Report on Oku Volcanic Field (Cameroon) — November 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 11 (November 1990)
Managing Editor: Lindsay McClelland.
Oku Volcanic Field (Cameroon) Working group assessment; temperature, ion concentration, and gas pressure increase
Please cite this report as:
Global Volcanism Program, 1990. Report on Oku Volcanic Field (Cameroon) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:11. Smithsonian Institution.
Oku Volcanic Field
6.25°N, 10.5°E; summit elev. 3011 m
All times are local (unless otherwise noted)
Research continues at Lake Nyos and on the broader problem of gas-charged lakes. Quoted material is from William Evans.
"The International Working Group on Crater Lakes (IWGCL) met 11-12 September in Nancy, France to discuss the hazards associated with Lake Nyos. Scientific teams from Cameroon, France, Germany, Japan, Nigeria, Switzerland, the UK, and USA attended the meeting, held at the 15th Colloquium on African Geology. The goal of the meeting was to prepare a document containing conclusions and recommendations regarding Lake Nyos that could be presented to the government of Cameroon. Each IWGCL member present could prevent the use of any statement in the document. At the conclusion of the meeting, there was unanimous agreement on the wording to be used. The following is a complete list of the conclusions and recommendations drawn up at the meeting."
Conclusions. 1) The 1986 gas disaster in Cameroon was caused by a massive release of CO2 from Lake Nyos. 2) Lake Nyos now contains ~300 x 106 m3 of CO2 and therefore remains dangerous. 3) This danger is increasing because CO2 is currently being added to Lake Nyos at a rate of at least 5 x 106 m3/year.
Recommendations. 1) Another gas disaster could occur at any time. Therefore the amount of CO2 in lake Nyos should be reduced as a matter of urgency. 2) Prior to controlled degassing, equipment should be installed for continuous monitoring of the stability of the lake. 3) Pipes should be installed to remove gas-rich water from the bottom of the lake. 4) Degassed water should be discharged outside the lake basin to avoid disturbing the natural stratification of the lake. 5) The rate of water extraction should not exceed the natural water recharge rate. Lowering the lake level would increase the risk of a gas release from beneath the lake. 6) Any system for gas extraction should be tested first at Lake Monoun. 7) The stability of the natural dam at the exit to Lake Nyos and the possibility of lowering the lake level should be further investigated. [A modified version of this document was published as Freeth and others, 1990].
"There is continued interest in the problems associated with Lake Nyos and its smaller analog, Lake Monoun. Cameroon, Nigeria, and several other countries maintain long-term research efforts. Scientists from the U.S. have made three field trips to Lake Nyos in . . . December 1989 and September and December 1990, to study the stability of the natural dam and the recharge rates of water and gas. The dam, which forms a spillway for the rainy season outflow, is 43 m wide at the top but is undercut several meters at its base. It maintains lake level at ~36 m above bedrock on the downstream side. The current rate of erosion due to water flow over and through the dam is unknown, but large-scale flooding and loss of life could occur when the structure ultimately fails. Dissolved salts and gases are transported into the lake by slightly thermal soda springs on the lake bottom at 210 m depth. Because of strong stratification of the water column, the incoming fluids are trapped in the deepest water layers. Studying the recharge process thus involves measuring temporal changes in bottom waters after the August 1986 gas burst when a partial mixing of the water column occurred. Near 200 m depth, water temperature has increased to 24.5°C, 1.0°C higher than in September 1986. Ionic strength has now risen from 0.015 to 0.024 at this depth. Total dissolved gas pressures, due mainly to CO2, have also increased. The highest pressure measured, 10.6 bar at 206 m depth, is almost 50% of the saturation pressure. When completely analyzed, these data will refine existing estimates of the various recharge rates."
Further References. Freeth, S., Kling, G., Kusakabe, M., Maley, J., Tchoua, F., and Tietze, K., 1990, Conclusions from Lake Nyos disaster: Nature, v. 348, no. 6298, p. 201.
LeGuern, F. and Sigvaldason, G., eds., 1989, The Lake Nyos event and natural CO2 degassing, I: JVGR, v. 39, nos. 2-3, p. 97-275 (15 papers); II: JVGR, v. 42, no. 4, p. 307-400 (10 papers).
Lockwood, J., Costa, J., Tuttle, M., Nni, J., and Tebor, S., 1988, The potential for catastrophic dam failure at Lake Nyos Maar, Cameroon: BV, v. 50, p. 340-349.
Nojiri, Y., Kusakabe, M., Hirabayashi, J., Sato, H., Sano, Y., Shinohara, H., Njine, T., and Tanyileke, G., 1990, Gas discharge at Lake Nyos: Nature, v. 346, no. 6282, p. 322-323.
Sano, Y., Kusakabe, M., Hirabayashi, J., Nojiri, Y., Shinohara, H., Njine, T., and Tanyileke, G., 1990, Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst: Earth & Planetary Science Letters, v. 99, p. 303-314.
Geologic Background. Numerous maars and basaltic cinder cones lie on or near the deeply dissected rhyolitic and trachytic Mount Oku massif along the Cameroon volcanic line. The Mount Oku stratovolcano is cut by a large caldera. The Oku volcanic field is noted for two crater lakes, Lake Nyos to the N and Lake Monoun to the S, that have produced catastrophic carbon-dioxide gas release events. The 15 August 1984, gas release at Lake Monoun was attributed to overturn of stratified lake water, triggered by an earthquake and landslide. The Lake Nyos event on 21 August 1986, caused at least 1,700 fatalities. The emission of ~1 km3 of magmatic carbon dioxide has been attributed either to overturn of stratified lake waters as a result of a non-volcanic process, or to phreatic explosions or injection of hot gas into the lake.
Information Contacts: (Conclusions and Recommendations document)M. Kusakabe, IWGCL, Japan; (Fieldwork)W. Evans, USGS; G. Kling, The Ecosystems Center, Woods Hole, MA; J. Lockwood, R. Schuster, and M. Tuttle, USGS.