Isluga () is a dormant stratovolcano in Chile's Tarapacá Region, close to the border with Bolivia. It is part of the Volcán Isluga National Park and lies in the Central Volcanic Zone of the Andes, one of several volcanic belts in the Andean Volcanic Belt. Isluga is an elongated mountain, whose summit reaches an altitude of above sea level, and features a summit crater. It forms a chain of volcanoes with several of its neighbours in the east. Sulfur mining took place on Isluga in the past. Polylepis trees growing at altitude form the highest timberline on Earth at Isluga.
The volcano began to form about half a million years ago during the Pleistocene in multiple stages, and there is evidence of historical eruptions which caused damage to neighbouring settlements. The last clearly attested eruption was in 1913, but more recent events in 1960 and 2005 are possible. Presently, there are conspicuous fumaroles on the volcano that generate gas plumes visible from the foot of the mountain. There are archaeological sites on the mountain and at its foot, and it has religious importance for the surrounding people.
Name
Isluga is the Spanish name of the mountain; the native names are Jiwq’ir qullu (Volcano Hill) or Jeoq'er Qollu, or Laram Qawani (Blue Valley), Laram Qhawani or Laram Kabani. These two names might refer to distinct aspects of the volcano, one to its physical and the other to its spiritual aspect. The etymology of Isluga is unclear, as it appears neither in the Quechua nor the Aymara language. One possible derivation is from "Islo", a plain in Alto Camiña. Other possibilities are that the name might come from the Kunza or Uru-Chipaya languages, the latter may contain a related word . Other spellings are Islonga and Isluca, Ilaga may be an alternative form, Isluya is a misspelling.
Geography Isluga lies in the commune of Colchane, of the Iquique Province of Chile's Tarapacá Region from the border with Bolivia and about east of Iquique. The town Enquelga is below the volcano at the foot of its lava flows. Other localities around Isluga are Chinchillani, Pampa Condorire and Payacollo northwest, and Carahuana/Caraguano, Chapicollo, Colchane Cotasaya, Enquelga, Escapiña, Isluga, Pisiga (on the Bolivian frontier) and Taipicollo south-southeast of Isluga.
The highest point of Isluga lies at altitude, rising about above the surrounding terrain. The volcano has at least three separate summit craters on a plateau west of the summit, which form an east-west trending line. The currently active crater is wide with steep rims, and is surrounded by a ring of volcanic scoria. Unconsolidated ash and pumice partially fill the crater, and pyroclastic rocks, sulfur crusts, scoria and volcanic ash crop out inside and in places where erosion has cut into the summit plateau. The summit plateau itself may be a former crater, which would make Isluga a somma volcano, and the summit of Isluga has been interpreted as a partially filled amphitheatre-shaped valley created by a lateral eruption to the northwest.
The volcano has the shape of a broad cone and consists of lava domes, conspicuous lobate blocky lava flows with well-developed forms such as levees and ogives, as well as lahars, pyroclastic flows, pyroclastic rock fragments and tephra. Lava flows are tens of metres thick and reach lengths of multiple kilometres. The edifice, which covers an irregular area of about , has a probable volume of . One report mentions the existence of a caldera. Isluga forms a horseshoe-shaped volcanic complex with the or high Quimsachatas northeast of the volcano, and the high Cerros Alpajeres form a lava dome that constitutes a western spur of Isluga.
There are no glaciers on mountains in the area, as the climate is too dry to allow their development, but some lava flows emplaced in the last 100,000 years bear evidence of glaciation. Lava flows have filled in some glacial valleys. Isluga is part of two watersheds, both of which eventually drain through Bolivia into the Salar de Coipasa: The Rio Todos los Santos to the northwest and the Isluga River to the south. Both originate in springs at the foot of the volcano, the waters that form the Isluga converge into Laguna Arabilla to form the river. The waters of the Isluga River during the 19th century were used for irrigation. Lake Parajaya lies north of Isluga.
Human activities
Agriculture in the valleys and pastures are the main economic activities in the area; tourist infrastructure includes camping, picnic areas and the hot springs of Enquelga but in 1988 tourism did not play much of a role yet. The volcano is within the Parque Nacional Volcan Isluga (PNVI), which disallows mining and restricts the extraction of natural resources.
The remote volcano can be accessed through the International Route 55, which is sometimes flooded out during bad weather. There is a trekking route leading to the summit of the volcano, which offers a panoramic view of the Altiplano. Sunburns, bad weather, animal (puma) encounters, and altitude sickness need to be watched for. park rules prohibit open fires, the introduction of exotic animals or disturbance of native animals, leaving the trails, damage or pollution to watercourses, the use of drones, noise, drug or alcohol use.
Sulfur mining on the Central Andean volcanoes, including Isluga, began during the 19th century. The sulfur deposits of Isluga were of regional importance and considered of high quality. Mining took place in the summit region, with activities attested in 1887; the sulfur was used for the processing of saltpeter. The volcano has been considered in evaluations of potential geothermal energy sources in Chile.
Geology
Off the western coast of South America, the Nazca Plate subducts under the South America Plate. This subduction process is responsible for the existence of the Andean Volcanic Belt, part of the wider Pacific Ring of Fire: Four gaps without volcanic activity subdivide the Andean Volcanic Belt into the Northern Volcanic Zone (NVZ), Central Volcanic Zone (CVZ), Southern Volcanic Zone (SVZ) and Austral Volcanic Zone (AVZ). The CVZ extends from southern Peru to northern Chile and includes more than a thousand volcanoes such as Taapaca, Parinacota, Guallatiri, Isluga, Tata Sabaya, Irruputuncu, Aucanquilcha, Ollagüe, San Pedro, Paniri, Uturuncu, Lascar, Cerro Overo, Socompa, Lastarria and the Altiplano-Puna volcanic complex. About 50 CVZ volcanoes were active during the Holocene but owing to their remote location their activity is poorly known. Lascar erupted more than 30 times since the 19th century, making it the most active volcano of the Central Andes.
The regional landscape is characterized by a tectonic depression to the west, the Occidental Cordillera in the centre, and the Altiplano to the east, with the focus of the CVZ in the Occidental Cordillera. Several cycles of volcanic and tectonic activity took place in the region from the Mesozoic to the Tertiary, followed by a state of quiescence interrupted only by major volcanic eruptions and characterized by water and glacial erosion. The terrain under Isluga consists of ignimbrites and volcanic rocks (mainly andesite, but also basalt and basaltic andesite), which formed during a Miocene flare-up of volcanic activity. In the early Pleistocene, volcanic activity settled into a steady state, during which it became concentrated in the volcanic arc of the Cordillera Occidental.
East of Isluga lie the volcanoes Cabaraya/Cabaray (), Separaya/Cibaray ( or ), Tata Sabaya (), Paryani/Sacasani/Saxani () and Villa Pucarani. The first four of which are situated on a geological lineament together with Isluga, Such east-west lineaments are common in the Chilean volcanic belt; whether the Isluga lineament is tectonically active is unclear. The lineament appears to separate the Antofalla craton of Proterozoic age to the north from the Chilenia terrane to the south. A long gap (Pica gap) in the volcanic arc begins south of Isluga, where Holocene volcanism is absent and there are several discontinuities in the geological structure of the Andes. Southwest of Isluga is the Puchuldiza geothermal field, with fumaroles and hot springs, which is sometimes linked to the volcano.
Composition
Isluga has erupted andesite, trachyandesite and trachydacite, which define a potassium-rich calc-alkaline suite. Component minerals of the rocks are amphibole, iron-titanium oxides and plagioclase, with biotite and pyroxene subordinate. Some rocks bear evidence of hydrothermal alteration, especially in the northern and eastern sector of the volcano. There are sulfur deposits on the slopes and the summit region both of Isluga and of neighbouring volcanoes.
Typical processes in the formation of CVZ magmas are the erosion of the continental crust at the trench, the absorption of crustal rocks by magma and the melting and dewatering of the downgoing oceanic crust, with crustal assimilation especially important. In the case of Isluga, magmas initially formed in the mantle and were mafic. As they ascended into the crust, garnet fractionation and the absorption of rocks from the lower crust took place. Under Isluga and Tata Sabaya they underwent further fractionation of plagioclase and pyroxene in the middle crust and of amphibole, biotite and plagioclase in the upper crust.
Eruption history
The volcano formed in several stages beginning 566,000±16,000 years ago. Initially, a pre-Isluga stage of volcanism ("Colchane Group") during the Pleistocene formed the volcanoes Cabaray, Quimsachata and Carcanchuni as well as the Cerro Blanco rocks south of Isluga; Isluga grew on top of rocks from this older stage. Radiometric dating has yielded ages of 500,000-100,000 years for rocks in the southeastern sector of Isluga, and of 100,000-50,000 years for the western and southwestern sectors. Two different staging systems have been proposed: The first envisages an Enquelga Unit, followed by the three Isluga Units and the Postavalanche Unit; the first crops out at the southern foot of the volcano while the other four units make up the bulk of the edifice. A major sector collapse to the northwest took place after the third stage; lava flows of the Postavalanche Unit cover the deposits of the debris avalanche. The second features a lower first stage, an upper second stage, followed by a third stage west and southwest of the summit and a fourth stage forming the summit region. The fourth unit, which is assumed to be of Holocene age on morphological criteria, bears evidence of sub-Plinian eruptions, far-reaching lahars and phreatomagmatic eruptions that spilled base surges over the summit.
Historical eruptions are known from the summit crater: August 1863. Weak eruption and its existence is contentious. August 1869 February 1878. Strong eruption with the emission of lava flows that allegedly destroyed numerous towns around the volcano. The existence of this eruption, which may be linked to a strong earthquake that year, is however contentious, and reports may reflect the effects of the earthquake rather than the volcano. 1885. Weak eruption. 1913. This is the most recent eruption of the volcano and was a phreatomagmatic eruption; it may be responsible for reports that light of volcanic activity was reflected in fumarole emissions in 1914.
Other, less certain eruptions are recorded from 1868 after the 1868 Arica earthquake, 1870, 1877, 1960, April 2002 and 2005, which may be a phreatic eruption linked to the 2005 Tarapaca earthquake. 19th century observers noted rumbling sounds, and earthquakes in the region were attributed to it. Seismic activity, while recorded and felt by people nearby, is relatively sparse at Isluga but with some variation in time (e.g volcanic tremors in 2013), and there is no evidence for ongoing deformation of the edifice.