Most people wouldn’t think of Argentina as a country of fire-spewing mountains, yet there is quite a list of extinct and not-so-sure volcanoes, and some can even be called outright active, although dormant at the moment. One of the latter group is Volcán Tromen in the province of Neuquén, at the northern end of Patagonia. Until two or three years ago, nobody would have thought that it could come to life again, now we learn that it has erupted just recently… I mean, a couple of centuries ago.
Tromen is an Andean back-arc volcano, part of the Tromen-Domuyo fold-and-thrust belt on the western margin of the Neuquén basin, some 150km east of the main volcanic chain that marks the border between Argentina and Chile (i.e. to the E of the Nazca-Southamerican-plate subduction zone). As the slab pushes down into the mantle it causes compression against the continental plate, uplifting and folding the old sediments into the Andes mountains. While the volcanoes of the main arc mainly formed on top of fault systems often created by extension of the crust (through uplifting), Tromen and its neighbors are thought to have grown in an area of thrust faulting – creased and folded by horizontal compression. This zone strikes oblique to the main calc-alkaline Andean orogenic belt, and its lavas strongly resemble those of intra-plate volcanics.
THE VOLCANOThe dark lava flows on its flanks give Tromen an impressive appearance. Hikers have described it as a moonscape, and warn visitors to bring really thick-soled boots as the sharp edged debris cuts them quickly. At the foot of the volcano lies the picturesce shallow Tromen lagoon (not to be confused with Tromen Lake, further south near Lanin volcano). In olden times Tromen was also called “Tomen” and “Punmahuida” by the Mapuche and Pehuenche people.
Geologically, it is one of the youngest volcanoes of the Andes, dating from the Pleistocene; its volcanic activity started more than 2 million years ago. The structure of the Tromen massif is complex due to its long history, which began during the final deposition of the Mesozoic sediments in the Neuquén Basin. It was followed by two periods of compressional tectonics.
Tromen is located in the southern part of the Tromen Volcanic Plateau, it is the highest of several polygenetic eruptive centres in a north-south line incl. Cerro Wayle, Cerro Negro del Tromen and Cerro Tilhue. Tromen has had effusive eruptions (domes and lava flows) and may have had explosive ones as well (although the ignimbrites of the area may have been produced by another nearby volcano).
Two types of Quaternary volcanism of two contrasting compositions have been recognised in the massif: rhyolitic and andesitic volcanism. The rhyolitic resulted in lava domes, lava flows and pyroclastic deposits, with ages ranging from 2.3 to 0.8 Ma, while the andesitic volcanism mainly resulted in lava flows and dykes of ages ranging from 1.9 Ma ago to the present. Both lavas of basaltic to more felsic compositions have erupted simultaneously from Tromen in several periods.
The main crater is of ~500m ⌀, surrounded by 9 small cinder cones, some with their own craters; its slopes are covered with native sulfur. The last flows follow the current topography faithfully, they do not show erosion and end in broad lobes. They have coarse surfaces creased by pressure ridges, showing the morphology of aa lavas and blocks. The young flows went only to about 6 km from the foot of the mountain. Lava flows of basalt or basaltic andesite cover the entire edifice. However, scientists were amazed to find that Tromen is not built up entirely of volcanic materials but its core consists of a huge ridge of pushed-up Mesozoic sediments, the outcrops of which reach an altitude of 3000 m on the volcano.
In such a compressional setting, the rising magma is expected to have formed as horizontal or gently dipping intrusive bodies or sills. It is thought that, under these conditions, Tromen volcano may not lie vertically above its deep source of magma: If magma rises along a thrust, it must move horizontally over a large distance, perhaps in the order of kilometers, which involves significant horizontal transport of magma within the crust. I’m being somewhat vague here, because there seems to be a lot of discussion on whether or not the thrust compression and E-W shortening of the crust are still happening today, if compressional or extensional forces have been at work, as well as disagreements on several other points of tectonic niceties. – Volcanology research seems to be relatively new to this area, while geologists have been in demand for a long time exploring Argentina’s rich treasures of the soil.
SOME CONFUSION – DETECTIVE WORK REQUIRED!
The dilemma with historical writings
Two historical eruptions are mentioned in the GVP database: in ~1751, with a VEI=3, and in 1822. Also, one v. Wolff attributed purported eruptions of Punmahuida in 1820, 1823, 1827, 1828 to Tromen without giving a source. Several researchers have wrongly interpreted a report given in the diaries (1751/52) of a German missionary, Bernhard Havestadt, as proof of a witnessed eruption. He wrote (in Spanish) about Tromen volcano: “February 7 , reached Tomen [Tromen], where there is a valley and a lake [Laguna del Tromen] at the foot of two volcanoes that are called Punmahuida no doubt because the smoke it sometimes cast is thick, black and abundant, that even at noon the place darkens and turns day into night. For eight days, I had to walk over such slags [today El Escorial] with great difficulty for me and the beasts, as they ruin their hooves.” To make confusion worse, the priest had marked Tromen on his map with a flame, implying that it did erupt. However, Leandro D’elia et al., in their study of Tromen historical eruptions (2014), dismiss him having seen an eruption, saying that Fr. Havestadt had only referred to the name of the mountain, which meant something like dark mountain.
This study undertook to determine the age of the youngest volcanic products on Tromen’s NW side by looking at relative stratigraphic order, internal geomorphological differences, compositional variations and change of eruption centers. They find that the “El Escorial” blocky lavas belong to a younger eruptive cycle. Luckily they found a piece of bone buried in an older flow deposit which then could be dated to roughly 500 years old, and this puts these lava emissions in a time frame between 1400 and 1752 (when Fr. Havestadt walked over the “El Escorial”).
Leandro D’elia et al. also established a second eruptive cycle “between 1820 and 1828”; it is based on their results of stratigraphical, geomorphical and geochemical evidence (which on their own would not provide such a narrow time window). They supported their findings by a letter that Eduard Poeppig (or Pöppig, a very knowledgeable German naturalist/scientist who lived in Antuco town for six months in 1827/28), wrote to A. v. Humboldt.
I have done a bit of research on Poeppig, and it turns out that he has never been further east than Antuco volcano in Chile (and was the first to climb it), and Tromen lies 116 km E of Antuco. Rather, he got his information on the volcano from another missionary, and compared and completed it with oral descriptions by locals he met in Antuco. He wrote, “This is a double mountain with two craters, only one of which is active. It made a big eruption in 1822, and a smaller between 1827 and 1828. It is located in the country of the Pehuenches, along the old road from Antuco to Pampas. It was well-known to the Pehuenches.” I guess that this is also the source v. Wolff got his dates from. Well, the years could or could not be right; given that the Aborigines had no writing and only reported things from memory, some doubt should be in order. Anyway, science says, Tromen erupted between 1400 and now, perhaps several times, so it is a volcano to reckon with.
The dilemma with the names
I got very confused when trying to identify the parts of Tromen volcano. Before I started on this post I made myself familiar with the maps and learnt that Tromen consists of four main parts, from N to S:
- A. Cerro Wayle, 3,044 m;
- (Lake Tromen)
- B. Volcán Tromen, 4,114 m;
- C. Cerro Negro del Tromen, 2,821 m;
- D. Cerro Tilhue, 2,056 m
First, I tripped over an annotated image in a scientific paper where the peaks seemed plainly named wrong. To clarify the matter, I looked up the GVP database and got still more uncertain: “The summit of Tromen is cut by two overlapping 3.5-km-wide calderas. The Pleistocene Volcán Cerro Negro del Tromen, with a 5-km-wide caldera, lies immediately north, and lava flows have partially overtopped the northern caldera rim“… yes, they stand out well on Google maps, dark and bare of greens. So, has the latest activity taken place on Cerro Negro d.T. then?
In contrast, another study states: “The oldest activity was recorded in the Cerro Negro del Tromen, and the most modern in the extreme north of the massif, the volcano Tromen, where Holocene lavas are found.” (J. Llambías et al., 2011). Now this seemed to be a clear message. I failed to recognise the calderas mentioned in the GVP description on a map, but clearly, they were wrong in putting C. Negro in the northern part of the massif. I found three or four images and descriptions named wrongly, and I found more than five with the names assigned as I think is right, incl. the available maps. Soo… the bucket of ash is at hand, in case I have to pour some on my head…
PARQUE PROVINCIAL EL TROMEN
Researching for this post I couldn’t help marvelling about the rich wild life in this high and dry steppe & stone area, Parque Provincial El Tromen, an important wetland reservation. Although humans are warned not to use the water of Laguna del Tromen because of its acidity, there are 26 species of water fowl living on it, including Black-necked swans, Chilean Flamingoes, coots, ducks, etc.; some endangered species among them. There are eight species of reptiles, including the endemic lizard of Tromen (Liolaemus punmahuida).
Among the mammals are the Patagonian piche (Pichi, a dwarf armadillo), foxes , skunks, pumas, chinchillóns – and the pampas cat (Oncifelis colocolo), also classified as near threatened. Pampas cats are yet another small felid species that have never been studied in the wild, and this lack of knowledge makes it very difficult to develop an effective management plan to protect them. Some energetic people have dedicated their lives to research and education about this beautiful but elusive wild cat of the Andes:
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 facts, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Tromen
– Historical eruptions of the volcano Tromen: geomorphological and […] (2014, Spanish)
– Crustal collapse in the Andean backarc since 2 Ma: Tromen volcanic plateau (2008)
– Volcanism in a compressional Andean setting: A […] study of Tromen volcano (2007)
– Tromen-Tilhue Volcanic Group (2011, PDF)
– Volcan Tromen (Blog The Earth Story)
– The Andean Geotrail – project
– “Volcanoes of Patagonian Argentina – 1 – Tromen” (Earth of Fire, Blog)
– “Coeval volcanic activity and tectonic shortening, Tromen volcano, Neuquén province”
– Havestadt’s “Mappa Geographica…”
– Maravillas del Tromen (RioNegro.com)
– Parque Provincial El Tromen
– In Search of the Wild Cats of the high Andes (blog post)
– Cat in Thin Air