Ruminarian III – Holey Musings (calderas)

Where to start where to start… It’s pretty messed up when you have two or three related topics buzzing around and you don’t know how to either piece them together, or how to talk about one without spoiling another.

I guess the first thing to talk about… are craters. When stuff is ejected from the ground, it will form a pile around the opening and material will stack up. If this is a vent or fissure, it eventually makes a scoria cone and over the millennia, it can grow into an actual volcano. That opening at the top, and the material piled around it… is not what I’m talking about. That’s a crater.

What I actually want to talk about is a caldera. But I had to get the two into focus. When a cavity forms and the overlying roof cannot support it’s own mass, it will subside or collapse into it. This is the general basics of how you get a caldera. As you can see, the formation of the two (crater vs caldera) is a bit different. If you go out searching the Internet for Caldera Types, one site you will hit will be IOP Science. According to “IOP Conf. Series: Earth and Environmental Science 3 (2008) 012021 doi:10.1088/1755-1307/3/1/012021” there are three general types of calderas; Summit calderas, Classic calderas and Graben calderas.

That last one should make your ears perk up.

explosive volcano-tectonic collapse structures from which large-volume, ignimbrite-forming eruptions occurred through several fissural vents along the graben master faults and the intra-graben block faults, causing the collapse of the whole graben or of a sector of it

For examples, they list Sierra Madre Occidental, La Pacana, some stuff in the Pyrenees, and Toba. (but they are not that sure about Toba). Either way, the system seems to erupt along the boundry of the graben, material evacuated from underneath, and the block(s) drop into the now emptied out space. It’s still a graben, but a somewhat more violently forming graben. Not really a volcano per se.. more like a chunk of land fell into a hole.

As noted in the various posts. Nisyros has had some attention, though mainly because we noticed it. The actual potential volcano mentioned by Prof. Dr. Övgün Ahmet of ERCAN is about 65 km further east between Simi and Bozburun. My estimate for the location is approximately 36.638°N – 27.956°E based on an eyeball of the seismic activity.

How this involves Nisyros.. well, that’s a bit iffy. Notice my plot from the other day about the Kos ring-fault. That spit of land east of Nisyros is Datça, a district of Turkey, and passes directly north of the suspected new volcano. It’s also part of the southern boundary of the Gokova Graben

I’m not saying that the Gokova Graben is a graben caldera, but having a set of large caldera structures on one end definitely makes you think about it. If Dr Ahmet’s volcano does get on with the show, it will be interesting to see how it plays out. Other than the oddball coincidence that I noted, there is no indication that anything catastrophic is in the works. It’s just a learning artifact when you discover something new.

Now.. back to the IOP article…

They mention that there are three collapse styles. Piston, Trap-door, and Piece-meal. Not that it’s is any great revelation, but it’s nice to have confirmation of the styles that we have heard about. Many times I have gone chasing after caldera outlines only to come up with semi-circles… such as in SE Spain:

The interesting part about these three, are that the two northern ones are mineralized, and the southern one is not. Mineralization of a caldera means that fluids percolating through the material have concentrated minerals into ore deposits that could be of value if mined. Los Frailes (the one that is not mineralized) could have produced up to about 51 km³ of material if my size estimate is correct. Part of the caldera is eroded out and open to the sea. The other ones… well, it’s hard to even guess at what the extents were. Logically, the opposite side of the chamber should represent the other boundary, but that adds more layers of uncertainty to a guestimate (wrap your noggin around that one) .. so I didn’t even try.

All three of these formed between 6.5 to 11.7 million years ago… or sometime towards the end of Bruneau-Jarbridge eruption and somewhat mostly during the Picabo and Twin Falls volcanic field events of the Yellostone Hotspot in North America. (no, not saying they are related… that’s just the time frame)

The driving mechanism of all of the Capo de Gata features appears to be driven by Eurasian Plate / African Plate activity and jostling in the Alboran Basin.

As a quick side note, the Carboneras Fault Zone bounds these systems on the west, and tracks out into the Alboran Basin. For the most part, it experiences left lateral motion.

Why did I mention these systems? Well, the two northern ones are possible examples of trap door calderas. Yellowstone (and a few others) we already know about… piston collapse. That’s where the roof comes down pretty much as one unit. Peicemeal is just that… sections drop… but not all at once. All types of calderas can experience each type of collapse.

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Figure 9 of “Overview of Production at the Mori Geothermal Field, Japan” Hanano et al (2005)

Now we get.. weird. Ever hear of a Maar? A Maar typically forms as a narrow cone eruption and usually involves magma interaction with ground water. Another similar shape, are Kimberlite pipes. The point of origin for Kimberlite pipes is much deeper than Maars, which is why you can get diamond bearing rocks from them. Yet they are similar in shape. Another place where you can see this, is in “Nigorikawa type” calderas. These are probably best thought of as a parent type for both Kimberlite Pipes and Maars, though likely they differ on scale. Sunagohara and Nigorikawa are examples of the structure. More of a breccia, ash and dome filled funnel than anything else.

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I’ve done a bit of estimating of the eruptive volume from various caldera sized. As you know, my formula only gets you into the ball-park as a rough estimate of what it may have produced. Why does it have a lot of slop? From the various types and styles that I have mentioned.

If you wish to play with it, here ya go.

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WARNING: Math here.

log(y) = C(0) + C(1)*log(x) + C(2)*(log(x))^2+ …
(log() = common logarithm)
Coefficients
C(00) = 0.582919757481
C(01) = 0.736205400529

Correlation coefficient is 0.891470429364
Standard error about the line (natural logarithm) = 0.689818420719

The Excel version is Km³ =10^(0.582919757481+0.736205400529*LOG(Ellipsoidal Area))

For the ellipsoidal area:
a=Long axis / 2
b=Short axis /2

Area = a * b * Pi()

END OF MATH STUFF, you may resume normal reading.

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It plots out like this:

It is important to realize that this formula does not mean that any one eruption was that size. Just that over the life of the caldera’s formation, that’s a reasonable estimate of what came out of the hole… that the roof then collapsed into. Different caldera styles can skew that value and make it even more inaccurate. It’s just for getting a ball park figure.

If you wish to take this a step further and estimate SO2 emission, I don’t recommend it. Taupo had a significant eruption 26,500 years ago, and the NGRIP ice core barely even blipped. Antarctica may show something, but I have yet to find it. Just because you have a large eruption, it doesn’t mean the SO2 bloom will be as large.

The formula is somewhat generic, using data that you guys have provided, backed up by some really nice lists such as the supporting table for “Sulfur dioxide initiates global climate change in four ways” by Peter L. Ward (2009)

Click to access SO2InitiatesClimateChange.pdf

Click to access Ward2009TableS1.pdf

And more on Los Frailes and the other two calderas mentioned in this article.

http://www.springerlink.com/index/j55110300r6777j7.pdf

And the IOP article of … dubious usefulness. (okay, it was a reference for the three types and styles, I’ll give ’em that.)

Click to access ees8_3_012021.pdf

Other things of three:

GEOLURKING

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And now the Sheepy Dalek part!

Name That Volcano Riddle!
I’d had a stressful day struggling with some cinematic effects software.
Back at home I tried to relax, a Jazz/Fusion band was playing softly in the background and a delicious bowl of Mexican food was steaming on the table.
Then the army decided to dispose of some explosive ordnance – so much for peace and quiet!!

Hints will be provided later. One Ding, One Point, One Volcano!

ALan´s Evil Riddle!

This old girlie apple on a fence, could be a great tragedy!
What am I?
What tragedy?

2 dings / 2 points.

Sissel and Spica

ps: We do not know the answers. Hints will be provided by riddlemasters.