Nyos lake

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Nyos lake
Lake nyos.jpg
Lake Nyos after the 1986 eruption
Geographical location Cameroon
Drain Kumbi; Mbum
data
Coordinates 6 ° 26 '16 "  N , 10 ° 17' 56"  E Coordinates: 6 ° 26 '16 "  N , 10 ° 17' 56"  E
Nyos lake (Cameroon)
Nyos lake
Altitude above sea level 1100  m
Maximum depth 208-210 m

The Lake Nyos is a crater lake in Cameroon (Central Africa). It is located in an old volcanic crater in the Oku volcanic area. The lake became famous through the Nyos tragedy, in which large amounts of carbon dioxide (CO 2 ) suddenly leaked from the lake in August 1986, killing around 1700 residents of the surrounding villages.

Geological conditions

Lake Nyos fills an almost circular maar - an explosion crater that is created when liquid lava meets groundwater. It is believed that the maar formed a maximum of 12,000 years ago during a volcanic eruption. It has a diameter of about 1800 m and is about 200 m deep. The area has been volcanically active for millions of years. After South America and Africa were separated from each other by plate tectonics about 110 million years ago , a rift valley , the Mbérégraben , formed in West Africa . The magma reaches the surface of the earth on a line that runs through the whole of Cameroon . The Cameroon Mountain is also on this line. Lake Nyos is surrounded by ancient lava flows and pyroclastic deposits.

The lake lies in the catchment area of ​​the Niger and drains into it in a north-west direction.

Saturation with gas

Lake Nyos is one of three lakes in the world that are known to contain carbon dioxide near saturation . The other two are Lake Manoun (also in Cameroon, about 100 km away) and Lake Kiwu between Rwanda and the DR Congo . A magma chamber under Lake Nyos is the source of carbon dioxide, which rises through the lake floor. An estimated 90,000 tons of CO 2 dissolve in the lake's water every year .

Solubility of CO 2 in water at different temperatures and pressures

The water in Lake Nyos is thermally stratified: layers of warm water on the surface lie over cold, denser layers on the lake floor. At a pressure of around 20 bar at a depth of 200 m, the cold water can store more than ten times as much CO 2 as surface water (see diagram). The constant supply of gas from the underground leads to a high level of CO 2 in the deep water over time.

Then when an event such as B. a landslide , a small earthquake , volcanic activity or a spontaneous outgassing due to the beginning of oversaturation mixes the water layers and saturated deep water reaches higher layers, this is suddenly oversaturated by pressure relief and temperature change and gas CO 2 . The gas-water mixture is specifically lighter than the surrounding water and shoots upwards. This leads to further pressure relief and mixing and thus further outgassing. This self-reinforcing process (overturning unstable equilibrium) causes large amounts of CO 2 to escape from the water in a short time .

Since CO 2 is heavier than air, the gas collects on the ground and flows as an invisible and odorless gas stream through the surrounding lowlands. An enrichment in the breath of more than 8 to 10%, which was probably present in the Nyos disaster, can lead to unconsciousness in humans within a short period of time and consequently to death.

The Nyos disaster

As early as 1984 there was a sudden outgassing of carbon dioxide at Lake Manoun in Cameroon, killing 37 people and which puzzled science for a long time.

Lake Nyos 8 days after the disaster. The shoreline of the lake that was flooded during the eruption is clearly visible.

On August 21, 1986 around 9:30 p.m., Lake Nyos suddenly released around 1.6 million tons of CO 2 . The gas flowed north into two nearby valleys, killing people and animals up to 27 km from the lake. About 1700 people and thousands of animals died.

The cause of this sudden outgassing is unknown. Most geologists suspect a landslide, some believe that a small volcanic eruption was the cause.

After the disaster, the affected villages were evacuated and the region declared a restricted area. In 2013 there were still 12,000 survivors of the disaster and their descendants in a total of seven reception camps. There are no basic health care, schools or other facilities in the camps.

Degassing project

Fountain (2006)

The extent of the disaster sparked numerous investigations into how to prevent it from happening again. Estimates of the amount of CO 2 entering the lake came to the conclusion that such outgassing could occur every 10 to 30 years.

As a solution, scientists proposed to lead pipes down into the deep layers of the lake and thus enable controlled and continuous degassing. A 14 cm thick polyethylene pipe installed by a team led by the French volcanologist Michel Halbwachs has been in operation since 2001 . It is attached to a raft at the surface, and weights at the bottom hold it in a vertical position. After the water flow was started once with a pump, it now runs independently: the water, which is approximately saturated with CO 2 , rises up the pipe from a depth of 200 meters. With a pressure decreasing from 20.78 to 0.88 bar, gas bubbles of carbon dioxide emerge (see diagram of the CO 2 solubility) and effect the delivery according to the principle of a mammoth pump : the water column, increasingly containing gas bubbles, develops significantly less hydrostatic in the pipe Pressure above 200 m than the compact water surrounding the pipe. On the surface, the two-phase mixture (water foam) shoots out of the pipe in a 40 meter high fountain while new seawater flows into the lower end of the pipe because it is pressed into the pipe by the water pressure of the sea depth. The CO 2 concentration in the air is not dangerous. It is hoped that this process will reduce the CO 2 concentration in the lake water to such an extent that similar catastrophes will no longer occur in the future (the pipe removes approx. 3 to 4 times as much gas from the lake as in the same period naturally added.)

In March 2011, the installation was expanded to include two more tubes. The total CO 2 content of the lake in June 2012 was around 40% lower than at the beginning of the degassing program in 2001.

Since the initially controversial project showed its first successes, a degassing of the Kiwu Lake in Rwanda is now also being considered.

Risk of dam breakage

When Dutch dyke experts surveyed the lake on behalf of the United Nations, they discovered that a natural dam could possibly break. The water masses would reach Nigeria, 100 km away, and up to 10,000 people could be killed. A 15 million dollar project, which is not universally recognized, provides for the water level to be lowered by 20 m in a short time by draining deep water in order to relieve the dam.

The Kashimbila Dam was also put into operation in 2015 to protect the lower settled people in the Katsina-Ala catchment area .

Myths

The anthropologist Shanklin reports on myths handed down by the locals, which, according to the description, could be based on earlier outgassing of the lake.

See also

literature

  • Henry Ngenyam Bang: 30 Years after the Lake Nyos Disaster: What Prospects for Rehabilitation in the Region? Book Venture Publishing LLC, 2016, ISBN 978-1-945496-20-2 .
  • Frank Westerman: The Valley of Death. Ch.links, Berlin 2018, ISBN 978-3-96289-012-4 .

Web links

Commons : Lake Nyos  - collection of images, videos and audio files

Individual evidence

  1. ↑ Prof. Joseph O. Ebeniro: GEOPHYSICS, a panacea for NATIONAL WEALTH AND SAFETY. (PDF; 2.6 MB) University of Port Harcourt, March 29, 2012, accessed on August 21, 2016 .
  2. ^ A b Preliminary report on the June 2012 field expedition to lakes Nyos and Monoun, Cameroon. (PDF) Japanese-Cameroonian SATREPS Project on Safety, Rehabilitation, and Development of the Lakes Nyos and Monoun areas in Northwest Cameroon., June 19, 2012, accessed on March 28, 2015 .
  3. ↑ Christoph Schmidt, Jean Pierre Tchouankoue, Peguy Noel Nkouamen Nemzoue, Félicité Ayaba, Siggy Signe Nformidah-Ndah: New thermoluminescence age estimates for the Nyos maar eruption (Cameroon Volcanic Line) . In: PLOS ONE . tape 12 , no. 5 , May 30, 2017, ISSN  1932-6203 , p. e0178545 , doi : 10.1371 / journal.pone.0178545 ( plos.org [accessed February 18, 2018]).
  4. ^ Russian general staff map . Retrieved August 21, 2016 .
  5. ↑ Monde Kingsley Nfor: Cameroon 27 years in detention. afrika.info, July 31, 2013, accessed August 21, 2016 .
  6. ↑ Ingo Knopf: Taming a Killer Lake. The first, November 13, 2015, accessed August 21, 2016 .
  7. ^ Dieter Lohmann: Pump and perpetual motion machine. In: The Knowledge Magazine. April 20, 2003, accessed August 21, 2016 .
  8. ^ Marguerite Holloway: Trying to Tame the Roar of Deadly Lakes. The New York Times, February 27, 2001, accessed August 21, 2016 .
  9. ↑ a b Jana Schlütter: Death from the depths. Die Zeit, January 26, 2006, accessed on August 21, 2016 .
  10. ↑ Abayomi Adebisi, Reginald Ikpeawojo, Imo E. Ekpo, Ali-Dapshima Abubakar: Progress on the Hydropower Component of Kashimbilla Multipurpose Dam Project, Nigeria. (PDF; 4.5 MB) 2015, accessed on August 21, 2016 (English).
  11. ^ Eugenia Shanklin: Exploding lakes and maleficent water in Grassfields legends and myth . In: Journal of Volcanology and Geothermal Research . tape 39 , no. 2-3 , 1989, pp. 233-246 , doi : 10.1016 / 0377-0273 (89) 90062-0 .