Diffuse seismicity and the active Galeras Volcano characterize the southern Andes of Colombia, whose inhabitants are prone to a high natural risk. We used field geological, structural, and quantitative morphological data to define the geometry and kinematics of late Quaternary faults and the volcanotectonic relationships in this area. The main NE-SW Buesaco Fault has a late Pleistocene-Holocene 29-km rupture length. Offset of dated landforms yields 188¿13 m of horizontal, right-lateral, long-term cumulative component of slip, the vertical component of uplift of the NW block being 67¿12 m. The long-term net slip rate is 1.48¿0.12 mm/yr. The other main structure, here named Aranda Fault, also strikes NE-SW and is 13 km long. Offset of landforms yields 160¿10 m of horizontal, right-lateral, long-term component of slip and 3.9¿1.9 m of vertical component of uplift of the NW block. The long-term net slip rate is 1.19¿0.08 mm/yr. On the Galeras edifice E-W to NW-SE faults have normal motions and slip rates from 0.02 to 0.23 mm/yr. Both the Galeras and a late Pleistocene pyroclastic cone lie along the ideal extension of the Buesaco Fault. Magma uprising in the crust can occur along the NE striking fractures when magma pressure (&sgr;mp) is greater than the normal-to-fault component of tectonic &sgr;1. Magma uprising in the uppermost crust (i.e., in the cone) can occur by rock cracking along E-W planes with &sgr;mp>&sgr;3 up to NW-SE planes with &sgr;mp greater than shear stress component of the horizontal greatest principal stress (&sgr;H max). ¿ 2000 American Geophysical Union