Abstract The Colombian volcano Galeras is one of the most active volcanoes in the world, and it has an andesitic composition and high explosive potential. We consider the period of 1989–2015, in which the volcano manifested frequent eruptions and high seismic activity. To study the temporal variations in the seismic structure beneath Galeras, we implemented an algorithm of repeated tomography using selected data sets with similar geometries. We obtained the variations in the seismic velocity structures beneath Galeras during the three time episodes of 1989–2000, 2001–2007, and 2008–2015. In the second episode, we observe a columnar anomaly with a high V p / V s ratio that possibly represents a conduit transporting a new portion of magma material from below. This result may indicate that in this period, which culminated with dome emplacement and collapse, the volcano was directly fed from deeper magma sources. In other episodes, the conduit is less visible in the tomography models, and we propose that the volcanic activity was merely controlled by shallow magma reservoirs at 2–4 km below the surface.