea level. Lake Kivu’s surface is just under a mile above sea level. These lakes are dangerous due to a combination of physics and geology. Lake Kivu poses a threat to 2 million people, and the reality of this threat was demonstrated by a 1986 limnic eruption in Cameroon’s Lake Nyos, where 1,800 people died.

Pressure, Volume, and Temperature

The root of the problem for those living on the shores of Lake Kivu is the amount of carbon dioxide and methane dissolved in the lake’s deep waters. Hidden below the lake’s surface are 60 cubic miles of dissolved CO2, 14 cubic miles of dissolved methane, and significant volumes of hydrogen sulfide, a highly toxic gas.

While the gas saturated water stays deep in the lake, there is no danger to the lakeshore villages and towns. Pressures at the bottom of the lake are high due to the weight of the overlying water column. But if the pressure at the bottom of the lake decreases or the temperature increases, then the danger level rises.

Readers who paid attention to physics in school will remember the Gas Laws developed at the end of the 18th century to explore relationships between pressure, temperature, and volume for ideal gases. These equations relate to an equation of state governing the behavior of gas in water. If the confining pressure drops, the gas comes out of solution, like popping the cork on a bottle of Taittinger Champagne.

The same equation of state applies to the bottom waters of Lake Kivu, and a pressure decrease lets CO2 and methane bubble out of solution and rise to the surface. The danger arises because CO2 is denser than air, and a cloud of CO2 accumulates on the lake surface. People suffocate when exposed to this cloud since the CO2 has displaced the oxygen needed for life. Following the Cameroon disaster, there were no flies on the dead bodies. They were also dead from lack of oxygen.

Pressure Drops and Gas Rises

Lake Kivu’s deep danger arises from two factors. Firstly, the lake is stratified, so water doesn’t circulate between its upper and lower levels. The second factor is that volcanic hot springs beneath the lake constantly feed the lake with a stream of CO2 and methane. These gases dissolve into the water under the deep lake’s high-pressure and low-temperature conditions. There they accumulate, waiting for a release to the surface.

The release of these gases requires changes in pressure conditions. Remember, the pressure is directly related to the weight of the vertical water column, so pressure changes require either removing water from the lake surface or raising the deep water to shallower depths.

Scientists speculate that a prolonged drought might cause enough surface water loss to trigger a limnic eruption at depth. Another source of concern is underwater landslides. Lake Kivu is on a plate boundary, and the rift zone is seismically active. Regular seismic shaking combined with steep slopes on the lake’s sides makes periodic underwater landslides likely. Such a slide may have triggered the Cameroon event.

A mass of rocks, sediments, and mud-laden water moving downslope displaces the water at the bottom of the lake and forces it upward. This sudden upward movement means less pressure from the water column above. Gases come out of solution, and a limnic eruption is born. It’s an event you don’t want to watch from the lakeshore. The white, misty cloud accumulating over the lake brings death when it blows in your direction.