Thursday, 29 November 2012

6. Limnic Eruptions and Exploding Lakes

Part of the plotline of “The Blue Angels” concerns itself with natural disasters, so I set myself the task of researching different types. We can all name some: earthquakes and tsunamis, volcanic eruptions, tornadoes and other types of storm, forest fires, landslips… the list goes on. The chances are that most of you will remember seeing reports of many of these on television, or in the newspapers, even if the exact date or location eludes you. Most of the world’s population lives in the shadow of at least one type of natural disaster and with climate change tightening its grip, that proportion is increasing all the time.

Of all the ways in which Mother Earth can kill, however, the phenomenon of the limnic eruption is one of the rarest - there have only been two in recorded history. What might be surprising then, is that both of these were within living memory, and both occurred within a short distance of each other. In a limnic eruption, vast volumes of gas are suddenly released from a body of water, suffocating surrounding animals and people. In 1984, Lake Monoun in Cameroon erupted, which killed 37 people. In 1986, neighbouring Lake Nyos erupted too, killing between 1700 and 1800 people. The second of these eruptions is known to have produced 80 million cubic metres of carbon dioxide. So, what is it about these two lakes that makes them so dangerous? Could the same thing happen at Windermere, Huron, Como, Baikal or Victoria?

What Makes a Lake “Explode”?

For a limnic eruption to occur, the lake must be saturated with dissolved gas. In Lakes Monoun and Nyos the gas in question was carbon dioxide, but at Lake Kivu (a limnically active lake that lies on the border between Rwanda and the DRC), methane is also a worry. The dissolved gas may come from volcanic sources under the lake bed, or from the decomposition of organic material, or it may be dissolved in the lake’s inflow. While the lake is building up to this point, it behaves like an unopened soda bottle: gas under higher pressure dissolves much more readily than gas at lower pressure. This is why bubbles in a soda can only form once the can is opened, as the release of pressure forces excess gas out of solution. In a lake, pressure increases steadily with depth, reaching a maximum at the lake bed. Gases such as carbon dioxide also dissolve more readily in cooler water, which is typically found at the lake bottom. Hence, a large deep lake can dissolved huge quantities of gas.

Once the lake is saturated with gas, it becomes unstable, only requiring a suitable trigger event to set off an eruption. In the case of Lake Nyos in 1986, landslides are suspected but a volcanic eruption, an earthquake or even wind and rain could trigger similar events in susceptible bodies of water. The trigger event displaces some of the gas-saturated water upwards, lowering its pressure and forcing it to release some of its dissolved gas as bubbles. These bubbles rise, pushing more water ahead of them, which in turn is forced to surrender some of its gas and so on. In this way, a column of rising gas quickly forms that pulls water and sediment from the bottom of the lake by suction, causing a runaway process. The released gas and sediment-rich water erupts onto the lake’s surface, giving it a boiling, dirty appearance. If the released gas is heavier than air (as is the case with carbon dioxide) then it cannot rise further and instead begins to spread out over the lake’s surface and ultimately onto surrounding land.

Fortunately, this type of eruption is rare. Firstly, there must be a source of gas, predisposing regions with volcanic activity. Secondly, lakes in the world’s temperate regions undergo regular “turn over” in response to seasonal surface temperature changes. These changes encourage water from different depths to mix, causing gases to be released much more gradually and preventing saturation. Finally, the lake must be deep in order to offer the high pressure, low temperature conditions that are favourable to limnic activity. Together, these factors imply that limnic eruptions are only possible in deep, stable, tropical volcanic lakes.

What are the Consequences of a Limnic Eruption?

As the released gas flows across the lake’s surface and onto neighbouring land, it pushes breathable air upwards. Carbon dioxide acidified body fluids and is toxic even at relatively low concentrations. Unlike other tasteless, colourless gases such as nitrogen, high levels in the blood causes the respiratory system to gasp, in an attempt to secure the required oxygen, thus speeding up its toxic effects.

At Lake Nyos, the gas cloud descended from the lake into a nearby village where it settled, killing nearly everyone. Fatalities were recorded as far as 16 miles from the lake. Some of the bodies exhibited a change in skin color, leading scientists to believe that the gas cloud may also have contained a dissolved acid, but this is uncertain. Many victims were found with blistered skin. This is believed to have been caused by low blood oxygen. At the lake’s edge, vegetation was damaged or destroyed by a five metre tsunami. Vegetation further away was largely unaffected, as would be expected from a temporary glut of carbon dioxide.

In one final twist, many victims were also found with what appeared to be frost bite: the gas released from the lake bottom would have been very cold. Victims would have experienced a sudden and prolonged wind of bitter cold “air”. Some survivors reported a smell of rotten eggs and feeling warm before passing out. This can be explained by the fact that, at high concentrations, carbon dioxide acts as a sensory hallucinogenic.

What Can Be Done?

In 1990, a team of French scientists began to experiment with a controlled degassing of Lakes Monoun and Nyos. Under this scheme, a long tube is inserted vertically into the lake, with its upper end standing clear of the lake’s surface. A small amount of gas-saturated water is then pumped upwards, causing it to give up its gas. The gas then rises as bubbles, pushing water ahead of it as before. In open water, this effect spreads outwards as well as vertically, to create the runaway reaction described above. Inside the pipe, however, this outward spread is not possible, and so fresh water is drawn into the bottom of the pipe to create a kind of siphon. Gas and water emerge from the top of the pipe and gas concentrations in the lake are slowly lowered, reducing the risk of a further eruption.

Degassing Lake Nyos

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