The area is part of the southernmost sector of the Andean Block in Colombia. Precambrian high grade metamorphic rocks, the La Cocha - Rio Tellez Migmatitic Complex, make up the center of the Cordillera Central-Oriental; Paleozoic low grade metamorphic rocks, the Buesaco Metamorphic Complex, crop out in the Cordillera Central; Mesozoic low to very low grade metavolcanic and metasedimentary rocks form the Cordillera Occidental and the basament of the Cauca-Patia Interandean Depresion; and the Tertiary sedimentary formations, Mosquera and Esmita, are molassic rocks which fill the Cauca-Pati'a Depression. AII of the rocks are intruded by plutons and sub-volcanic bodies of intermediate composition. Two stages can be distinguished in the Cenozoic volcanism of southwestern Colombia: the first and extinct stage is connected in space and time with the sub-volcanic activity already mentioned; the second includes the present active volcanism. In the study area, both stages are present; the Galeras Volcanic Complex exemplifies both, ot which the Galeras Volcano, an active strato-volcano or caldera-volcano that presents hazards to the I ives and economic activity of the inhabitants of its slopes and adjacent areas, belongs to the final stage. The chemical composition of major elements of the Galeras Volcanic Complex lavas show that they are calc-alcaline andesites with little differentitation; geochemical behavior of these elements and sorne traceelements (Li, V, Cr, Co, Ni, Cu, Rb, Sr, Se, Y and Zr), as well as the presence of olivine xenocrysts and glomerocrysts as show thatthese lavas formed by fractional cristalization; the mineralogy, the geotectonic setting and isotopic ratio values of 143Nd/144Nd y 87Sr/86Sr, y de 18O also help to deduce that the lavas of the Galeras Volcanic Complex are orogenic andesites with little continental contamination derivated from magmas formed at a destructive plate margin (island are or inmature continental margins), supplied from the mantlewedge and the oceanic crust of the Nazca Plate which is subducting the Southamerican Plate.