Meanwhile, over at Rabaul

Don’t shoot it Martha, it’s bigger than both of us!!

“Do like the gallant Roman soldier who, at the destruction of Pompeii, died in his sentry box. Remain at your post. When the critical moment approaches you may — to cheer yourself up — sing ‘Let me like a soldier fall.”

Well, talk about futile last commands. This order was apparently given to soldiers prior to the 1937 eruption of Rabaul. Perhaps we should add human imbecility to the list of volcanic hazards in addition to gas and ash fallout.

 

(picture and quote taken from http://press.anu.edu.au/wpcontent/uploads/2013/12/ch063.pdf
Note that the vent shown in the following video is Tavurvur which
is the small steaming cone in the left of the picture from 1937.)

Recently we have talked a lot about explosive volcanism in association with Bardarbunga. Here is a great example of what an explosive eruption can look like, albeit a very small one as far as volcanoes go (warning, earplugs recommended):

Rabaul Caldera
To say Tavurvur sprang back into life on August 29  would be a gigantic misrepresentation of the facts. Not only was the eruption not really what you would describe as a jaunty pas-de-chat across Blanche Bay but Tavurvur has also been merrily erupting away almost continuously since 1994 and actually started long before that.

(credits: https://pbs.twimg.com/media/BwLaXo0CQAABVV2.png)

In fact Tavurvur is just one satellite cone of the Rabaul caldera complex and a small and young one at that. According to Nairn et al. “Geology and Eruptive History of the Rabaul Caldera Area, Papua New Guinea”, volcanism at Rabaul goes back to at least 0.5 Ma.  There have been multiple pumice forming eruptions and caldera collapses probably accompanied many of the larger eruptions. Last century there were two eruptions that nobody here would hesitate to describe as major eruptions: 1937 and 1994. Both of these destroyed the town and resulted in loss of life (507 in 1937 (Wikipedia) and there were four fatalities in 1994 although the town had been evacuated beforehand). However, as is so often in the scheme of things, what for us is major, is just trifling for nature. It is thought that eruptions the size of 1937 and 1994 occur every 30 to 60 years, in other words, just daily business as usual for Rabaul. The last caldera forming eruption, on the other hand, was 1400 years BP with another one occurring about 7000 years ago and the larger Kulau Ignimbrite 20,000 years ago. Needless to say there have been many more large pyroclastic eruptions in between though not large enough to lead to caldera collapse. No wonder then, that Rabaul qualifies as one of the Decade Volancoes (see Henrik’s great post on these: https://volcanocafe.wordpress.com/2012/12/25/the-decade-volcano-programme/

http://www.volcano.si.edu/volcanoes/region05/new_brit/rabaul/3607_11s.jpg)

Rabaul, however, is just one of four large calderas located on the Gazelle Peninsula, the northeastern tip of New Britain.  Obviously the tectonic setting, like elsewhere in New Britain, is very conducive to caldera volcanism. One look at the maps shows us why.  To the south of New Britain the Solomon Sea Plate  is subducting steeply under the South Bismarck Plate (there is actually a spreading center in the Woodlark basin – slab push / slab pull?? you choose). This subducting margin is the main source of the island arc volcanism that lines the northern coast of New Britain.

tectonic map: http://volcano.oregonstate.edu/vwdocs/volc_images/southeast_asia/papua_new_guinea/PNG1.jpg

However, directly to the east of Rabaul, the North Bismarck Plate is sliding up and around the South Bismarck Plate in response to the wider collision of the Pacific Plate and the Australian plate. Here we see strike-slip earthquakes and extensional faulting. This suggests that there is a fairly abrupt eastern edge to the Solomon Sea Plate as it subducts under the South Bismarck Plate, almost reminiscent of a slab window or tear. In other settings, such tears are conducive to pooling of volatiles and melt and it is precisely along this margin that we find the North/South lineament of the four major volcanic centers on the Gazelle peninsula. On top of this slab tear/margin scenario I suspect that the line of four calderas indicate one of those extensional settings in the above map (i.e. the little white rectangles where the plates are moving away from each other). Acocella and Spinks did some work on this kind of thing in New Zealand and came to the conclusion that shear (the strike-slip faults) forms a constraint on large shallow magma bodies forming whereas extension facilitates it. Given this setting, the prevalence of caldera volcanism on the Gazelle peninsula makes more sense and extension provides a good explanation of the slightly NE trend to the lineament that stands about 60° off the line of the volcanic arc elsewhere on New Britain. Well it was a nice theory. Unfortunately the local geography skewers the idea. There is indeed a local extensional graben but it is aligned NNW, not NNE and it is about 20 km west of Rabaul. Drat. Guess I have to go back to the drawing board on that one.

http://press.anu.edu.au/wp-content/uploads/2013/12/ch063.pdf

Moving on to the form of volcanism at Rabaul, we see it generally takes two forms. The first is regular basaltic and andesitic eruptions of lava and scoria that build up nice symmetrical cones and shower more distal areas with ash. Over time such eruptions can have a major impact on the local environment. The larger eruptions involve uplift of the caldera floor, which strands local fish, and the ash buries buses in ash, destroys local towns and creates great backdrops for soliders to take photos to send back home. That kind of thing. The second form of volcanic activity is more intermittent explosive eruption of dacite and, more rarely, rhyolite. These eruptions dump large quantities of pumice and coat the surrounding countryside with ignimbrite and plinian fall deposits. Only the largest of these lead to caldera collapse. It is this type of explosive eruption that Nairn was referring to when he said there have been at least five and possibly as many as nine ignimbrite forming eruptions over the last 20,000 years. These eruptions can be pretty violent. Walker also had a look at Rabaul and came to the conclusion that the Rabaul ignimbrite was deposited by “very high velocity pyroclastic flows” which devastated the surrounding area out to at least 50 km from source.”

credits: http://press.anu.edu.au/wp-content/uploads/2013/12/ch063.pdf

But, as stated above, ignimbrite forming eruptions are only the top end of the scale at Rabaul. To quote Nairn again, “At least fifteen smaller dacitic pryroclastic eruiptions occurred during the interval between 20ka [i.e. the major Kulau ignimbrite eruption] and 1400 yr B.P. [i.e. the Rabaul ignimbrite].” Ok, adding in the ignimbrite eruptions, that is about one major eruption every thousand years. Not that dramatic considering how short our lives are. However, seven of these occurred in the last 5600 years of that period, culminating in the Rabaul ignimbrite. The Rabaul ignimbrite was the largest and most energetic eruption since the Kulau ignimbrite.

(credits: http://www.eqh.dpri.kyoto-u.ac.jp/~mori/rabaul/rabaul.html)

Subsequently, there were no notable dacitic eruptions until 1937. However, recent uplift (more than 1 meter was measured between 1973 and 1981 prior to the 1994 eruption) and the short interval between 1937 and 1994 indicate the reservoir is once again charged. That implies the system required roughly 1400 years to produce a significant quantity of dacite to induce two eruptions over a space of 50 years. That is pretty rapid recharge and I think it is safe to assume this is just the beginning of a new series.  So watch this space! While Bardarbunga is still working out what it wants to do, it pays to bear in mind the world is studded with volcanoes, many of which are quite restless and can pose a much higher risk than Bardarbunga. While researching this article I came upon this gem from the 1937 eruption. It is the earliest photo that I have seen of volcanic lightning. And what a beauty it is!

Credits: http://press.anu.edu.au/wp-content/uploads/2013/12/ch063.pdf

Addendum: Similkimeen found an absolute classic of Duk Duk dancers dressed up like volcanoes that just has to be up there with the best volcano / cultural photos out there.

And MikeD found this wonderful book on the region:
http://press.anu.edu.au/apps/bookworm/view/Fire+Mountains+of+the+Islands/10911/ch03.xhtml

References: This article draws heavily on my layman’s reading of Geology and Eruptive History of the Rabaul Caldera Area, Papua New Guinea released by Nairn, McKee, Talai and Wood in 1995.Any mistakes or misrepresentations are entirely mine. I highly recommend reading the original to get a better idea. The other major source was a wonderful historical document from Australian National University: Calderas, Ignimbrites and the 1937 Eruption at Rabaul: 1914–1940 to be found at http://press.anu.edu.au/wp-content/uploads/2013/12/ch063.pdf This is a great historical impression of the eruption in 1937, comparable to that one from Taal someone posted a couple of years back. There is also a wonderful thesis out there if you can find it from Herman Patia at the Australian National University. Chris Newhall (yes that Chris Newhall) did a short précis on it too: http://www.geo.mtu.edu/volcanoes/rabaul/rabaul.usgs.html And finally anything with Chris McKee’s name on it is worth its weight in gold as he is at, as I understand it, the Rabaul Volcano Observatory.   Bruce Stout Friday Riddles are up!  This set has a chemistry theme!  Click the tab at the top of the page.