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.

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