An island of glass - the explosive history of Pantelleria, Italy

By @volcanologist

If you’ve heard of Pantelleria, it may be because you’ve seen A Bigger Splash, or read about it in a travel magazine extolling its yet undiscovered virtues (unless you are Madonna or Georgio Armani who have holiday homes there). If you’re a geologist, you may know the name from Pantellerite, the name given to peralkaline rhyolites, which are so abundant on the island. Or perhaps, the enigmatic mineral Aenigmatite, also known as Cossyrite after Cossyra, the ancient name of Pantelleria. What you may not know is that Pantelleria is an island of glass and has a history of catastrophic, caldera forming eruptions. Our recent work has pieced together the island’s explosive past in a new, comprehensive volcanic history.

The last eruption to occur at Pantelleria was a submarine basaltic eruption that occurred 4 km NW of Pantelleria in 1891. The eruption lasted for 9 days and produced floating scoria bombs which eventually exploded and sank. But this gentle, effusive basaltic activity is not typical of the island’s history, instead the island more commonly erupts trachyte and rhyolite, regularly covering the island in volcanic glass of one form or another. For the last 46,000 years, eruptions have mostly been small (strombolian in scale) with local pumice fall deposits and lavas. Around 46,000 years ago, a large eruption occurred generating a hot,sticky pyroclastic density current which covered the island in a welded, rheomorphic ignimbrite known as the Green Tuff. The Green Tuff blanketed the island, and covered the older deposits. The older rock record was known to include eruptions of at least the same size as the Green Tuff, but as their deposits are best exposed in dramatic sea cliffs it has been difficult to piece together this story of explosive eruptions until now.

Panoramic view of a section of the sea cliffs on Pantelleria showing the complicated and largely inaccessible geology: a succession of laterally discontinuous lavas and pumice deposits can be seen, draped by ignimbrites at the top. White and cream units are either non-welded pumice fall or PDC deposits.

Our study brings together field volcanologists, palaeomagnetists and experts in radiometric dating to put together the complete story of the pre-Green Tuff eruptions. The field volcanologists carried out detailed studies of the rocks left behind by these older eruptions, interpreting the rocks in order to understand the processes that formed them. The palaeomagnetists used palaeomagnetic data as a correlation tool, to help match up some of the deposits where these couldn’t be easily traced in the field. Finally, we used Ar/Ar radiometric dating so that we know when the different eruptions occured.

General vertical stratigraphy of ignimbrite-producing eruptions on Pantelleria

We find that the island’s history is dominated by large ignimbrite-forming eruptions. Ignimbrites are the deposits of pyroclastic density currents; dramatic hot flows of gas, ash and rocks which can travel at speeds up to 450 mph (to learn more about ignimbrites and how we use them to reconstruct PDCs, read this blog). Some of the ignimbrites are related to eruptions which resulted in caldera collapse. This is where the magma reservoir underneath the volcano is evacuated so dramatically during an eruption that the roof of the reservoir collapses - the volcanic edifice disappears into the space created by erupting the magma. Caldera collapse eruptions are thought to be some of the biggest, most violent eruptions that a volcano can produce. We found that Pantelleria had experienced at least five of these catastrophic eruptions. The Green Tuff eruption is commonly thought to have ended with a caldera collapse event, but recent work suggests that this isn’t the case. These large eruptions occur every few thousand years up to a gap of around ~40 kyr - we found that there didn’t appear to be any cyclicity or pattern to the timing of the eruptions.

Sea cliffs at Scauri. The bump on the right is a small local centre, draped by ignimbrites.

We also found that in between these large eruptions the island was far from quiet. Small scale eruptions were producing eruption columns that covered local areas with pumice, or perhaps generated small lava flows. These ancient small eruptions are very similar to the activity that has occured on the island since the Green Tuff. So then, is the island currently in a phase of activity so very different to the island’s volcanic past? Probably not. Studies on the last 46,000 years have suggested a decrease in eruptive activity, particularly in the last 15,000 years. But we cannot assume that no large, catastrophic eruption will occur in the future. Importantly though, there is no current evidence that an eruption is expected imminently.

Nina J. Jordan, Silvio G. Rotolo, Rebecca Williams, Fabio Speranza, William C. McIntosh, Michael J. Branney, Stéphane Scaillet, 2018. Explosive eruptive history of Pantelleria, Italy: Repeated caldera collapse and ignimbrite emplacement at a peralkaline volcano. Journal of Volcanology and Geothermal Research, 349, 47-73. https://doi.org/10.1016/j.jvolgeores.2017.09.013.

A PDF is available at http://www.sciencedirect.com/science/article/pii/S0377027317300781; from the University of Leicester or Hull’s repository; or by emailing Nina Jordan or Rebecca Williams.

Kinematic indicators in the Green Tuff Ignimbrite: can they tell us about the timing of caldera collapse?

By Dr Rebecca Williams (@volcanologist) & Jodie Dyble

In the summer of 2014 I have had a Nuffield Foundation student, Jodie, working with me towards a Gold CREST Award, which we blogged about the other week. Here, I’m going to talk a bit about the research she did.

Jodie looked at the Green Tuff Ignimbrite on the island of Pantelleria, Italy. The Green Tuff Ignimbrite is a rheomorphic ignimbrite which was emplaced during an eruption about 45 thousand years ago. An ignimbrite is the deposit from a pyroclastic density current. Rheomorphic means that the deposit was still hot when it was formed, so that the shards of ash welded together and was able to be deformed ductiley. Rheomorphic ignimbrites are common on places like Gran Canaria, in the Canary Islands (where the classic work of Schmincke & Swanson 1967 was done) and the Snake River Plain in the western US. You can get two types of rheomorphism, that which occurs during deposition of the ignimbrite (e.g. the overriding current exerts a shear on the underlying deposit) and rheomorphism which occurs after the deposit has been fully formed (e.g. the deposit starts slumping under gravity). I’m avoiding using primary vs secondary here, as actually the historical meaning of those words and their relative timings can be difficult to disentangle. For a very good, concise overview take a read of (Andrews & Branney 2005). Either way, rheomorphic structures within the deposit like lineations, folds, tension gashes and rotated crystals or clasts, can tell us about this sense of movement. Volcanologists interpret these kinematic indicators in the same way a structural geologist would interpret verging folds, or rotated porphyroclasts in a mylonite (e.g. Passchier & Simpson 1986). You can even determine the direction a pyroclastic density current flowed if you map out these kinematic indicators across the ignimbrite (e.g. Andrews & Branney, 2011).

Schematic diagram of the development of rheomorphic structures in a syndepositional shear zone during the deposition of an ignimbrite. Taken from Andrews & Branney, 2005.

The Green Tuff eruption was said to have been a caldera forming eruption, but the details of this have been debated. Two different calderas have been proposed: the Cinque Denti caldera (Mahood & Hildreth 1986) and the Monastero caldera (Cornette et al. 1983; Civetta et al. 1988). These share the same scarps to the east, west and south but while the Cinque Denti caldera has exposed scarps in the north (the Costa di Zinedi scarp, the Kattibucale scarp and the Cinque Denti scarp), the Monastero caldera has a buried northern scarp. During my PhD on the Green Tuff (Williams 2010; Williams et al. 2014) I found that the Costa di Zinedi scarps, the Kattibucale scarps and the Cinque Denti scarps were extensively draped by the Green Tuff, right down to the bottom of the exposed caldera walls.

The map shows the two different proposed calderas for the Green Tuff eruption. Panoramics and sketches show the draping Green Tuff down the three disputed scarps. Localities used in this study are highlighted. From Williams, 2010.

What Jodie set out to determine this summer was when that draping occurred. My work on the chemical stratigraphy of the Green Tuff already determined that those drapes represented the earliest part of the eruption. So, did caldera collapse happen after the deposition of the Green Tuff and did those drapes represent the rheomorphic slumping of the deposit down a newly formed caldera wall? Or, did the caldera wall exist before the emplacement of the Green Tuff, and those drapes represent a deposit formed by an overriding current? In the field, macro indicators (such as large scale folds) suggested that the deposit slumped down the caldera wall. We went in search of micro kinematic indicators to see if they would tell the same story.

 Some of the micro-kinematic indicators seen in the thin sections from the Green Tuff Ignimbrite, including verging folds and rotated clasts (δ and σ–objects). From Dyble & Williams, 2015.

What Jodie found was compelling evidence for upslope flow in the thin sections that she analysed. Thus, those deposits were formed by the Green Tuff pyroclastic density current flowing up the caldera scarps, depositing and shearing the underlying deposit as it went. Which means that those caldera scarps must have existed before the Green Tuff ignimbrite did, so we support the idea that those scarps had nothing to do with the Green Tuff eruption. We think that’s pretty neat and we’re presenting the work at the Volcanic and Magmatic Studies Group annual conference, which in January 2015 will be held in Norwich. Jodie has already made the poster we’ll be presenting as part of the assessment required to achieve a Gold CREST Award, so we’ve decided to publish that online before the conference. I’d like to thank Jodie for some stellar research this summer, despite only having done 1 year of Sixth Form (AS level) geology (she’s 17!), and answering some questions I’ve been pondering for about 6 years. Hopefully, this data will go into a couple of papers I’m working on too!

Andrews, G. & Branney, M., 2005. Folds, fabrics, and kinematic criteria in rheomorphic ignimbrites of the Snake River Plain, Idaho: Insights into emplacement and flow. In J. Pederson & C. . Dehler, eds. Interior Western United States: Field Guide 6. Bouldor, Colorado: Geological Society of America, pp. 311–327.

Andrews, G.D.M. & Branney, M.J., 2011. Emplacement and rheomorphic deformation of a large, lava-like rhyolitic ignimbrite: Grey’s Landing, southern Idaho. Geological Society of America Bulletin, 123(3-4), pp.725–743.

Civetta, L. et al., 1988. The eruptive history of Pantelleria (Sicily Channel) in the last 50 ka. Bulletin of Volcanology, 50, pp.47–57.

Cornette, Y. et al., 1983. Recent volcanic history of pantelleria: A new interpretation. Journal of Volcanology and Geothermal Research, 17(1-4), pp.361–373.

Dyble, J.A., Williams, R., 2015. Micro kinematic indicators in the Green Tuff Ignimbrite: can they tell us about caldera collapse? VMSG Meeting, Norwich, 5th-7th January 2015. http://dx.doi.org/10.6084/m9.figshare.1160476

Mahood, G. & Hildreth, W., 1986. Geology of the peralkaline volcano at Pantelleria, Strait of Sicily. Bulletin of Volcanology, 48, pp.143–172.

Passchier, C. & Simpson, C., 1986. Porphyroclast systems as kinematic indicators. Journal of Structural Geology, 8(8), pp.831–843.

Schmincke, H. & Swanson, D., 1967. Laminar viscous flowage structures in ash-flow tuffs from Gran Canaria, Canary Islands. The Journal of Geology, 75(6), pp.641–644.

Williams, R., 2010. Emplacement of radial pyroclastic density currents over irregular topography: The chemically-zoned, low aspect-ratio Green Tuff ignimbrite, Pantelleria, Italy. University of Leicester. http://dx.doi.org/10.6084/m9.figshare.789054
Williams, R., Branney, M.J. & Barry, T.L., 2014. Temporal and spatial evolution of a waxing then waning catastrophic density current revealed by chemical mapping. Geology, 42(2), pp.107–110.

How do you become a volcanologist?

Researcher profile: Dr Rebecca Williams (@volcanologist)

How do you get to be a volcanologist? That’s a question I get asked a LOT. And a question that I’m happy to talk to anybody about, because I think it’s the best job in the world. It’s a question that I never had anybody to ask it to, when I was thinking about what career I might want to have. Through my GCSEs I got more and more interested in physical geography and my rock collection at home was growing (on the journey back from a Girl Guides camping trip, the coach driver asked me “what have you got in here, rocks or something?!” as he loaded my bags. He was stunned when I replied “yes, actually”). For a GCSE project we did an information pamphlet for the people of Naples about the volcano Vesuvius. Could you do this as a job?!

Pantelleria caldera lake - studying volcanoes means travel to some beautiful places.

But when I met with the ‘careers guidance’ teacher at school, they didn’t know what you could do to study volcanoes and geology. “Perhaps you could be a geophysicist?!” Well that was a word I’d heard of, being an avid Time Team watcher, so I thought that it sounded like a good idea. I chose my A levels based on that careers advice and started collecting university prospectuses based on who offered geophysics, but found myself narrowing down my UCAS choices by who emphasised volcanology on their courses.

Working at HVO as a gas geochemist.

The promise that ‘some of our undergraduates have volunteered at the Hawaii Volcano Observatory (HVO)’ made me head off to Royal Holloway to do a BSc in Geology. By that point I’d had a Nuffield Science Bursary and been awarded a Gold Crest Award for a summer’s work experience at TGS-NOPEC, where I discovered that geophysics probably wasn’t for me. But I knew that studying Geology would be ace, and I wasn’t wrong. My degree instilled a love of fieldwork, a sense of travel and adventure and a never ending curiosity about rocks: where did they come from? how they were formed? I entered my 3^rd year not really knowing what career I’d end up having, but knew I wanted it to be geology related. I applied constantly to the HVO until they finally offered me a placement. So, a week after graduation I flew to Hawaii where I worked as a gas geochemist for 6 months. This was not only an amazing experience (walking on lava flows, contributing to important science, hiking across volcanic terrain, snorkelling at the weekends) but also the moment when I realised that I could be a volcanologist as a career.

My path to volcanology wasn't always linear. For a while I worked as a PADI Divemaster.

On return from HVO I spent a year and a half working at the Hydroactive Dive Centre as a PADI Divemaster. I spent this time saving up and applying for Grad School so I could get a Master’s degree in Volcanology. I was awarded a teaching assistantship to study at the University of Buffalo in the USA. Here, my interest in hazardous volcanic flows developed, starting with my Master’s research on lahars.  Developing and driving my own research was something I’d really enjoyed so I then searched high and low for a great PhD project so I could continue doing volcanic research. I returned to the UK to do my PhD at the University of Leicester on pyroclastic density currents.

Logging volcanic deposits in the field

After my PhD I sailed as an igneous petrologist on an IODP expedition, and held a series of short-term teaching contracts at Leicester. This post-doc time of anyone’s life can be tough – when you’re never sure if that holy grail of an academic job can be found. I stuck it out, worked hard, juggled a part-time job as a teaching fellow and a part-time research job and gained some invaluable experience. Then, a year ago I made the move to Hull as a lecturer in geology, undertaking research in volcanology and now hold a permanent position. I made it. I’m a volcanologist. Now, I'm training up a new generation of budding geographers, geologists and hopefully, a volcanologist or two.

What's that coming over the hill...?

Understanding how pyroclastic density currents behave through time and space.
by Rebecca Williams (@volcanologist)

Pyroclastic density currents are flows of searing hot gas, ash and rocks that swoop down the sides of a volcano during the most violent eruptions. They can travel at speeds of up to 450 mph and can be as hot as 1000°C. Historically, they have been responsible for over 90,000 deaths, and so are the most deadly volcanic phenomenon.  In order to try and reduce the risk to people from these hazardous flows, we strive to understand how they behave. The trouble is though, is that they are very difficult to observe. Even small flows, such as those recently seen at Sinabung (Indonesia) and Tungurahua (Ecuador) volcanoes are hidden in a cloud of ash. The largest types of these currents however, have not been observed at all, and no instrument exists that can be deployed (and survive) to record them. So, we try to understand the processes in these currents by studying their deposits.

 

A typical ignimbrite as found on Tenerife. You can study the changes in size and quantity of pumices (yellowish, bright bits) and lithics (dark bits) to infer current dynamics. You can also interpret sedimentary features such as low-angle cross bedding (on a level with the scale) or if the deposit is massive (as it is towards the top of the photo). Photo credit: Rebecca Williams

 The deposit of a pyroclastic density current is called an ignimbrite. We can infer quite a lot about a current by studying an ignimbrite – such as which direction it was going, whether it was turbulent and dilute or whether the flow was dense and more full of blocks of rocks than it was gas and ash. At a single location, we can study the vertical section through a deposit and see how the ignimbrite changes from the bottom to the top. This tells us how the current was changing from its start (the bottom of the deposit) to its end (the top of the deposit), as the deposit builds up through time, recording the changing dynamics of the flow. What we need to know though, is how the current behaves through time everywhere it flows. Did it flow down that valley at the same time that it flowed down this one? Did it go over that big hill at the start of the eruption, or did it have to bury some of the topography to make it easier to flow over? Of particular interest are the very large flows that form a circular deposit around a volcano – are these formed by a radially expanding current that flows in all directions around a volcano all at once? This is much harder to do: there is often nothing in the deposit that tells us where we are in time in the current's duration. So, we can’t join up all those vertical sections to understand how the current behaved through time and space all around the volcano. That is till now, as published in the February issue of Geology.

 

A 'typical' section of the Green Tuff Ignimbrite - it's not typical! You can see it doesn't look like the Tenerife ignimbrite - that is because it is welded. The ash and pumice were so hot when they were deposited they welded together into glass. You can still see lots of primary sedimentary features though, including imbrication as seen here - the current was flowing from right to left. Photo credit: Rebecca Williams

The study here maps out the chemical composition of an ignimbrite which was deposited from a single, sustained pyroclastic density current and reveals how the current behaved through time and space. The ignimbrite used in the study, The Green Tuff Ignimbrite on Pantelleria, Italy, is a circular ignimbrite. It is chemically zoned, that is the chemical composition of the deposit is different at its top than it is at the base, and the composition changes gradually between the two extremes. This is because the magma chamber that the current was erupted from was zoned. At the top of the chamber the magma was more evolved than the magma at the bottom. So, when the volcano erupted it progressively withdrew magma of gradually changing composition and this was recorded in the volcanic deposits. This then, gives us a timeline through those deposits – we can say whether that chemistry was erupted at the beginning, the middle, or the end of the eruption.

The type section of the Green Tuff Formation and representative graphs of the changing composition. Copyright Geology. Full figure caption: http://geology.geoscienceworld.org/content/42/2/107/F1.expansion.html

  What we found in the Green Tuff was that the chemical change was gradual, so we were able to divide the unit into 8 different compositions, and these were equivalent to 8 different time steps through the eruption. Once we’d determined this chemical stratigraphy, and defined it at a type locality (a place that records the entire compositional change) we then went and mapped the entire ignimbrite. I spent over 6 months spread over 3 years logging, mapping and sampling the Green Tuff Ignimbrite and analysed over 500 samples to determine their chemical composition. I then input all this data into ArcGIS so I could see what the spatial extent of each of the 8 compostional zones were. I produced maps of these different zones and these maps represent the footprint of the current that formed the Green Tuff at a snapshot in time during the eruption.

Footprint maps of the Green Tuff pyroclastic density current based on zironium (Zr) compositional zones. Copyright Geology. Full figure caption: http://geology.geoscienceworld.org/content/42/2/107/F2.expansion.html

By analysing these maps we realised that, contrary to assumption, the current did not flow in all directions all at once. Instead, at the start of the current's duration it didn’t get very far at all, and only flowed in certain areas around the island. As the current continued it flowed further in more directions, but was often deflected or reflected around topographic barriers such as hills and old caldera walls. However, at the current's peak, it DID flow in all directions all at the same time. If there had been humans living here at the time, there would have been no escaping the devastating currents. The current continued, but the directions it travelled in decreased and it wasn’t able to go as far. It stopped being able to top over those hills and eventually decreased til it was able to flow only short distances from the vent before stopping altogether. We think that this all happened within about one and a half hours.

This has changed how we think about pyroclastic density currents and will impact on the way we model them for hazard assessments. We should think about these currents as dynamic, and able to change rapidly – each of the 8 zones represents only around 11 minutes. A current that was initially quite slow, restricted to a small area and unable to flow up hills, changed and was able to flow in all directions and over 500 m hills in less than an hour. 

 

The Green Tuff ignimbrite drapes the La Vecchia caldera wall - both the inward dipping caldera scarp and the seaward-dipping older stratigraphy - at Scauri, Pantelleria. Photo credit: Rebecca Williams

On the other hand, without knowing that the current changed through time, we might have overestimated how big it was. You can use the maximum distance that the current travelled and the volume of the deposit to estimate how much material was coming out of the volcano with time (mass flux). If we had used the maximum values and assumed that was what the current was like for its entire duration, we would have massively overestimated the eruption's mass flux.

Zoned ignimbrites such as the Green Tuff are not rare, therefore we hope that this technique can be applied to other deposits so we can continue to advance our understanding of pyroclastic density currents. 

This blog is a summary of this open-access paper:

Williams, R., Branney, M.J., Barry, T.L., 2014. Temporal and spatial evolution of a waxing then waning catastrophic density current revealed by chemical mapping. Geology. 42, 2, 107-110. http://geology.geoscienceworld.org/cgi/content/full/42/2/107?ijkey=y6rKLpaDrGsCQ&keytype=ref&siteid=gsgeology