Stratovolcano in Bolivia and Chile
Ollagüe () or Ullawi () 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 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, 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, 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 . 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 , 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 ; 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 . 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.
Local.
Ollagüe is a stratovolcano and lies isolated slightly east of the main volcanic arc. The volcano is usually covered with snow, which together with yellow and red colours gives Ollagüe a "beautiful" appearance. Other than some past glacial activity, the arid climate of the Altiplano region has kept erosion rates low, meaning that the volcanic edifice is well preserved. On the other hand, lack of erosion also means that relatively little of its internal structure is exposed.
Ollagüe has two summits, Ollagüe South is high and Ollagüe North . Southwest of the summit is the summit crater below the summit with a narrow opening towards the south, which forms the Quebrada El Azufre. The rim of the crater culminates into high Ollagüe South. The western rim is formed by several lava domes. These lava domes feature landslide deposits and lava flows that emanate from the foot of the dome. Originally they were considered to be a single lava dome, before it was found that the dome is formed by four individual domes. Just north of the summit crater lies another semicircular crater rim which encircles the summit crater on its northern side and whose high point is high Ollagüe North. The northeastern part of the edifice is old and affected by glaciation and the development of gullies, while the southwestern part has experienced younger activity and flank collapses. The volume of the well exposed edifice is about covering a surface area of . Ollagüe rises about above the surrounding terrain. 
The volcano has a number of adventive vents on its slopes, especially the northwestern and southeastern slope. These include Ch'aska Urqu on the southeastern slope and La Celosa (; also known as El Ingenio) on the northwestern. They lie at distances of and from the summit vent, respectively. The alignment of these subsidiary vents with the summit vents suggests that a N55°W striking lineament influenced their eruption; such channelling of magma along radial fractures has also been observed on other volcanoes such as Medicine Lake volcano, Mount Mazama and South Sister. A normal fault runs across the main edifice but is not aligned with these adventive vents, and the Pastos Grandes-Lipez-Coranzuli lineament intersects with the volcanic arc at Ollagüe. Fault scarps are found on the northwestern and southeastern side of the edifice. Overall, northwest trending lineaments exercised a strong influence on the tectonic development of Ollagüe, and may be the path that feeder dykes of the more recent eruptions followed. The basement undergoes extension perpendicularly to the lineament.
A wide phreatomagmatic vent named La Poruñita lies on the western slope, on the deposit formed by the sector collapse. It lies at an elevation of , is constructed out of tephra and formed on the sector collapse deposit. Farther up on the edifice, two cinder cones are found just north and west of the highest summit of Ollagüe.
Older volcanic centres around Ollagüe are Cerro Chijliapichina southwest (also known as Cerro Peineta), Cerro Canchajapichina south and Wanaku east of the volcano. These centres are unrelated to Ollagüe and were deeply affected by glaciation. On the eastern foot the Carcote ignimbrite crops out, a 5.9–5.5 million years old ignimbrite that is part of the Altiplano–Puna volcanic complex. These ignimbrites form the basement in much of the region. The Carcote ignimbrite originally formed a plateau that extended around the volcano. Off the western foot of Ollagüe lies a smaller volcanic centre that forms an effusive shield.
The Salar de Ollague is located due north, while the Salar de San Martin lies southwest and Salar de Chiguana northeast of Ollagüe. They are situated at elevations of . The Salar de San Martin and the Salar de Ascotán farther south form a northwest–southeast trending graben delimited by the same normal fault that crosses the edifice of Ollagüe. A ring plain formed by debris shed from Ollagüe surrounds the volcano.
Glaciation.
Presently, high insolation and evaporation as well as the dry climate prevent the formation of glaciers or the existence of a snow cover. Ollagüe lies in one of the driest regions of South America. Thus, the present-day snowline is higher than the volcano. Underground ice deposits have been found on Ollagüe; presumably they form through evaporation cooling.
Ollagüe has experienced glacial activity. Moraines are found on top of young lava flows and glacial valleys cut into the slopes. On the western side, there are remnants of a moraine girdle, which reaches an elevation of on the southwestern foot of the volcano. Another possibly separate moraine girdle has been reported in the summit region, at elevations of about . This moraine is thought to have been formed during the Little Ice Age. The Pleistocene snowline may have occurred at elevations of . 
Debris avalanche.
A major sector collapse occurred on the western flank of the edifice, with the deposit formed by the collapse extending west from it. Debris from the collapse flowed for into the Salar de San Martin/Salar de Carcote, which slowed down the landslide. Only the distal sector of the collapse deposit is still visible; the parts higher up on the edifice have been buried by more recent lava domes and lava flows. The distal segment is also slightly raised compared to the more proximal parts. The collapse deposit covers a surface area of and has a hummocky appearance, similar to the collapse deposit formed by the 1980 eruption of Mount St. Helens. The avalanche deposit separates the Salar de San Martin from the Salar de Ollague.
The younger debris avalanche deposit has a volume of about . It was believed that it occurred about 600,000–400,000 years or 800,000 ± 100,000 years ago, but dating of the andesites cut by the collapse yielded a maximum age of 292,000 ± 25,000 years ago. Later the deposit was covered by lake deposits and debris from the piedmont, and evaporites accumulated in depressions within the deposit. Several lake terraces are set into the avalanche deposit, with the traces of the highstand of Lake Tauca being recognizable; thus the sector collapse predates the highstand.
Andesitic lava bombs on top of the deposit may indicate that an eruption occurred during the collapse. Indeed, pyroclastic materials have been found at the foot of the volcano within the collapse deposit, where they fill small depressions. These materials are formed by several units of pumice and ash, generated by fallout and lava dome collapses.
The sector collapse was probably caused by the edifice oversteepening as it grew, with Ollagüe reaching a critical height before the collapse. Magma pressurization probably triggered the failure, as the remnants of a lava lake in its summit indicate that magma pressure in the edifice was high at the time of the collapse. Conversely, hydrothermal alteration