I thought I might take the opportunity to write a few posts about my first love – pyroclastic density currents (PDCs). These are basically particle-laden flows, rich in ash and pumice, which form from explosive eruption columns or the collapse of lava domes. There’s some great videos of these things on Youtube, from small pulsing flows on Merapi, to much larger flows such as the fatal Unzen flow which famously killed Maurice and Katia Kraft, along with 41 others in 1991.

PDCs are very complex things to try and understand; they transfer a lot of their energy through particle collisions, with a gaseous interstital fluid providing very low viscosity. That means they don’t behave like normal Newtonian fluids such as water. In fact, we really don’t understand the physics behind how these things behave – there’s thousands of papers, numerous research groups, and entire journals specifically aimed at trying to develop a better understanding of granular physics. It’s not even just restricted to volcanism; grain silos, industrial hoppers, snow and rock avalanches, even how your cereal settles in its box – these are all controlled by granular interactions.

If you watch that Unzen video, you’ll see that the flow inflates and accelerates as gas is either entrained from the atmosphere, or the volcanic material itself exsolves it. You’ll also see that they move phenomenally fast, yet come to a surprisingly sudden end. They’re able to flow up hill in places. So it was that my PhD project was aimed* at developing a new numerical model to investigate deposition from these flows to try and understand their flow mechanics better. Several models already exist, but they have a range of flaws and limitations, some of which I was hoping to investigate.

I was desperate for some real-world datasets which we could ground-truth our numerical model against, and so we went looking for a field analogue. We knew that for our modelling approach to work we needed detailed data on deposit geometry, as well as the physical properties of the materials deposited. We knew a good DEM existed of Ngauruhoe, which Gert Lube and Shane Cronin at Massey had collected a year or two before. Most importantly, Ngauruhoe had been picked because it had a number of exceptionally preserved pyroclastic deposits from some small-volume eruptions in the mid 70’s.

So it was that I was lucky enough to get permission to go and spend a week data and sample collecting on the flanks of one of the most beautiful and youngest (it’s only 2500 years old) composite cone volcanoes in the world. This is probably better known to many of you as the volcano used by Peter Jackson for a lot of the Mount Doom footage in the Lord of the Rings films.

Ngauruhoe sits at the edge of the Mangetepopo valley – a drainage basin which feeds from the numerous vents of Tongariro volcanic complex, just a few miles from the much larger and more imposing Rhuapehu volcano. My targets were the 1975 pyroclastic deposits on the North Western flank of the volcano. These intercalate with a number of older leveed lava flows. They’re actually incredibly difficult to distinguish from each other on the photo above, but are basically the majority of the darker flow units reaching down toward the valley floor.

The main objective was to identify which of the flow units would be the best for modelling, and – having done that – collect samples from various points on the selected flow for physical analysis back in the lab. With the volcano forming part of a national park, as well as a dual-status World Heritage Site (for both its Maori cultural importance, and it’s outstanding nature as a volcanological and biological park) getting permission to sample is no small feat.

Gert and a number of his colleagues from Massey were kind enough to spend a day giving me an orientation on the volcano, and then it was just me, a rucksack, a spade and some sample bags.

The interesting thing about the pyroclastic deposits here is how small they are. In truth, they have a lot more in common with a dry-rock avalanche than what most volcanologists would consider a pyroclastic flow deposit. We are generally trained to look for poor sorting, with lots of pumice and ash, with channel filling morphologies, probably associated with some cross-bedded surge or well sorted fall deposits. These things are really quite different.

Most pyroclastic flows are thought to be gas-fluidised. That is – as well as the pumice and ash, they have a large volume of hot gas keeping the solid particles from interacting too much. That means they flow for ages, and in some cases travel for miles more than expected, even going uphill. These 1975 deposits are from far more unsteady dense flows. The beauty of these is that we have a much better idea of what was going on inside the flow to be able to model it. Hence, for this project, they were a perfect analogue for the first stage of the project **

Having decided on which flow was the most interesting and feasible to model, I will now take you on a whistle-stop tour of a dense pyroclastic flow deposit, from distal to proximal end.

1. The distal toe.

The parts of the flow which travelled furthest. The flow lobe is comprised of an upper surface of the large scoriaceous blocks, lying above a much finer grained internal material. There were two of these flow lobes, one deposited on top of the other, suggesting that the flow had two discrete pulses of material within it (material has travelled in from the upper right in this photo).

2. The feeder channel

Feeding the depositional lobes, the flow ran through these leveed channels. The levees are made up of large scoriaceous blocks on the upper and outer edges, with the finer fill material making up the bulk of the rest. This bimodal size distribution was pretty consistent throughout the deposit. The blocks are mostly scoria with low vesicularity, with fewer red agglutinate blocks which had been eroded from the upper reaches of the volcano. There’s about a meter of relief between the levee edges and channel centre.

3. Upper reaches

Higher up the flanks, where the slope angle was approaching 25 degrees the channels lost most of their sinuosity, increased the levee height, and also demonstrated a number of bifurcations – again suggesting multiple flow pulses.

4. Proximal deposit

Then, as you approach the 30 degree upper slopes the levees disappear entirely. This following shot is from a couple of hundred meters below the summit. You’ll notice that you can see the flow in the middle distance, but it’s obscured in the foreground by scree (you might just make out a shallow leveed channel to the left – the paler channel to the right is a recent feature formed by meltwater flow).

Above this point the flow is completely indistinguishable from the surrounding scree.

These relatively low volume deposits (<1,000,000 m3) are a window into the most dense, least fluidised end of the PDC spectrum. They have more in common with rock-falls than what most would consider a pyroclastic flow, but are an excellent case study in what happens when gas fluidisation is not a major factor in flow propagation. In a future post I’ll take you through a deposit from the other end of the spectrum.

For more information on the 1975 PDCs at Ngauruhoe I would strongly recommend you check out this JVGR paper by Lube et al: http://www.sciencedirect.com/science/article/pii/S0377027306004239

*As many of you who have gone through the PhD process will understand, these projects can sometimes suffer from feature-creep. Mine was particularly susceptible, as it was unfunded, not part of a larger research group, had 3 supervisors (each of whom have very different expertise), and it turned out once I’d got digging into the literature that there was not enough suitable published data for us to use as baseline testing for a new model. As such, a project aimed at numerical modelling soon became one based in analogue experiments in order to create well constrained baseline data. Once the experiments started throwing up unexpected results, I ended up down a whole other rabbit hole. But that’s for another blog post.

** See above.

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