Schütze, C.

An extensive self-potential survey was carried out in the central volcanic complex of Tenerife Island (Canary Islands, Spain). A total amount of ~237 kmof profileswith 20 mspacing betweenmeasurementswas completed, including radial profiles extending from the summits of Teide and Pico Viejo, and circular profiles inside and around Las Cañadas caldera and the northern slopes of Teide and Pico Viejo. One of themain results of this mapping is the detection ofwell-developed hydrothermal systemswithin the edifices of Teide and Pico Viejo, and also associated with the flank satellite M. Blanca and M. Rajada volcanoes. A strong structural control of the surface manifestation of these hydrothermal systems is deduced from the data, pointing to the subdivision of Teide and Pico Viejo hydrothermal systems in three zones: summit crater, upper and lower hydrothermal systems. Self-potential maxima related to hydrothermal activity are absent from the proximal parts of the NE and NW rift zones as well as from at least two of the mafic historical eruptions (Chinyero and Siete Fuentes), indicating that long-lived hydrothermal systems have developed exclusively over relatively shallow felsic magma reservoirs. Towards Las Cañadas caldera floor and walls, the influence of the central hydrothermal systems disappears and the self-potential signal is controlled by the topography, the distance to thewater table of Las Cañadas aquifer and its geometry. Nevertheless, fossil or remanent hydrothermal activity at some points along the Caldera wall, especially around the Roques de García area, is also suggested by the data. Self-potential data indicate the existence of independent groundwater systems in the three calderas of Ucanca, Guajara and Diego Hernández, with a funnel shaped negative anomaly in the Diego Hernández caldera floor related to the subsurface topography of the caldera bottom. Two other important self-potential features are detected: positive values towards the northwestern Santiago rift, possibly due to the relatively high altitude of the water-table in this area; and a linear set ofminima to thewest of Pico Viejo, aligned with the northwestern rift and related to meteoricwater infiltration along its fracture system.

assess the stability of the flanks of this volcanic edifice during such a crisis. To provide a response to this question, we analyzed a detailed fluid flow mapping plus the reiteration of a profile located in the vicinity of the active vents using the self-potential method, temperature data, soil-gas (CO2) measurements, and electric resistivity tomography. Coupling the interpretation of these methods that are sensitive to the flow of gas and water in the ground indicates the position of areas of mechanical weakness. In addition, they can be used to monitor the change in the discharge of fluids associated with these features before and during the 2002–2003 eruptive crisis. Our results emphasize the importance of old structural boundaries, such as the Large Fossa crater, in the development of the new set of fractures observed during the 2002–2003 eruptive crisis. Between October 2002 and January 2003, the use of CO2 soil-gas technique evidenced an increase in the discharge of CO2 outside the Large Fossa crater boundaries, along the failure boundary of the southern Sciara del Fuoco area. Self-potential and temperature measurements made before the 2002–2003 eruptive crisis reveal significant changes along the main structural boundaries of the Fossa area. The development of these anomalies is interpreted as an increase of the permeability of the structure from May 2000 to May 2002. Between January 2003 and March 2003 the reiteration of self-potential, temperature, and CO2 measurements shows an increase of fluid discharge along weakness planes located inside the Large Fossa crater boundary. They evidence no change outside this structural boundary. The importance of the Large Fossa crater boundary in controlling the deformation and fluid flow from January to March 2003 has been attested by the development of the fractures inside the Large Fossa crater boundary, and also with a network of electrooptical distance measurement stations located inside and outside this ancient crater. This multidisciplinary approach to fluid flow assessment before and during an eruptive crisis is complementary to geodetic measurements of the deformation of the edifice. It demonstrates for the first time the powerful potential of combining electrical resistivity tomography, self-potential, temperature, and soil CO2 measurements in assessing the position of the planes of mechanical weakness in a volcanic edifice.