The summit of the volcano has steep slopes inclining from 20 to 30 degrees. At lower elevations, the slopes become less steep; their inclination is about 10 degrees. From there on, foothills stretch almost to the edge of the Magdalena River, north of the volcano and the Cauca River to the west. On the two major sides of the summit, headwalls show where past rock avalanches occurred. At times, ice on the summit has melted, generating devastating lahars, including the continent's deadliest eruption in 1985. On the volcano's southwest flank is the pyroclastic cone La Olleta, which is not currently active, but may have erupted in historical times.