The Colombian Andes is an active volcanic chain produced by the present subduction of Nazca Plate under South America plate. Presently there are 15 active volcanoes in Colombia, among which is the Ruiz-Tolima volcanic massif. This massif comprises five different formerly ice-clad stratovolcanoes ("nevados"): El Ruiz, El Cisne, Santa Isabel, El QuindÃo, and El Tolima (figure 2). Nevado del Ruiz is the highest and most extensive stratovolcano in this massif. It rises more than 5,300 m above mean sea level, and supports an ice cap that had an area of 21.3 km2, measured on a 1976 Landsat image. The snowline is at an altitude of 4,900 m on its west flank and 4,800 m on the east flank.
The northern Andes is one of the three major volcanic provinces of the Andean orogenic belt. The volcanic northern Andes is part of a regional complex within which The Nazca, Cocos, Caribbean and Southamerican plates are in close dynamical interaction. The Benioff zone beneath the modern arc is about 150 km in depth, typical for andesitic volcanism. The crustal structure beneath the Cordillera Central (volcanic arc) is no well known in Colombia. In the vicinity of Ruiz volcano Bouguer gravity anomalies appears to be about -100 mgal. It suggests that the crustal section beneath Ruiz is either comparatively thin or anomalously dense when compared with that of the Galeras region, being -250 mgal, and 45 km thick (James D. E., Murcia, L. A. 1984).
The basement complex of the Cordillera Central consists dominantly of crystalline metasedimentary rocks, probably of Paleozoic or late Precambrian age. Mesozoic granodioritic batholits and sedimentary, metasedimentary, and volcanic sequences crop out on both flanks of the cordillera. An impresive series of large N-NE trending faults, chief among them the well known Dolores-Romeral-Palestina megashear, run longitudinally trough the Cordillera Central. The location of a least some of the modern volcanoes appears to be structurally controlled by these fault elements (James D.