Ollagüe (Spanish pronunciation: [oˈʎaɣwe]) or Ullawi (Aymara: [uˈʎawi]) is a massive andesite stratovolcano in the Andes on the Bolivia–Chile border, within the Antofagasta Region of Chile and the Potosi Department of Bolivia. Part of the Central Volcanic Zone of the Andes, its highest summit is 5,868 metres (19,252 ft) above sea level and features a summit crater that opens to the south. The western rim of the summit crater is formed by a compound of lava domes, the youngest of which features a vigorous fumarole that is visible from afar. Ollagüe is mostly of Pleistocene age. It started developing more than one million years ago, forming the so-called Vinta Loma and Santa Rosa series mostly of andesitic lava flows. A fault bisects the edifice and two large landslides occurred in relation to it. Later two groups of dacitic lava domes formed, Ch'aska Urqu on the southeastern slope and La Celosa on the northwestern. Another centre named La Poruñita formed at that time on the western foot of the volcano, but it is not clear whether it is part of the main Ollagüe system. Activity at the summit continued during this time, forming the El Azufre sequence. 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.
Name The original Aymara name of the volcano was Ullawi. It is derived from Aymara ullaña to see, to look at, to watch, and wi which is a nominalizing suffix to indicate a place, thus "viewpoint". The common name is Ollagüe. Other alternate names are Oyague, Ollagua and Oyahué.
Geography and geomorphology Ollagüe straddles the border between Chile and Bolivia, with most of the edifice lying on the Bolivian side. The Chilean portion lies in the commune of Ollague, in the El Loa province of the Antofagasta Region, while the Bolivian segment lies in the Potosi department. Towns and human sites close to Ollagüe are Amincha, Buenaventura, Cosca, El Chaco, Ollague and Santa Rosa. Ollagüe has a railway station, and the main road of Ollagüe runs along the western foot of the volcano. The first documented ascent was in 1888. The mountain reportedly can be climbed from the eastern side. The occurrence of warning signs about minefields has been reported.
Regional Ollagüe is part of the Central Volcanic Zone (CVZ), one of the volcanic arcs that exist in the Andes. The Andes have segments with volcanic activity and segments without; volcanic activity occurs only where the angle of subduction is relatively steep. There are four such segments, the Northern Volcanic Zone, the CVZ, the Southern Volcanic Zone and the Austral Volcanic Zone. The subducted part of the plate (slab) loses water as it sinks into the mantle, and this water and other components migrate into the mantle that lies between the subducted plate and the overlying crust (mantle wedge) and cause the formation of melts in the wedge. The CVZ is located between 16° and 28° southern latitude, on the western margin of South America. At this latitude, 240–300 kilometres (150–190 mi) west of the CVZ, the oceanic Nazca Plate subducts steeply beneath the continental South America Plate in the Peru–Chile Trench. East of the CVZ lies the Altiplano, a plateau with average elevations of 3,800 metres (12,500 ft). The CVZ contains about 1,100 volcanoes of Cenozoic age, including Parinacota, San Pedro and Tata Sabaya. Many volcanoes in the CVZ have summit heights exceeding 5,500 metres (18,000 ft), forming the Occidental Cordillera of the Andes at these latitudes. About 34 of these volcanoes are considered to be active; most of the volcanoes have not received detailed scientific reconnaissance. A notable feature of the volcanoes of the CVZ is that they formed over a fairly thick crust, which reaches a thickness of 70 kilometres (43 mi); as a consequence contamination with crustal material has heavily affected the magmas that formed the volcanoes. The crust is not uniform along the length of the south-central CVZ because the northern segment is of Proterozoic and the southern of Paleozoic age. The Central Andes formed first during the Paleozoic–Eocene and were worn down by erosion during the Oligocene. The recent volcanic activity started during the Miocene and includes major ignimbrite eruptions of dacitic to rhyolitic composition; such large eruptions began 23 million years ago and caused the formation of calderas like Galán. The total volume of this formation exceeds 10,000 cubic kilometres (2,400 cu mi). Stratovolcanoes also began to form 23 million years ago, although most were constructed in the last 6 million years. They are volumetrically much smaller and were formed by magmas whose composition ranges from basaltic andesite to dacite. Finally, small alkaline volcanic centres are found primarily in the back-arc region and appear to be young. A notable trait of the Central Andes are the long strike-slip faults that extend from the Eastern Cordillera northwest through the Altiplano into the volcanic arc. These include from north to south the Pastos Grandes–Lipez–Coranzuli, Calama–Olacapato–El Toro, Archibarca–Cerro Galan and Chulumpaja–Cerro Negro lineaments. Monogenetic centres are aligned on these faults.
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