Lake Nyos

Lake Nyos
Lake Nyos -
Location Northwest Province
Lake type Meromictic, crater lake
Primary inflows subaquatic source
Basin countries Cameroon
Max. length 2.0 km (1.2 mi)
Max. width 1.2 km (0.75 mi)
Surface area 1.58 km2 (390 acres)
Max. depth 208 m (680 ft)
Water volume 0.15 km3 (120,000 acre·ft)
Surface elevation 1,091 m (3,580 ft)

Lake Nyos is a crater lake in the Northwest Province of Cameroon. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity. A natural dam of volcanic rock hems in the lake waters.

A pocket of magma lies beneath the lake and leaks carbon dioxide (CO2) into the water, changing it into carbonic acid. Nyos is one of only three known lakes to be saturated with carbon dioxide in this way, the others being Lake Monoun, at a distance of 100 km SSE, and Lake Kivu in Rwanda. On 21 August 1986, possibly triggered by a landslide, the lake suddenly emitted a large cloud of CO2, which suffocated 1,700 people and 3,500 livestock in nearby villages.[1] Though not completely unprecedented, it was the first known large-scale asphyxiation caused by a natural event. To prevent a repetition, a degassing tube that syphons water from the bottom layers of water to the top allowing the carbon dioxide to leak in safe quantities was installed in 2001, though additional tubes are needed to make the lake safe.[2]

Today, the lake also poses a threat due to its weakening natural wall. A geological tremor could cause this dike to give way, allowing water to rush into downstream villages all the way into Nigeria.

Contents

Formation and geological history

Lake Nyos is a couple of kilometres south of the dirt road from Wum, about 30 km to the west, to Nkambe in the east. Villages along the road in the vicinity of the lake include Cha, Nyos, Munji, Djingbe, and Subum. The lake is 50 km from the Nigerian border to the north, and lies on the northern slopes of the Massif du Mbam, drained by streams running north then north-west to the Katsina-Ala River in Nigeria, part of the Benue River basin.[3]

Lake Nyos fills a roughly circular maar in the Oku Volcanic Field, an explosion crater caused when a lava flow interacted violently with groundwater. The maar is believed to have formed in an eruption about 400 years ago, and is 1,800 m (5,900 ft) across and 208 m (682 ft) deep[4]. The area has been volcanically active for millions of years — after South America and Africa were split apart by plate tectonics about 110 million years ago, West Africa also experienced rifting, although to a lesser degree. The rift is known as the Mbéré Rift Valley, and crustal extension has allowed magma to reach the surface along a line extending through Cameroon. Mount Cameroon also lies on this fault line. Lake Nyos is surrounded by old lava flows and pyroclastic deposits.

The lake waters are held in place by a natural dam composed of volcanic rock. At its narrowest point, the wall measures 40 metres high and 45 metres wide.

Gas saturation

Lake Nyos is one of only three lakes in the world known to be saturated with carbon dioxide—the others are Lake Monoun, also in Cameroon about 100 km away, and Lake Kivu in Rwanda. A magma chamber beneath the region is an abundant source of carbon dioxide, which seeps up through the lake bed, charging the waters of Lake Nyos with an estimated 90 million tonnes of CO2.

Lake Nyos is thermally stratified, with layers of warm, less dense water near the surface floating on the colder, denser water layers near the lake's bottom. Over long periods, carbon dioxide gas seeping into the cold water at the lake's bottom is dissolved in great amounts.

Most of the time, the lake is stable and the CO2 remains in solution in the lower layers. However, over time the water becomes supersaturated, and if an event such as an earthquake or volcanic eruption occurs, large amounts of CO2 may suddenly come out of solution.

The 1986 disaster

Lake Nyos as it appeared less than two weeks after the eruption; August 29, 1986. Areas once covered with vegetation are especially visible from the divide between photos to the extreme right.
A cow suffocated by gases from Lake Nyos

Although a sudden outgassing of CO2 had occurred at Lake Monoun in 1984, killing 37 local residents, a similar threat from Lake Nyos was not anticipated. However, on August 21, 1986, a limnic eruption occurred at Lake Nyos which triggered the sudden release of about 1.6 million tonnes of CO2. The gas spilled over the northern lip of the lake into a valley running roughly east-west from Cha to Subum, and then rushed down two valleys branching off it to the north, displacing all the air and suffocating some 1,700 people within 20 km of the lake, mostly rural villagers, as well as 3,500 livestock. Worst affected villages were Cha, Nyos, and Subum.[5] About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gases.[6]

It is not known what triggered the catastrophic outgassing. Most geologists suspect a landslide, but some believe that a small volcanic eruption may have occurred on the bed of the lake. A third possibility is that cool rainwater falling on one side of the lake triggered the overturn. Whatever the cause, the event resulted in the rapid mixing of the supersaturated deep water with the upper layers of the lake, where the reduced pressure allowed the stored CO2 to effervesce out of solution.

It is believed that up to a cubic kilometre of gas was released. Because pure CO2 is denser than air, the gas flowed off the mountainous flank in which Lake Nyos rests and down two adjoining valleys in a layer tens of metres deep, displacing the air and suffocating all the people and animals before it could dissipate. The normally blue waters of the lake turned a deep red after the outgassing, due to iron-rich water from the deep rising to the surface and being oxidised by the air. The level of the lake dropped by about a metre, representing the volume of gas released. The outgassing probably also caused an overflow of the waters of the lake. Trees near the lake were knocked down.

Degassing

French scientists working on degassing Lake Nyos.

The scale of the disaster led to much study on how a recurrence could be prevented. Estimates of the rate of carbon dioxide entering the lake suggested that outgassings could occur every 10-30 years, though a recent study shows that release of water from the lake, caused by erosion of the natural barrier that keeps in the lake's water, could in turn reduce pressure on the lake's carbon dioxide and cause a gas escape much sooner.

Several researchers independently proposed the installation of degassing columns from rafts in the lake. The principle is simple: a pump lifts water from the bottom of the lake, heavily saturated with CO2, until the loss of pressure begins releasing the gas from the diphasic fluid and thus makes the process self-powered.[7] In 1992 at Monoun, and in 1995 at Nyos, a French team demonstrated the feasibility of this approach. In 2001, the US Office of Foreign Disaster Assistance funded a permanent installation at Nyos.[8]

More pipes are expected to be needed to make the lake safe: the original French estimates called for a total of five, and the current OFDA project calls for an additional two pipes, each with ten times the capacity of the single current pipe.

Following the Lake Nyos tragedy, scientists investigated other African lakes to see if a similar phenomenon could happen elsewhere. Lake Kivu in Rwanda, 2,000 times larger than Lake Nyos, was found also to be supersaturated, and geologists found evidence for outgassing events around the lake about every thousand years. The eruption of nearby Mount Nyiragongo in 2002 sent lava flowing into the lake, raising fears that a gas eruption could be triggered, but fortunately it was not, as the flow of lava stopped well before it got near the bottom layers of the lake where the gas is.

Weakening dam

On 18 August, 2005, Dr. Isaac Njilah, a geologist at the University of Yaoundé, suggested that the natural dam of volcanic rock that keeps in the lake's waters could collapse in the near future. Erosion has worn the dam away, causing holes and pockets to develop in the dam's upper layer, and water already passes through the lower section. Meanwhile, landslides have reduced dam strength on the outside. Seismic activity caused by the lake's volcanic foundation could thus cause the lake wall to give way, resulting in up to 50 million cubic metres of water flooding downhill into areas of the Northwest Province and the Nigerian states of Taraba and Benue. Dr. Njilah estimates that the area is home to more than 10,000 people.

The Cameroonian government, speaking through Dr. Gregory Tanyi-Leke of the Institute of Mining and Geological Research, acknowledges the weakening wall but denies that it presents any immediate threat. A United Nations team led by Olaf Van Duin and Nisa Nurmohamed of the Netherlands Ministry of Transport and Public Works inspected the dam over three days in September 2005 and confirmed that the natural lip had weakened. Van Duin believes that the dam will breach in the next 10 or 20 years.

One possible means of averting such a catastrophe would be to strengthen the lake wall, though this would take much time and money. Engineers could also introduce a channel to allow excess water to drain; if the water level were lowered by about 20 metres, the pressure on the wall would be reduced significantly.

See also

References

  1. ↑ New York Times, "Trying to Tame the Roar of Deadly Lakes", February 27, 2001. [1].
  2. ↑ Degassing Lakes Nyos and Monoun: Defusing certain disaster, Proceedings of the National Academy of the Sciences, October 4, 2005, vol. 102, no. 40, 14185-14190
  3. ↑ "Map of Wum, Cameroon". MSN Encarta World Atlas.
  4. ↑ Smithsonian Institution Global Volcanism Program Website, 1999
  5. ↑ Jill Smolowe: "Cameroon the Lake of Death". Time Magazine, Monday, Sep. 08, 1986.
  6. ↑ Lake Nyos disaster, Cameroon, 1986: the medical effects of large scale emission of carbon dioxide?, BMJ, Volume 298, 27 May 1989, [2]
  7. ↑ Degassing Lake Nyos project http://pagesperso-orange.fr/mhalb/nyos/nyos.htm
  8. ↑ Degassing the "Killer Lakes" Expedition 2001

External links