The mazuku
The word “Mazuku” comes from Kinyabwisha, a dialect from Rwanda, and indicates areas where both human and animals mysteriously die during the night (Verschuren, 1965). In the field, these areas were found to be dry gas vents that correspond to depressions where carbon dioxide, being heavier than air, accumulates by gravity in often lethal concentration (15 to >80 %vol. of CO2). According to isotopic analyses, CO2 originates from the Upper Mantle (Vaselli et al., 2003). Mazuku kill humans and animals every year, though this phenomenon is known since a long time in the Virunga Volcanic Province.
Mazuku in inhabited areas are located along the northern shore of Lake Kivu, between Sake and Goma. Due to the exponential demographic growth in this area, mazuku represent an increasing hazard that seriously threaten humans, especially refugees and new inhabitants who, unlike the local population, are not aware of the danger. Mazuku kill tens of people every year, which make them the most important natural hazard for the area in terms of human loss.
Preferential location
Mazuku in the Sake-Goma area are usually located within a 3 km wide strip along the shore of Lake Kivu, which corresponds to the area where volcanic cones have a phreatic or phreatomagmatic origin. This observation suggests a possible link between mazuku and a hydrothermal system and/or groundwater (Capaccioni et al., 2003 ; Vaselli et al., 2003).
Location map of the mazuku along the northern shoreline of Lake Kivu. (c) B. Smets, RMCA – 2010
Mazuku often appear at foot of lava flows where the superposition of lava flows creates morphological depressions. Depressions where gas accumulates can also be formed by collapsed lava tunnels, or open cracks (open faults, cooling fractures, etc.) that provide a preferential access to the surface for gas and a depression for gas accumulation.
The spatial distribution of mazuku also suggests a possible link with large structures such as volcanic fractures and faults, but it is not always well evidenced due to the lava flow accumulation that usually hides those structures (Smets et al., 2010).
Presence of Radon
Radon (222Rn) is also detected inside mazuku. Radon is a radioactive noble gas (T=3.8 d) that originates from radium (226Ra), a member of the natural 238U decay series. Uranium and radium concentration vary with the specific site and geological material. Radon migrates through pores in soil, fractures in rocks and along weak zones like shears, faults, thrust, …. As radon concentrations are extremely small (typically 1 x 10-17), radon needs a carrier gas (soil-air, CO2, CH4 ….) for transport over long distances.
Gas dynamics
Field observations show that atmospheric pressure, wind and rainfall can strongly influence gas concentration in mazuku. These meteorological parameters can modify the danger level of a given mazuku. Consequently, the human perception of the risk in a mazuku varies accordingly.
Subsurface CO2 concentrations in a mazuku located in Goma, along Lake Kivu. LEFT IMAGE: Gas concentrations in absence of wind. RIGHT IMAGE: Gas concentrations when wind is blowing. These variations can strongly influence gas concentrations, the level of the hazard, and the human perception of danger. (c) B. Smets, RMCA – 2010.
First data from the continuous gas flux monitoring in the mazuku “Le Chalet” show daily variations in gas concentrations measured in the subsurface and in open air of the mazuku. Data also reveal that solar irradiance is another parameter that influences atmospheric CO2 concentrations. The study of all variations in gas concentration is still in progress and no link with volcanic activity has been established so far.
CO2 and Radon evolution of the mazuku “Le Chalet” from 9th to 14th August 2009. Variation of charging the battery indirectly gives information on the evolution of solar irradiance and reveals that solar irradiance is another parameter able to strongly influence atmospheric CO2 in a mazuku. (c) B. Smets & A. Kies – 2010.
References
Capaccioni B., Vaselli O., Santo A.P., Yalire M.M. (2003) – Monogenic and polygenic volcanoes in the area between the Nyiragongo summit crater and the Lake Kivu shoreline. Acta Vulcanologica 14-15, 129-136.
Smets B., Tedesco D., Kervyn F., Kies A., Vaselli O., Yalire M.M. (2010) – Dry gas vents (”mazuku”) in Goma region (North Kivu, Democratic republic of Congo): formation and risk assessment. Journal of African Earth Sciences 58 (5), 787-798.
Vaselli O., Capaccioni B., Tedesco D., Tassi F., Yalire M.M., Kasereka M.C. (2003) – The “Evil’s Winds” (Mazukus) at Nyiragongo volcano (Democratic Republic of Congo). Acta Vulcanologica 14-15, 123-128.
Verschuren J. (1965) – Un facteur de mortalité mal connu, l’asphyxie par gaz toxiques naturels au Parc National Albert, Congo. La Terre et la Vie 3, 215–237.
