17 August 2011
Last week I happened to be watching the National Geographic Channel and caught their new program, “How to Build a Volcano”. Being somewhat interested in volcano-building processes myself, I sat down with a pad of paper and got ready to take notes for a review.
The show started off with an exciting idea: bring together a special effects team and a group of volcanologists and try to replicate volcanic processes on a large (but controllable scale). Thus, building a volcano. The four volcanologists (Mike Manga and Ben Andrews of UC Berkeley, Josef Dufek of Georgia Tech, and Ed Llewellin of Durham University) worked with special effects expert Max MacDonald to create a 10-meter-high volcano in a Canadian quarry (and we all know from Mythbusters that anything involving an abandoned quarry is also going to involve explosions).
The construction methods involved were pretty much what you might expect from a scaled-up version of a paper-mache tabletop volcano, although in this case the group used shipping containers, mesh, burlap, and spray-concrete. But I noticed one interesting thing right off: This volcano wasn’t exactly to scale. And by “not to scale”, I mean “45 degree slopes”. In most volcanoes, slope steepness is limited by the “angle of repose“, or the angle at which things are too unstable to stay put. This angle is rarely greater than 30 degrees. But Mount Boom, as the group called it, had implausibly steep slopes. The volcano admittedly looked pretty cool for a one-day construction job, but if the volcanologists and special effects folks were trying to make the setup as true to nature as possible, they didn’t do a very good job. (After the main edifice was constructed they did extend the slopes a bit with dirt and gravel, but in volcanic processes, that first drop is pretty important, and oversteepening the slopes can give moving material a lot of extra
The volcanologists had two goals in the program: to model pyroclastic processes (namely a pyroclastic flow), and to test whether slug flow was possible in a lava-filled conduit. The edifice of Mount Boom was only really necessary for the first, but used for the second as well, since it would look pretty darn cool to film both there. (Another nitpick: it wasn’t clear from the narration or the explanations the volcanologists were giving that they wanted to observe gas slugs in a conduit, and the narrator kept going on about lava flows.)
While the experiments were being set up, the volcanologists trucked off to some real volcanoes to illustrate what they wanted to model. In one case, a group visited Mount St. Helens, and in the other, went to Stromboli. Mount St. Helens was predictably scenic but not particularly active, but Stromboli was supposedly surprising to the group that visited. Instead of the ‘usual’ Strombolian eruptions, the summit craters were extruding lava flows and experiencing more violent Strombolian eruptions. (The show played on this to make it seem like no one knew what they were going to find up there, which – despite the cloudy conditions – I highly doubt was true. Stromboli is one of the best-observed volcanoes in the world, and I would be surprised myself to find that the volcanologists had made the trek to the top without having first looked into the current eruption conditions.) Note: I deserved to be surprised – apparently this actually was the case! See Dr. Ed Llewellyn’s commentary in this post.
The experiments themselves are pretty clever. In the pyroclastic flow reproduction, fly ash, sand and rocks (as well as colored balls for tracers) were heated and then blasted from the volcano with compressed air and explosives. It wasn’t a sustained eruption, but it was enough to produce a column of material that then collapsed and formed a ‘pyroclastic flow’ on the lower slope Mount Boom. The eruption was recorded with a FLIR thermal camera and a video camera, slides were laid out to catch fallout, and posts were placed in the path of the flow and covered in sticky tape to collect samples. In the ‘Strombolian’ eruption, a boiler was set up to ‘erupt’ heated, pressurized water; a gas trap within the boiler was able to release ‘slugs’ of carbon dioxide into a clear column of water, with the goal of seeing if the gas would maintain its coherence through the length of the conduit. The experiment proved that in a cylindrical conduit, it was possible to expel a slug of gas and burst it at the surface, creating the spray of ‘lava’ that characterizes a Strombolian eruption.
The experiments – difficult setup aside – were pretty straightforward and performed well for only a few days’ worth of setup, but there were some problems which could make it difficult to use the results in a publication (if that was ever the goal of the investigation).
Something which could be due to some poor editing (or ignoring the scientists’ input) are the narrator’s comments on the results of the pyroclastic flow experiment, which are extremely misleading. In the show, the narrator says that pyroclastic flows have been thought to be relatively “simple currents” – which is incorrect. Volcanologists have thought for decades that pyroclastic flows are complicated phenomena, encompassing elements of granular and fluid flows. Current pyroclastic flow models (such as TITAN2D and VolcFlow) assume this in order to choose the equations which define the flow behavior. Saying otherwise ignores the research that’s been – and is being – done to work out how different parts of a flow (the rocky basal avalanche, the overlying turbulent cloud of ash and gas, the mixing zone between the two, etc.) interact.
The experiment shown on “How to Build a Volcano” may have produced the first artificial pyroclastic flow of such a large size, but the results
aren’t didn’t seem to me to be particularly groundbreaking. I’m curious why the volcanologists depicted in the show didn’t object more strenuously to this inaccurate characterization, but to give them the benefit of the doubt, they probably weren’t involved in the scripting or editing process. On a positive note, using sticky tape and colored balls for markers in the PF was a cool idea, and something that you couldn’t get away with in an actual flow. Instead, scientists have to use rock types, mineralogy, textural, and photo/video evidence to map flow paths. It’s also totally impossible to sample a flow as it’s happening, which the sticky tape allowed the volcanologists to do.
The gas “slugs” simulation is interesting, but didn’t appear to be
not very accurate. (Note: There’s a lot more background to this than I realized – and scaling concerns – so again, check out Dr. Llewellyn’s comments here.) An experiment which was trying to approximate the behavior of gas in a magma should have used something much more viscous than water. This would be a bit difficult, especially if you had some sticky substance gunking up the boiler, but as it was presented, this experiment didn’t try very hard for an accurate model. Also, the model raises some questions because it assumed that the “conduit” was a lovely smooth-sided tube, which probably wouldn’t be the case in a natural system. What happens if those gas slugs run into rough walls, or obstructions? The results of the gas slug experiment were visually appealing, but seemed pretty preliminary – not as final as the narrator made them seem.
I suppose it’s too much to expect a TV show to be totally accurate when it comes to scaled-down natural systems, but I was confused at the way the experiment was being portrayed – as if it was rigorous enough to be used for significant research. There are some problems that would have been fairly easy to fix, but as it was presented in the show, the experiment was neat but more of a visual spectacle than anything else. The idea of bringing special effects experts into the mix was something that might be useful to pursue – especially for a setup like Buffalo’s new large-scale volcanological experimental facility. All in all, it was an entertaining show – I certainly wouldn’t pass up the chance to build a giant model volcano – but the science
needed appeared to need a little work.