That the question even exist is a bit of an oddity in modern volcanology, after all we have known amply how to take ejecta-depth measurements to create Isopac-maps since 1956, more about that below.
The reason that this question has prevailed is that there are anomalies in the numbers proven for the eruption, compared to the results that these numbers give. In other words, the figures point in one direction, and the conclusions drawn in another direction. And the variance is big, and nobody has succeeded to explain to me why X suddenly becomes a Y in the other end.
What if there was a work in Volcanology of such standing that it is the Golden Standard to be measured against, like the kilogram and the meter references that used to be in the vaults of the French Academy of Science? Or, something as momentous as Albert Einstein’s theories of Relativity?
Thankfully there are two of those in volcanology, one is Iain Carmichaels work on Thingmuli (reference below), the other is Thorarinsons work on the eruption of Hekla in 1947-48 (reference below). All later works in volcanology rely on these two.
Carmichael made petrochemistry a staple diet of volcanology, and Thorarinson showed that Isopac-maps was a reliable way to determine size of an eruption. In this case I will invoke Thorarinson to split the difference on Puyehue Cordón-Caulle, below abbreviated as PCC before I write Cauliflower by mistake again.
Now let us talk about the eruption, and compare it to Hekla, I will be doing that from the principal paper about PCC, by Pistolesi et al (reference-link below). But before we start there is something that I must state from the paper. The authors in words talk about the eruption as a VEI-4, but in numbers it becomes a VEI-5. This is something that must be reconciled.
Puyehue Cordón-Caulle 2011 in numbers
The eruption started on the fourth of June 2011, and the bulk of the ash was produced in the first 48 hours (roughly 75 percent). The eruption was declared over on the 21st of April in 2012.
The height of the ash column from sea level during this period was 11 to 14 kilometres, so roughly 9 – 12 kilometres of columnal height from the vent.
Now, let us look at the depth of the ash. There are 13 layers of tephra that are quite distinct from each other, either from changes in wind-direction, or changes in petrochemistry. The by far thickest is the layer 1 from the first 48 hours.
Isopac-measurements was taken at 70 different spots out to a distance of 240 kilometres, ranging in depth from 0.1cm to 30cm. To do this well you do a random dispersal pattern and stick to that without any cherry-picking.
We know for a fact that ash ended up in several places from the initial blasts across the southern South America. Nothing to odd about that.
But, there is something mentioned in the paper that is borderline bizarre. And that is a claim that ash would have travelled across the southern hemisphere to reach the continent again on the 18th of June. That seems improbable due to the low columnal height.
I do know that there are several good professional meteorologists in here, one of them could perhaps explain this with a jet-wind or some such, I surely can’t.
Now, the suspicion about cherry-picking. “At section 7 (Fig. 1, 240 km from the vent), the Unit I deposit, collected on a tombstone relatively sheltered from winds…” (Page 6; Pistolesi et al). So, to find the ash they needed to search for a wind-sheltered spot to even be able to take samples and measure the depth? Intriguing.
The reason for them cherry-picking is that this is basically not a paper about the amounts on explosive ejecta volumes, it is a paper on stratigraphic layering. As such they needed to find as thick layers as possible to tell them apart.
At the same time in Argentine they estimated the eruption to be 100 million cubic metres in explosive ejecta, giving the eruption as just precisely a VEI-4. This is quite interesting, and I will get back to it in the conclusion.
Now let us go back in time, to a place where volcanologists had lunch between exploding vents.
A musical interlude: History of Science
Back in the day scientists wrote in an easy style, not that unlike an action novella. This is why Sigurdur Thorarinson comes across like an eighty’s movie action hero in an extremely serious academic paper. As I read his works, I always feel that he walked around with a catchy eighties theme-song echoing between the volcanoes of Iceland.
The reason for this is simple, back in the day it was popular to go to lectures in what was new in science, and what was newly discovered. Also, people read a lot of books about science, and it was almost mandatory to write popular books about your discoveries.
That made the style in the scientific papers easier too, because if your audience fell for you and made you a star, well then, they would read your papers too.
Einstein is a great example of this tradition, you can even today go and buy a book that he wrote explaining relativity written in simple sentences, and that uses parables and analogies to explain the concepts. If you wish you can obviously go deeper in some more mathematical examples.
He also did massive lecture tours that was completely sold out. Who would’ve thunk that scientists once made more cash on being stars than they got from their University salaries?
But, even for that time Thorarinson was a bit of a character. The part where he and colleague walked up Hekla during the eruption and stands between two explosively erupting vents at the top while bus sized lava bombs bop down around him definitely made him into a legend.
In part why so many people are sceptical of science today is because they never get into close contact with scientists. People tend to trust people they have met, regardless if they have understood everything, and science have forgotten their obligation of educating and entertaining the people.
Some obviously have this skill; Albert is a stunning example of this. Whenever he gets around to writing that definitive coffee table book about astrophysics, well I will be the first to pester him for having my copy autographed. It will be a stellar read.
Now dear reader, how about I shut up and return to what I was writing about?
Hekla 1947-48 in numbers
Thanks to Oddur, an intelligent and reliable man (he was a farmer in Iceland, and a lot of other farmers have saw the same thing), we do know that the eruption started at 06.41 local time on the 29th of March in 1947. We also know that it started with Hekla pushing up a 100 to 200-metre-tall spine extrusion at the Toppgigur (top vent).
The spine was probably the leftover lava in the Toppgigur from the last eruption. Please feel free to consider the force needed to lift a huge honking lava plug 100 metres up into the air in two minutes. After that the well shaken warm champagne bottle was uncorked.
At 06.51 a powerful earthquake was felt from Hekla and the mountain split across its length in a large explosive eruption. At both the north and the south ends lava spewed forth, but between that ash, tephra, lapilli and lava bombs hurled out. Lava bombs was later found more than 30 kilometres from this initial blast, so Oddur was lucky to not be hit out in his field.
During the first two hours the ash column reached a height of 27 kilometres with a +/-10 percent accuracy, well into the troposphere. During these two hours 90 percent of all ash of this 13-month long eruption was ejected. The total amount of explosive ejecta was 220 million cubic kilometres, so during the first two hours 0.198km3 was coughed up.
That leaves a columnal height of 26 kilometres with the same uncertainty as above. The column height is given from a photograph taken 120 kilometres away, witness statements from the farmers, and a pilot. It was also noted in degrees from the horizon by two ship captains.
The total lava amount was 1km3, making this the largest Icelandic eruption until Grimsvötn 2011.
During the eruption and in the year after Thorarinson went around all of Iceland taking hundreds of ash depth measurements out on the fields. If any sample was sheltered or unusually deep he noted that and did not use them.
He found that ash depths in the wind direction during the first two hours of the eruption (NNE) was between 1 and 50 centimetres, he never got lower readings since he ran out of Iceland to measure from (in other words, he hit the coastline).
He then made a map with isopac-contours drawn on it, and from that he could accurately calculate the amount of explosive ejecta.
I could obviously go on and on about the various figures and comparing them, showing that Pistoleti et al, are talking out their wazoo. Problem is that I am quite convinced that the mistake is an honest one.
In the text they talk about small to medium sized explosive eruptions in the range of VEI-3 to VEI-4, and they seem to state that this was a VEI-4. I am therefore quite convinced that a human error somewhere crept in and they got a decimal-comma moved in the wrong direction.
The text parts fit quite a lot better if it is taken into the context of 100 million cubic metres (0.1km3) of total ejecta. And that is after all what the Argentinians came up with.
Now that I have given you an example of a larger and more powerful eruption, the Argentinian volcanic authorities calculations (based on Chilean raw data), the logical steps, the data, and so on.’, you can yourself do the calculations if you wish. I am not about to beat a dead horse anymore Fennimore.
The pertinent articles
Sigurdur Thorarinson; The eruption of Mt. Hekla, 1947 – 1948 (1950). Private copy.