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. File:Summit View of Ollagüe from Bolivia(Jan2012).jpg|Ollagüe summit from Bolivia File:Ollagüe Summit(Jan2012).jpg|View from the summit of Ollagüe to southwest 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 and were filled with water during the late
Pleistocene-early
Holocene wet episode "
Lake Tauca". 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 avalanches Two major but hardly distinguishable
sector collapses 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 while part of the deposit shows evidence of water interaction; thus the sector collapses predate the highstand but one of the collapses took place when the lake was water-filled. 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 – which tends to weaken the stability of a volcanic edifice – was not involved in the onset of instability. The northwest–southeast cutting fault probably additionally destabilized the edifice, allowing it to fail into a southwestern direction. A previous southwesterly tilt of the basement also assisted in focusing the failure into that direction. The sector collapse formed a wide collapse scar on the upper western flank, although the summit itself was probably unaffected. This scar however was later filled by subsequent volcanic activity and modified by glaciation and is thus not conclusively identifiable. Two old sector collapses occurred during the older stages of volcanic activity. Their collapse scars are noticeable on the southeastern-southern and northwestern areas of the summit. The first is high and long, the second long and many high. Hydrothermally altered
breccia with block sizes of several from the first collapse fills a valley on the western slope of the volcano. Compared to the younger collapse, they are much narrower and have a highly unusual rectilinear form. These collapses occurred about 450,000 years ago along the strike of a normal fault that cuts across Ollagüe. Like in the young collapse, the summit was unaffected. The lava domes that form the western rim of the summit crater have been subject to smaller sector collapses as well. File:Volcán de Ollagüe, Bolivia, 2016-02-03, DD 90.JPG| File:Volcán de Ollagüe, Bolivia, 2016-02-03, DD 91.JPG|Fumarole clearly visible File:Volcán de Ollagüe, Chile, 2016-02-09, DD 72.JPG| File:Volcán Ollague.jpg| File:Volcán Ollagüe - panoramio.jpg| File:Volcán de Ollagüe, Bolivia, 2016-02-03, DD 80-88 PAN.JPG|Panoramic image, with Ollagüe in the centre == Composition ==