This phase of edifice growth was interrupted by a major collapse of the western flank of Ollagüe. Debris from the collapse spread in the form of hummocks down the western slope and into an adjacent salt pan, splitting it in two. The occurrence of this collapse was perhaps facilitated by a major crustal lineament that crosses Ollagüe from southeast to northwest. Later volcanic activity filled up the collapse scar, forming the Santa Cecilia series. This series includes lava flows as well as a compound lava dome on the western rim of the summit crater, which represent the youngest volcanic activity of Ollagüe. While there is no clear evidence of historical eruptions at Ollagüe, the volcano is considered to be potentially active and is monitored by the National Geology and Mining Service (SERNAGEOMIN) of Chile. Hydrothermal alteration has formed sulfur deposits on the volcano, which is the site of several sulfur mines. Later glaciations have formed moraines on the volcano.