Geochemistry, Mineralogy, Petrology & Volcanology

Geochemistry, Mineralogy, Petrology & Volcanology

Fire, Fog, Frost, Famine – French Revolution? The Lakagígar eruption in Iceland, 1783-1784 [Part 1]

Fire, Fog, Frost, Famine – French Revolution? The Lakagígar eruption in Iceland, 1783-1784 [Part 1]

“On the 8th of June 1783, at Whitsun, there gushed forth from the mountains behind the summer pastures a fire which devastated land, cattle and humans with its effects, both nearby and far away”, wrote Reverend Jón Steingrímsson of Kirkjubæjarklaustur in his autobiography [2]. The “fire” which welled up from a volcanic fissure now known as Lakagígar (the craters of Mount Laki) was the biggest flood basalt eruption in written history, and led to the worst natural catastrophe Iceland ever endured. Within three years, the population dropped by about 20%. But was it merely the volcano that killed these people – or are humans also to blame?

PART I: The Fury of the Torn Earth

The Fires of the Skaftá River

Map of the surroundings of the Lakagígar. See here for an interactive map of Iceland.


Strange phenomena preceded the fiery outburst: a burning island had appeared southwest of Reykjavík due to a small submarine eruption, foul-smelling floods had surged down the glacial river Skaftá, and a slight bluish haze had been observed over the vast glacier Vatnajökull (signs of unrest at the volcanic system of Grímsvötn). From mid-May onwards, earthquakes shook the district of Síða, slowly increasing in strength. And then, on the sunny morning of Whitsun, a dirty, whirling plume emerged from the uninhabited mountains, spread southwards over the settlements, with growling thunders and showers of black ash. However, after a few hours, southerly winds drove the ash cloud back into the mountains. Reverend Jón was able to conduct his church service under clear skies, and the people hoped that the worst was over…

The opposite was the case. The river Skaftá dried up, and a few days later, a flood of fire emerged from its gorge. The lava followed the riverbed and slowly spread over the lowlands, burning down farms and mowing lands. Other farms were flooded by boiling-hot water where the lava dammed up the tributaries of the Skaftá. Within two weeks, the Lakagígar discharged some 6km3 of lava; in total, the eruption would eject 15km3.

By July 20th, the lava had advanced eastwards in the Skaftá riverbed, threatening the little parish church of Kirkjubæjarklaustur. Reverend Jón led his congregation to what he believed would be the last service conducted in this church. It was hazy and dark, except for bolts of volcanic lightning. The earth shook and trembled, and the bells echoed and wailed with the cracks of thunder, while Reverend Jón gave his sermon, telling his parishioners to submit to God’s will. When they left the church and headed to the tip of the lava flow, fearing for the worst, they found that the lava had miraculously stopped. The people believed that Reverend Jón had performed a miracle, halting the lava by the force of his prayers. Jón himself was convinced that God had stopped the lava, but being scientific-minded as well as deeply religious, he tried to figure out how God had achieved this, and concluded that the little rivers coming down from the hills must have cooled and solidified the lava [1].

The tip of the lava stream which followed the Skaftá riverbed and “miraculously” came to a standstill during Reverend Jón’s sermon on July 20th, 1783. The lava is now thickly covered by green moss. This location is still known as “Eldmessutangi” (the land spit of the fire sermon). Photo credit: Claudia Wieners

After the “fire mass”, the volcanic eruption seemed to abate. Only small amounts of lava trickled down the Skaftá valley. But nine days later, the northern part of the Laki fissure started to erupt, sending yet another flood of lava down the gorge of the river Hverfisfljót. The terrified inhabitants of the Síða district feared getting caught up between two fiery streams, but luckily the lava flows stopped just before the coastal marshes, leaving the travelling routes open. No one was killed by the lava, but when the eruption finally ceased in February 1784, 37 farms had been devastated by lava flows, dammed-up rivers, ash fall and sand storms [5] – almost 1% of all Icelandic farms.

The loss of farms was a tragedy for the local population, but the volcanic fog and ash caused a national catastrophe.

Foul Fog and Black Snow

Lava fountains, lava flows and plume formation during the fissure eruption at Holuhraun (Sept. 2014). The Lakagígar fissure was much longer and emitted roughly 10 times as much lava. Photo credit: Peter Hartree

The Lakagígar eruption was mainly effusive, but there was also some explosive activity due to violent degassing and interaction between lava and ground water. This generated a volcanic plume which covered the island in a thick haze and may have reached the stratosphere. It was this fog which gave the period after the eruption its name: “Móðuharðindin” – the Haze Hardships.


The formation of a volcanic plume. 1) Magma with dissolved gasses rises to the surface. Degassing starts due to decreasing pressure. 2) Violent degassing and evaporation of heated ground water thrust lava fragments into the air, leading to lava fountains, ash formation and a high eruption plume (up to about 12 km). 3) In addition, gases are slowly released from solidifying lava streams, contribution to a local haze.

The gases released by the eruption caused widespread destruction. H2S (hydrogen sulfide) smothered the whole of Iceland with a sickening smell of rotten eggs. But the 120 megatons of SO2 (sulfur dioxide) had worse effects [4]. The SO2 reacted with water vapour to form H2SO4 (sulfuric acid). H2SO4 attracts water molecules, forming little droplets which caused the haze. The sun appeared blood red or was blocked altogether. Acid rain burnt holes into the skin of newly shorn sheep and caused irritated eyes and lungs in humans. It also destroyed the vegetation. Within a week, the grass became withered and yellow. The timing of the eruption could hardly have been worse: it started at the beginning of the hay harvest season. Farmers throughout Iceland could harvest only meagre amounts of hay, and what they got was spoilt by ash. Some tried to rinse it, but to no avail. Reverend Jón noted that the hay burnt with a blue flame – like sulfur.

The Lakagígar also released 8 megatons of fluorine [4]. Most of it got adsorbed to the surface of fine ash particles which spread over most of the country. The rain washed the fluorine into the ground, poisoning the grass and the grazing animals. The sheep near Lakagígar perished within two weeks, the cows and horses survived somewhat longer. By the winter, the fluorine poisoning had reached wide parts of the country, killing 80% of the sheep, 40% of the cows and 50% of the horses. Wild birds perished in flocks; so did the fish in lakes and rivers. Only marine life seemed unaffected, and there is no evidence of massive human death caused directly by the haze [3].

Beyond the shores of Iceland, nobody knew what was going on. Due to the island’s isolation, it took two and half months for the first sketchy news of the eruption to reach Copenhagen.

The volcanic fog travelled much faster. Owing to sinking air masses in a high pressure area, the foul fumes accumulated over Europe by the end of June [4]. The reddish sunlight, acid-damaged plant leaves, and a smell of sulfur led many to believe that doomsday was approaching. Others tried to find a natural explanation and suggested that the “sun smoke” was a consequence of the strong earthquakes in southern Italy in February 1783 [6]. Ultimately, by the time that the news from Iceland reached Europe, the strange fog had disappeared and the interest in understanding its causes had subsided.

In Great Britain and France, the haze was followed by an increase in mortality rate (11500 excess death in Britain). Some scientists link these deaths directly to the Icelandic “killer cloud”. Although aerosols from Icelandic volcanoes could cause increased mortality in Europe, some scholars point out that in 1783, the mortality in England only peaked a few months after the fog was gone. More importantly, it seems unlikely that the Lakagígar haze caused massive poisoning in England and France while it did not do so in Iceland, where the concentrations were much higher.

Frost and Floods

The winter of 1783-84 was very cold in Iceland, Europe and North America [4]. The harbour of New York was frozen, and in mainland Europe rivers froze and towns were covered in snow. When the thaw came at the end of February, the melting snow over frozen, impermeable ground caused severe flooding. To make things worse, the ice on the rivers broke open and piled up in dams which at the end collapsed under the pressure of the flood, causing surges of water and ice floes. In Germany, more then twenty bridges were destroyed.

Painting of the ice flood in Würzburg (Germany) in the river Main. Men stand on the bridge, trying to push ice floes through the arches to prevent them from piling up. Cannons are fired on the ice to make it break quicker. Image source: Wikipedia

In Iceland, the sea-ice from Greenland blocked several harbours, not only in the North, but even in the normally warmer Southeast. The snow in the South melted only in May, and even by June only the topmost inch of the ground had thawed, hampering the growth of grass. In Northern Iceland, the snow remained till June.

Several observers, including Benjamin Franklin and the Icelandic medical student Sveinn Pálsson, blamed the unusual fog for the cold, postulating that it had blocked the sun’s rays. Indeed, it is now known that major stratospheric eruptions can cool the climate for a few years (the most well-studied example being the Pinatubo eruption 1991). However, most of the aerosol from the Lakagígar likely remained in the troposphere and should have rained out rather quickly. Climate models, when fed with aerosol concentrations representative for the Lakagígar eruption, mostly do show significant cooling over the Northern hemisphere, but mainly in autumn 1783, not in the following winter. So while it is plausible that the Lakagígar haze caused severe cooling, it is not yet proven.

In summary, while the immense lava streams ejected during the Lakagígar eruption only had a regional impact, the impacts of the volcanic gases were certainly felt over large parts of the Northern hemisphere. The gases from Lakagígar created a volcanic haze which caused panic and possibly health problems in Europe, but may also have had severe impacts on climate. However, the societal impacts of the eruption were nowhere as disastrous as in Iceland, and in Part II I will examine the aftermath of the Lakagígar eruption in detail.


Blog post written by Dr Claudia Wieners. The next installment is coming soon! 

Claudia is a postdoctoral researcher at the Institute for Marine and Atmospheric research, Utrecht (Utrecht University, Netherlands). She did a PhD on the possible influence of the Indian Ocean on El Niño. Recently she started working on the interaction between climate, economy and policy, focusing on the potential role of (sulfur) geoengineering. She is also involved in the Centre for Complex System Studies in Utrecht. 


Sources and further Reading

[1] Jón Steingrímsson’s famous account on the eruption is available for free in Icelandic: An English translation was made by Keneva Kunz under the title “Fires of the Earth”.

[2] His autobiography is available for free in Icelandic: An English translation with useful comments was made by Michael Fell under the title “A very present help in trouble: The autobiography of the Fire Priest”

[3] A good overview of the Lakagígar eruption is presented in “Eruptions which shook the world” by Clive Oppenheimer, chapter 12.

[4] Scientific details on the eruption and the haze were mainly taken from this paper: Thordarsson and Self, “Atmospheric and environmental effects of the 1783–1784 Laki eruption: A review and reassessment”,

[5] The description of the consequences of the Lakagígar eruption in Iceland and the intervention (or lack thereof) by the Danish government is based on this book: “Skaftáreldar 1783-1784: Ritgerðir og heimildir” (The Skaftá Fires 1783-1784: Articles and Sources) edited by Gísli Ágúst Gunnlaugsson. It is in Icelandic but has English abstracts.

[6] The consequences of the Lakagígar haze in Europe are described in Eyþór Halldórsson, “The dry fog of 1783: Environmental Impact and Human reaction to the Lakagígar Eruption”,

[7] The Icelandic Trade Monopoly is discussed in Gísli Gunnarsson, “Monopoly trade and economic stagnation: studies in the foreign trade of Iceland 1602-1787”,;view=1up;seq=37. The book also deals with the economic side of the Lakagígar eruption (chapter 9).

Building a lava dome: one block at a time

Building a lava dome: one block at a time

Lava domes form when lava is extruded from a volcanic vent, but is too viscous to flow far away. Think of thick treacle that does not flow as easily as runny honey, and so when it is extruded, it forms a “lava pile” around the vent. Lava domes commonly form within the crater of a larger volcano (e.g. Mt. St. Helens), but can also stand alone or form part of a “dome complex”.

Soufrière Hills Volcano, Montserrat (taken in 2016). Credit Claire Harnett

A lava dome can take on a variety of shapes and sizes dependent upon many factors, including lava chemical composition, viscosity and extrusion rate. Some of the different types of lava domes range from “pancake domes”, generally short and wide, to “spiny domes”, where lava cools and breaks into blocks of rock. Volcanologists have observed over time that every lava dome is different, even at the same volcano or during the same eruption. The number of factors that influence the behaviour of a lava dome makes it difficult to understand the processes occurring during lava dome growth. There is still a lot of research necessary before we can think about forecasting behaviour during a dome-building eruption.

Lava dome hazards

There are many hazards associated with dome-building eruptions, and they are often linked to explosive volcanic activity. In the same way that landslides occur in a slope made of soil, lava domes can collapse if they become oversteepened. Lava dome stability can also be affected by intense bouts of rainfall, as the rainwater can be pressurised when it comes into contact with the hot dome (temperatures at eruption are generally in excess of 850˚C), thus turning the rainwater into steam and forcing the solidified lava to fracture apart, causing collapse. Other causes for collapse include a build-up of gas within the dome, or a change in direction of the lava extrusion.

Pyroclastic flows from dome collapse in Montserrat, June 1997. Credit BGS.

A number of things can happen when a lava dome collapses, ranging from small scale rockfalls, to debris avalanches, to pyroclastic density currents (turbulent mixtures of gas and rock that can travel at speeds of over 100 kilometres per hour). Any of these can be devastating to communities surrounding the volcano, as pyroclastic density currents cannot be redirected using the same methods that have been utilised for lava flow diversion.

Hazards for scientists

Despite how dangerous these events can be to populated areas, very little is known about various causes of a collapse. This is mainly because the internal structure of a lava dome is uncertain. We think there is a ductile, frictionless core that is surrounded by a solid, friction-controlled rind (think of a soft caramel encased by hard chocolate). A dome often also includes talus aprons, which are piles of loose rubbly material that accumulate due to rockfalls that occur during dome growth.

Research into lava dome collapse can be split into three main areas: field observations, analogue and laboratory experiments, and numerical modelling. Field observations of real life environments and landscapes are vital for creating conceptual models of a lava dome.

Conceptual diagram of internal structure, and photo of the dome at Mt. Unzen (2016). Credit: Claire Harnett

The missing particle of the puzzle

After fieldwork at both Mt. Unzen and Soufrière Hills Volcano, my research has focused on numerical modelling of lava dome collapse. The aim is to build a computer simulation of a lava dome, and then change conditions in the model to create various “model scenarios” for collapse. This allows us, for example, to change the gas pressure (something we know to cause collapse at a real dome) in our synthetic lava dome, and vary the time period over which forces are exerted upon the dome.

To do this, we use discrete element method (DEM) modelling. DEM considers the simulated lava dome as an assembly of individual particles/blocks, which means that the model can evolve dynamically and the particles act according to gravity and their interactions with neighbouring particles. We generate a particle based material that represents lava. We then give the particles a speed and a direction to simulate extrusion, and we control the point at which parts of the material start to behave as a solid (i.e. representing a realistic cooling/solidification process).

Discrete element model of a very simplistic lava dome. Credit: Claire Harnett

Although these models are still in development, we are able to replicate the growth patterns observed at lava domes. For example we see when a lava dome reaches its maximum height (determined by the rock properties), it starts to grow outwards rather than upwards. We aim to create software that can further our understanding of the processes occurring within a dome, with the ultimate goal of use in observatories. Scientists monitoring the volcano could then input what they see occurring during a volcanic eruption, and have a greater chance of predicting what will happen next.

These models are proving to be a really exciting and innovative way to study lava dome growth and collapse, and will reveal some of the mysteries of lava domes and the dangers they pose.

A movie of lava dome emplacement, where red material acts as ductile core, and grey material acts as solid rock. 

Blog post written by Claire Harnett. For more information, keep your eyes peeled for the paper coming soon! Or get in touch with Claire on Twitter.

Living with volcanic gases

Living with volcanic gases

Professor Tamsin Mather, a volcanologist in Oxford’s Department of Earth Sciences reflects on her many fieldwork experiences at Masaya volcano in Nicaragua, and what she has learned about how they effect the lives of the people who live around them. 

Over the years, fieldwork at Masaya volcano in Nicaragua, has revealed many secrets about how volcanic plumes work and impact the environment, both in the here and now and deep into the geological past of our planet.

Working in this environment has also generated many memories and stories for me personally. From watching colleagues descend into the crater, to meeting bandits at dawn, or driving soldiers and their rifles across the country, or losing a remotely controlled miniature airship in Nicaraguan airspace and becoming acquainted with Ron and Victoria (the local beverages), to name but a few.

I first went to Masaya volcano in Nicaragua in 2001. In fact, it was the first volcano that I worked on for my PhD. It is not a spectacular volcano. It does not have the iconic conical shape or indeed size of some of its neighbours in Nicaragua. Mighty Momotombo, just 35 km away, seems to define (well, to me) the capital Managua’s skyline. By comparison, Masaya is a relative footnote on the landscape, reaching just over 600 m in elevation. Nonetheless it is to Masaya that myself and other volcanologists flock to work, as it offers a rare natural laboratory to study volcanic processes. Everyday of the year Masaya pumps great quantities of volcanic gases (a noxious cocktail including acidic gases like sulphur dioxide and hydrogen chloride) from its magma interior into the Nicaraguan atmosphere. Furthermore, with the right permissions and safety equipment, you can drive a car directly into this gas plume easily bringing heavy equipment to make measurements. I have heard it described by colleagues as a ‘drive-through’ volcano and while this is not a term I like, as someone who once lugged heavy equipment up 5500 m high Lascar in Chile, I can certainly vouch for its appeal.

Returning for my fifth visit in December 2017 (six years since my last) was like meeting up with an old friend again. There were many familiar sights and sounds: the view of Mombacho volcano from Masaya’s crater rim, the sound of the parakeets returning to the crater at dusk, the pungent smell of the plume that clings to your clothes for days, my favourite view of Momotombo from the main Managua-Masaya road, Mi Viejo Ranchito restaurant – I could go on.

But, as with old friends, there were many changes too. Although in the past I could often hear the magma roaring as it moved under the surface, down the vents, since late 2015 a combination of rock falls and rising lava levels have created a small lava lake visibly churning inside the volcanic crater. This is spectacular in the daytime, but at night the menacing crater glow is mesmerising and the national park is now open to a stream of tourists visiting after dark. Previously, I would scour the ground around the crater for a few glassy fibres and beads of the fresh lava, forced out as bubbles burst from the lava lake (known as Pele’s hairs and tears after the Hawaiian goddess of the volcano – not the footballer) to bring back to analyse. Now the crater edge downwind of the active vent is carpeted with them, and you leave footprints as if it were snow. New instruments and a viewing platform with a webcam have been put in, in place of the crumbling concrete posts where I used to duct-tape up my equipment.

Footprints in the Pele’s hair. Image credit: Tamsin Mather

This time my mission at Masaya was also rather different. Before I had been accompanied solely by scientists but this time I was part of an interdisciplinary team including medics, anthropologists, historians, hazard experts and visual artists. All aligned in the shared aim of studying the impacts of the volcanic gases on the lives and livelihoods of the downwind communities and working with the local agencies to communicate these hazards. Masaya’s high and persistent gas flux, low altitude and ridges of higher ground, downwind of it, mean that these impacts are felt particularly acutely at this volcano. For example, at El Panama, just 3 km from the volcano, which is often noticeably fumigated by the plume, they cannot use nails to fix the roofs of their houses, as they rust too quickly in the volcanic gases.

The team was drawn from Nicaragua, the UK and also Iceland, sharing knowledge between volcano-affected nations. Other members of the team had been there over the previous 12 months, installing air quality monitoring networks, sampling rain and drinking water, interviewing the local people, making a short film telling the people’s stories and scouring the archives for records of the effects of previous volcanic degassing crises at Masaya. Although my expertise was deployed for several days installing new monitoring equipment (the El Crucero Canal 6 transmitter station became our rather unlikely office for part of the week), the main mission of this week was to discuss our results and future plans with the local officials and the communities affected by the plume.

Some of the team working on Masaya: Left to right Dave Lynch, Evgenia Ilyinskaya, Rachel Whitty, Peter Baxter, Tamsin Mather, Jennifer Le Bond, Gudrun Halla Tulinius and Sara Barsotti. Image credit: Tamsin Mather

Having worked at Masaya numerous times, mainly for more esoteric scientific reasons, spending time presenting the very human implications of our findings to the local agencies, charged with monitoring the Nicaraguan environment and hazards, as well managing disasters was a privilege. With their help we ran an information evening in El Panama. This involved squeezing 150 people into the tiny school class room in flickering electric light, rigging up the largest TV I have ever seen from the back of a pick-up and transporting 150 chicken dinners from the nearest fried chicken place! But it also meant watching the community see the film about their lives for the first time, meeting the local ‘stars’ of this film and presenting our work where we took their accounts of how the plume behaves and affects their lives and used our measurements to bring them the science behind their own knowledge.

Workshop at El Panama. Image credit: Tamsin Mather

Watching the film it was also striking to us that for so many of this community it was the first time they had seen the lava lake whose effects they feel daily. Outside the school house there were Pele’s hair on the ground in the playground and whiffs of volcanic gas as the sun set – the volcano was certainly present.  However, particularly watching the film back now sitting at home in the UK, I feel that with this trip, unlike my others before, it is the people of El Panama that get the last word rather than the volcano.



Blog written by Tamsin Mather (University of Oxford). You can follow the project on Twitter @UNRESPproject. UNRESP project is funded by GCRF Building Resilience programme. 

A little fracture can go a long way: How experiments illuminate our understanding of volcanic eruptions

A little fracture can go a long way: How experiments illuminate our understanding of volcanic eruptions

What controls how violently a volcano erupts?

Stratovolcanoes like Mount St Helens (USA), Gunung Merapi (Indonesia), or Volcán de Colima (Mexico) tend to erupt in two distinct ways: effusively and/or explosively. Effusive eruptions are eruptions where lava is extruded without any major explosions. Although effusive eruptions can be dangerous, at stratovolcanoes they tend to be restricted to volcanic craters and loss of life and property is relatively limited. On the other hand, explosive eruptions tend to be more catastrophic and can devastate local populations and land. What’s peculiar is that stratovolcanoes often switch their eruptive behaviour between effusive and explosive (sometimes during the same eruption!) and volcanologists aren’t totally sure why.

Stratovolcanoes can erupt both explosively and effusively. Mount Sinabung in Indonesia (L) erupting explosively in 2015. Mount St. Helens in the USA (R) in 2006 during a period of effusive eruption. Sinaburg photo credit: SUTANTA ADITYA/AFP. Mount St. Helens photo credit: Willie Scott, USGS.

A lot of processes can affect eruption style, but one that volcanologists spend a lot of time thinking about is how gases build up in and are released from the volcanic system.

Experimental volcanologists, like me, try to address this problem by conducting scalable lab experiments that help reveal the pathways that gases take to the surface. We try to understand how the permeability of the volcanic system – the ease with which the volcanic system allows gas to move within it – changes over time.

Fear the shear

As rising magma travels up through the conduit on its way to the surface, the rigid conduit walls try to slow it down: magma tends to rise more slowly at the conduit’s edges than at its center. A knock-on effect of this is that the stresses acting on the magma – in this case, shear stresses – are largest at the conduit margins. These stresses tend to stretch the magma, sometimes resulting in fracture development and, because of this, a lot of volcanic outgassing (gas release from the system as a whole) is observed at conduit rims. One spectacular example of this type of outgassing event is at Santiaguito volcano in Guatemala where you can sometimes see volcanic gases escaping in an annulus around the conduit:

Shear fractures pose a conundrum: they’re not optimally oriented to direct gases accumulating in the centres of volcanic conduits up towards the surface. To understand how magma deformation can produce escape pathways for gas similar to those seen at Santiaguito volcano, we ran a series of laboratory experiments in which we sheared magma at temperature and pressure conditions relevant to volcanic systems. Using a deformation apparatus, we could shear cylindrical samples of a two-phase magma – containing bubbles and melt – by twisting them.

Our goal was to see if we could make the samples permeable simply by deforming them in shear.

Schematics of our samples before deformation (L) and after deformation (R). The black circles in the starting material are argon bubbles; the white space is the melt. After deformation, the bubbles are stretched in the direction of deformation (as indicated by the red line). The picture shows a sample after deformation; the entire sample surface is covered by an iron jacket that isolates the pore fluid (inside of the sample) from the confining pressure (outside the sample). Image credit: Alex Kushnir.


Experiments with a twist

To keep things simple we used a synthetic glass with a composition comparable to natural volcanic products. We also used argon to create bubbles instead of water (since dissolved water can change the melt viscosity and we wanted to keep this parameter constant!). Most importantly of all: we checked that all our samples were impermeable before deformation. In practice, this meant that no gas could travel from the top of the sample to the bottom of the sample.

We found that for samples deformed at low shear strain rates (i.e. low twist rates), the bubbles became very stretched in the direction of shear but never coalesced and, so, the samples never became permeable.

For samples deformed quickly (i.e. high twist rates), bubbles became stretched and again did not coalesce. However, at these high twist rates fractures developed perpendicular to the overall foliation!  Thus the samples deformed at the high twist rates became permeable.

These fractures are well known to structural geologists: Mode I (or extension) fractures. They open in response to a tensile force, which acts to pull the magma apart in the direction of shear. In our study, only samples that were sheared quickly contained these Mode I fractures and became permeable.

Scanning Electron Microscope images of the magma deformed at low twist rates (L) and at high twist rates (R). The arrows show the sense of direction (the top of the photos is being moved to the right) and the dotted red line shows the foliation in the samples. The fracture in (R) is oriented at approximately 90° to the overall sample foliation. Photo credit: Alex Kushnir.


Big picture from a tiny sample

We confirmed something that volcanologists have known for a long time: when you deform magma slowly, it tends to relax and flow in response to the applied stress.  But if you deform that same magma quickly, it tends to break.

What’s novel about our study is how the magma broke. If you superimpose a shear zone onto conduit margins, things start to look a bit like this:

Idealized (and definitely not to scale!) schematic of shear-induced fractures near conduit margins. Given the general sense of shear at conduit margins when magma rises, Mode II (or shear) fractures are typically oriented approximately parallel to the conduit margins and directed slightly into the conduit. Mode I (or extension) fractures, on the other hand, are oriented such that they can potentially direct gases from near the centre of conduits upwards and outwards to the surface. Image credit: Alex Kushnir.

In general, the shear fractures run parallel to the conduit margins, which is great for getting gas out of the system, but not so great for grabbing the gas from the centre of the magma column, where it’s expanding and accumulating. That’s where these handy Mode I fractures might come into play. They’re oriented in an optimal direction for sampling the gas in the interior of the magma column and directing the gas upward and outward toward the conduit margins, where they can join a high permeability highway to the surface.

Of course, our samples are tiny compared to a volcano and we aren’t yet able to say how prevalent this process may be. However, our experimental approach shows that you can start to learn a lot about a big problem from a centimeter-scale sample!

Blog written by Alex Kushnir. For more information check out the recent paper. Or get in touch with Alex on twitter.