Abstract
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.
Highlights
Dynamics at other lakes, in Africa and other continents, are often over-interpreted as if they should be Nyos-type lakes when CO2 degassing occurs in those lake areas; this view might be most prudent, in case of doubt, it turns out to be unrealistic in some cases (e.g. Rouwet et al, 2019)
Out of respect for Lake Nyos, “the mother of volcanic lakes,” here we first provide an update on the postPliocene volcanic lakes located on the African continent, Madagascar and minor islands (Annobon, Bioko, Tristan da Cunha, Karthala, Mohéli Island and Mayotte, Figure 1), presented as VOLADA_Africa 2.0
Their nearness to the ocean (
Summary
The Cameroonian “killer lakes” Nyos and Monoun (Western Africa, Figure 1) are reputable for having induced the boom in volcanic lake studies since the late 1980s (e.g. Kling et al, 1989; Giggenbach, 1990; Giggenbach et al, 1991; Freeth, 1992; Evans et al, 1993, 1994; Freeth, 1994; Martini et al, 1994; Zhang, 1996, 1998; Viollier et al, 1995, 1997; Aeschbach-Hertig et al, 1996, 1999), to such a degree to have introduced a “Nyos bias” – for the good and the bad – on how to cope with lakes in volcanic craters in terms of hazard assessment and risk mitigation strategies (Rouwet et al, 2015a, 2019; Rouwet, 2021). Out of respect for Lake Nyos, “the mother of volcanic lakes,” here we first provide an update on the postPliocene (i.e. contemporaneous) volcanic lakes located on the African continent, Madagascar and minor islands (Annobon, Bioko, Tristan da Cunha, Karthala, Mohéli Island and Mayotte, Figure 1), presented as VOLADA_Africa 2.0 This version aims to 1) supply an updated list of the geo-referenced volcanic lakes, 2) shed light on the type of each lake, following classical (Pasternack and Varekamp, 1997; Varekamp et al, 2000) and novel (Christenson et al, 2015) classification schemes, 3) show a realistic picture on the level of study of each volcanic lake in Africa, regarding volcanological research sensu lato, and 4) provide a hazard assessment related to volcanic lakes in Africa. We suggest strategies for future research and monitoring setups for those lakes we deem as potentially hazardous, or as peculiar for other reasons from a volcanological point of view
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