This really should be Ruminarian VI, but I got delayed/sidetracked in writing it. (or just plain burnt out). Someone posted a Vanuatu quake and… well, they are sort of common. Poking around at it I ran across Ambrym, and one of my discarded post ideas came back to lend an intro to this. For the sake of sanity, I hopped out of the Ruminerian sequence to the coordinating Dragon won’t pop a gasket trying to keep it straight. Ruminerian VI will logically follow V when I get around to finishing it.
I’ve sought calderas now for several weeks… I find them quite entertaining. I always get a hoot when some media bobble head yammers about “Supervolcanoes.” I’m not a geologist, and I also detest the term. My preference is “large caldera structure.” One or two media oriented sites stated that there were six known “supervolcanoes,” which is total B/S when you get right down to it. Some in the geologic community have actually adopted the term, more out of making something useful out of it rather than fight the phrase. There still isn’t a clearly defined set of standards that make one eruption super and another not super. I’m pretty sure that the residents of Yakima Washington thought Mount St Helens was pretty “super” at the time it went off… though even with the widest pseudo definition, Saint Helens was far from a “Supervolcano.” I’m sure the residents around Merapi feel the same way about their recent volcano encounter.
One breakpoint that I’ve seen mentioned, is 500 km³ of ejecta. Okay… then what about the 200 km³ eruptions? They aren’t “super?” At 200 km³ I’m pretty sure that those affected may not really give a rats arse what the definition is. So… if you break it down into “large caldera” events… they are everywhere.
The one I’m going to look at shows up in earthquake lists all of the time. It’s the system that usually pops up with the Vanuatu quakes. The name of it… is Ambrym. Mt Marum and Mt Benbow are in a state of eruption… but they are just features inside the Ambrym caldera. At 12 x 14km, Ambyrm caldera has an extent of about 132 km². It’s last major event was about 50 AD ± 100 years with a bulk Tephra volume of 7.0 (±1) x 10^10 m³. That’s not DRE (Dense Rock Equivalent), that’s bulk. You can estimate the DRE by multiplying that against the ratio of bulk density vs rock density… or about 1000 kg/m³ over 2700 kg/m³. That’s about 0.3704. That gives us a rough DRE of about 2.59 x 10^10 m³. (GVP’s generic conversion routine when you don’t have the actual densities)
Doing a quick conversion to km³, it comes out to about
2.59 km³ 25.9 km³. Not small, but hardly that impressive. Far less than what the 132 km² area of the caldera hints at. Using that formula that you guys here on VC helped to gather the data for, it comes out at 155 km³ of material associated to a caldera of that size.
Since the caldera is supposed to have formed in conjunction with that eruption, there seems to be a huge disconnect with the formula. Something on the order of around a factor of six. Why the disconnect? Because Ambyrm caldera is a collapse structure… not an eruption structure.
McCall et al note:
“The Ambrym caldera appears to have formed by quiet subsidence or by subsidence accompanied by eruption of scoria lapilli little or no different from the material that was erupted prior to and subsequent to caldera formation.”
And given that their observations indicate that the caldera walls have a youthful appearance (little erosion in a tropical environment) it is easy to see where they came up with that conclusion.
They also note that the non-collapsed slopes tend to represent the pre-collapse gradient… working off of that, I calculate that the summit max height could have been as high as about 1600 meters.
So, where is all that missing 129 km³ of material? It’s not really missing. That was part of what built the island to begin with.
Now for something that hasn’t been touched on since way before Eruptions moved to its current site. One of the users there, “Passerby” and I would trade info back and forth about processes. My knowledge was inferior to his, but I did pay attention to the stuff he put forth. One of the ideas is best described as “pleating.” Passerby noticed that in some of my plots, the quaked in the Benioff zone (aka Wadati–Benioff zone) seemed to describe a shape similar to the folding of a piece of fabric as it hit the floor… bending back and forth. This is from the nose of a subducted plate encountering resistance (either from density or from buoyancy) and folding up like slow moving molasses/taffy. It’s really remarkable when you think about it, and actually comes from the fact that rock is not the hard substance that we think it is when you get to really high pressures and temperatures. It oozes. Carrying this a bit further, as a plate subducts, it doesn’t do so in a simplistic fashion. It can bend, fold, pleat or move in any fashion that you can imagine a semi-ridged slab of not quite ridged material can. Clay, Fudge, toffee… all show similar movement characteristics.
You can see this best illustrated of you yank out all of the quakes above 50 km and look at just those associated with the Benioff zone in the area of Ambrym. Notice that the subducting slab is folding and pleating as it goes down. This is a combo graphic of a fitted poly-sheet to those quakes. This indicates roughly where the quakes are at based on what their history was, and fills the gaps so that we can mentally picture where that surface it at.
“The Geology and Geophysics of the Ambrym Caldera, New Hebrides” McCall et all (1969)