There is a group of volcanoes that I’ve always wanted to talk about. It is a surprisingly little known, little studied location. However, about a decade ago, thanks to advances in technology, one of the volcanoes jumped from being practically uncharted to becoming moderately famous, when using satellites scientists were able to detect inflation around a very remote lake of the Chilean Andes. Laguna del Maule. But Laguna del Maule is far from the only interesting volcano here. In fact, I find some of its obscure neighbours to be even more fascinating.
Subduction zones tend to have a line of volcanoes that run parallel to the trench above the subducting plate. What is so special about the area of Laguna del Maule is that instead of a simple line, you have three different volcanic provinces with different eruption styles, and lava chemistries. There is the typical arc, with mostly intermediate andesitic magmas and stratovolcanoes. Then, behind the arc, to the east, lies another chain of volcanoes, these are dominated by felsic magmas, rhyolites, and rhyodacites. And even further into the backarc another volcanic province appears, made up of mafic magmas, mostly alkaline basalt, that form massive shield volcanoes and vast plateaus of lava.
The back-arc mafic province is known as the Payenia Volcanic Province. Its landscape is spectacular when seen from above, in Google Earth. Countless monogenetic shield volcanoes crowned with steep-sided cinder cones tower hundreds of meters above the deserted plains below. The land is covered by either basaltic lava flows, ashes from Calabozos and Laguna del Maule, or worn down granites and rhyolites from a Permian-Triassic silicic large igneous province. Grey ash, black scoria, pink granites, and a harsh, remote, human-deserted landscape stretch for 400 kilometres of the Payenia Volcanic Province.
The Payenian volcanism
The oldest Payenian volcanism forms extensive lava plateaus about 24-20 million years ago, which form a platform on which the later Payún Matrú shield volcano was built, and also extend north to Sierra de Palaoco. There are no visible central volcanoes that erupted these lavas in the Payún Matrú area, unless it was a volcano buried under Payún Matrú itself. In the Palaoco area there are three radiating dike swarms with a radius of 5-10 km, and also a separate laccolith, Bayo de la Batra, which is 3 x 2km in size and surrounded by updomed sediment layers. Each of the swarms and laccoliths may have been a small central volcano. Their age is not known, but because of their extensive erosion, that has completely removed their volcanic edifices, they might be 20 million years old or so.
Next volcano was Sierra de Huantraico, a spectacular radiating dike swarm, with a radius of 24 km, located at the southern end of the province. Dikes radiate from the ancient, deformed, and eroded summit of what must have been a huge shield volcano, probably similar to Auca Mahuida, given the great extent of its dikes. The dikes seem to follow a triple rift configuration. The age of its lavas is 18-17 Ma.
A second volcano, Cerro Bayo, formed to the NW of Huantraico, and was active a little later, around 16-15 Ma. The radiating dike swarm only reaches to 8 km. Cerro Bayo was possibly more of an explosive stratovolcano, I suspect, although it hasn’t been studied in detail. The intrusive core of this ancient volcano makes a small conical hill, whose nature makes me intrigued.
The Payenia was largely quiet between 15 Ma and 8 Ma. Although some activity did take place in the Cerro Nevado Volcanic Field, building small central volcanoes. Around 8 Ma there was a resurgence in activity. Both in the area of Sierra de Palaoco, and in the southern part of the volcanic province, in Sierra de Chachahuén.
At Chachauén volcano, the main activity lasted 7.9-5.6 Ma. This volcano has a small dike swarm, mostly less than 4 km in radius. The swarm is highly exceptional though, most of the dikes are white coloured, probably felsic composition, unlike typical dark dikes, and some are very thick. The thickest dikes are over 50 meters wide. I have spent a lot of time looking at intrusions from Southern Andes volcanoes, and these are probably the thickest dikes I’ve seen. This volcano produced large volumes of dacite volcanism, including abundant pyroclastic flows. My guess is that Chachauén was a field of fissure-fed dacitic lava domes, and was also highly explosive, producing ignimbrites fed by the lava dome collapse and/or by plinian eruptions. The composition of its dacitic and rhyolitic lavas are interesting. In a potassium versus sodium plot, the lavas match closely with that of Laguna del Maule, Puelche, and Tromen, they share the same slight sodium depletion and alkalinity. The lavas of Chachauen are different from the more alkaline (higher in both sodium and potassium) trachytes of Auca Mahuida and Payún Matrú, which are also more towards the sodic side of things, even though Chachauen is right between these two giants.
Around 2 million years ago, a major resurgence of volcanism started across the Payenia. Since then, practically every volcanic area in the Payenia has seen effusive eruptions, consisting of cinder cones and monogenetic shield volcanoes, of alkali basalt composition. The first major activity constructed the shield volcano of Auca Mahuida. The major shield erupted mostly from 1.8 Ma to 1.2 Ma, and has a total volume of roughly 900 km3. Lavas range in composition from alkali basalts to trachytes. But the evolved types seem very minor. There are only a few minuscule lava domes near the summit. A crater sits at the peak of the volcanic range, with a diameter of 1.5 km. It was probably the site of explosive eruptions with evolved compositions. Two rift zones extend to the east and west of the summit in the shape of a butterfly`s wings. Numerous cones and a few monogenetic shield volcanoes erupted great quantities of basaltic lava from its rift zones.
The E-W direction of Auca Mahuida’s rift of is the same as that of Chachauén, Payún Matrú, and Llancanelo volcanoes to the north. It is also perpendicular to the subduction. The orientation of dikes perpendicular to the volcanic arc and subduction zone reveals that the stresses during the volcanic activity have been compressive, the subduction pushes against the continent and squeezes the rock. The compression would close cracks that are parallel to the subduction and only allow for intrusions to be perpendicular to the arc. Like the rift zones of the shield volcanoes, that are perpendicularly oriented.
Volcanism during this time reached into every corner of the Payenia. It reactivated the Chachauén volcano, although with minor volumes. Cinder cones erupted from the east and west sides of this mountain. Palaoco also reactivated. Three clusters of cinder cones and shields formed next to each of the ancient radiating dike centres of Palaoco, and another cluster next to the Bayo de la Batra laccolith, all of them together amounting to 100 km3 of alkali basalts. This young Palaoco volcanism is known as the Llancanelo Volcanic Field. The last Llancanelo eruptions look very young and I guess must have happened less than 100,000 years ago. They consist of massive flows of pahoehoe lava formed in very long-lived effusive eruptions.
Around 1.3 Ma, an explosive stratovolcano grew near the centre of the Cerro Nevado Volcanic Field. This heralded the most spectacular phase of Payenia Volcanism, when monogenetic shield volcanoes started erupting all over an area 150 km across. This activity took place around 1.3-0.8 Ma judging from the few ages available. Some of these monogenetic shields have individual volumes of up to 20 km3 and are up to 700 meters high. Isolated conical mountains or chains of dark cinder cones and shields dwell in these forsaken lands. Lava flows descended their flanks and inundated the surrounding plains. They probably formed in single long-lasting eruptions, like the shields in Mexico and the Cascades. The composition seems to have been alkali basalt. Total erupted volume is difficult to estimate, maybe 900 km3 or less.
After 0.8 Ma, activity migrated away to the distant northern reaches of the Payenia. During 0.7-0.4 Ma activity was seemingly restricted to the Northern Mendoza Volcanic Field. Small-volume volcanism took place, building mafic lava domes, scoria cones, and a few small shields. About 50 km to the NW of the Northern Mendoza Volcanic Field lies Caldera Diamante, a massive circular collapse of 16 x 20 km wide, formed during a VEI-7 eruption at 0.45 Ma. It is probably not coincidental that the Northern Mendoza Volcanic Field was active next to Caldera Diamante during its build-up to the caldera-forming eruption.
Following this 400,000 year vacation to the north, the southern portion of the Payenia reawakened, with the construction of its most impressive central volcano yet. The great shield of Payun Matrú celebrated this resurgence by making the longest known lava flow during Quaternary times, the 180-km long Pampas Onduladas flow. To reach this length, the flow benefited from a continuous gently sloping ground, a high fluidity, and a very large volume of 7 km3. The eruption was long-lived, with low but sustained eruption rates, making inflated pahoehoe lavas. It was probably erupted somewhere from the east rift zone of Payún Matrú, but the original vent is buried under the shield volcano. Pampas Onduladas is the oldest dated eruption of Payún Matrú, around 370,000 years old. Since then Payún Matrú has grown into a 1200 km3 giant, a massive pile of lava flows erupted from an E-W trending rift zone system.
Payún Matrú is a fun volcano. It erupts variable types of lava simultaneously. Mafic alkali basalts and trachybasalts from its two long rifts, and highly evolved, viscous trachyte from the summit. It has done all kinds of eruptions.
Around 270,000 years ago Payun Matru grew a satellite stratovolcano, Payun Liso. As is typical of stratovolcanoes, Payun Liso erupted intermediate composition lavas, trachyandesites and basaltic-trachyandesites. It built a beautiful symmetrical cone. Its summit crater must have produced vulcanian explosions and subplinian eruptions.
168,000 years ago, the top of Payun Matrú volcano collapsed in a massive VEI-6 eruption of trachyte composition, making the Portezuelo Ignimbrite, and making an 8 km wide circular caldera.
Trachyte eruptions have continued to issue from the caldera. The eruptions happen through circumferential fissures around the west rim of the collapse. Because the fissures have opened outside the caldera, it shows that they are fed by cone sheet type intrusions, this type of intrusion, cone sheets, can be pictured as the petals of a flower, inward dipping sheets of magma that radiate from the magma chamber towards the surface. Lava issued from many vents along these fissures.
I was personally surprised to see how variable the thickness of the trachyte lava is. Some of the trachyte flows are 5 meters thick or less, while other flows are over 100 meters in thickness. Sometimes the same vent erupted lavas with dramatic differences in flow thickness. Because of this, I started to doubt it was a compositional difference. After some exploring, I found that the thicker flows were associated to longer more complicated flows, usually with multiple lobes, while the thinner flows were usually found next to vents that only erupted a single sheet of lava, and usually for a short distance. Because the more complicated flows must have formed during longer eruptions with waning eruption rates, I think the thickness is controlled by the eruption rate, and distance flowed by the lava. It might be similar to the pahoehoe aa duality of basaltic lavas, pahoehoe is thinner, while aa is thicker although they have the same composition. The trachytes of Payún Matrú formed very thin flows when they were erupted rapidly during the opening of circumferential fissures, which would be a form of evolved sheet pahoehoe, but formed thick flows during slower eruptions, which would be the evolved equivalent of aa. The reason these flows grew thicker is likely because of increased crust thickness damming the flow and holding up the melt.
The trachyte fissures not only follow circumferential directions around the caldera but also a slightly radial pattern from a large crater on the west side of caldera, which might represent some kind of central vent of Payún. This crater erupted the most voluminous trachyte flow, which I estimate has a volume of 2 km3, is also the thickest flow and one of the most complex ones, with multiple lobes. The eruption that formed this flow seems to have started highly explosive, maybe subplinian, and built a large pumice cone around the vent. This flow is dated at 7000 years ago. Some other trachyte fissures seem to me that they overlie the pumice from this 7000-year-old eruption and are probably younger. Overall, it looks like trachyte eruptions at Payún are increasing in frequency, most of the flows being very young looking.
The youngest eruptions of the shield volcano are basaltic. They happened on the west rift and showered black scoria on top of the big flow and the other circumferential trachytes. The basalt events were fast and remarkably explosive. Multiple vents erupted from fissures producing tall curtains of fire and pitch-black scoria. Some of these eruptions concealed the earth in a mantle of darkness, of black basaltic pyroclasts, for up to 10 kilometres downwind from the erupting cones. Floods of aa lava rapidly spread over the landscape around the fissures. One of the latest Holocene eruptions happened offset from the rift, to the north, at low elevations, and because of this it effused a particularly large lava flow with a volume of 0.85 km3. Although the other eruptions were also very substantial.
Tromen volcano is located at the intersection between the Payenia Volcanic Province and a north-south chain of volcanoes that includes Domuyo and Laguna de Maule rhyolite systems. This chain starts at Tromen and continues north to Laguna del Maule, including numerous stratovolcanoes, mostly inactive and eroded, but the way that the edifices are partly preserved means they probably date to the last few million years or so.
They are two other similar chains that run parallel more to the north, one ends at Calabozos volcano, the other at Overo volcano. Their significance seems enigmatic to me. Some authors believe that the subduction was shallower during recent times, ~5 million years ago, and led to the formation of many volcanoes further inland, then steepened drawing closer to the ocean, to the trench. But to be true, evidence indicates the volcanic arc has actually shifted inland since 15 million years ago. The 16 Ma Melado batholith near Laguna del Maule is actually closer to the trench than any of the present-day active volcanoes. In the area of Diamante Caldera, several dated plutons show a progressive inland retreat of volcanism from 15 Ma to 5 Ma, and seemingly continuing to present, since present volcanism has receded even further than these plutons. This retreat amounts to 80 km in the area of Diamante Caldera.
Widespread uplift that has brought up these underground pluton intrusions to the surface, and has formed tall mountain ranges, intense intra-arc seismicity, and evidence of a compressive stress from perpendicular-oriented rift zones in the back-arc, are all signs to me that the subduction is gradually shallowing, and is pushing up this area of the Andes. Some of the tallest mountains in the Andes are here, including Aconcagua (6961 m) which is 170 km north of Diamante Caldera and located within the Pampean flat-slab, a volcanic graveyard of the Andes. The Pampean flat-slab seems to be eating into this region from the north at Tupungato volcano, to the south at Lonquimay. This whole 570 km section of the volcanic arc having experienced some retreat from the trench since Miocene, most intense near the northern end. The lineaments of volcanoes behind the volcanic arc must have been erupted in back-arc positions, Including Tromen.
Tromen has seen multiple stages of activity. First stage between 2.3-2 Ma involved rhyolite eruptions, lava flows, domes and widepread pyroclastic deposits. It is not impossible a caldera collapse may have taken place, although evidence is not clear. If the rhyolite magma chamber collapsed, it would explain why rhyolite volcanism suddenly vanished. Following this stage, from 2 to 1.8 Ma, there were eruptions of andesites and trachyandesites which built a complex of overlapping stratovolcanoes. At present this stratovolcano complex is deeply dissected. A prolonged dormancy came until a new episode of volcanic activity happened in 1.2-0.8 Ma. This episode involved only minor eruptions, rhyolite lava domes which were erupted from the lower flanks of Tromen over an area 20 km across. Then Tromen entered another prolonged dormancy. Some basaltic eruptions happened to the north of Tromen during this time, but these seem more closely associated to late activity of the Wayle and Los Patos stratovolcanoes to the north.
Very recently there was a reactivation, building a small stratovolcano on the north slopes of Tromen. Using topographic contours I’ve done a rough estimate of this new cone’s volume. I was interested because the cone seem to be made of multiple overlapping structures, erupted from several vents. I pictured the edifice as multiple overlapping cones and shields of lava and estimated their respective volumes. The map below shows the results of this investigation of mine. Phase of activity 1 constructed a conical stratovolcano of 10 km3. This phase was the most explosive, with likely vulcanian and subplinian activity. The lavas from this phase are relatively well preserved and I think they must be very young, maybe post-glacial, less than 16,000 years or so. If that’s true, then the whole edifice is postglacial. Afterwards eruptions happened from peripheral vents around the cone. Each eruption was long-lasting and effusive, building shield like structures, built of many overlapping tongues of relatively viscous trachyandesite lava. Some eruptions may have lasted several years or decades.
The last activity of Tromen was within historical times. Eruptions are reported in 1820, 1822, 1823, 1827, and 1828. The 1822 event, is described as a great eruption, and the 1827 event as a smaller one. No more details seem to be known. The youngest lava flows of Tromen are 5, 6 and 7, the three are extremely young looking of a similar dark-brown colour, and perfect preservation. My speculation is as follows. Flow 5, with 0.1 km3, formed in 1820. Flow 6, which has 1.1 km3, was the grand eruption of 1822-23, gas-rich ashy explosions from an upper vent of the fissure seem to have showered the flow 5 cone in fine ash, but do not cover flow 7. Flow 7, with 0.3 km3, would have been the eruption of 1827-1828, which stratigraphically overlies flow 5. The three eruptions were sustained effusive eruptions. 7 and 5 had strombolian activity at the vent. Flow 6 partitioned gas and magma, gas blew into ashy explosions from an upper vent, and lava issued from a lower vent.
Domuyo is a huge mountain. It rises to 4,702 m. There is no other peak anywhere near as high around it, so Domuyo is the ruler here. Hence is it sometimes called the “Roof of Patagonia”. This is a volcano enveloped in mysteries and an awful lot of misconceptions. Domuyo is often described as a caldera or as a stratovolcano. There is no stratovolcano here. There is no caldera here either. But if it is not stratovolcano, and it is not a caldera, then what is it? The mountain is mostly made of Mesozoic marine sediments. But how can a volcano be made of sediments? Let’s first start with how we found out Domuyo was an active volcano.
Some volcanic rocks do happen in Domuyo. A series of rhyolite pyroclastic flow deposits mantle the lower western flank on the mountain. Lava flows then erupted on top of the pyroclastic flows. The eruption history is not known in great detail, but the last eruption had been roughly dated at 110,000 years ago. As such, it was natural to assume the volcano was extinct.
The first clue that there was something special going on with Domuyo came from a 2014 article which looked at Domuyo’s hydrothermal system. The western flank of the mountain, the same area of the lava flows, has several thermal springs and small geysers. There were also hydrothermal explosions in 2002, 2007, and 2012. Three of the thermal springs continuously discharge boiling water, which is thought to have been at 220ºC before emerging to the surface, while other thermal springs discharge colder water. The article measured the thermal energy released from the springs at ~1.1 GW, which the authors believe is the second highest measured heat flux from a hydrothermal system after Yellowstone. As such, they proposed Domuyo might not be a dead volcano after all.
Second clue came from a 2019 article where, using the satellite-based InSAR technique, they found that Domuyo was inflating. The uplift was roughly centred at the peak of the mountain and spanned an area ~20 kilometres across. The roof of the Patagonia was going up! It rose for five years, starting in 2015, and stopping by the start of 2020, reaching a total of 60 cm of uplift. Models show the source of this deformation to be somewhere 4-7 km under Domuyo, probably a sill-like magma chamber.
But there is more. As I’ve mentioned, the rocks of Domuyo are mostly sedimentary, with some interbedded igneous intrusions. The rocks are mapped as marine sediments of Jurassic and Cretaceous age. One may wonder how these marine sediments ended up at the top of the highest peak in the whole region and at the very epicentre of a magma driven inflation. I have been inspecting the mountain in Google Earth, and it is quite clear that Domuyo is a very rare structure. Sediment layers are exposed in the various glacial valleys that cut into the massif, and they always dip away from the summit of Domuyo at very steep angles. It seems as if the whole mass has been uplifted into a huge dome about 15 kilometres in diameter, which matches very well with the area that was inflating in 2015-2020. I assume that even its very name alludes to this shape. Domuyo likely comes from “domo”, which means dome in Spanish.
One of the traceable contacts between rock layers rises up from 2200 m elevation to 3600 m as you get closer to the summit. However, the contact can’t be traced all the way to the peak due to erosion. I expect that the total amount of uplift at Domuyo is of at least 2 kilometres. Doing a simple area per height calculation, I get that a rough volume of 150 km3 of magma would have been responsible for this making this dome. This kind of deformation is seen around or above many ancient plutons, where erosion has dug down to the level of the intrusions and has exposed concentric layers of rock arranged around the fossil magma chambers, much like a cut onion, which were displaced upwards by the intruding magma. This is also seen in resurgent domes at calderas or whenever a volcano inflates. A volume of magma that intrudes into the crust must displace the rock away to open up space. This is usually upwards, although it can also be sideways in places where there is extension, such as divergent plate boundaries, or volcanoes that are falling apart sideways. As such, it is likely that a very large shallow magma chamber underlies Domuyo, probably capable of a VEI-7 eruption, which makes this system a dangerous one. Hopefully, this volcano will get abundant research in the future, although its remote location and challenging mountainous terrain doesn’t help much. However, it hasn’t erupted in 110,000 years, so it doesn’t seem like much of an immediate threat.
It is surprising though that Domuyo’s dome nature doesn’t seem to be discussed in scientific literature much. There are only a few passing mentions of this uplift. I’ve read one article that refers to Domuyo, and also nearby Cerro Palao volcano, as granitic laccoliths, while another article speaks of it as an structural dome.
Next to Domuyo lies another smaller uplift structure, called Cerro Palao. It is possibly about 11 kilometres in diameter and with an uplift of about 1 km or more. It is not as spectacular as Domuyo, but you can still see the concentric marine sedimentary layers arranged around the mountain.
It is unclear whether Cerro Palao is active or not. There are numerous lava domes scattered over the landscape to the north of Palao. They are bright, probably rhyolites or dacites, and heavily eroded. They seem considerably older than the last flows of Domuyo. The domes are found along a number of red scoria cones of probably basaltic or basaltic andesite composition, which time has beaten down to irregular mounds of oxide-red scoria.
There are a lot of interesting volcanoes in this region. So far we’ve seen much of the province already. But a few important volcanoes are lacking. In the future I will do an article where I look at Laguna del Maule, Calabozos, and the nearby systems.
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