Lake Nyos

Lake and vicinity from Landsat 8, 2014

Lake Nyos lies within the Oku Volcanic Field, located near the northern boundary of the Cameroon Volcanic Line, a zone of volcanoes and other tectonic activity that extends southwest to the Mt. Cameroon stratovolcano. The field consists of volcanic maars and basaltic scoria cones.[6]

Lake Nyos is located south of the dirt road from Wum, about 30 km (19 mi) to the west, to Nkambé 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 (31 mi) from the Nigerian border to the north, and lies on the northern slopes of the Massif du Mbam, drained by streams running north, then northwest, to the Katsina-Ala River in Nigeria which is part of the Benue River basin.[7]

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 a maximum of 12,000 years ago, and is 1,800 m (5,900 ft) across and 208 m (682 ft) deep.[8][9] 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.

Although Nyos is situated within an extinct volcano, magma still exists beneath it. Approximately 80 kilometres (50 mi) directly below the lake resides a pool of magma, which releases carbon dioxide and other gases; the gases then travel upward through the earth. The fumes then dissolve in the natural springs encircling the lake, ultimately rising to the surface of the water and leaching into the lake.[10]

The lake waters are held in place by a natural dam composed of volcanic rock. At its narrowest point, the wall measures 40 metres (130 ft) high and 45 metres (148 ft) wide.

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, and Lake Kivu in the Democratic Republic of Congo. 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 CO
2.

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 CO
2 remains in solution in the lower layers. However, over time, the water becomes supersaturated, and if an event such as an earthquake or landslide occurs, large amounts of CO
2 may suddenly come out of solution.

1986 disaster

Main article: Lake Nyos disaster

Lake Nyos as it appeared less than two weeks after the eruption; August 29, 1986.

Although a sudden outgassing of CO
2 had occurred at Lake Monoun in 1984, a similar threat from Lake Nyos was not anticipated. However, on August 21, 1986, a limnic eruption occurred at Lake Nyos, triggering the sudden release of about 100,000–300,000 tons[11] (some sources state as much as 1.6 million tonnes)[12] of CO
2. This gas cloud rose at nearly 100 kilometres per hour (62 mph) and[6] spilled over the northern lip of the lake into a valley running roughly east–west from Cha to Subum. It then rushed down two valleys branching off to the north, displacing all of the air and suffocating 1,746 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock. The villages most affected were Cha, Nyos, and Subum.[13]

Cattle suffocated by carbon dioxide from Lake Nyos

Scientists concluded from evidence that a 100 m (330 ft) fountain of water and foam formed at the surface of the lake. The huge amount of water rising suddenly caused much turbulence in the water, spawning a wave of at least 25 metres (82 ft) that would scour the shore of one side.[14]

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. Others still believe there was a small earthquake, but as witnesses did not report feeling any tremors on the morning of the disaster, this hypothesis is unlikely. 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 CO
2 to effervesce out of solution.

It is believed that about 1.2 cubic kilometres (0.29 cu mi) of gas was released.[15] 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 and trees near the lake were knocked down.

Degassing

See also: Limnic eruption § Degassing

The scale of the 1986 disaster led to much study on how a recurrence could be prevented.[16] 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. These use a pump to lift water from the bottom of the lake, heavily saturated with CO
2, until the loss of pressure begins releasing the gas from the diphasic fluid, thus making the process self-powered.[17] In 1992 at Monoun, and in 1995 at Nyos, a French team directed by Michel Halbwachs demonstrated the feasibility of this approach. In 2001, the U.S. Office of Foreign Disaster Assistance funded a permanent installation at Nyos.[18]

In 2011, two additional pipes were installed by Michel Halbwachs and his French-Cameroonian team to assure the complete degassing of Lake Nyos.[19]

Degassing pump schematic

Following the disaster, scientists investigated other African lakes to see if a similar phenomenon could happen elsewhere. Lake Kivu, 2,000 times larger than Lake Nyos, was also found 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 it was not, as the flow of lava stopped well before it got near the bottom layers of the lake, where the gas is maintained in solution by the water pressure.

In 2005, 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 m³ (1.8 billion ft³) of water flooding downhill into areas of the Northwest Province and the Nigerian states of Taraba and Benue.

The Cameroonian government, speaking through 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 believed that the dam would breach within the next 10 to 20 years.[20]

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 m (66 ft), the pressure on the wall would be reduced significantly.[21]

Despite the risks from carbon dioxide and collapse of the lake's retaining wall, the area is being resettled. Settlers cite the wish to return to ancestral lands (although some are newcomers) and the great fertility of the land as reasons for their return.[22][23]

Nyos: Η τελετή της αθωότητας [Nyos: The ceremony of innocence] (2016), a novel by Basileios Drolias focusing on the lake Nyos disaster.

Stikvallei [Choke Valley] (2013), a non-fiction account of the lake Nyos disaster by Frank Westerman.

• Carbon sequestration – Capture and long-term storage of atmospheric carbon dioxide
• Limnic eruption – Type of volcanic eruption
• Limnology – The science of inland aquatic ecosystems
• Mazuku – A pocket of carbon dioxide–rich air that can be lethal
• Meromictic lake – Permanently stratified lake with layers of water that do not intermix

• "Cameroon scientist denies dam about to collapse" (August 23, 2005). Reuters.
• Cotel A (1999), A trigger mechanism for the Lake Nyos disaster, American Physical Society, Division of Fluid Dynamics Meeting, November 21–23, 1999
• Decker, R. and Decker, B. (1997) Volcanoes, 3rd edition, WH Freeman, New York.
• Musa, Tansa (September 28, 2005). "Cameroon dam could collapse in 10 years-UN experts". Reuters.
• Musa, Tansa (August 18, 2005). "Cameroon dam nears collapse, 10,000 lives at risk". Reuters.
• Sano Y., Kusakabe M., Hirabayashi J. et al. (1990), Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst, Earth and Planetary Science Letters, v. 99, p. 303–314
• Sano Y., Wakita H., Ohsumi T., Kusakabe M. (1987), Helium isotope evidence for magmatic gases in Lake Nyos, Cameroon, Geophysical Research Letters, v. 14, p. 1039–1041
• Stager, J.C. (1987), "Silent Death from Cameroon's Killer Lake", National Geographic, September 1987

Wikimedia Commons has media related to Lake Nyos.

• Lake Nyos Degassing: in 1995 Michel HALBWACHS with his french scientist team found the solution for lake Nyos degassing problem
• Lake Nyos incident
• BBC News 'On This Day' article
• BBC news article about the degassing of Lake Nyos
• BBC Horizon episode Killer lakes
• USGS information
• Defusing Africa’s Killer Lakes – Smithsonian
• Gas threat grows from Cameroon's lethal lakes — The Guardian
• Lake Nyos – The Vanguard, Cameroon
• BBC News 27 Sep 2005: Action needed on deadly lakes
• Mechanics of the switching on of the trigger mechanism of limnological catastrophes – Latvian research by Nataliya Anatolievna Solodovnik and Anatoliy Borisovich Solodovnik
• BBC Article on Lake Kivu A similar problem at Lake Kivu and plans to deal with it
• The Strangest Disaster of the 20th Century reprinted from Uncle John's Bathroom Reader: World of Odd.