It’s always good to have a spare volcano in your desert – for scenery, for tourists, for sulfur, for energy, whatever – as long as it doesn’t erupt. There are far more volcanoes in the world which have not erupted in the last 10 000 years than those considered presently active. With most of them we will never know whether they are truly extinct or just keep their heads down. And then there are those that constantly wave a flag: Hey, I may have been dormant for a few ten thousands of years, but I’m not dead! Such sign could be some earthquakes telling of magma moving deep down. Or, like in the case of Ollagüe, a couple of permanent fumaroles blowing steam up high as a reminder.
Ollagüe (also Oyahué or Ullawi), with its hight of 5,868 m a.s.l, is a massive andesite and dacite stratovolcano in the Andes. I straddles the border between the Antofagasta region of Chile and the Potosi Department of Bolivia. Or, in geographical terms, it sits between the N-S-trending Western Cordillera and the Bolivian Altiplano. Ollagüe is serving well its role as spare volcano: it has delivered its sulfur for ages, and it greatly enhances the surreal landscape of the salt lake plains Salar de Ollagüe and Salar de San Martin Carcote (elevation of ~3690 m a.s.l.), as well as the bone-dry and salty grassland (puna) at the edge of the Atacama desert.
The area has scarce rainfall and an extreme thermal regime, in which besides human beings only lamas and donkeys have been able to maintain themselves. As you look further around and to the east on the satellite images you also see hundreds of volcanoes, cinder cones, lava flows, collapse deposits etc.. The next active neighbors of Ollagüe are the volcanoes Olca-Paruma, 50 km to the N, and San Pedro, 67 km S.
The Andes have segments with volcanic activity and segments without; volcanic activity occurs only where the angle of subduction is relatively steep. Ollagüe is part of the Central Volcanic Zone (CVZ). Here the Nazca Plate subducts steeply beneath the South America Plate, creating the Peru-Chile Trench. The CVZ represents the
active volcanic arc, ca. 120 km above the subducted slab and 240-300 km east of the trench.
One feature of the volcanoes of the CVZ is that they formed over a fairly thick crust, which can be ~70 km thick.
As a consequence, rising magmas re-melt and incorporate a lot of crustal materials along the way, thus producing various types of lava in the volcanoes (crustal contamination; magma mixing and magma mingling).
In the Miocene, major ignimbrite eruptions of dacitic to rhyolitic composition started from 23 million years ago. Most stratovolcanoes were constructed in the last 6 million years. They produced much smaller volumes and were formed by magmas with compositions between basaltic andesite and dacite.
Ollagüe is capped by a summit crater, 1 250 meters across, that is broadly eroded to the south. There are also two older craters on the summit.
The western rim of the main crater is formed by a compound of lava domes, the youngest of which features a vigorous fumarole that can be seen over large distances. One can recognize at least four lava domes that become younger to the south-east.
Around Ollagüe volcano some solitary and well-preserved small volcanic centers can also be seen. They are evidence of the great variety of past events: dacite lava domes and flows, basaltic andesite scoria cones, rings and lava flows, as well as a number of silicic lava domes and flows. 16 eruptive centers have been found on and around Ollagüe, all located along a 4-km-wide strip that crosses the entire edifice from SE to NW.
Pyroclastic deposits are very rare in the lava sequences of the Ollagüe cone, indicating that Ollagüe’s eruptive activity was mainly effusive. Three silicic lava flows from late post-collapse eruptions first seemed to be very young, but that was not so: they do show evidence of glaciation and therefore pre-date the last glacial advance at about 11 000 years ago.
The volcano is of Quaternary age; it grew upon the 5.5 Ma Carcote Ignimbrite, a tens-of-metre-thick, massive, resistant and lithified deposit. Its geological history has four stages of volcano building, separated by three main events of deformation and lateral collapse of the cone.
In its main growth phase the volcano piled up andesitic–dacitic lavas and pyroclastic deposits. Later, after a lateral vent pierced the side of the volcano, a collapse of the western volcano sector produced the hummocky terrain of a debris avalanche some 300 ka. Post-collapse lavas, now mainly andesites, were erupted on top of the avalanche deposits. The presently active summit dome grew within the collapse amphitheatre.
Research around Ollagüe has much focussed on the flank collapses that have happened probably in three places. During the evolution of Ollagüe volcano, these collapses proceeded from creep-like movements of the flank to the final catastrophic landslide of the SW sector of the volcano.
It was found that Ollagüe volcano has grown on a base much affected by extensional tectonics, and that a regional N-W-striking extensional fault system bisects the volcanic edifice. Successive fault offset below the volcano caused an increase in cone deformation and development of fractures.
The two older failure scarps on Ollagüe volcano have an unusual trend: they are straight lines – instead of the usual horseshoe shape. No examples had been documented before of flank or sector collapses with such geometry of collapse scars. The reason would have been the fault: As the volcano had grown across the said fault, the latter propagated upwards into the volcanic rock during following tectonic events. So it fractured the mountain inside along the fault’s direction, making it prone to failure with the next eruptions or earthquakes. In fact, it is thought that deformation from rising magma made the cracks slip in the end.
IS OLLAGÜE REALLY STILL ACTIVE?
Regional tectonics have caused successive deformations and collapses, in short, a weakness of the edifice. This explains geometric and compositional variations in the shallow magma feeding system of the volcano. During Ollagüe’s history, the feeding system shifted from a central conduit to lateral vents along the volcano’s south-eastern and north-western flanks (tree-like shape).
The lateral vents form an alignment with the central summit crater zone, depicting a NW-trending zone of weakness crossing the entire volcano. This zone of weakness is thought to be the expression of a sort of volcanic-rift, linked to regional tectonics. So, we have fracturing by regional tectonics, rifting and multiple pre-historic flank failures creating a new magma system. This all is suggesting that monitoring the volcano for further instabilities would be a good idea.
Moreover, the youngest summit dome produces continuous fumarolic activity. It is round-ish in plan view, about 850 m in diameter and 400 m high. Active emissions of gas occur from some fumaroles along a flank-failure wall that have exposed the core of the dome. In 2012, the plume had a relatively low SO2 content. A climber reports a second plume on the summit in 2014. SERNAGEOMIN reports microseismicity is present in the volcano.
A reactivation of this volcano would, with high probability, come with an extrusion of domes or viscous lavas. Possible pyroclastic current flows would be mainly directed towards the western flank, where the international route CH-21 is located and the railroad transits to Bolivia. A major eruption, though not very probable, could affect the town of Ollagüe.
While none of the usual sources have any evidence of historical eruptions at Ollagüe, one paper (G. Tamburello et al., 2014) state that “The last eruption occurred in 1903”. Also there were unconfirmed reports of activity in 1879, 1887 and 1927. Anyway, with good reason the volcano is considered to be potentially active and is continuously monitored by SERNAGEOMIN.
Sulfur Mining, Trains and Roads
With the construction of a railroad the opening and expansion of mining operations was much encouraged. Immediately below Ollagüe’s summit, sulfur was mined from the beginning of the 20th c. until 1976 (others say, mines are still working?). Only Quechua Indians from Bolivia or Peru were employed, as only they were accustomed to work at such heights. I guess they also had to build the access roads…
Some of the world’s highest roads go up the mountain in countless serpentines. In the western flank of the volcano, one leads from Buenaventura to an altitude of 5672 m. A former miner’s camp can be seen there at 4292 m altitude. Another road leads directly from Buenaventura from the west into the southern flank and there to 5500 m altitude. And a third, on the Bolivian side of the railway, goes from the east side of the volcano to an altitude of 5494 m.
Also an 11 km long cableway was constructed for material transport, overcoming an altitude of 1000 m. Just look at the images… and imagine the tremendous amount of back-breaking work that must have been done here!
The website “DangerousRoads” has a description, images and a video on the descent on one of those roads. “…the road to the summit is an old road on the north side to a disused sulphur mine. It’s extreme. Only 4wd vehicles with high clearance. The road ends at 5.705 m (18,717 ft) above the sea level. This road has humbled many egos. It’s not for the sissies and shouldn’t be attempted by novice drivers. The road is in dreadful condition and requires strong nerves to negotiate it. The gravel road is extremely narrow and bordered by a drop of hundreds of meters (many hundreds of feet) unprotected by guard rails.” This video by ‘venomancio’ kept me glued to the screen for all of its 15 minutes.
This unbelievable plant caught my eye the first time I read about this area:
Yareta or Llareta (Azorella compacta) is a tiny but woody-hard flowering plant native to the arid Puna grasslands. It is found at 3200-4500 m altitude, forming dense mats. It also grows very slowly, at a rate of 1-1.5 mm per year, and thus many yaretas have been calculated to be over 3,000 years old.
Inside the cushions a considerable amount of dead material accumulates with time. This detritus, the growth habit, the content of resinous components and the extremely dense growth all serve as adaptations to the extreme conditions of the highlands – they reduce water losses and help regulate the temperature. And they make this plant useful for men:
In those treeless heights, yaretas are collected by locals for heating and cooking, after they have been dried for 6 months. It is also a traditional medicine, and the ash is used as fertilizer.
Unfortunately, it was also used as fuel by railways and in the mines. Large quantities of this fuel were needed in the drying process, calcination, smelting, electric generators and in general steam machinery. The plants were collected in an ever more expanding area. From 1915-1958 one of the many nearby mines alone consumed 1000 tons of yareta per month, and so burned about half a million tons of these extremely slow-growing plants.
Although the use as fuel is now prohibited, for the local population, there are no alternatives to burning yareta. Decades of stress have markedly reduced plant populations, especially in Chile, and it is now believed that very few of the larger and older specimens remain.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Ollagüe
– SERNAGEOMIN, Volcan Ollagüe
– Volcanic and magmatic evolution of Volcán Ollagüe […] (1993, paywalled)
– Ollagüe Commune, Geography & History
– Mountain Forecast
– […] lateral collapses: insights from Ollagüe volcano […] (2006, paywalled)
– Faulting-assisted lateral collapses and influence on shallow magma feeding system at Ollagüe volcano (2008, paywalled)
– Gas emissions from five volcanoes in northern Chile […] (2014)
– Lavas at Volcan Ollagüe and the Origin of Compositional Diversity
– Miocene–Quaternary structural evolution of the Uyuni–Atacama region […] (2009, paywalled)