Observations of a liquid sulphur flow on Mount Lastarria, northern Chile.

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.

Remote sensing reveals sulphur flows on the fumarolic field 1, which occurred on an undetermined date between 2016 and 2022 (2016–2022 flows) and January 2019 (2019 flows). The upper row is the overview map, and the lower row is the close-view (A), (B) 2016 Pleiades image shows the presence of old sulphur flows and the fumarole field as bright pixels (C), (D) 2022 Pleiades image shows new sulphur flows as bright pixel flow-like structures. Panel (D) shows the position of the 2016–2022 and 2019-flows, which emerged at 5100 and 5114 m above sea level, respectively. Inostroza et al. (2023).

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. 

Pool and flows of molten sulphur observed in January 2019 at Lastarria volcano. (A) A molten sulphur pool at 158 °C feeding two channels is shown in panel (B). (C) Front of the sulphur flow with a delta-like morphology, reaching up to 108 cm wide and a temperature of 124°C. Panels (D), (E) show the sampling procedure and measured temperatures of molten sulphur. White arrows point to the direction of the flow while red dots show the sampling site. Inostroza et al. (2023).

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.

Sequence of the four sulphur flows identified during the April 2019 field excursion. They are sourced from the same sulphur pool. It is important to note that only sulphur flow number 1 was observed live while flows number 2, number 3, and number 4 occurred after fieldwork. Sulphur flows are marked in different colors according to their distribution and contact relationship. Four control points (CP) show scales of 50 cm according to the image perspective. Inostroza et al. (2023).

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.

Back-scattered electron-images (left panels) and energy dispersive X-ray spectroscopy-chemical maps (right panels) of the sulphur flow sample (F2) showing their textural and morphological characteristics. These images show the reticulated growth of sulphur (a)–(d) with significant amounts of rock fragments (RF) in red to orange colours (e), (f). Inostroza et al. (2023).

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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Understanding the unexpected volcanoes of Pluto.

Pluto is the largest body in the Kuiper Belt, with a diameter of 2377 km and a density which suggests a structure comprising about 300 km of water-rich ice overlaying a rocky core. It is not predicted to have high levels of internal radiogenic heating, and, while it is thought that the moon Charon was formed in a collision event, any residual heating from this event, or subsequent tidal interactions, should have dissipated billions of years ago. Surface temperatures on Pluto range between 35 and 60K (between -238 and -213°C). The surface pressure on Pluto is about 10 μbar, precluding the existence of any form of liquid there. At the temperatures and pressures found on the surface of Pluto water should be an immobile bedrock. Adding sufficient amounts of ammonia to water can lower its freezing point by about 100 degrees, and other substances could conceivably lower it slightly further, but depressing the freezing point of water enough for it to retain any fluidity on the surface of Pluto is highly unlikely. Nevertheless, when the New Horizons space probe visited the dwarf planet in 2015, it found abundant signs of resurfacing, which are difficult to explain without the presence of liquid water beneath the surface of Pluto.

In a paper published in the journal Nature Communications on 29 March 2021, a team of scientists led by Kelsi Singer of the Southwest Research Institute discuss the possibility of cryovolcanism (ice volcanoes) as an explanation for the observed features on the surface of Pluto.

Singer et al. examine an area to the southwest of the Sputnik Planitia ice sheet (an ancient impact basin with a diameter of about 1000 km), which they view as a possible volcanic terrain. The most prominent features of this region are a series of large rises or mounds of material separated by broad depressions, the two largest of which form annular features with deep central depressions, and are named Wright Mons and Picard Mons. Wright Mons stands roughly 4-5 km high, and has a diameter of about 150 km, giving it a volume of roughly 24 000 km³, while Picard Mons appears to be about 7 km high, and to have a diameter of about 225 km.

 

Panchromatic basemap mosaic of the putative cryovolcanic terrains; dashed red curve indicates the transition from directly sunlit terrain to haze-lit terrain. Simple cylindrical projection. Image shown with north up, and the lighting direction is indicated here with the large arrow at the upper left. Singer et al. (2022).

Wright Mons is surrounded, and partially covered, by a hummocky terrain, which has a typical wavelength of 6-12 km, but reaches up to 20 km between hummocks in places. The hummocks are generally interconnected rather than singular, and have flattened or rounded tops. In the images the northern flank of Wright Mons, and the plain to the north of the mountain, appear to be smoother and less hummocky, but Singer et al. suggest that this is probably an artifact of the lighting direction. The hummocks appear to have no preferred direction with regard to the summit, and closer examination shows smaller structures, such as boulders, blocks, slabs, and ridges, with scales of 1-2 km, superimposed on top of the hummocks.

 

Features of Wright Mons and the surrounding terrain. (a) Wright Mons region with features labelled, (b) high-resolution topography for Wright Mons, (c) zoom of region with smaller dome named Coleman Mons (label 'D'), undulating, hummocky terrain on the flanks of Wright Mons and the superposed smaller-scale (1–2 km) ridges or boulders, (d) topographic profile of Wright Mons and adjacent rise as shown by the line A to A’ in panel (a). All images are from the New Horizons observations on a simple cylindrical projection. The large arrow in the upper left of panel a indicates the direction of incoming sunlight. Figure shown with north up. Singer et al. (2022).

The slopes of the flanks of Wright Mons average about 3-5°, but reach 10° in places. The mountain has a central depression roughly 40-50 km across and about 4 km deep (i.e. taking it down to the level of the surrounding terrain). The central depression of Picard Mons appears to be larger still, with a more u-shaped profile. The structure of these bodies is unlike anything seen in terrestrial or Martian volcanoes, with the central caldera(?) taking up a third of the structure's area and being as deep as the mountain is tall. The inner slopes of the crater appear to have the same hummocky profile as the outer slopes, with no collapse terraces, or similar structures, as might be expected on a volcano in the Inner Solar System. The northern and southern faces of Wright Mons, and the nearby ridge of the Medial Montes all appear to have similar profiles, with a steeper northern face and a more shallowly inclined southern face, both of which are covered by hummocks.

 If the interior depressions of Wright and Picard montes are collapse structures, formed within formerly dome-shaped structures, then they would represent a loss of about half the mountains' total masses, which seems highly implausible. 

 

Topographic profile comparison. (a) Mauna Loa (subaerial portion only) compared with Wright Mons (roughly N-S through the feature), upper panel is without vertical exaggeration, and lower panel is shown at 10× vertical exaggeration. Mauna Lau continues for approximately 6 km below the ocean surface. This figure illustrates how Wright Mons is very dissimilar to Mauna Loa and that if Wright was originally more similar to a shield volcano it would have had to have lost more than 50% of its volume from the central region in order to attain its current appearance. (b) Martian shield volcanos from the Tharsis region provide additional examples of large volcanos, some with more advanced caldera collapse. Several of these also show signs of later embayment, thus they also may not represent the full original height of the features. The more typical collapse terraces can be seen in the Martian calderas. Note that none of these examples are scaled for gravity, they are shown at their original scales. Singer et al. (2022).

A few other depressions of varying size are scattered across the area around Wright and Picard montes. These are generally not circular in shape, which rules out an impact origin. Some of these structures may be related to collapse at fault faces, while others might be depressions between areas of uplift.

No vents or fractures from which effusive material might escape can be seen in this landscape, not any sign of flow through which the presence of such structure might be inferred, although this might be an artefact of the resolution of the images (234–315 m per pixel), which prevents the detection of small structures. Nor are there any signs of explosive eruptions, such as fall deposit patterns (either radial or directional), or steeper cones. The extent of this terrain is unclear, as it continues southward until it disappears within a low-light haze. The absence of any cratering on or around Wright Mons gives it a maximum age of 1-2 billion years, and potentially considerably younger.

Methane, nitrogen, and water ice have all been observed on the surface of Pluto, in high-volume, concentrated deposits, so Singer et al. consider whether these might make up the structures of Wright and Picard montes, and the surrounding terrain. Data provided by the Linear Etalon Imaging Spectral Array instrument on New Horizons suggests that nitrogen- and methane-rich complexes of nitrogen, methane, and carbon monoxide cover much of the surface of Pluto. These are likely to sublimate and redeposit on a seasonal cycle (which on Pluto implies a 248 Earth year cycle), or a multi-million-year cycle determined by obliquity/precession cycles. In darker areas with low albedos these volatiles are either not being deposited, or not being retained, with the upshot that these areas are dominated by the non-volatile water ice 'bedrock'. Such areas include the dark equatorial band of Pluto, as well as the terrain around Wright and Picard montes. In this later area, deeper areas are dark, implying water ice, whereas higher elevations are more reddish, suggesting the presence of methane ice. This is probably a thin layer of snow, with the bulk of the structures composed of water ice. The area shows a very different topographic profile to the 'bladed terrain' found at high altitudes close to Pluto's equator, which is thought to be formed by the condensation and sublimation of thick methane deposits.

 

Comparison of Bladed Terrain and Wright Mons region. (a) The bladed terrain deposits form one of the highest elevation areas on Pluto (on the far eastern side of the hemisphere observed by New Horizons during closest approach) and are hypothesized to be a sprawling, concentrated deposit of methane ice, with the bladed texture forming due to sublimation of methane. It may also exist in large areas on the 'ar side' of Pluto that was only observed at low resolution by New Horizons. (b) Although Wright Mons has several different scales and styles of textures, it is not covered by the distinctive blades that characterize the bladed terrain. The longitude and latitude extents are as follows: panels (a) approximately 220–243°E and approximately 10–26°N; panel (b) approximately 165–175°E and approximately 19–26°S. The large arrows in the upper left indicate the approximate direction of the incoming light. Singer et al. (2022).

Patches of nitrogen ice can also be seen in the area around Wright and Picard montes, both in thin layers on the peaks, similar to the distribution of methane, and in depressions where it has apparently pooled. Although conditions on Pluto are cold enough to make water ice an immobile bedrock, nitrogen here is much closer to its melting point, and therefore likely to be capable of some sort of glacier-like plastic flow, which means that nitrogen ice is unlikely to be able to form tall topographic features.

 

Colour information for the Wright Mons region. Darker/lower albedo, redder patches exist primarily on north-facing slopes but there are also more subtle differences in albedo and redness across the region. The region labelled 'A' represents a redder unit transition to less red units at lower elevation (described in the text and methods). The longitude and latitude extents of the image are approximately 160–182°E and 13–31°S. Singer et al. (2022).

For these reasons, Singer et al. assume that the bulk of the Wright and Picard montes structures, and the surrounding topographic features, are composed of water ice, although these is a potential for other materials to have played a role in their formation and evolution. Notably, ammonia has been detected in areas in Pluto's Northern Hemisphere where cryofluids are thought to have erupted through extensional fractures. No ammonia has been detected in the study area, but its presence could potentially be masked by the presence of methane ice. There are also some darker areas on some north-facing slopes, which may indicate the presence of tholins (disordered and insoluble carbon-rich macromolecular materials).

The ices of the Wright Mons region have a slightly reddish tinge to them, indicating the presence of methane, while the terrain to the north is significantly redder, and that to the west is much less red. This may indicate that these regions are of different ages, having been emplaced at different times, from varying source reservoirs, although their precise age-relationship is impossible to determine.

Singer et al. note a number of features which they  believe indicate suggest the study area was not formed by the erosion of an older, elevated terrain. The area around Wright and Picard montes represents a large area of terrain lacking in craters, suggesting that the area was resurfaced in a single or series of events rather than as a consequence of gradual evolution. The distinct hummocky pattern seen in this terrain covers both the plains and the raised topographic features, and is unlike anything seen elsewhere on Pluto. The raised topographic features of the region are variable in elevation, whereas topographic features made from similar materials and exposed to the same conditions tend to erode down to similar heights.

Examination of the available data leads Singer et al. to conclude that Wright Mons, and probably also Picard Mons, formed by the merging of multiple separate rises into a single structure, and that it shares a structure essentially similar to the other topographic features of the area. They note that a smaller dome-like feature, named Coleman Mons, may provide an example of the emplacement of how such structures form. This dome is about 25 km in diameter and 1.5 km high, a structure which could be achieved by water or water-ammonia ice being deposited around a central vent, but would be beyond the ductile strength of softer ices such as nitrogen or methane.

Smaller dome-like feature named Coleman mons. (a) Topography overlain on base image of feature, (b) base image alone, (c) topography alone, (d) perspective view of dome and pit with no vertical exaggeration, (e) view inside the pit, (f) topographic profiles as shown in panels (a)-(c) with 3× vertical exaggeration. Singer et al. (2022).

The hummocky profile of the flanks of Wright Mons and the surrounding terrain suggests some form of viscous flow is occurring. Singer et al. suggest three different possibilities for the formation of these features, namely; the (1) creation of individual small volcanic domes, the (2) viscous extrusion of rapidly cooled lavas analogous to pillow lavas, or the (3) compression of viscous material with a frozen skin analogous to pahoehoe, viscous pressure ridges, or funiscular terrain on Enceladus.

The formation of ridges from pillow-lava like extrusions would require lava ice to be extruded at similar rates and for similar durations, both on the flanks of Wright Mons and on the surrounding plains. Such an even rate of extrusion across a varied terrain seems highly unlikely, and is at odds with observations of volcano behaviour both on Earth and elsewhere in the Solar System. It is possible that the hummocks were formed first, and then uplifted, but this would require an implausibly large mass of material being emplaced from below. Likewise, if the hummocks are contractional or compressional features, then this would imply a high-viscosity layer 8-13 km thick, which is in itself implausible, as well as a force capable of compressing such a layer, for which no obvious cause exists.

In addition, the extrusion of material to the surface to form any of these features would require a system of deep fractures through which the material could escape. Such structures have been seen elsewhere on the surface of Pluto, but not within the subject area, with the only possible fractures being along the large scarp which separates the Wright region from the plateau to the west, and another possible scarp further to the northwest, although other fractures could be hidden beneath extruded material on the surface.

None of the conventional models used to explain the emplacement of volcanic terrains on Earth and other Solar System bodies examined to date appears to fit well with the observed features on Pluto. Furthermore, the features of the area around Wright Mons appear to be quite different from anything seen elsewhere in the Solar System. No obvious vent regions can be seen, and though the surface does appear to be subject to viscous flow, no indication of directionality can be observed, making it difficult to explain the formation of the topology of the region. However, the constructs are compatible with a cryovolcanic origin, fuelled from multiple subsurface sources where the sources are below the constructs. 

The low surface temperature on Pluto, combined with the low predicted heat flux within the Dwarf Planet's interior, makes it very hard to account for the mobilization of subsurface fluid comprised largely of water, but the relative youth of the terrain suggests that a heat-source must be available. While in itself apparently unlikely, such an unexplained heat-source is would help to explain other areas of Pluto with young surfaces comprised of volatile ices, such as Sputnik Planitia.

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Understanding the origin of the 2018 Kīlauea eruption, and the implications of this for future forecasting.

Kīlauea’s 2018 flank eruption, on the volcano’s Lower East Rift Zone, produced approximately one cubic kilometer of lava and was the most destructive volcanic event in the past 200 years in Hawaiʻi, with over 700 structures destroyed. The accompanying collapse of the summit caldera was one of the largest at Kīlauea in centuries, triggering small explosions and tens of thousands of earthquakes that damaged nearby infrastructure. Kīlauea has one of the most comprehensive volcano monitoring networks on Earth, and the 2018 eruption provides an excellent opportunity to understand the complex processes that culminate in destructive flank eruptions, the timescales of priming and triggering in open-vent basaltic systems, and how forecasting such extreme events might be improved.

Prior to 2018, the most recent eruptions from the Lower East Rift Zone occurred in 1955, 1960, and 1961, and in 2014–2015 lava from the long-lived Puʻu ʻŌʻō eruption in the Middle East Rift Zone reached the outskirts of Pāhoa. Based on this historical activity and geologic mapping, volcanologists have long known that the Lower East Rift Zone is at high risk for lava inundation. Likewise, several large collapses of the summit caldera floor have occurred over the past 200 years, primarily during the 1800s. Despite the recognition of long-term hazard in these areas, short-term eruption forecasting must address factors of immediate relevance to hazard mitigation and is largely predicated on interpretation of geophysical and geological monitoring data.

In a paper published in the journal Nature Communications on 6 November 2020, Matthew Patrick of the Hawaiian Volcano Observatory, Bruce Houghton of the Department of Earth Sciences at the University of Hawaiʻi at Mānoa, Kyle Anderson of the California Volcano Observatory, Michael Poland and Emily Montgomery-Brown of the Cascades Volcano Observatory, Ingrid Johanson, also of the Hawaiian Volcano Observatory, Weston Thelen, also of the Cascades Volcano Observatory, and Tamar Elias, agian of the Hawaiian Volcano Observatory, characterise the changes at Kīlauea in the weeks to years leading up to the 2018 eruption.

Patrick et al. highlight that the short-term cause was an increase in magma pressure due to a backup of magma in the shallow plumbing system, which ultimately drove magma into the lower flank of the volcano. Several processes, however, likely primed the magmatic system for years prior to the eruption. A cascading series of events caused a relatively small change atKīlauea’s long-term East Rift Zone eruptive vent to lead to historic consequences at the summit and a major destructive effusive eruption on the lower flank. The 2018 activity highlights the challenges in forecasting the form and timing of low-likelihood, large-volume eruptions that result from a cascade of interconnected processes.

 

Location map for Kīlauea volcano. (a) Map of Kīlauea Volcano, on the Island of Hawaiʻi. The Puʻu ʻŌʻō eruption (1983–2018) produced a 144 km² lava flow field in the middle East Rift Zone. The May-September 2018 eruption occurred on the Lower East Rift Zone, roughly 40 km from the summit caldera. A large portion of the summit caldera floor subsided during the 2018 eruption. UWEV, PUOC, JCUZ, and JOKA are continuous Global Positioning System stations. The star shows the epicenter of the May 4 Mw 6.9 earthquake. (b) Schematic structural map of Kīlauea Volcano, showing the summit region and two rift zones. The mobile south flank exhibits steady southeast motion, and is tightly coupled with the rift zone magmatic system. (c) Small ash-rich explosive event at the summit, during the collapse of Halemaʻumaʻu crater, on May 15, 2018. (d) Fountaining (about 50 m high) at fissure 8, the dominant vent in the Lower East Rift Zone on June 5, 2018. Residences in Leilani Estates subdivision are visible in the background. Patrick et al. (2020).

Forecasting volcanic eruptions remains fundamentally challenging, despite ongoing improvements in our ability to measure and understand the processes that prime and drive eruptive activity. Once unrest is detected, volcano observatories must provide actionable assessments to emergency managers and the public that include the likelihood and potential timing of an eruption, its location, and ideally its scale and style. Even with robust monitoring networks and a strong understanding of a volcano’s geological and historical activity, these questions usually cannot be answered with confidence. Magmatic systems are highly complex and cannot be directly observed; therefore, volcanologists infer subsurface processes from often sparse monitoring data and idealised models. In addition, while magmatic systems may recharge and prime over extended intervals, the events that ultimately trigger an eruption occur over much shorter timescales, for example, the collapse of the north flank of Mount St Helens in 1980. These limitations create significant uncertainty in forecasting scenarios.

Explosive eruptions may exemplify the common picture of volcanic hazards, but effusive (lava-producing) events, typical at basaltic systems, can also be dangerous and destructive. In human terms, the stakes of forecasting can be particularly high when lava effusion occurs low on a volcano’s flank, where population centers are common. Eruptions at Nyiragongo (Democratic Republic of Congo) in 1977 and 2002, Etna (Italy) in 1669 and 2001–2002, Piton de la Fournaise (Réunion Island) in 1977, Mauna Loa in 1926, 1950, and 1984, and Kīlauea in 1960 and 1983–2018 are part of the long historical record of the risk to society posed by flank effusions and emphasise the critical importance of accurate forecasts during these events.

Kīlauea’s magma originates in the mantle and rises into a reservoir complex beneath the summit caldera at depths of 1–5 km.The summit reservoir system supplies magma to summit vents and laterally to rift zones that radiate to the east and southwest. Eruptions can occur at the volcano’s summit, where lava lakes have been common over the past 200 years, and/or along the rift zones; Kīlauea’s East Rift Zone has been especially active since the 1950s. Magma can also be stored within the rift zones, but the volume and geometry of this storage remains uncertain, especially below about 3 km depth. Further complicating the picture, Kīlauea’s south flank moves seaward at rates of up to about 8 cm a year, imparting extensional stresses on the rift zones and facilitating rift zone magma transport, dike intrusions, and fissure eruptions. Likewise, magma injected into the rift zones can produce stresses that trigger south flank motion and earthquakes. Thus, the tectonics of the volcano’s south flank are tightly coupled with the magmatic system of the East Rift Zone.

Prior to 2018, the most recent eruptions in the Lower East Rift Zone occurred in 1955, 1960, and 1961 with eruptions focused on the Middle East Rift Zone and summit during the 1960s and 1970s. Kīlauea erupted nearly continuously at or near the Puʻu ʻŌʻō eruption site in the Middle East Rift Zone from 1983 until the onset of the 2018 Lower East Rift Zone eruption. For most of that time, slow-moving lava flowed south to the ocean, producing a 144 km² flow field. In 2008, a lava lake formed in Halemaʻumaʻu crater, at the volcano’s summit 20 km uprift from Puʻu ʻŌʻō, and persisted until the start of the 2018 eruption.

Following several weeks of pronounced pressurization of the magmatic system at both the East Rift Zone and summit eruptive vents, a small, brief fissure eruption occurred on the west flank of the Puʻu ʻŌʻō cone on 30 April 2018. Over the next few days, earthquakes migrated eastward into the Lower East Rift Zone and rift-normal displacements were recorded by Global Positioning System instruments, signaling large-scale injection of magma downrift of Puʻu ʻŌʻō. Magma reached the surface in Leilani Estates subdivision on 3 May marking the onset of the Lower East Rift Zone eruption. The next day, a Magnitude 6.9 Earthquake occurred on Kīlauea’s south flank, the largest earthquake in Hawaiʻi in 43 years. The Earthquake involved southward displacement of the mobile flank and is thought to be a consequence of stress induced by the East Rift Zone injection. Throughout May, 24 short-lived fissures developed in the Lower East Rift Zone, but activity focused on fissure 8 by the end of that month. The fissure 8 lava flow reached the ocean at the eastern tip of the island in early June, destroying several subdivisions and establishing a stable lava channel that persisted for two months.

The Lower East Rift Zone eruption drained magma from the summit reservoir, 40 km away, at rates exceeding 100 m³ per second, causing rapid summit deflation. By mid-May, the summit lava lake had drained and the floor of Halemaʻumaʻu crater disintegrated in a piece-meal fashion, accompanied by several small explosive events. Summit collapses eventually involved larger portions of the caldera floor in June–July, with episodic piston-like failures that released energy equivalent to Magnitude 5.2–5.4 Earthquakes at intervals of 20–50 hours. Significant lava effusion on the Lower East Rift Zone ended on 4 August, roughly coincident with the end of summit collapse, although minor activity continued sporadically within the Lower East Rift Zone eruptive vent for the next month. Since September 2018 there has been no eruptive activity at Kīlauea, although ongoing inflationary ground deformation and a subsurface mass increase since late 2018 indicate that magma is refilling the summit and East Rift Zone.

 

Deformation and seismicity at Kīlauea’s summit, 1980–2020. (a) Summit deformation (UWT radial ground tilt and and UWEV northward GPS displacement) showing deflation of the summit reservoir following the onset of the Puʻu ʻŌʻō eruption, interrupted by several years of inflation due to a surge in magma supply from the mantle. From 2010 to early 2018, the summit experienced sustained inflation, terminated by the 2018 Lower East Rift Zone eruption. (b) Located deep crustal earthquakes (magnitude 1.7 and greater) beneath the summit (5–15 km depth), showing lower crustal swarms in the 1980s and 1990s that were not associated with changes in eruptive activity. Patrick et al. (2020).

Kīlauea’s summit magma reservoir complex deflated for two decades following the onset of the Puʻu ʻŌʻō eruption in 1983 as magma drained from the summit to supply the eruption, then inflated from roughly 2003–2007 in response to a surge in magma supply. Sustained deflation returned to the summit with the opening of a new vent near Puʻu ʻŌʻō in July 2007. Inflation recommenced in late 2010 and was followed by a brief deflationary episode due to the formation of a new vent near Puʻu ʻŌʻō in March 2011. Inflation continued into 2012 and subsequent years, likely caused in part by an increase in East Rift Zone vent elevation, and was accompanied by a net rise in the Halemaʻumaʻu lava lake (a proxy for magma reservoir pressure). Inflation and lava lake rise rates at the summit increased in 2016 but leveled off in 2017.

Geodetic data provide evidence for episodic magma transport downrift of Puʻu ʻŌʻō and into the Lower East Rift Zone during the years prior to 2018. Leveling and Global Positioning System data suggest a pause in long-term Lower East Rift Zone subsidence at the eastern tip of the island during 2003–2007, perhaps due to the arrival of magma in the Lower East Rift Zone linked to a surge in magma supply to Kīlauea. Beginning in 2013, pauses in subsidence, and in some cases uplift, were observed at campaign and continuous Global Positioning System stations downrift of Puʻu ʻŌʻō (for example, at JOKA). These changes lasted until the 2018 eruption. The early 2013 change in deformation style at Global Positioning System stations downrift of Puʻu ʻŌʻō roughly coincided with changes in the summit and Puʻu ʻŌʻō eruptions. In January 2013, the Halemaʻumaʻu lava lake rose 50 m over 10 days, and the Puʻu ʻŌʻō lava lake rose several tens of meters, to one of the highest levels of lava in the crater in years.

 

Long-term changes on Kīlauea, 2009–2018. (a) Elevation of the lava lakes at the summit and Puʻu ʻŌʻō, as well as the elevation of the vents at or near Puʻu ʻŌʻō. “E” notes times of eruptive vents forming on the East Rift Zone (at or near Puʻu ʻŌʻō), and “I” notes the time of intrusions at the summit-most eruptions and intrusions were preceded by rapid summit inflation and lava lake rise and an increase in shallow summit and upper East Rift Zone earthquakes. The Kamoamoa eruption is noted specifically due to the broadly similar precursors it shared with the 2018 eruption. (b) Northward displacement of summit Global Positioning System station UWEV, and line-length change between Puʻu ʻŌʻō stations PUOC and JCUZ, showing a long-term inflationary trend at both eruption sites. (c) Shallow (less than 5 km depth) summit and upper East Rift Zone Earthquakes (Magnitude 1.7 and greater), which often increase in rate during summit pressurisation. (d) Displacement of Global Positioning System station JOKA, in the middle-lower East Rift Zone, showing the onset of uplift in early 2013. Patrick et al. (2020).

South flank motion continued at a steady rate in the years prior to the 2018 eruption. Displacement rates were greatest (about 8 cm per year) in coastal areas southeast of the summit, with rates diminishing to under 1 cm per year south of the Lower East Rift Zone. Transient displacements occurred at semi-regular intervals during slow slip of the south flank of the volcano.

Ground deformation data indicate that Kīlauea’s shallow magma system, from the summit to Puʻu ʻŌʻō, showed low rates of deformation during 2017 and the first two months of 2018, but high rates of pressurization were recorded starting in mid-March 2018. Lava lakes at both the summit and Puʻu ʻŌʻō rose to unusually high levels during that time period, confirming pressurisation in the magma system. The summit lava lake produced the largest overflows on the Halemaʻumaʻu crater floor observed during the 10 years of the summit eruption. The small lava lake in Puʻu ʻŌʻō also rose to unusually high levels in April 2018, and the adjacent crater floor was lifted up by roughly 15 m. An increase in shallow (less than 5 km depth) Earthquakes at the summit and Upper East Rift Zone also occurred in April; similar Earthquakes have commonly been associated with summit pressurisation. 

Field observations and thermal satellite data indicate that the eruption at Puʻu ʻŌʻō was waning during early 2018, despite the pressurization in the summit-East Rift Zone magmatic system. In September–October 2017 the flow of lava at the ocean entry weakened, and the ocean entry shut down in mid November. From this time onwards, lava breakouts showed diminishing reach from the vent. MODVOLC thermal satellite data showed reduced radiant heat flux from the lava flow field55 during March and April, and sulphur dioxide emission rates from Puʻu ʻŌʻō, used as an indicator of lava eruption rates, were unusually low in April. These observations are all consistent with a reduction in the eruption rate from Puʻu ʻŌʻō during this time.

 

Short-term changes preceding the 2018 eruption. (a) Displacement of summit and Puʻu ʻŌʻō Global Positioning System (GPS), 2013–2018 showing the 2014, 2016, and 2018 vent openings. The 2014 and 2016 vents opened on Puʻu ʻŌʻō, while in 2018 a minor vent opened on Puʻu ʻŌʻō but was followed by a larger eruption on the Lower East Rift Zone (LERZ). (b) Displacement of summit and Puʻu ʻŌʻō Global Positioning System from September 2017 to April 30, 2018, showing inflationary changes starting in March 2018. 'VANs' shows the dates of Volcanic Activity Notices issued by Hawaiian Volcano Observatory; the 17 April Volcanic Activity Notices noted the ongoing pressurization and forecast that a new vent could form on the East Rift Zone (ERZ), while the 1 May Volcanic Activity Notices noted magma moving east of Puʻu ʻŌʻō and forecast that a vent could form downrift. (c) Surface elevation of the lava lakes at the summit and Puʻu ʻŌʻō, showing an abrupt rise in March–April. (d) Shallow (less than 5 km depth) summit and Upper East Rift Zone earthquakes, which commonly increase in rate during summit pressurization. (e) Indicators of flow activity on the Puʻu ʻŌʻō flow field. 'Breakout distance' shows the distance of the farthest surface lava breakouts from the Puʻu ʻŌʻō vent, measured along-tube, and shows a gradual retreat of breakouts upslope from November 2017 to April 2018. MODVOLC radiant heat flux, an indicator of surface flow activity, also decreased by April. Sulphur dioxide emissions from Puʻu ʻŌʻō exhibited unusually low values in April. Patricj et al. (2020).

The rate of lower crustal earthquakes beneath the summit (5–15 km depth) increased in November 2017 and remained elevated into early 2018, with several swarms in March 2018. These rates were higher than in the previous 10 years, during which time these earthquake swarms were commonly associated with brief summit deflation episodes.

Visual observations of changes preceding the 2018 eruption. (a) Normal summit lava lake levels vs. (b) unusually high lake levels due to pressurisation in April 2018. The lake is about 300 m long. (c) Normal lava lake elevation in Puʻu ʻŌʻō vs. (d) unusually high lake elevation due to pressurization in April 2018. The lake is roughly 50 m long. (e) Ocean entry activity typical of the preceding year, with numerous lava streams and a moderate steam plume vs. (f) an inactive ocean entry in November 2017 as East Rift Zone surface breakouts retreated upslope. Patrick et al. (2020).

Long-term, seaward sliding of Kīlauea’s south flank may have gradually set the stage for the 2018 Lower East Rift Zone eruption. Sliding of the south flank causes a corresponding increase in extensional strain in the shallow rift zone (under 3 km), which consequently reduces the magma overpressure required for downrift propagation of a magma-filled crack. Along the Middle East Rift Zone, where steady south flank motion amounts to several cm per year, 'passive' intrusions have been known to occur as magma from the Middle East Rift Zone conduit rises toward the surface in response to rift zone extension, for example, in 1997 and 1999. South flank slip rates are much lower in the Lower East Rift Zone than in the Middle East Rift Zone, suggesting that it might take decades for enough extension to accumulate along the rift to facilitate intrusions. Consistent with this idea, a roughly 50-year recurrence interval has been estimated for major flank slip and rift opening events over the past 200 years at Kīlauea. By 2018, 57 years had passed since the previous Lower East Rift Zone eruption and 43 years since the last major south flank earthquake, thus, the Lower East Rift Zone may have been poised for failure. This scenario has similarities with the 2004–2005 eruption of Etna Volcano, which was triggered by extension resulting from long-term motion of the eastern flank of the volcano. Likewise, the 2018 eruption of Ambrym (Vanuatu) was facilitated by tectonically induced extensional stresses that prompted magma flow into the rift zone.

Intrusions and eruptions at Kīlauea are frequently preceded by increases in magma pressure, and Kīlauea’s magma system was unusually pressurised before the onset of the 2018 eruption. Inflation at the summit began in 2010 and was sustained through 2016. By 2018, tilt and Global Positioning System data suggested the system was at its highest level of pressurisation in at least 20 years. Puʻu ʻŌʻō was likewise in a prolonged inflated state since 2010. The highly pressurised magma system would have increased the likelihood of an intrusive event and provided a greater head to drive magma into the Lower East Rift Zone.

In addition, data suggest that leakage of magma downrift of Puʻu ʻŌʻō was occurring by early 2013. The unusually high lava column in Puʻu ʻŌʻō in January 2013 may have provided sufficient overpressure at depth to open or expand a pathway downrift of Puʻu ʻŌʻō, which enabled gradual magma migration during 2013–2018. Could the additional magma flow and heat transfer downrift of Puʻu ʻŌʻō after 2013 have facilitated the 2018 injection of magma into the Lower East Rift Zone? Campaign Global Positioning System data collected annually since 1995 in the Middle and Lower East Rift Zone suggest that at least one period of downrift magma transport occurred prior to 2007, with no accompanying eruption. It is possible that periods of slow magma transport downrift of Puʻu ʻŌʻō have occurred regularly in the past, but this remains poorly understood.

Kīlauea’s magma system began to pressurize much more rapidly during March–April 2018, shown by inflation, rising lava lake levels, and increasing shallow summit and upper East Rift Zone Earthquakes. Increased magma pressure at Kīlauea is sometimes associated with higher eruption rates from East Rift Zone vents, while at other times it is associated with a decrease in eruption rates. In the latter instances, as during early 2018, pressurisation may be explained as the result of reduced output at the Puʻu ʻŌʻō vent, causing magma to backup in the system. The cause of the reduced output is not well understood but, like in 2018, previous instances of backups at Puʻu ʻŌʻō occurred after the vent persisted for several years, suggesting that the shallowest conduit feeding the vent may tend to atrophy due to reduced transport efficiency over time. One strong possibility is that a restriction of some form develops in the shallowest conduit connecting the Puʻu ʻŌʻō reservoir to the vent feeding the lava flow field, inhibiting conduit flow. The efficient hydraulic connection between Puʻu ʻŌʻō and the summit results in magma accumulation throughout Kīlauea’s shallow magma system. Historically, rapid inflation from this process has culminated in the formation of formation of new Middle East Rift Zone vents. 

Kīlauea’s shallow magma system is also known to pressurise in response to increases in magma supply from the deeper magmatic system, but we see no clear evidence for an increase in deep magma supply in early 2018. Changes in magma supply had been documented in the mid-2000s and had a significant and direct impact on summit inflation and Middle East Rift Zone eruptive activity. During early 2018, however, the Middle East Rift Zone eruption rate decreased, rather than an increase as might be expected from an increase in deeper magma supply. Furthermore, there was no change in the character of the deeper portion of the magma reservoir complex (3–5 km depth), as had occurred during the mid-2000s. Carbon dioxide emission rates, previously used as a proxy for deep magma supply rates, were not available in the years immediately prior to 2018 due to challenging measurement geometry. There was an increase in lower crustal (5–15 km depth) earthquakes in late 2017 to early 2018 relative to the previous decade, but previous work has shown that these lower crustal earthquakes, common in the 1980s and 1990s, are not clearly related to changes in eruptive activity, and their source mechanism remains ambiguous. In the context of activity that has occurred at Kīlauea since 2008, when the geometry of the magmatic system was most similar to early 2018, an increase in magma supply is not needed to explain the inflation and seismicity during that time.

On 30 April 2018, a small intrusion occurred into the west flank of the Puʻu ʻŌʻō cone, similar to the culmination of previous episodes of rising pressure that created new vents on or around Puʻu ʻŌʻō. This event, however, coincided with a larger injection of magma far downrift of Puʻu ʻŌʻō, creating the first large-scale magmatic episode in the Lower East Rift Zone in 57 years.

What changed in the plumbing system to allow large volumes of magma to enter the Lower East Rift Zone? A persistent feature must have existed in the rift zone that prevented significant downrift magma transport past Puʻu ʻŌʻō during the 35 years of magma supply to the vent. Localised barriers to magma transport have been previously hypothesised in the East Rift Zone based on seismic data, and for diking events at other volcanoes. Multiphase mixture models suggest that a section of rift east of Puʻu ʻŌʻō was exceptionally dense, perhaps making it difficult for new cracks to initiate or propagate downrift. Vents have opened slightly east of Puʻu ʻŌʻō several times during the eruption, but were fed by very shallow dikes that probably emanated from the Puʻu ʻŌʻō feeder system, implying that any long-lived barrier to downrift magma transport was rooted deeper, in the main East Rift Zone conduit. The 2013 and onwards deformation downrift of Puʻu ʻŌʻō (station JOKA) suggests that such a barrier may have been leaky, raising questions on how the feature may have evolved, or degraded, over three decades. The  barrier must have been sufficiently resilient, however, to shunt the majority of magma to Puʻu ʻŌʻō despite numerous disruptions during the 35-year eruption. In addition, the entire magmatic pathway downrift of Puʻu ʻŌʻō, having been largely abandoned for decades, may have been so poorly developed as to permit nothing more than a trickle of magma prior to 2018.

Whether due to a localized barrier near Puʻu ʻŌʻō or to the intrinsic resistance to flow in the largely abandoned, vestigial pathway east of Puʻu ʻŌʻō, downrift flow might have been impeded prior to 2018 simply because magma overpressure was insufficient to initiate new cracks. The long-term pressurization of the system, coupled with the short-term perturbation of 30 April, may have finally exceeded the threshold needed to overcome this resistance to flow. Flow into the Lower East Rift Zone was probably aided by long-term dilation of the rift zone due to south flank motion. The combination of rift dilation and magma pressurisation may simply have reached a critical threshold by late April 2018.

There remain unanswered questions in this conceptual model that deserve further study, particularly with regard to the exactfailure process that allowed magma to move east of Puʻu ʻŌʻō. Nonetheless, the onset of the 2018 eruption can be adequately explained by intrinsic magmatic and tectonic processes. Recent work has proposed that heavy rainfall triggered the 2018 eruption, based on a purported lack of significant precursory inflation. The high rates of widespread inflation and lake level rise in the weeks prior to the 2018 eruption, however, indicate that increasing magmatic pressure was the dominant driver, and extrinsic triggers such as rainfall are not required to explain the eruption.

Patrick et al.'s conceptual model explains the buildup to the 2018 activity at Kīlauea, but why was the eruption so large? Previous work has demonstrated that eruptions along Kīlauea’s Lower East Rift Zone tend to be infrequent but relatively large (0.1–0.3 km³ in 1790, 1840, 1955, 1960). This is likely due to the lower elevation of Lower East Rift Zone vents, which require lower overpressures to drive flow, and can drain magma storage zones more thoroughly (including magma stored in the East Rift Zone). Indeed, for 18 East Rift Zone eruptions in the 20th century, summit deflation (a proxy for pressure change) scaled inversely with vent elevation. The volume of the 2018 eruption (approximately one cubic kilometer), however, was large even by Lower East Rift Zone standards, so other factors, noted below, must also have contributed.

An open question regards the role of the May 4 Magnitude 6.9 south flank Earthquake in influencing the magnitude of the Lower East Rift Zone eruption. The timing of the Earthquake, days after the onset of magma moving into the Lower East Rift Zone, suggests that the intrusion triggered the earthquake by stressing the south flank, as proposed for previous episodes where rift zone intrusions apparently induced strong south flank Earthquakes. Did the Magnitude 6.9 Earthquake dilate the East Rift Zone and enable higher rates of magma transport? An apparent increase in the rate of summit drainage after the Earthquake supports the notion that the Magnitude 6.9 Earthquake boosted transport rates in the magmatic system. If the Magnitude 6.9 Earthquake did enhance magma transport to the Lower East Rift Zone, this might, in part, explain the comparatively large erupted volume that triggered structural failure at the summit. In comparison, the 1955 and 1960 Lower East Rift Zone eruptions occurred in a similar area of the rift zone but were much smaller (0.1–0.3 km³), were not associated with south flank Earthquakes with Magnitudes greater than 6, and did not produce large-scale collapse at the summit.

The role of the collapsing caldera at the summit of Kīlauea in the evolution of the eruption also requires further study. Episodic failure of the rock above the summit magma reservoir renewed the pressurisation of the reservoir with each collapse and produced transient increases in eruption rate; these events therefore probably played a role in sustaining the eruption. A quasi-exponential decay of pressure in Kīlauea’s deeper summit magma system over the course of the eruption, however, suggests additional summit processes also affected the magnitude of the event.

How did relatively minor events at Puʻu ʻŌʻō progress to the historic scale of the 2018 eruption? We propose that the 2018 eruption of Kīlauea began and evolved as a cascading series of events, which was difficult to anticipate due to the complexity of the system. Cascading sequences are intrinsic to volcanic eruptions and can occur over a wide range of spatial and temporal scales. In an idealized explosive eruption, for example, pressure in the reservoir drives magma towards the surface, and decreasing pressure eventually allows gas exsolution to occur. Bubble growth then enhances ascent rates and leads to fragmentation, producing an eruption. The resulting eruption hazards may also occur in a cascading manner.

 

Schematic showing the changes leading to the 2018 eruption. (a) Cross-section of Kīlauea from the summit down the East Rift Zone, prior to 2018. Two simultaneous eruptions were occurring (summit and Puʻu ʻŌʻō). (b) Proposed changes at Puʻu ʻŌʻō that led to the 2018 eruption. A restriction between the Puʻu ʻŌʻō magma reservoir and lava flow vent is hypothesised to have reduced lava flow effusion rate, causing magma to backup and accumulate in the magmatic system. This produced pressurization at Puʻu ʻŌʻō, and the summit via the East Rift Zone magma conduit. (c) Onset of the 2018 eruption sequence at Puʻu ʻŌʻō. Overpressure produced a local intrusion on the west flank of Puʻu ʻŌʻō and initiated the larger injection of magma into the lower East Rift Zone. Magma flow into Lower East Rift Zone triggered drainage of the Puʻu ʻŌʻō magma reservoir, causing crater floor collapse and termination of the lava flow vent. Patrick et al. (2020).

In the 2018 eruption of Kīlauea the cascade sequence was a chain of events that was unforeseen at the onset of unrest. The magmatic system may have been primed for years due to (a) gradual dilation of the rift zone due to south flank motion, (b) the prolonged inflated state of the magmatic system, and/or (c) slow magma leakage into the Lower East Rift Zone. The cascade was set in motion in late 2017 and early 2018 as a restriction developed in the vent conduit supplying magma to the Puʻu ʻŌʻō lava flows (step 1), reducing lava outflow (step 2) and causing magma to backup and pressurize the system (step 3), opening a pathway and/or clearing a barrier near Puʻu ʻŌʻō that allowed a larger scale magma migration east into the Lower East Rift Zone (step 4). Magma reached the surface as a Lower East Rift Zone eruption (step 5). The input of magma into the Lower East Rift Zone imparted stress on Kīlauea’s south flank, which triggered the Magnitude 6.9 Earthquake (step 6), relieving confining stress on the rift zone which, in turn, may have enhanced magma transport to the Lower East Rift Zone eruption site (step 7). The Lower East Rift Zone eruption removed magma at a high rate from the summit magma reservoir (step 8), causing collapse of the caldera floor (step 9) that led to small explosions (step 10) and maintained magma reservoir pressure, in part sustaining the eruption. This convoluted sequence links a relatively small change near Puʻu ʻŌʻō to major, destructive lava effusion on the Lower East Rift Zone (20 km downrift) and historic changes at the summit (20 km uprift), all enabled by an efficient hydraulic connection along the East Rift Zone.

 

Overview of precursors and 2018 eruption stages. (a) Prior to 2018, summit and Puʻu ʻŌʻō eruptions were ongoing, and were jointly supplied magma from the summit reservoir complex. (b) During early 2018, a restriction near the Puʻu ʻŌʻō lava flow vent caused magma to backup, driving concurrent inflation at the summit and Puʻu ʻŌʻō, and rising lava lakes at both sites. (c) This pressurization reached a critical threshold on April 30, 2018, when a small intrusion occurred at Puʻu ʻŌʻō and a larger intrusion was initiated that migrated into the lower East Rift Zone. (d) The lower East Rift Zone (ERZ) intrusion created an eruption on the lower flank of the volcano, which led to substantial draining of the summit magma reservoir, driving caldera collapse, and small explosions. Pateick et al. (2020).

It should be possible to learn for the future by examining how Hawaiian Volcano Observatory scientists assessed the activity as it was unfolding. The long-term inflation at the summit and uplift east of Puʻu ʻŌʻō for example, at station JOKA), were recognized by Hawaiian Volcano Observatory scientists as indicating system-wide inflation, but the gradual nature of the trends did not clearly point to short-term hazard.

The short-term precursors in March–April, pressurization at both the summit and Puʻu ʻŌʻō, were identified immediately. During this phase of pressurisation, the consensus among Hawaiian Volcano Observatory staff was that a new vent would most-likely form on or near Puʻu ʻŌʻō, following patterns in 2011, 2014, and 2016. With the inflation persisting into April, Hawaiian Volcano Observatory issued a Volcanic Activity Notice on 17 April that stated 'Observations…during the past month suggest that the magma system beneath Puʻu ʻŌʻō has become increasingly pressurized. If this activity continues, a new vent could form at any time, either on the Puʻu ʻŌʻō cone or along adjacent areas of the East Rift Zone.' At that time, a primary concern was that such a vent might appear on the north side of the Puʻu ʻŌʻō cone, sending lava into a catchment that could eventually reach populated areas, as happened during the 2014–2015 Pāhoa lava flow crisis. A Volcanic Activity Notice issued on 24 April highlighted the increased pressurisation and high level of the summit lava lake, and the possibility of a new vent forming on or near Puʻu ʻŌʻō.

The expected local intrusion occurred at Puʻu ʻŌʻō at approximately 2.20 pm Hawai'i Standard Time on 30 April, creating a brief fissure eruption and small flows on the west flank of the cone. What was not expected, however, was the continuation of Earthquakes and further magma injection downrift of Puʻu ʻŌʻō that commenced within the subsequent hours. The earthquakes reached the area of Highway 130, 18 km east of Puʻu ʻŌʻō, by midday on1  May. This recorded the first major movement of magma into the Lower East Rift Zone since the 1960s and was an unambiguous signal that larger, and potentially more hazardous, changes were underway. On 1 May (4.54 am Hawai'i Standard Time), the Hawaiian Volcano Observatory issued a Volcanic Activity Notice alerting the public of the evolving hazard and stating that an outbreak of lava in a new location was one possible outcome. The focusing of earthquakes beneath Leilani Estates on 2 May, and the opening of small ground cracks on that day, suggested that an eruption could occur in this area. On 2 May (7.23 pm Hawai'i Standard Time) the status report was updated to indicate that an outbreak of lava from the Lower East Rift Zone remained a possible outcome of the continued unrest. The Lower East Rift Zone eruption began 21 hours later, at about 5.00 pm Hawai'i Standard Time on 3 May.

The Volcanic Activity Notices and status reports released during the precursory phase in April did not forecast significant hazards at the summit, based on lack of recent precedent. Previous intrusions in the area of Puʻu ʻŌʻō, such as in 2011, produced significant summit deflation but no large-scale structural changes to the caldera floor, nor summit explosive activity. The 1955 and 1960 Lower East Rift Zone eruptions caused localized sagging and disintegration of the Halemaʻumaʻu crater floor, but not large-scale caldera collapse or explosive activity. Once the 2018 Lower East Rift Zone eruption commenced and the Halemaʻumaʻu lava lake began rapidly draining, however, Hawaiian Volcano Observatory recognized the possibility of explosive events, similar to those in 1924, which also followed lake draining and a Lower East Rift Zone intrusion. On 9 May a Volcanic Activity Notice stated that the dropping lava level 'raised the potential for explosive eruptions in the coming weeks.' Relatively minor explosive activity began in mid-May and continued throughout the month.

The onset of Kīlauea’s 2018 eruption was forecast accurately in the weeks leading up to the event, but its location and size were not. What can volcanologists learn from these events when responding to future activity at Kīlauea and other volcanoes?

First, the 2018 eruption serves as a cautionary tale against overreliance on recent volcanic activity as a guide for future behavior. Kīlauea’s Puʻu ʻŌʻō eruption had persisted for decades despite numerous perturbations of the magmatic system and appeared to be a testament to the stability of eruptive activity in the Middle East Rift Zone. Based on parallels with magma injections in 1991, 2011, 2014, and 2016, inflation in early 2018 suggested only a new Middle East Rift Zone intrusion or formation of a new vent at Puʻu ʻŌʻō, a fundamental change to the eruption was not expected. In retrospect, the March–April inflation and the sequence of events that was anticipated to result from it served as a point of focus and may have distracted from consideration of Kīlauea’s broader geologic record, which includes four Lower East Rift Zone eruptions in the past 200 years, one (1840) of which triggered collapse of the caldera floor. Several additional large collapses of the caldera floor occurred in the 1800s; however, unlike 2018, none of these previous Lower East Rift Zone events or summit collapses occurred in the midst of an ongoing, multiyear Middle East Rift Zone eruption.

Humans may naturally focus on obvious changes and most likely outcomes at the expense of less obvious changes and less likely outcomes. The predilection to see the future as similar to the immediate past can be considered a kind of tunnel vision, which can have detrimental effects on unbiased, comprehensive consideration of information and illustrates a challenge of forecasting volcanic eruptions using short- to intermediate-term pattern recognition. The risks of tunnel vision may be alleviated in part by considering the broader geologic history of a volcano, which can serve as a useful reminder that other (possibly much larger) outcomes are also possible, even if unlikely. These possible outcomes must be considered during each new phase of evolving unrest, even if a previously recognised pattern appears to be repeating itself.

Evident precursors to the 2018 eruption were relatively small and provided a deceptive underestimate of the scale of the impending eruption. Thus, the 2018 eruption highlights the challenge of forecasting complex cascading sequences of events. At Kīlauea, the extensive rift zone magmatic system has previously exhibited complex interactions with the summit reservoir. In 1924, an intrusion in the Lower East Rift Zone withdrew magma from the summit reservoir and caused the floor of Halemaʻumaʻu crater to collapse, followed by a series of summit explosions. At Stromboli Volcano, increasing magma pressure at the volcano’s summit has previously triggered flank eruptions, leading to rapid decompression of the deeper reservoir and dangerous paroxysmal explosions. The 1980 Mount St. Helens eruption began with a Magnitude 5.1 Earthquake and landslide of the unstable, bulging flank, which then exposed a cryptodome of magma and removed overburden from the central conduit, resulting in a lateral blast and Plinian explosion. In each of these examples, major aspects of the eruption could not be anticipated in a straightforward manner from the immediate signs of unrest. Whether in a cascading sequence or not, unforeseen volcanic events are common from a global perspective, reinforcing the need for implementing forecasting frameworks that account for remote outcomes.

It may never be possible to determine if or when a particular event, such as a small magma injection, might trigger a sequence of cascading events and culminate in a large eruption. Volcanic systems are highly nonlinear and behave chaotically and unpredictably; however, small events can only trigger large events if the state of the volcanic system, such as the volume and pressure of eruptible magma and tectonic stress state, permits it. An important focus of future work should thus be to better understand when systems may be primed such that a small trigger can result in a large eruption. These conditions may be characterised using monitoring data together with conceptual and mathematical models, and interpreted in light of geological and historical records, which can be used to make inferences on the types and recurrence rates of future activity.

Quantitative hazard forecasting tools, such as probabilistic event trees and Bayesian belief networks, allow scientists to rigorously integrate information from geologic mapping, monitoring data, models, and even expert opinion, to obtain probabilistic assessments of possible future activity. In some cases, these tools can be used to obtain not only a forecast of future activity, but also quantitative insight into the state of the volcanic system (e.g. whether or not magma is ascending). Also, importantly, the utilisation of these tools requires careful analysis and discussion of possible outcomes and may thereby reduce the tendency towards tunnel vision. The simple act of carefully discussing possible outcomes may bring a greater awareness of the possibility of low-probability high-impact events and help observatory scientists consider a broader range of outcomes. It should also encourage vigilance for the prospect that ostensibly small changes at a volcano could, given the right circumstances, evolve into much larger and more hazardous activity. Patrick et al. also advocate for observatory scientists to become familiar with these tools, and to use them to develop basic long-term forecasts that can be modified as needed, well before the onset of a volcanic crisis.

Finally, Patrick et al. emphasise that volcanologists must remain humble no matter how sophisticated our data and models become. Volcanoes often erupt in unexpected ways. Stromboli, for instance, has been studied for centuries, and the onset of flank effusion is now recognized by Italy’s volcano monitoring agency as a possible precursor to paroxysmal explosions, an outstanding example of eruption forecasting based on monitoring data, the historical record, and an understanding of the volcanic system. Nonetheless, two paroxysmal explosions in 2019 (one fatal) occurred in the absence of flank activity. As with Kīlauea’s 2018 eruption, these events highlight the limits of current understanding even at relatively well-studied volcanic systems.

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