Key lecture: Volcanological framework of Costa Rica and its volcanic lakes G.E. Alvarado 1, 2, 3, 4, G.J. Soto 1, 2, 4, 5, R. Mora 3, 4, P. Ruiz 6, C. Ramírez 3, 4, A. Vargas 1, J.F. Fernández 7 1: Área de Amenazas y Auscultación Sísmica y Volcánica, Instituto Costarricense de Electricidad 2: Escuela Centroamericana de Geología, Universidad de Costa Rica 3: Centro de Investigaciones en Ciencias Geológicas, Universidad de Costa Rica 4: Red Sismológica Nacional (RSN: UCR-ICE), Costa Rica 5: Terra Cognita Consultores, Costa Rica 6: Rutgers University, New Jersey, USA 7: Laboratorio Químico, Instituto Costarricense de Electricidad Costa Rica is located in a convergent margin, where three plates (Caribbean, Cocos and Nazca) and a microplate (Panama) interact. Hence, there is a complex young geology, since no rocks of pre-Mesozoic age are known. Magmatic provinces can be summarized in: a) ophiolitic complexes of different origins (200-40 Ma), b) volcano-sedimentary basins (100-0 Ma) including several primitive island arcs (125-100 Ma, 75-40 Ma), c) the first in situ arc (29-12 Ma), d) the second volcanic front (7-2 Ma), and the present volcanic front (2-0 Ma) with many eruptive foci, which have developed in three main stages. Orogenic plutonic rocks are Late Oligocene to Pliocene (29-2.1 Ma), mainly Miocene to Pliocene. In the present volcanic front, at least 9 volcanoes are definitively known to have erupted during the last 10 ka, but additionally at least 4 more could have had Holocene activity. Monogenetic vents are both isolated or part of the huge present volcanic massifs, structurally aligned. Arcuate grabens on volcanic summits (often misinterpreted as calderas: e.g., Poás and Tenorio), and horseshoe-shaped sector collapse amphitheaters are present in several volcanoes (e.g., Cacao, Miravalles, Irazú and Turrialba). Volcanoes such as Poás and Rincón de la Vieja have shown periodical eruptions characterized by short-lived (few hours to several days), violent (vulcanian and phreatic) eruptions, or clusters in periods of 10-70 years (Rincón de la Vieja and Irazú), while others clearly erupted once, and after that, have had long periods of inactivity previous to next eruption, from more than one century (i.e., Turrialba, as today), to several centuries (as Barva) to several thousands years (i.e., Congo). Hazard assessments have focused on the historically (after 1700 AD) active volcanoes (Rincón de la Vieja, Arenal, Poás, Irazú and Turrialba), and lesser on dormant volcanoes (Hule and Barva). Such hazard assessments, though, must be forwarded on other volcanoes without historical activity (like Orosí, Tenorio, Platanar and Porvenir), for a better volcanic risk management. At present, only six volcanoes (Rincón de la Vieja, Miravalles, Arenal, Poás, Irazú and Turrialba) have minimum monitoring systems in operation (seismic stations, geochemical and visual monitoring, geodetic control) for observing “normal” activity and unrest, though most monitoring is not in real time (only seismic records in some cases). Some volcanoes that could be classified as dangerous, and then represent an unpredictable potential hazard for future 1 eruptions, remain poorly understood and virtually unmonitored, situation that must be further tackled in a close future. The necessity for a better volcano knowledge in Costa Rica is mainly because several volcanoes are near densely populated urban areas (>1.5 million people), and also because they have become major destinations for ecologically-curious tourists from throughout the world, who visit the volcanoes, their rain forests, volcanic lakes, hot springs and other related attractions. For instance, about 4.2 x 105 tourists (43% foreign) visit the Costa Rican volcanoes every year. Poás, Irazú and Arenal are the top three spots for tourism. Other volcano-related products have also a tremendous positive impact in Costa Rican society and economy, as the exploitation of aggregates for construction, huge volcanic aquifers, and geothermal energy (up to 15% of the total electrical power produced in the country). Costa Rica has at least 5% of the hot hyperacidic lakes in active volcanoes of the world: Poás (21- 84° C, pH 0-1.8), and Rincón de la Vieja (31-47° C, pH 0.2-1.2), with ever changing colors from milky white to mustard or aquamarine. There are also cold crater lakes, as Irazú presently (13-15° C, pH 3-5) which has been warmer previously (in 1991-92 was 25-29° C, pH 3.0-3.5) due to a mild fumarolic input. Irazú lake has shown as well, drastic changes of colors varying from blood-red, to green to mustard. Other cold crater lakes are Santa María, Tenorio, Chato, Botos, Barva and Danta, located at the summit of dormant volcanoes. There are also lakes residing into the Holocene maars of Hule and Río Cuarto, which have presented overturning events, with sudden changes in their color to reddish, cyclically repeated through the last decades, causing massive fish deaths. Many other lakes in volcanic environments though, have been formed by damming by lava flows, lahars, debris avalanches or other geological processes, as for instance Los Jilgueros, Peje, Cedeño, Copey and Bonilla, among several others. 2 Hydrogeochemical study of Azorean lakes: monitoring active volcanoes P. Antunes , J. Cruz, R. Coutinho, F. Pedro, J. Fontiela Centro Vulcanologia e Avaliação de Riscos Geológicos (CVARG), University of the Azores [email protected] The Azores archipelago is located in the North Atlantic Ocean, about 1600 km from Europe and 2200km from North America, between the latitudes 37º-40ºN and longitudes 25º-31º W. Made by nine islands and several islets of volcanic origin, the archipelago occupies a flat area of approximately 2332 km2 and presents, in general, an WNW-ESE orientation. In the majority of the islands numerous lakes can be observed, whose physical characteristics are conditioned by the specific geologic setting. However, these water bodies are located predominantly inside explosion craters. A total of 88 surface lakes are distributed throughout the islands of São Miguel, Terceira, Faial, Flores and Corvo as well an 2 small cave lakes (Graciosa and Terceira islands). The total water volume stored in the crater lakes is about 90x106 m3, 93% of which in São Miguel island. The lakes on Flores Island contribute with 5% of the total water and the remaining 2% correspond to the lakes located in Terceira, Faial and Corvo islands. Several depth profiles was made between 2002 and 2007 in 6 lakes in São Miguel island, 1 lake in Terceira and Graciosa Island (caves lakes), 6 lakes in Pico Island and 5 lakes in Flores island. In general, sampled water are cold (5.2 ºC – 23.5 ºC) and correspond mainly to sodium chloride and sodium bicarbonate types. The water from Furna do Enxofre cave lake (Graciosa) is mainly from the magnesian bicarbonate type. Some of these lakes are eutrophic and in summer, water density stratification of thermal origin appears, surface pH reaches 10 and the hipolimnion is slightly acid, with the minimum value of 5.93. During this period, the bottom of the lakes shows a maximum concentration of CO2 (80 mg/L in crater lakes and 456 mg/L in Furna do Enxofre lake), and in some cases is coincident with a slight increase of the electric conductivity. Results suggest, in general, three different mineralization processes that occur in these systems: (1) caves lakes of Furnas do Enxofre, Algar do Carvão (Terceira Island) and Furnas lake (São Miguel Island), characterize a group in which the dissolution of silicate minerals controls water chemistry. This process is related with the water contamination by volcanic fluids, situation that is obvious in Furna do Enxofre lake. (2) The multivariate analysis suggests a second group of lakes, without dominant mineralization process [Sete Cidades, Santiago Congro (São Miguel Island), Funda, and Negra lakes (Flores island)]. Nevertheless, water are influenced by sea salts of marine transport, as well as by water-rock interaction. (3) The third group consists of small lakes systems from Pico Island, Flores Island, and Corvo island, except the Fogo Lake (São Miguel Island). This group of lakes have the less mineralized water and the mechanism that control the chemical composition is related, mainly, with contamination of marine salts. 3 Origin of fish kill events at Lake Averno, Campi Flegrei, Italy R. Avino, S. Caliro, G. Chiodini, C. Minopoli Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli, Osservatorio Vesuviano, Naples, Italy Lake Averno is situated in the homonymous crater in the northwestern sector of the Campi Flegrei active volcanic system in Campania region, Italy. The lake was affected in the past by a series of a fish kill events, the last of which occurred in February 2005. The origin of the event was investigated by means of a geochemical survey performed few days after the occurrence of the phenomenon. The survey revealed that the lake water was unstratified chemically and isotopically as a result of lake overturn. In fact, in contrast to the February 2005 results, data collected in October 2005, shows the Lake Averno to be stratified, with an oxic epilimnion (surface to 6 m) and an anoxic hypolimnion (6 m to lake bottom at about 33 m). Chemical and isotopic compositions of Lake Averno waters suggest an origin by mixing of shallow waters with a Na– Cl hydrothermal component coupled with an active evaporation process. The isotopic composition of Dissolved Inorganic Carbon, as well as the composition of the non-reactive dissolved gas species again supports the occurrence of this mixing process. Decreasing levels of SO4 and increasing levels of H2S and CH4 contents in lake water with depth, strongly suggests that anaerobic bacterial processes are occurring through decomposition of organic matter under anoxic conditions in the sediment and in the water column.