Abstract
The Azores archipelago is composed of nine volcanic islands located at the triple junction between the North American, Eurasian and Nubian plates. Nowadays the volcanic activity in the archipelago is characterized by the presence of secondary manifestations of volcanism, such as hydrothermal fumaroles, thermal and cold CO2-rich springs as well as soil diffuse degassing areas, and low magnitude seismicity. Soil CO2 degassing (concentration and flux) surveys have been performed at Pico, Faial and Sao Jorge islands to identify possible diffuse degassing structures. Since the settlement of the Azores in the 15th Century these three islands were affected by seven onshore volcanic eruptions and at least six destructive earthquakes. These islands are crossed by numerous active tectonic structures with dominant WNW-ESE direction, and less abundant conjugate NNW-SSE trending faults. A total of 2855 soil CO2 concentration measurements have been carried out with values varying from 0 to 20.7 vol.%. Soil CO2 flux measurements, using the accumulation chamber method, have also been performed at Pico and Faial islands in the summer of 2011 and values varied from absence of CO2 to 339 g m-2 d-1. The highest CO2 emissions were recorded at Faial Island and were associated with the Pedro Miguel graben faults, which seem to control the CO2 diffuse degassing and were interpreted as the pathways for the CO2 ascending from deep reservoirs to the surface. At Sao Jorge Island, four main degassing zones have been identified at the intersection of faults or associated to WNW-ESE tectonic structures. Four diffuse degassing structures were identified at Pico Island essentially where different faults intersect. Pico geomorphology is dominated by a 2351 m high central volcano that presents several steam emissions at its summit. These emissions are located along a NW-SE fault and the highest measured soil CO2 concentration reached 7.6 vol.% with a maximum temperature of 77 oC. The diffuse degassing maps show that anomalous CO2 degassing areas are controlled essentially by the tectonic structures and the lithology of the sites since the youngest volcanic systems are characterized by very low CO2 emissions.
Highlights
Carbon dioxide (CO2) together with water vapor (H2O) and sulfur dioxide (SO2) are usually the most abundant gases released to the atmosphere during volcanic eruptions (Giggenbach, 1996; Fischer and Chiodini, 2015 and references therein)
Soil CO2 concentration ranged between 0 and 20.7 vol.%, with the maximum values recorded at Faial Island (Table 2)
Anomalous soil CO2 degassing areas were identified at Faial, Pico, and São Jorge islands and a total of 15 diffuse degassing structures (DDS) were defined based on the integration of soil CO2 flux and concentration measurements
Summary
Carbon dioxide (CO2) together with water vapor (H2O) and sulfur dioxide (SO2) are usually the most abundant gases released to the atmosphere during volcanic eruptions (Giggenbach, 1996; Fischer and Chiodini, 2015 and references therein). During non-eruptive periods, gas emissions are frequent in many volcanic systems through the presence of permanent fumaroles, and due to the existence of thermal and cold CO2-rich springs, as well as soil diffuse degassing emissions (Allard et al, 1991; Chiodini et al, 1998) In the latter, the most studied gases released through volcanic soils are usually CO2 and the radioactive gas radon (222Rn); in some cases the CO2 diffusely emitted by the soils is of similar magnitude to the CO2 released from fumaroles and crater plumes (e.g., Allard et al, 1991; Chiodini et al, 2010a; Viveiros et al, 2010; Pedone et al, 2015). Based on the release of soil gases in confined areas, Chiodini et al (2001) named the anomalous CO2 degassing areas, where hydrothermal/volcanic CO2 is released, as diffuse degassing structures (DDS), whose shape depends on morphological, geological, and structural factors, such as the topography, existence of lithological heterogeneities, and presence of faults/fractures (Schöpa et al, 2011; Peltier et al, 2012; Pantaleo and Walter, 2014)
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