Volcanic lakes are complex natural systems and their chemical composition is related to a myriad of processes. The chemical composition of major, minor, Rare Earth Elements (REE) and physico-chemical parameters at the hyperacid crater lake of Rincón de la Vieja volcano (Costa Rica) are here investigated during February 2013–August 2014. The study of the lake chemical composition allows to identify the main geochemical processes occurring in the lake and to track the changes in the volcanic activity, both important for active volcanoes monitoring. The total REE concentration (REE) dissolved in the crater lake varies from 2.7 to 3.6 mg kg−1 during the period of observation. REE in the water lake samples normalized to the average volcanic local rock (REEN-local rock) are depleted in light REE (LREE). On the contrary REEN-local rock in the solids precipitated (mainly gypsum/anhydrite), from lake water samples in laboratory at 22°C, are enriched in LREE. The low variability of (La/Pr)N-local rock and (LREE/ HREE)N-local rock ratios (0.92–1.07 and 0.66–0.81, respectively) in crater lake waters is consistent with the low phreatic activity (less than 10 phreatic eruptions in 2 years) observed during the period of observation. This period of low activity precedes the unrest started in 2015, thus, it could be considered as a pre-unrest, characterized by infrequent phreatic eruptions. No clear changes in the REE chemistry are associated with the phreatic eruption occurred at mid- 2013. The results obtained investigating water-rock interaction processes at the Rincón de la Vieja crater lake show that rock dissolution and mineral precipitation/ dissolution are the main processes that control the variability of cations composition over time. In particular, precipitation and dissolution of gypsum and alunite are responsible for the variations of REE in the waters. Despite the low variations of (La/Pr)N-local rock and (LREE/HREE)N-local rock ratios, this study allows to suggest that REE can be used, together with major elements, as practical tracers of water-rock interaction processes and mineral precipitation/ dissolution at active hyperacid crater lakes over time, also during periods of quiescence and low phreatic activity.

In this article, volcanic lakes that have shown sedimentological evidence of limnic eruptions (i.e., Nyos-type gas bursts) are reviewed. Indeed, to better assess “Nyos-type lakes” related hazards, paleolimnology offers a promising tool to trace the evidence of potential ancient Nyos-type gas explosions. After gas bursts from Lakes Monoun and Nyos in 1984 and 1986, respectively, multiple paleolimnological approaches have been applied to several lakes assumed to be Nyos-type Around the world,. Only 3 lakes in Europe (i.e. lakes Pavin in France, Albano and Monticchio in Italy and one in Africa (i.e. Lake Kivu in D.R. Congo) evidenced markers of limnic eruptions in their sedimentary archives. These features include reworked sediments with reversed ages, brown colors of sedimentary deposits, gas-rich sediments, iron hydroxide-rich sediments, strong Ti and Fe enrichments, sedimentary hiatuses, absence of seismic evidence in the sedimentary record, and significant change in geochemical signature. The dating of these sedimentary deposits has made it possible to determine the ages of the events and their recurrence. This has led to associating these markers with evidence of limnic eruptions, even though some lakes are in temperate climates that favor seasonal overturning of lake waters and thus gradual release of accumulated gas. There is still no agreement on the dynamics and causes, and the scientific debate remains open since there is no concrete reference event in historical time. Lakes Monoun and Nyos, the first and only lakes exploded in recent history, could therefore be considered as natural laboratories to better understand limnic eruptions in lakes around the world. Unfortunately, the well-studied aspects of these Cameroonian “killer lakes” are based more on the dynamics of the explosions, hazard assessment based on water chemistry, and gas releases, rather than on the possible similar behavior in the recent geologic past by applying a combination of old and new limnological approaches. In addition, as the first natural laboratory, Lake Monoun features several advantages, including smaller surface area, shallower depth favorable for coring, easy access, and negligible gas content after artificial degassing since the early 2000s.

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.

Volcanic lake research boosted after lethal gas burst occurred at Lake Nyos (Cameroon) in 1986, a limnic rather than a volcanic event. This led to the foundation of the IAVCEI-Commission on Volcanic Lakes, which grew out into a multi-disciplinary scientific community since the 1990s. We here introduce the first data base of volcanic lakes VOLADA, containing 474 lakes, a number that, in our opinion, is surprisingly high. VOLADA could become an interactive, open-access working tool where our community can rely on in the future. Many of the compiled lakes were almost unknown, or at least unstudied to date, whereas there are acidic crater lakes topping active magmat- ic–hydrothermal systems that are continuously or discontinuously monitored, providing useful information for volcanic surveillance (e.g., Ruapehu, Yugama, Poás). Nyos-type lakes, i.e. those hosted in quiescent volcanoes and characterized by significant gas accumulation in bottom waters, are potentially hazardous. These lakes tend to remain stably stratified in tropical and sub-tropical climates (meromictic), leading to long-term build- up of gas, which can be released after a trigger. Some of the unstudied lakes are possibly in the latter situation. Acidic crater lakes are easily recognized as active, whereas Nyos-type lakes can only be recognized as potentially hazardous if bottom waters are investigated, a less obvious operation. In this review, research strategies are lined out, especially for the “active crater lakes”. We make suggestions for monitoring frequency based on the principle of the “residence time dependent monitoring time window”. A complementary, multi-disciplinary (geochemis- try, geophysics, limnology, statistics) approach is considered to provide new ideas, which can be the bases for fu- ture volcanic lake monitoring. More profound deterministic knowledge (e.g., precursory signals for phreatic eruptions, or lake roll-over events) should not only serve to enhance conceptual models of single lakes, but also serve as input parameters in probabilistic approaches. After more than 25 years of pioneering studies on rather few lakes (~20% of all), the scientific community should be challenged to study the many poorly studied volcanic lakes, in order to better constrain the related hazards.

Assessing Radiative Power (RP) output is essential for monitoring and understanding volcanic systems. While Mid‐Infrared channels are used to assess thermal outputs at volcanoes exhibiting effusive activity, Thermal Infra Red (TIR) bands are better suited for measuring moderate‐to‐low‐temperature(≲600K) features, such as those associated with hydrothermal activity. However, failure to meet key assumptions in TIR‐based calculations results in up to a ∼90% RP underestimation of ≲600 K sources. We thus introduce theTIR‐ based Volcanic Radiative Power (VRPTIR) method to accurately retrieve RP from single‐band TIR (10.5–12 μm) spectral radiance at systems dominated by surface temperatures of ≲600 K, that is, crater lakes and fumarole fields, achieving an uncertainty of ±35%. Comparison with ground truth for Ruapehu, El Chichón, Taal, Vulcano, Puracé, Poás, and White Island demonstrates the accuracy of VRP TIR in quantifying thermal output and detecting subtle variations in volcanic activity. This exportable method will facilitate compilation ofglobal RP inventories for moderate‐to‐low‐temperature volcanic systems.

Assessing Radiative Power (RP) output is essential for monitoring and understanding volcanic systems. While Mid‐Infrared channels are used to assess thermal outputs at volcanoes exhibiting effusive activity,Thermal Infra Red (TIR) bands are better suited for measuring moderate‐to‐low‐temperature(≲600K) features, such as those associated with hydrothermal activity. However, failure to meet key assumptions in TIR‐ based calculations results in up to a ∼90% RP underestimation of ≲600K sources. We thus introduce theTIR‐ based Volcanic Radiative Power (VRPTIR) method to accurately retrieve RP from single‐band TIR (10.5–12 μm) spectral radiance at systems dominated by surface temperatures of ≲600 K, that is, crater lakes and fumarole fields, achieving an uncertainty of ±35%. Comparison with ground truth for Ruapehu, El Chichón, Taal, Vulcano, Puracé, Poás, and White Island demonstrates the accuracy of VRPTIR in quantifying thermal output and detecting subtle variations in volcanic activity. This exportable method will facilitate compilation of global RP inventories for moderate‐to‐low‐temperature volcanic systems.