Volcanic Lakes in Africa: The VOLADA_Africa 2.0 Database, and Implications for Volcanic Hazard

Volcanic lakes pose specific hazards inherent to the presence of water: phreatic and phreatomagmatic eruptions, lahars, limnic gas bursts and dispersion of brines in the hydrological network. Here we introduce the updated, interactive and open-access database for African volcanic lakes, country by country. The previous database VOLADA (VOlcanic LAke DAta Base, Rouwet et al., Journal of Volcanology and Geothermal Research, 2014, 272, 78–97) reported 96 volcanic lakes for Africa. This number is now revised and established at 220, converting VOLADA_Africa 2.0 in the most comprehensive resource for African volcanic lakes: 81 in Uganda, 37 in Kenya, 33 in Cameroon, 28 in Madagascar, 19 in Ethiopia, 6 in Tanzania, 2 in Rwanda, 2 in Sudan, 2 in D.R. Congo, 1 in Libya, and 9 on the minor islands around Africa. We present the current state-of-the-art of arguably all the African volcanic lakes that the global experts and regional research teams are aware of, and provide hints for future research directions, with a special focus on the volcanic hazard assessment. All lakes in the updated database are classified for their genetic origin and their physical and chemical characteristics, and level of study. The predominant rift-related volcanism in Africa favors basaltic eruptive products, leading to volcanoes with highly permeable edifices, and hence less-developed hydrothermal systems. Basal aquifers accumulate under large volcanoes and in rift depressions providing a potential scenario for phreatomagmatic volcanism. This hypothesis, based on a morphometric analysis and volcanological research from literature, conveys the predominance of maar lakes in large monogenetic fields in Africa (e.g. Uganda, Cameroon, Ethiopia), and the absence of peak-activity crater lakes, generally found at polygenetic arc-volcanoes. Considering the large number of maar lakes in Africa (172), within similar geotectonic settings and meteoric conditions as in Cameroon, it is somewhat surprising that “only” from Lake Monoun and Lake Nyos fatal CO2 bursts have been recorded. Explaining why other maars did not experience limnic gas bursts is a question that can only be answered by enhancing insights into physical limnology and fluid geochemistry of the so far poorly studied lakes. From a hazard perspective, there is an urgent need to tackle this task as a community.

Carbon Dioxide in Lake Nyos, Cameroon, Estimated Quantitatively From Sound Speed Measurements

Gases dissolved in the deep water of lakes can pose a hazard when extreme concentrations are reached. A sudden release of large amounts of gas can cost the lives of humans living in the neighbourhood, as happened at Lake Nyos in 1986. Since 2001, the gas risk at Lake Nyos has been mitigated by induced degassing, but the lake continues to be supplied by CO2, and a regular survey needs to be implemented to guarantee safe conditions. Frequent sampling of this remote lake requires an enormous effort, and many analytical techniques are very difficult to run at the lake site. In this contribution, we combined a commercially available sound speed sensor with a CTD (electrical conductivity, temperature, depth) probe to obtain an indirect but quantitative estimate of carbon dioxide concentrations with fine depth resolution (decimetre scale). Dissolved carbon dioxide increases sound speed but does not contribute to electrical conductivity. Hence the difference between measured and calculated (on the base of electrical conductivity, temperature and pressure) sound speed gives a quantitative indication of dissolved carbon dioxide. We infer the vertical distribution of dissolved CO2 and hence continue the survey of the progress of the intended degassing. In conclusion, we present an easy to implement method for very high CO2 concentrations in deep lakes, and we highly recommend the implementation of the sound speed-CTD probe combination at Lake Nyos and at other gas-laden volcanic lakes, as such an approach could safeguard the people living in the area with acceptable cost and effort for the operators. In this manner, alarming CO2 concentrations in deep parts of lakes can be detected in a timely fashion.

THE 1986 LAKENYOSGAS DISASTER IN CAMEROON:CHALLENGES AND IMPLICATIONS ON SURVIVORS AND THE LIVESTOCK ECONOMY

This paper seeks to examine how the Lake Nyos gas disaster of 21 August 1986 affectedlivestock production and cattle rearing within the environment of Subum, Cha and Nyos. It brings to the forefront, the post-disaster effects onthe pastoral cattle rearing community. Thepolicies and strategies put forth by the various stakeholdersat the national and international levels have partially addressed some of the worries posed by this disaster, even thoughthe area is still in dire need of government and humanitarian support to improve onthe sector.The paper argues that, the resettlement of these livestock breeders could improvethis sector and render the environment sustainable for human habitation. Gleaned from a wide range of primary and secondary sources, the paper concludes that there isabsolute need for the government of Cameroon to tackle these long-term difficulties faced by these communities and to develop an effective livestock policy geared towards improving the livelihoods of the people around Lake Nyos.

THE 1986 LAKENYOSGAS DISASTER IN CAMEROON:CHALLENGES AND IMPLICATIONS ON SURVIVORS AND THE LIVESTOCK ECONOMY

This paper seeks to examine how the Lake Nyos gas disaster of 21 August 1986 affectedlivestock production and cattle rearing within the environment of Subum, Cha and Nyos. It brings to the forefront, the post-disaster effects onthe pastoral cattle rearing community. Thepolicies and strategies put forth by the various stakeholdersat the national and international levels have partially addressed some of the worries posed by this disaster, even thoughthe area is still in dire need of government and humanitarian support to improve onthe sector.The paper argues that, the resettlement of these livestock breeders could improvethis sector and render the environment sustainable for human habitation. Gleaned from a wide range of primary and secondary sources, the paper concludes that there isabsolute need for the government of Cameroon to tackle these long-term difficulties faced by these communities and to develop an effective livestock policy geared towards improving the livelihoods of the people around Lake Nyos.