FIELD ACTIVITIES /
Permits to sample three volcanic lakes will be granted by the organization and field activities may comprise:
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Collection of lake water samples;
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Gas flux measurements;
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Geophysical measurements;
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Other field activities according to the participants interests.
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Gas sampling and potential geophysical measurements in the fumarolic areas on the shore of Furnas Lake will also be possible.
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GENERAL INFORMATION /
The Azores archipelago is located in the North Atlantic Ocean between 37º to 40ºN latitude and 25ºto 31ºW longitude, about 1500 km west of mainland Portugal. The archipelago is comprised of nine volcanic islands, spread along a ~640-km-long WNW–ESE trending zone. Geographically, the islands can be categorized into three groups: the Western Group, composed of the islands of Corvo and Flores; the Central Group, encompassing five islands (Terceira, Faial, Pico, São Jorge, and Graciosa); and the Eastern Group, including the islands of Santa Maria and São Miguel, the latter where the workshop will be held.
São Miguel is the largest island of the archipelago (area of 744 km ) and has 138,138 inhabitants. The geology of São Miguel is dominated by three quiescent central volcanoes (Sete Cidades, Fogo and Furnas), associated with highly explosive eruptions of trachytic magmas and linked by two fissure zones (Booth et al., 1978; Moore, 1990; Guest et al., 1999; Pacheco et al., 2013). Two inactive volcanic systems (Povoação and Nordeste) are located on the eastern side of the island (Duncan et al., 2015).
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Location of the Azores archipelago in the North Atlantic Ocean and framework of São Miguel Island with the location of the three main lakes, Sete Cidades, Fogo and Furnas.
AREAS TO BE VISITED DURING THE CVL11 /
SETE CIDADES VOLCANO /
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Sete Cidades Volcano occupies the westernmost sector of the island and has an area of 110 km , a subaerial volume of 45 km , and a summit caldera ~ 5 km in diameter. In addition to Sete Cidades Lake, volcanic structures such as pumice cones, maars and domes are present within the caldera. No subaerial volcanic eruption occurred in this volcanic system since the settlement of the islands, in the 15 Century; however, three submarine eruptions were reported (one in 1638, and two in 1811). The only visible gas emissions in this volcanic system are associated with thermal springs located offshore.
Sete Cidades Lake (261 m above sea level) has a total surface area of 4.39 km and, due to morphological features, is subdivided into two interconnected water bodies, known as Blue Lake (Lagoa Azul; 3.58 km ) and Green Lake (Lagoa Verde; 0.81 km ). Blue Lake has a maximum length, width, and water depth of 2600, 2100, and 29.5 m, respectively, while Green Lake has a maximum length, width, and water depth of 1540, 770, and 24.5 m, respectively (Andrade et al., 2020b). The total estimated water storage of Sete Cidades Lake is ~58×10 m (Blue lake: ~47.4×10 m ; Green lake: ~10.6×10 m ; Pacheco et al., 2018).Green and Blue lakes are monomictic in character. Waters are cold (mean=17.6 ºC), weakly mineralized (EC mean=116 μS/cm) and with neutral pH (mean=7.7). The majorion relative composition in decreasing order is Na > Mg > Ca > K for cations and Cl > HCO > SO for anions. The lake waters are mainly of the Na–Cl type, which is likely caused by sea salt input, arising from seawater spraying. Values for the CO fluxes range between 0.3 and 17.2 g m d (Blue lake), and from 2.1 to 17.9 g m d for the Green lake. The most relevant degassing areas are found associated with the massive presenceof macrophytes and algal blooms. The measured values, supported by isotopic data of δ C suggest one single CO source, which is probably of biogenic nature. The estimated total CO emission varied between 5.8 t d (Green lake) and 24.9 t d (Blue lake) (Andrade et al., 2020b).
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Sete Cidades Lake inside the caldera of the Sete Cidade Volcano.
Santiago Lake is a 0.26 km maar crater lake with a maximum depth of 33 m, located in the southeastern part of the summit caldera at about 400 m a.s.l. (Andrade et al., 2019). From the storage volume, equal to 5.3x10 m , and the runoff inflow 2.17x10 m yr , the residence time was estimated as ~24.5 yr.
Santiago Lake is a monomictic lake, being usually stratified during late spring and summer, with low mineralized waters of meteoric origin (EC mean=128 μS/cm), being the temperature in the range between 12.4°C (winter period) and 23.8°C (summer period), and with neutral pH (mean=7.2). The main water type is Na-HCO , and the relative major-ion composition is HCO > Cl > SO > F for anions and Na > K > Mg > Ca for cations.
CO flux values measured in the lake surface range between 0 and 34.8 g m d , presenting a mean value of 8.3 g m d . The higher CO fluxes were measured during the winter surveys (5.6 t d ), while the lowest values (0.2 t d ) were recorded in the summer. These seasonal differences observed are associated with the monomictic character of the lake, as the CO is not able to ascend to the surface when the water column is stratified during the warmer period (Andrade et al., 2019).
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Santiago Lake inside the caldera of the Sete Cidade Volcano.
FURNAS VOLCANO /
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Furnas Volcano is the easternmost dormant central volcano of São Miguel Island. Growth of Furnas volcano started about 100,000 years ago, mainly due to explosive eruptions that emitted trachytic pumice deposits. Subsequent stages of caldera collapse produced a caldera complex (Guest et al., 1999; 2015). Two subplinian volcanic eruptions occurred since the settlement of the island, in 1439-43 and in 1630. Secondary manifestations of volcanism include fumaroles, thermal and cold CO -rich springs, as well as several diffuse degassing areas both inside the caldera and on the south flank of the volcano (Cruz et al., 1999; Viveiros et al., 2010). One of the four fumarolic fields is located in the north part of Furnas Lake.
Furnas Lake is located inside the youngest caldera of Furnas Volcano and has a surface area of 1.87 km , a maximum length of 2025 m and a maximum width of 1600 m. The lake surface is 280 m above sea level and the estimated water storage is equal to 14x10 m (Pacheco et al., 2010). The deepest part of the lake (15 m) is located in the northcentral sector (Andrade et al., 2016).
Lake water temperature is cold, with temperature values between 14ºC in winter period and 19ºC in early autumn. Temperature values along the water column suggest a monomictic character. Water chemistry depicts a relative composition in decreasing order of Na > K > Ca > Mg for cations, and HCO ->Cl > SO for anions. Furnas Lake may be classified as a non-active lake, due to the neutral-diluted character of the waters, which is shown by conductivity and pH, respectively, in the range of 152 to 165 μS cm and 5.3 to 8.7.
Measured CO flux values from Furnas Lake ranged between 0 and 20,960 g m d , with a mean value of 424 g m d . The total amount of diffuse CO emitted to the atmosphere was estimated between ~52 and ~600 t d , respectively, associated to early autumn and winter period. A magmatic CO origin is confirmed by isotopic analysis of δ C.
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Furnas Lake and one of the hydrothermal fumarolic fields located inside the caldera of the Furnas Volcano.
FOGO VOLCANO /
Fogo Volcano (also known as Água de Pau), is a depressed area of about 4.8 km , with a maximum diameter of 3.2 km and walls as high as 370 m, inside which a pumice cone and several domes can be observed (Wallenstein et al., 2015). Fogo Volcano dominates the central portion of the island, with an area of 132 km and a subaerial volume of about 44 km and corresponds to a complex volcanic system dated from > 200 ka (Muecke et al., 1974). The last magmatic events occurred already after the settlement of the island on 1563 and corresponded to a subplinian intracaldera eruption followed by a basaltic flank episode. A phreatic intracaldera episode was recorded in 1564 (Wallenstein et al. 2015).
Nowadays volcanic activity is characterized by the presence of fumarolic fields and thermal and cold CO -rich springs, all located on the north flank of the volcano (Caliro et al., 2015; Cabral et al., 2015). Several soil diffuse degassing areas have also been identified in this volcanic system, mainly on the north flank of the volcano (Viveiros et al., 2015). No visible hydrothermal manifestations are found inside the caldera. A geothermal power plant is located in the north flank of this volcanic system and produces about 50% of the Island’s energy.
Fogo Lake is located at an altitude of 580 m and has a surface area of 1.53 km , a length of 985 m, and a width of 2280 m (Andrade et al., 2020a). The maximum depth is 31.6 m and the storage volume is estimated as equal to 23.4x10 m . The watershed that drains to the lake has an area of 5.04 km , and the total inflow was estimated as 4.07x10 m /a (AHA-DRA, 2015). Water temperature of the Fogo Lake is cold (mean=16.5°C), and depict a monomictic behaviour, with water column stratification during summer
resulting in higher carbon dioxide concentrations in the hypolimnion. Samples present an acid to neutral pH (mean=7.04), and in general correspond to very diluted and weakly mineralized waters (mean EC=48 μS/cm). The prevailing hydrogeochemical facies are of the Na-Cl type, which derives from atmospheric marine salts contamination, major-ion content variation with depth shows low variation, along the water column.
Measured CO flux values are relatively low, ranging from 0 to 28.4 g m d (mean = 2.6 g m d ). The very low values measured in the central area of Fogo Lake are associated with the monomictic character of the lake. Statistical analyses suggest that the CO emission is associated with a single source of CO , probably biogenic; in agreement with the water δ C isotopic content. The estimated total CO emissions from the Fogo Lake was approximately 4.2 t d .
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Fogo Lake inside the caldera of Fogo Volcano.