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.

High school students as research partners: working with Nuffield Placement Students

 by Jane Bunting (@DrMJBunting) and Rebecca Williams (@Volcanologist)


Meanwhile, back in the lab...

This week, the blog is back indoors, where Jane and Rebecca are spending August helping some Sixth Form students get a taste of 'real science' in the summer before they apply for University.  Five students have placements with us in GEES through the Nuffield Foundation Research Placements Scheme, which will enable them to be assessed for a British Science Association CREST Gold Award.

Rebecca did a Nuffield Placement herself in the summer after her first year of A Levels.  Neither the Nuffield scheme or the CREST Awards had been done before at Rebecca’s school. An eager biology teacher, Dr Bridgeman, had heard of the scheme and so started it up that year with Rebecca and two of her school friends being the first students to go through it. They weren’t provided with placements, but rather had to find them for themselves. At the time, Rebecca knew she enjoyed Geography, Science and Maths. She was also a bit obsessed with Time Team and she has blogged before about how her journey into geology really started by wanting to be a geophysicist. The only company she could find locally which did geophysics was a consultancy company for the oil and gas company, TGS-Nopec (as they were then known). Rebecca wrote a letter (no email back then!) asking if they would take her on as a work experience student and was delighted when they did. It was a phenomenal experience. Rebecca worked on a project called ‘Hydrocarbon prospectivity along the eastern seaboard, offshore northwest Europe’. She doesn’t have a good memory, but the report is sat next to her as she types this – a testament to how important the experience was. Rebecca found that the geophysical interpretation of the seismic lines wasn’t what interested her. Rather, it was the geology – how is the oil formed, where does it come from, where is it stored, how is it trapped and where can it be found? When Rebecca then had to fill out her UCAS application a month or so later, it was geology degrees she applied to, and not the geophysics that she thought she was going to do, and the rest, as they say, is history. The Nuffield Scheme really did change Rebecca’s path in life. The results from that project were eventually presented by TGS-Nopec at the PETEX Conference – the premier oil and gas conference!

Students doing placements work with a supervisor for 4-6 weeks on a 'real' research project - one where the supervisor doesn't know what will happen or be found out.  The students are expected to read around their topics, contribute to discussions about the design of experiments or studies, plan their own time, learn to use different pieces of equipment, collect data and interpret it, and produce a report and a talk or poster at the end of the placement - of course there is lots of help available, from the supervisor, from technical staff, from other students and researchers in related fields, but it is still quite a challenge.  This year's students all seem to be making the most of it, and are filling their lab notebooks with lots of lovely data.

Tinashe weighing an ear of wheat

surface of a wheat leaf: the 'squashed donuts' are the stomata

Jordan, Leah, Charlotte and Tinashe from local sixth forms at Wyke and Sirius Academy are all working with Jane and Lindsey Atkinson (@LJA_1), who also blogs here, on a pilot study of the effects of small climate changes on spring wheat, which is linked to a bigger project being run by the Network Ecology Group called "The impacts of climate-warming on farmland food-webs and ecosystem services".  In this project, 24 plots are marked out in a field of spring wheat.  Half of these are warmed by 2 ^oC, the sort of change in summer temperature which we are likely to see in our region within the next century according to predictive models.  Since the warming will dry out the soil, half of the warmed plots and half of the non-warmed plots are also given some extra water, so some plots are warmer and drier, and some are just warmer.  We're studying wheat plants collected from the different plots in the field experiment, and also growing our own in the controlled environment rooms in the GEES building, where special lights on timers mimic day and night cycles, the room temperature is controlled, and neither rabbits nor aphids can snack on the growing leaves - the indoors experiment should therefore help us understand how the plants respond to the climate changes without the rest of the food web complicating the picture.  Jordan is studying how biomass allocation varies (essentially 'plant budgeting', looking at how plant resources are divided between light capture, water capture and reproduction).  Charlotte is looking at the effects of the climate changes on the grain yield of the wheat plants.  Leah and Tinashe are looking in more detail at whether the plants can adapt to grow in different conditions by varying the number of stomatal cells in their leaves (an introduction to studying stomata aimed at students can be found here). 

Jordan and Leah cutting up wheat plants

These data, along with other aspects of the plants being measured by Jane and Lindsey, will form the basis for an initial paper on the response of this important crop plant to anticipated climate changes (which of course will get blogged about here) and for a grant application to extend the work; we need to show that our experiments will produce interesting results before we can ask for funding, so these projects are playing an important role in helping us develop this research area.

Jodie uses a digital camera to photograph her thin sections

Jodie joins us from Hessle High School and Sixth Form College. Jodie is interested in geology and chemistry so we’re convincing her that volcanology is an excellent subject! Jodie is doing a research project on the Green Tuff Ignimbrite from Pantelleria with Rebecca. In particular she is looking at thin sections of the ignimbrite to look for features that she can use to interpret how the ignimbrite was formed. This project is a continuation of a long-running project that started with Rebecca’s PhD in 2006. It’s a small, but important part of a much bigger research jigsaw, and the results look promising! We’ll be blogging more about the project next week. If the results look good, Jodie and Rebecca will be presenting the research at the UK’s volcanology conference which this year is hosted in Norwich; Jodie is getting real experience of working on a research project at the cutting-edge of Rebecca’s science.

The Nuffield Schemes offer a wonderful opportunity for students to try out real science; it's very different from school!  For us, it's an excellent way to communicate with the next generation of scientists and consumers of scientific findings, and gives the students involved a taste of scientific work, a boost for their university or job applications and helps them make better course and career choices.  If you're a student reading this, ask your teachers about the scheme or go to this link.  If you're a scientist, we urge you to consider taking on placement students through the scheme - it might even help you get that crucial bit of data to progress your research next summer.

Work experience of a different kind - research interns in the Dept of GEES

This week, we have a guest blog written by Jessica Keeble and Richard Caley who joined us this summer to gain some experience in what its like to do academic research and the highs and lows of lab work.

As two Geology with Geophysics students in between our second and third years at the University of Leicester, we found ourselves with a rather long summer break which needed to be filled. After spending 4 weeks in the field carrying out coursework we came to the Department of Geography, Environment and Earth Sciences at the University of Hull to try and be helpful and learn one or two things! We were very lucky to spend 5 weeks in the department and although there were times where we could have cried (lab equipment can really try your patience sometimes!), we had an absolutely fantastic time with lots of great photo opportunities so we thought we would share our experiences (and funny pictures) with you.

Rich feeling very smart in his lab coat whilst filtering water samples

In the first two weeks of our work experience we learnt two things: 1) there is a LOT of paperwork and introductions to be made before you can even start being helpful; 2) sometimes being helpful means filtering a lot of water… Yes, in the first couple of weeks we did filter a lot of water. This was for Dr Karen Scott who is helping out on a project associated with The Deep (@TotEnvSimulator), which is looking at the effects of sediment concentrations on sedimentation and erosion in fluvial environments. Filtering the samples received from The Deep was quite a lengthy process, however, we did feel pretty cool getting to wear lab coats and doing actual work (not just making cups of tea/carrying out a lot of photocopying!). Filtering the water produced small disks of sediment trapped on filter paper, which when collected together looked more akin to a Dulux colour chart than a scientific experiment. Nonetheless, it wasn’t just sample preparation we were involved with. No, we were then introduced to the delights of the QICPIC machine.  The QICPIC measures the size and sphericity of the sediment grains and when it works, is a really neat piece of kit. However, working with the QICPIC machine was one of those times were we could have cried – it was very temperamental and there was a lot of stopping/starting and cleaning equipment involved (we really sympathise with anyone who has to use one of these machines for labwork/project work!).

"All hope is lost" - Jess contesting with the latest tantrum the QICPIC is throwing

In between filtering water samples and testing our patience with the QICPIC, we catalogued the extensive geological map collection the department owns (and when we say extensive, we are talking about >2000 maps), and organised a section of the sample collection which had been locked away for twenty-odd years. Admittedly, cataloguing the maps wasn’t the most exciting job in the world but we did subject the department to some interesting music (along with Jess’s rather out-of-tune renditions) so luckily the job didn’t take too long! On the other hand, organising the sample collection was really enjoyable and we unearthed some absolutely brilliant samples which gave rise to the unfortunate picture of Jess below.

 "Maybe Palaeo isn't so boring after all!" - Jess with one of her favourite samples from the collection

The main project we were involved in during our time in the department was testing out a handheld-XRF machine to determine its suitability for use in the field.  This involved getting up at an ungodly hour to collect the equipment and receive an induction at the Scarborough campus first, followed by many days of testing samples. Firstly we tested manufacturer's standards to determine the accuracy of the equipment, followed by testing our own standards and then some of Dr Williams and Dr Rogerson’s rock samples. Experiments included: rough vs. smooth surfaces to determine whether the XRF could be used on weathered rocks in the field; carrying out multiple tests on the same sample point to determine accuracy and reliability of the XRF; and carrying out tests on multiple sites on the same sample to determine how well the XRF deals with heterogeneity. Once we had completed the experimental work, we then began the rather extensive task of writing up our results (and to this day we are continuing to finish up the report we started to create – don’t worry Becky, we’ll have it with you very soon!)

The Handheld XRF kit - yes it does look just like a supermarket scanner, but it can't check the prices of your groceries for you

We were incredibly lucky to have such an active role in the work the department carries out and feel really privileged that, despite being only Second years, we were trusted to carry out experimental work and our opinions are truly taken into consideration! Despite the, at times hilarious, low points (running a whole test cycle on the XRF when we had forgotten to even add a rock sample; the QICPIC failing us for what felt like the hundredth time, and dealing with rush hour traffic on a bus) we had an absolutely brilliant time in the Department and gained a real insight to the life of an academic.

I'd like to add a huge thank you to Jess and Rich for their invaluable help in a number of research projects this summer.

Work experience in an academic department is an excellent way to add to a CV, gain skills outside of your degree programme and to trial what research is like if you're thinking of doing a PhD. If you would like some research work experience, get in touch!