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.

Christmas is a time for...conferences.

By Rebecca Williams (@volcanologist)

Christmas for academics is a time for family, eating a bit too much and relaxing before the inevitable marking chaos starts during the January exam period. Some use the time away from teaching to go on field trips or to write a paper that’s been neglected during a semester. For many, it’s also conference season.  Many of the Geological Society of London’s specialist groups hold their conferences during the Christmas ‘vacation’. This year for me was a double whammy, kicking off the holidays with the British Sedimentological Research Group (BSRG) meeting here at the University of Hull, and then heading up to Edinburgh to start the New Year at the Volcanic and Magmatics Study Group (VMSG) meeting.

Left: The BSRG field trip to Flamboro Head (photo credit to Dan Parsons @bedform)
Right: The BSRG conference dinner at The Deep, Hull (photo credit to Rob Thomas)

It was quite poignant to have BSRG here at Hull. In 1989, the Annual General Meeting (AGM) was planned at Hull, but the Geology department fell foul of the Government’s Earth Science Review and was closed. BSRG instead went to Leeds. This year, Geology is back in Hull. 2013 saw the start of BSc Geology with Physical Geography and 2014 will see the start of single honours BSc Geology. The conference kicked off with an excellent keynote speech from Dan Bosence who summarised a lifetime’s work on Carbonate Depositional Systems and our latest knowledge on these systems. That started a session of plenary lectures on novel advances in Sedimentology. This was my first BSRG conference and so it was great to listen to the latest cutting-edge research on such a wide range of sedimentological settings: from submarine channels to microbial biofilms to mass extinctions related to Chinese volcanism via drones (sorry, Unmanned Aerial Vehicles). The plenary set the tone for the rest of the conference. Given that I was still teaching during this week and that I was swamped in marking, I missed the apparently brilliant ice breaker in the Brewery Wharf microbrewery on Wednesday night and the conference dinner in the The Deep Aquarium, Hull’s premier tourist attraction. I was able to present some of my work on pyroclastic density currents (essentially, a summary of my latest paper – blog coming soon!) in the “Turbidites, Debrites and Mass Flows – dynamics and deposits” session. It was the first time I’d presented this work to sedimentologists, which was a bit nerve wracking. I got some great feedback though so it was worth the nerves! I also missed out on the field trip to the spectacular exposures along the Yorkshire Coast between Bridlington and Scarborough, led with help from the Hull Geological Society.

Top left: One of the groups on the VMSG field trip at Holyrood Park, on their way to Salisbury Crags
Top right: Hutton's section at Salisbury Crags, note the broken sedimentary 'bridge'.
Bottom left: The winning University of Hull duck - see this Storify/Twitter summary of the Volcanic Duck Race
Bottom right: VMSG conferences (and geologists in general?) now go hand in hand with a good Ceilidh.

This year is VMSG’s 50^th Birthday and the anniversary year started off with a bumper conference in Edinburgh. This year saw 5 days of volcanology and magmatic conferencing, starting off with a trip to blustery Arthur’s Seat. We got to see some classic geology including Salisbury Crags and Hutton’s section where he demonstrated that intrusions were once liquid magma that had forced their way into surrounding sediments and then crystallised, leading in part to his 1788 book Theory of the Earth and his recognition as the Father of Geology. The conference itself was opened with the Hallimond Lecture given by Sally Gibson on ‘Continental rifting and mantle exotica’ as part of a session on Intraplate and Continental rifting dedicated to the late Barry Dawson. The theme of the conference was 50 years of VMSG and so the sessions followed those at the very first VMSG conference (then VSG) with some modern sessions such as Planetary Volcanism and Volcanic Hazards and Risks, reflecting some of the new directions the science is now going in. I presented my on-going work on the Louisville Seamounts, drilled during IODP Expedition 330 (some of which I’ve blogged about before). Highlights of the conference included the VMSG Award Lecture from Jon Davidson on ‘Interrogating crystals to understand magma systems’ with a plea for people to not forget and abandon fundamental petrology, and the VMSG Lifetime Achievement Award Lecture from Steve Sparks on a lifetime’s work of ‘Integrating disciplines, models, experiments and observations to understand volcanic processes’ with a special shout-out to all the young researchers who will be the future of VMSG research. The ice breaker saw the official launch of Volcano Top Trumps with the excellent Volcanic Duck Race which was won by our University of Hull duck! We were also treated to a dinner and ceilidh at Our Dynamic Earth, Edinburgh’s excellent interactive exhibit about the planet we live on. To finish up the conference, the microanalysis facilities at the University of Edinburgh opened their doors to us and delivered an excellent workshop on techniques such as the Ion Microprobe (SIMS) and the Electron Microprobe.

Both of these conferences have the same ethos at their heart: they showcase outstanding UK research on an increasingly international stage, are supportive of the student members with significant proportions of oral presentations given by students, and they are always full of social occasions in which to meet with old and new colleagues. Whilst they seem to cover completely different specialisms, for a volcanologist who looks at volcaniclastic deposits, there is a strong cross over...perhaps in the future there is potential for a joint session at one or the other of these conferences? I certainly hope so.

This is a fantastic video of the Volcanic Duck Race filmed by Thermal Vision Research Ltd.