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In central Africa is a deep lake that has a dangerous propensity to explode
By Onlykhabar News Media
Wed 21 October, 2020 ( about 5 years ago )
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Lake Kivu is one of Africa’s strangest bodies of water.
An unusual set of properties make it an intriguing subject for scientists, as well as a potential source of both peril and prosperity for the millions of people living nearby. This process, known as convection, generally keeps the surfaces of deep lakes warmer than their depths. Typically, when water at the surface of a lake is cooled – by winter air temperatures or rivers carrying spring snowmelt, for example – that cold, dense water sinks, and warmer, less dense water rises up from deeper in the lake. Kivu doesn’t behave like most deep lakes. But at Lake Kivu, circumstances have conspired to block this mixing, giving the lake unexpected qualities – and surprising consequences.
The resulting stresses thin the Earth’s crust and trigger volcanic activity, creating hot springs below Kivu that feed hot water, carbon dioxide and methane into the lake’s bottom layers. Straddling the border between Rwanda and the Democratic Republic of the Congo, Kivu is one of a string of lakes lining the East African Rift Valley where the African continent is being slowly pulled apart by tectonic forces. Microorganisms use some of the CO2, as well as organic matter sinking from above, to create energy, producing additional methane as a by-product. These gases would ordinarily bubble out of the water, but Kivu’s great depth – more than 460 metres (1,500ft) at its deepest point – creates so much pressure that they remain dissolved. The deeper water is also saltier due to sediment raining down from the upper layers of the lake and from minerals in the hot springs, which further increases the density. This mixture of water and dissolved gases is denser than water alone, which discourages it from rising.
The result, says limnologist Sergei Katsev of the University of Minnesota Duluth, is a lake with several distinct layers of water of sharply different densities, with only thin transition layers between. The layers can be separated roughly into two regions: one of less-dense surface water above a depth of about 60m (200ft) and, below that, a region of dense saline water that is itself further stratified, says Alfred Wüest, an aquatic physicist at the Swiss Federal Institute of Technology in Lausanne. But Lake Kivu is more than just a scientific curiosity. Its unusual stratification and the CO2 and methane trapped in its deeper layers have researchers worried that it could be a disaster waiting to happen. About 1,400 miles (2,250km) north-west of Kivu, a crater lake in Cameroon known as Lake Nyos similarly accumulates and traps large amounts of dissolved gas – in this case CO2 – from a volcanic vent at the bottom of the lake. On 21 August 1986, the lethal potential of that gas reservoir was made tragically evident.
Possibly due to a landslide, a large amount of water was suddenly displaced, causing the dissolved CO2 to mix rapidly with upper layers of the lake and release into the air. A large, deadly cloud of the gas asphyxiated an estimated 1,800 people in nearby villages. Nyos is a relatively small lake, measuring a little more than a mile long (1.6km), just under a mile wide and less than 210m (700ft) deep. Kivu is 55 miles (89km) long, 30 miles (48km) across at its widest point and more than twice as deep as Nyos. Events like this are called limnic eruptions, and scientists fear that Kivu may be ripe for a similar, even deadlier event. Because of its size, Katsev says, Kivu “has the potential for a major, catastrophic limnic eruption where many cubic miles of gas would be released”. About 14,000 people lived near Nyos at the time of the eruption. More than two million live in the vicinity of Lake Kivu today, including roughly one million residents of the city of Bukavu, in the Democratic Republic of the Congo.
The resulting stresses thin the Earth’s crust and trigger volcanic activity, creating hot springs below Kivu that feed hot water, carbon dioxide and methane into the lake’s bottom layers. Straddling the border between Rwanda and the Democratic Republic of the Congo, Kivu is one of a string of lakes lining the East African Rift Valley where the African continent is being slowly pulled apart by tectonic forces. Microorganisms use some of the CO2, as well as organic matter sinking from above, to create energy, producing additional methane as a by-product. These gases would ordinarily bubble out of the water, but Kivu’s great depth – more than 460 metres (1,500ft) at its deepest point – creates so much pressure that they remain dissolved. The deeper water is also saltier due to sediment raining down from the upper layers of the lake and from minerals in the hot springs, which further increases the density. This mixture of water and dissolved gases is denser than water alone, which discourages it from rising.
