Ubinas (5675 m a.s.l.) is known as the most active volcano in southern Peru during historical times, with 23 minor eruptive or intense fumarolic crises since 1550. Due to its intense explosive recurrence and to the high proximity of the homonymous village (4000 inhabitants), only 6 km far away from the summit crater, Ubinas appears as one of the highest priority in volcano monitoring in southern Peru. The purpose of this survey has been (1) in a first step to evidence the geochemical characteristics of the hot springs located around the volcano in order to define (2) in a second step an appropriate fluid geochemical monitoring program. (1) The analysis of the waters collected around the volcano displays evident geochemical trends that can be interpreted as mixing processes between 3 end-members: fresh meteoric waters, magmatic fluids, sometimes interacting with a deep chloride reservoir....

Misti volcano (16°18'S, 71°24'W and 5822 in elevation) is located 17 km from Arequipa (-800000 inhabitants). It is characterized by two concentric summit craters. The youngest one hold lava dome, 130m wide, with fumarolic activity and temperature higuer than 200°C, geochemical techniques turn out to be very effective for the monitoring of active volcanoes. The ascent of a magmatic gases and their interactions with aquifers induce changes in the chemical composition and physical properties of the waters that can be sampled at the surface.The geochemical monitoring of the Misti volcano began in 1998 with a systematic sampling of the hot spring of "Charcani V", located to 6 Km of the active crater at 2960 meters in elevation.

El volcán Ubinas (16° 22’ S, 70° 54’ W; 5672 m.s.n.m.) ubicado en el sur del Perú y dentro de la ZVC o Zona Volcánica Central de los Andes (Fig. 1), es considerado como el más activo del Perú, con hasta 23 erupciones menores registradas históricamente en los últimos 450 años. Estudios geológicos recientemente realizados sobre este volcán han puesto en evidencia su condición de peligro potencial (Rivera, 1997; Rivera et al., 1997). Poco se conoce, sin embargo, acerca de su estructura interna. Este trabajo muestra los resultados de la aplicación de diferentes métodos geofísicos, geoquímicos, de la realización de mediciones de temperatura del suelo, y observaciones detalladas del interior del cráter activo, realizadas en el volcán Ubinas entre 1997 y 1999. Tales resultados han permitido proponer un modelo de estructura y de circulación de fluidos propio a este volcÃ...