Native Sulphur, which is to say pure sulphur found as a mineral deposit, is a common phenomenon on volcanoes, particularly those with fumarolic activity (the venting of sulphur-rich gasses). Flows of liquid sulphur are a much rarer and less well-understood occurrence, making them of particular interest to volcanologists when they are found. Sulphur has at least 30 allotropes (allotropes are different forms of the same element, for example graphite and diamond are both allotropes of carbon), more than are known for any other element, and each of these allotropes has its own set of physical properties, including melting point, which can make its behaviour as an element difficult to predict. Furthermore, the viscosity of liquid sulphur varies significantly over quite small temperature ranges, reaching its minimum viscosity at 159°C, and becoming up to four orders of magnitude more viscous by 160°C, due to polymerisation, This also leads to sharp changes in the colour of liquid sulphur in response to temperature changes, with the liquid typically being yellowish at temperatures of less than 160°C, reddish between 160°C and 250°C, and brown-to-black ar temperatures above 250°C.
Fossil sulphur flows are known from a number of volcanoes, including Mauna Loa in Hawai'i, the volcanoes of the Galapagos Islands, Momotombo in Nicaragua, and mounts Tacora, Guallatiri, Irruputuncu, Aucalquincha, Ollagüe, Bayo, and Lastarria in Chile. Mount Lastarria is home to some particularly impressive fossil sulphur flows, with one example one reaching 350 m in length.
The first observation of an active sulphur flow took place on Shiretoko-Iozan volcano in Japan in 1932, where a 1400 m long slow was observed. More recent flows have been observed on Mount Turrialba in Costa Rica, one of which reached 175 m in length in 2012. Lakes of molten sulphur have also been observed in the craters of several volcanos, including Mount Poas in Costa Rica, Kusatsu-Shirane in Japan, and Mount Copahue, on the border between Chile and Argentina. Molten sulphur has also been observed on the Daikoku submarine volcano in the Mariana Arc.
Flows of liquid sulphur are typically reddish in colour. This is unsurprising when the sulphur has a temperature of around 200°C, however, this colour was also observed Poás and Hakone volcanoes at temperatures of 116–159°C. It has been suggested that this might be due to impurities in the sulphur, with arsenic, iodine, chlorine, and hydrogen sulphide being suggested as substances which could potentially changing the colour of the sulphur flow. Chemical studies of fossil lava flows have revealed significant quantities of arsenic, gold, molybdenum, nickel, and lead, all of which would have changed the properties of the melt, while a molten flow on Mauna Loa was reported to be enriched in molybdenum, tungsten, bismuth, mercury, gold, and copper, albeit at lower concentrations than the solidified lava flows.
Sulphur is naturally deposited around fumaroles on volcanoes, and liquid flows of sulphur were presumed to be formed when such deposits are heated past their melting points. However, studies of these systems have suggested an alternative route for liquid sulphur formation, involving reactions between sulphur dioxide, hydrogen sulphide, and water.
Liquid sulphur is not present at all volcanoes, and therefore presumably requires a very specific set of circumstances to form. Its presence is frequently linked to eruptions, particularly phreatic eruptions (explosions caused by liquid water coming into contact with hot magma), making the phenomenon more than a matter of abstract interest to volcanologists. Liquid sulphur was observed on Mount Lastarria in northern Chile for the first time in January 2019, a volcano which has not erupted in recorded history, but which has recently been undergoing ground deformation, and changes in the composition of the gasses it emits, presenting scientists with a unique opportunity to study the phenomenon and the conditions under which it occurs.
In a paper published in the journal Frontiers in Earth Science on 23 June 2023, Manuel Inostroza of the Millennium Institute on Volcanic Risk Research, Bárbara Fernandez of the Departamentode Ciencias Geológicas at the Universidad Católica del Norte, Felipe Aguilera, also of the Millennium Institute on Volcanic Risk Research and the Departamento de Ciencias Geológicas at the Universidad Católica del Norte, Susana Layana, again of the Millennium Institute on Volcanic Risk Research, Thomas Walter and Martin Zimmer of the GFZ German ResearchCentre for Geosciences, Augusto Rodríguez-Díaz of the Instituto de Geofísica at the UniversidadNacional Autónoma de México, and Marcus Oelze, also of the GFZ German Research Centre for Geosciences, and of the Bundesanstalt für Materialforschung und-prüfung, present the results of a study of the liquid sulphur flows observed on Mount Lastarria in January 2019, and of the solidified flows that were found on subsequent visits to the volcano in April 2019 and February 2020.
(A) Location map of the Central Volcanic Zone (CVZ) of the Andes, including the Northern, Southern, and Austral Volcanic Zones (NVZ, SVZ, and AVZ, respectively), showing the Lastarria volcano as a red triangle. (B) General view of the northern side of Lastarria volcano, including the four fumarolic fields (F1-F4). (C) Drone photographs of the fumarolic field 1 (F1), showing the location of the 2019 sulphur flows, a pool of molten sulfur in the upper part of the fumarolic field 1, and undocumented sulphur flows accounted between 2016 and 2020. Inostroza et al. (2023).
Mount Lastarria, along with Mount Espolón and the Negriales lava field, forms the Lastarria Volcanic Complex, which is in turn part of the Lazufre Volcanic Area, along with Cordón del Azufre and Bayo volcanoes. Lastarria is built up from a series of basaltic andesites to dacitic lava flows and domes, in addition to block and ash and fallout deposits. Evidence suggests the volcano has gone through ten periods of significant activity, the oldest of which began about 260 000 years ago.
Recently, radar monitoring of Lastarria has suggested that it is inflating by about 3 cm per year. This has been attributed to moving magma beneath the volcano, with two centres of hydrothermal fluid circulation detected beneath the volcano, one at a depth of 7-15 km, and the other much shallower, at about 1 km.
Lastarria is currently undergoing vigorous and persistent fumarole activity, which is also a symptom of magmatic and hydrothermal fluids moving close to the surface, producing about 800 tonnes of sulphur dioxide per day. Gasses are emitted from the volcano at about 408°C, and combination of substances including sulphur dioxide, hydrochloric acid, hydrofluoric acid, hydrogen sulphide, and methane. The gasses are thought to be evolving from a hydrothermal system fed by precipitation, but since 2012 the composition of the fumaroles has been steadily shifting towards something closer in chemical make-up to a magma source, which suggests that a significant amount of gas is entering the hydrothermal system from a pressurised magma chamber.
The northern flank of Lastarria is largely covered by vast yellow crusts and alteration zones, the products of the fumarole activity, which are formed by materials precipitating out from the gas emissions. These deposits are notably colourful, with a wide variety of colours (white, yellowish, orange, reddish, and grey) produced by the different sulphate, sulphite, halide, borate, and native elements present. The area is home to deposits of arsenic, lead, and thallium minerals, as well as high concentrations of selenium, cadmium, zinc, and copper. Also noteworthy are a series of fossil sulphur flows, ranging from 220 m to 350 m in length, and known to have been laid down prior to 1964. These flows are typically a pale yellow in colour, with ropey textures, and contain large amounts of rock fragments, which would suggest the flows were both cool and not particularly viscous when they were active.
Satellite images of the area where the sulphur flows occurred were obtained both before (2016) and after (2022) the event by the Pleiades Satellite, and the area was visited while the flow was occurring, enabling images to be taken, and videos made, of the flows with a mobile phone, and the direct measurement of temperature with a probe. Samples were collected from the flow in January 2019, while it was molten, and again in April 2019, when it had cooled and solidified.
Two separate sulphur flows were observed on Mount Lastarria, the first was observed only in the satellite images, and happened at some point between 2016 and 2022 (and probably between January 2019 and February 2020) at an altitude of about 5100 m. This flow comprised several overlapping units, with a maximum length of about 55 m, and a maximum width of about 5.3 m. This flow emerged from a 16 m wide fumarole cluster, and descended to about 5074 m, implying a dip angle of about 25°.
The second flow originated at 5114 m, and was directly observed in January and April 2019, This flow comprised four sub-flows, the first of which was flowing when the site was visited in January 2019. This first sub-flow reached 9.5 m from its source, and was 108 cm wide and an average of 3 cm thick, although the thickness increased lower down, reaching about 4 cm close to the lower front of the flow. This flow was a dark brown towards its centre, fading to a pale grey around its edges. This flow appears to have followed well-defined channels for much of its route.
Sub-flow 2 flowed over the top of sub-flow 1, splitting into two branches, which reached 12 m and 9.8 m from the source, with a maximum width of 80 cm and a maximum thickness of 5 cm. Sub-flow 3 flowed over the top of the previous flows, reaching about 7.8 m from the source, 1 m wide, and 7 cm thick. Finaly sub-flow 4 reached 7 m from the source, and was only 18 cm wide and 1 cm thick. The total volume of material in the flows was estimated at 1.45 m³.
The flow observed in January 2019 was reddish brown in colour, and bubbled continuously, due to gas entering the pool from below. This pool was measured at 158°C, and was overflowing in two places, producing flows which travelled down channels with an average angle of descent of 11-15° at a speed of 6.9 cm per second. The flow slowed and eventually stopped as the slope flattened out, and the sulphur piled up, cooling to form a greyish crust with a rope-like texture. The temperature at the front was measured at 124°C. The sulphur flowed faster in the centre, leading to the formation of levees at the edge of the flow, and the overall development of a delta-like form. The surface was uneven, and blocky in places, due to the different velocities at which the molten sulphur was moving within the flow, with the maximum observed velocity being about 40 cm per second.
Arsenic was the most abundant trace element, reaching more than 40 000 parts per million in the pool from which the flows originated, and more than 6000 parts per million in flow number 1. The elements lead, bismuth, copper, zinc, rubidium, zirconium, antimony, and tin were all present at between 1 and 56 parts per million, while the elements lithium, cadmium, thorium, uranium, colbalt, scandium, nickel, gallium, and niobium were present at concentrations of less than 1 part per million.
The second flow was sampled in April 2019, when it had cooled and solidified. The samples collected were comprised of orthorhombic sulphur crystals, with degrees of crystallinity (a measurement of the proportion of ordered molecules) of 45% in the pool sample and 65% in the flow sample. Analysis of the samples showed the presence of aluminium, arsenic, iron, potassium, oxygen, silicon, titanium, iodine, and lead, with sulphur clearly being overwhelmingly the most common element, with sulphur crystallization providing the shape of the crystals, although small amounts of crystalline arsenic, iodine, and lead were present. Fragments 20–200 µm in size and largely comprised of silicon and oxygen, with trace amounts of iron, potassium, aluminium, and titanium were interpreted as being fragments of rock which had been incorporated into the melt. The trace amounts of arsenic, iodine, and lead are interpreted as a product of magmatic degassing.
Sulphur samples taken from this second flow were found to be slightly depleted in the isotope sulphur³⁴, although not as much so as samples taken from Mount Poás in Costa Rica. This value is close to that found at many volcanoes in Japan, as well as the Campi Flegrei in Italy.
Temperatures of 140°C and 158°C were recorded in pools of molten sulphur on Mount Lastarria, while a sulphur flow was recorded to be 124° C. For comparison, a temperature of 124.7°C was recorded from a pool of yellow molten sulphur on the Hakone volcano in Japan. There is a close relationship between temperature and viscosity for sulphur. Pure sulphur will melt at 119°C, and become steadily less viscous as it is heated, until it reaches 159°C, rapidly becoming much less viscous as the temperature rises beyond this point, due to polymerization processes within the melt. The inaccessible nature of the locality meant that it was not possible to measure the viscosity of the liquid sulphur on Mount Lastarria, although there were clearly differences in viscosity within the observed flow, which could potentially have been due to temperature differences, particularly as the sulphur flows became visibly more viscous further away from the pools from which they originated, where they would have been expected to have been cooling down.
The flow observed in January 2019 was enriched in a variety of elements, most notably arsenic, with the remaining elements sortable into two groups, one of which was present at very low levels, and the other at higher levels. The elements present at only very low levels, niobium, caesium, uranium, zirconium, thorium, rubidium, cobalt, lithium, gallium, vanadium, strontium, and scandium, all have very high evaporation points, producing almost no volatile material at temperatures of less than 400°, as well as an affinity with silicate melts, but nor sulphur-rich volcanic fluids. These elements are proposed to have entered the fluid through the melting of rock fragments.
The second group of elements were present at higher levels, and include arsenic, bismuth, tungsten, tin, cadmium; lead, copper, molybdenum, and zinc. These elements with evaporate if heated sufficiently, and have been found within fumarole emissions on Mount Lastarria. Arsenic was present at notably higher levels than any of the other elements, which corresponds to reddish deposits found around fumeroles on the volcano, which are also often highly enriched in sulphur. Tin, cadmium; and lead are known to be particularly prevalent in fumarole emisions on volcanoes associated with subduction zones, which includes all volcanoes in the Andes. The high levels of these elements in the sulphur flows supports the idea that the sulphur flows formed by the melting of fumarole deposits, or at least reached the surface from the same source.
Arsenic, tin, tungsten, and copper have previously been found at significant quantities in liquid sulphur flows, and in liquid sulphur at the bottom of acid crater lakes. Mercury, molybdenum, gold, copper, and iron have also been found at significant quantities in floating sulphur spherules in volcanic crater lakes. The ability of sulphur to 'scavage' these chalcophile elements as it passes over deposits which contain them while in a liquid or even gaseous state is well documented, so the presence of such elements in liquid sulphur flows is unsurprising, although too few trace element studies of liquid sulphur flows have been made to make a detailed comparison between them at this time. The trace elements were found to be present within the sulphur pools at a much higher concentration than in the flows, which was a surprise, given the similarity in colour of the two. Inostroza et al. suggest that this difference in concentration is strong evidence for the elements being introduced to the pools continuously via the fumarole gasses bubbling through them from a deeper volcanic source.
The total volume of sulphur in the flows is estimated at 1.45 m³, much more modest than the previously observed sulphur flows on volcanoes such as Azufre, Shiretoko-Iozan, and Poás, and indeed fossil sulphur flows on Lastarria itself. However, Inostroza et al. note that the two flows (January 2019 and 2016-2022) are almost certainly part of the same cycle of volcanic activity, which we have no reason to believe has ended, so it is quite possible that further flows will build up this volume to a much higher level. Pure sulphur has a density of 2.07 g per cm³, but the material in the flows on Mount Lastarria contains significant impurities, both in the form of dissolved elements (mostly arsenic) and as fragments of rock, so Inostroza et al. calculate its density as 1.43 g per cm³, which would give the 1.45 m³ of material in the flows a mass of 2.07 tons.
Whilst sulphur flows are known from several volcanoes, direct observations of active flows is very rare. One of the notable findings of Inostroza et al.'s study is the slow rate at which the sulphur flowed, with an average of 0.069 m per second. This is much lower than the rate predicted by models and experimental methods. Based upon this observation, Inostroza et al. calculate that the total volume of sulphur in the four flows would have taken 408 minutes (or 6 hours and 48 minutes) to emplace.
Elemental sulphur can be formed at low temperatures in a variety of ways. Sulphur dioxide will react with hydrogen sulphide to give elemental sulphur and water. Sulphur dioxide will also react with water to give sulphuric acid and elemental sulphur, or sulphuric acid and hydrogen sulphide. It has generally been assumed that elemental sulphur found on subaerial volcanoes is formed by the reaction of sulphur dioxide and hydrogen sulphide, while sulphur found on submarine volcanoes is the result of sulphur dioxide reacting with water. Fumaroles on Mount Lastarria have been the subject of long-term monitoring, enabling useful observations about the sulphur flows to be made; in 2019 the proportion of both sulphur dioxide and hydrogen sulphide in the emitted gasses increased sharply, strongly supporting the idea that the sulphur present was produced by a reaction between these two gasses. However, this increase did not happen in 2019, but in 2014, and was present in all subsequent samples, suggesting that the lava flows could potentially have happened at any time after that point.
Thus, the chemical conditions for the formation sulphur appear to have been present on Mount Lastarria for a long period of time, but the appearance of sulphur flows has remained sporadic. This suggests that the liquid sulphur on volcanoes does not simply form by the melting of fumarole deposits. On other volcanoes, sulphur flows have often occurred close to eruptions, which could suggest that such flows are caused by some physical change on the volcano, such as the opening of a new fissure. Mount Lastarria has not undergone an eruption in recorded history, but has been expanding in recent years, so the opening of a new fissure would not be an implausible event.
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