Tuesday, September 29, 2009

More On the Gigavolcano

Busy news day... Samoa got double-walloped first by an 8.3 magnitude earthquake, then by a tsunami, with implications for the Oregon coast. (That last link is a very interesting article illustrating how even a very minor tsunami can be dangerous.) Also, the millionaires on the Senate Finance Committee have decided that profitable insurance companies are more important than affordable health care. A gleeful Charles Grassly announced that the next target was the socialist and government-run FBI. "I'm tired of seeing the small-business and entrepreneurial orientation of bank robbers and kidnappers thwarted by government interference with the free market."

Snark aside, the point is, I'm not going to get through a great deal of my news today... I still have 450+ items in my RSS.

But I do want to get back to the teravolcano, and its peta-eruptions, because its 25 km of megaplumbing is ultrafascinating.

I had not heard of this Permian volcanic system before the recent news, and my original intention was to highlight and discuss points in the press release that were interesting and engaging to me. Sadly, I made the mistake of doing a Google for "Ivrea-Verbano Zone and Serie dei Laghi of northwest Italy." Oh my. There's a lot of information to digest.

For example, here we have a very recent field guide to the area (1MB pdf)- a geologic overview rather than a specific trip with stops- dated September 18, 2009.
Together with the metamorphic and igneous rocks of the adjacent Serie dei Laghi (SdL), collectively forming the Massiccio dei Laghi (Boriani et al., 1990), the rocks of the region record Palaeozoic accretion, metamorphic and magmatic processes, the effects of the Hercynian orogeny, post-orogenic magmatic underplating and associated lithospheric stretching and thinning, Mesozoic extension and effects associated with the position of the region in Alpine tectonism (e.g. Handy et al., 1999) (Fig.3).
To the extent I have a particular interest in a specific sub-discipline of geology (I don't really), I would probably have to say igneous petrology is it. Petrology is the study of how rocks are formed; igneous petrology deals specifically with rocks that cool and crystallize from a molten state. The above implies, to me at least, that we're dealing with hot-spot volcanism. The ability to look at the compositional variations, along with temperature and pressure as recorded in minerals, from the top of the mantle to the surface of the earth is as exciting as anything I can conceive. And for those of you into structure, there are some diagrams that almost qualify as pornography (see, for example, the bottom of page 8 and top of page 9).

Another article seems to confirm the hot-spot idea in this diagram of the conjectured structural-thermal evolution of the region.

The point is, there seems to be quite a bit of relatively accessible information on this exposure. In the abstract to the scholarly article in question, Quick et al. claim that "Correlation of ages of volcanic and middle to deep crustal plutonic rocks suggests that they constitute an unprecedented exposure of a subcaldera magmatic plumbing system to a depth of 25 km, and points to a cause and effect link between intrusion of mantle-derived basalt in the deep crust, and large-scale, silicic volcanism." Now this is exactly what has been assumed since my puppy days, back in the early 80's, with respect to Yellowstone. Magma with a basaltic composition rises to the lower crust from the mantle, then some combination of magmatic differentiation (some components solidify earlier, leaving the residual magma with a different composition) and crustal melting (mantle basaltic magmas have temperatures of ~1200 degrees Celsius; crustal rocks melt around 800 C) result in granitic-rhyolitic magma that is relatively enriched in volatiles. In large amounts, this is exactly what's required for explosive volcanism.

So if this is what has been hypothesized, why is this discovery important? Because it's an actual confirmation of the model, not just a theoretically plausible conjecture. I have described the process of doing geology as predicting the past. At first, that seems oxymoronic, but what we do is painstakingly assemble all sorts of data, try to put it together into a coherent picture, then develop a model that explains how that picture came to be. We test the model by saying "If that model is accurate, then we should be able to find A, B and C." Then we go look for A, B and C. In other words, the model involves predictions (A, B, C) that should have already happened at some time in the past, but no one has looked for them. So if we find A, B, and C, it strongly supports our model. And that appears to be exactly what happened in this case.

The next point that is of interest is that the spatial distribution of magma below a volcano is an extremely important, but difficult to investigate, aspect of how and why volcanoes erupt. A recent Accretionary Wedge post by Tuff Cookie talks more about this issue. Certainly there are lots of places around the world where one can observe shallow to deep components of volcanic "plumbing," but the ability to see a section of a single volcanic system, essentially a snapshot in time, through an essentially continuous depth of 25 km (15 miles) is indeed unprecedented, as far as I know. One of the reasons I was so disparaging of the lack of tectonic setting is that each volcano has its own "personality," architecture, eruptive style, compositional range and so on. But not even describing whether the volcano was the result of extension, a hot-spot or subduction would be like describing an organism's behavior without specifying whether the organism was a plant, animal or bacterium. This is one single volcano, which undoubtedly has its own quirks and in many ways is different from any other volcano that has existed. Nevertheless, it can serve as a model for some aspects of many volcanoes, and as a better model for a few.

I knocked the idea of calling this caldera a "supervolcano;" it's nowhere near big enough. I also mocked the very idea of "supervolcano." There are big eruptions, little eruptions and enormous eruptions. Do we really need a special category for the largest few dozen eruptions in the planet's history? Would it make sense to describe the very smallest eruptions as "microvolcanoes?" If your goal is to rivet people to the tube, so they'll sit through the commercials, sure, it's a useful ploy. But in terms of science, it's a minor distinction of degree. Yes, we are particularly fascinating by the extremes, but it doesn't really serve any purpose to set those extremes apart in their own group. Silver Fox made much the same comment in an Accretionary Wedge post a few months ago:
Geology and volcanology have seen some warranted increase in notoriety and recognition since the release of movies like Supervolcano, and since the publication of supervolcano articles in popular scientific magazines like Scientific American and Discover, however overdramatized or inexact the popular renditions might be. The Wikipedia article Supervolcano, for example, says that "Supervolcanoes are relatively new to science; they were previously unknown because they do not fit the stereotypical model of volcanoes." This statement is incorrect: very large-scale explosive caldera-forming eruptions have been known to geologists for quite some time. Hey, I knew about them way back in the dark ages, before I reached the age of thirty!
I know that the enormous eruptions at Yellowstone had been well-documented by the early 80's when I started in geology, and I think it's important to keep in mind that only 15 years earlier, plate tectonic theory was only starting to be widely accepted in the US.

I have been pondering today if I could think of a way in which you could get a "supervolcano" without also getting a caldera. I think the key is that supervolcanoes are explosive. One might erupt an enormous amount of lava in a flood basalt eruption, but that's not explosive (which is the reason the basalt flow illustrating the press release is ludicrous: that kind of lava, and particularly that kind of flow, couldn't be associated with a supervolcano). As the media and pop culture have defined supervolcano though, I think such an eruption would necessarily involve a caldera. But to reiterate a point from yesterday, calderas come in a range of sizes, from a km or so, up to Yellowstone's 45 X 85 km, and perhaps even larger. Only the very largest of those constitute supervolcanoes. One proposed formal definition would define these as erupting a volume of ejecta with a dense rock equivalent of 1000 cubic km or more. Under this definition, even the Long Valley eruption, with a volume of 600 cubic km, falls short- better than halfway there, but not much.

All that said, calderas of any size are formed in dramatically violent eruptions, and any discovery that leads to a better understanding of such events is just fine by me. The "Rosetta Stone" metaphor was a quote from the lead author, and I still feel like it's unjustified hyperbole. Nevertheless, the point he was trying to make was that having an actual physical layout for the system underneath the vent gives us a better way to draw geophysical (heat flow, gravity, seismic, inflation-deflation, and so on) inferences about the activity of magmatic systems where we don't know what the physical layout looks like. So there's some validity to it, but tossing out a term like Rosetta Stone to a staff PR writer is inviting trouble.

I could probably go on, but the point is this: there's lots and lots of fascinating, amazing and important stuff out there in the world. People who write press releases are not on salary to represent that stuff in any accurate, knowledgeable or meaningful way. They get their paychecks for making their employer look good. As I said yesterday, they may from time to time get things right, or write a story about an interesting topic, but it's purely coincidental. I don't hold their ignorance of anything other than how to write a fluffy, catchy story against them. They're doing the job they're being paid to do, and they have the background and knowledge base that the job requires them to have.

But I get pretty furious with "journalists" who have learned to type ctrl "a", ctrl "c", ctrl "v", and think that constitutes doing their job.

1 comment:

Silver Fox said...

Thanks for the link and quote! And great continuation of your original article.