South American Supervolcanoes: Cerro Galán

You might have heard that supereruptions are VEI 8 events, right at the top of the scale.

Could there ever be a magnitude 9 eruption?

Yes, one did happen about 28 million years ago in what’s now Colorado:

La Garita, long extinct, wasn’t a hot-spot volcano like Yellowstone. Rather, it was part of an ancient ignimbrite flareup that hit North America back then, similar to the more recent one in the Andes.

Put a day pack* in between that espresso cup and the 55-gallon drum and you’ll have Cerro Galán — the Central Andes supervolcano we’re going to look at today.

What is Cerro Galán?

Galán sits on the Puna Plateau in northwestern Argentina, not far from the big Altiplano calderas, including La Pacana in northern Chile and Pastos Grandes in Bolivia.

It’s hard to talk about this volcano in lay terms, partly because it’s fairly remote and unfamiliar to most of us.

But most of the difficulty comes from a need to translate technical jargon into plain English. Scientists are fascinated by Galán and there’s a lot of in-depth debate about it in the literature I’ve checked.

Why are some of the best minds on the planet so invested in this Argentinian supervolcano?

Perhaps because it was one of the first calderas they recognized.

Geoscientists spotted Cerro Galán in the late 1970s while examining satellite images.

It looks a lot different from space. This is a Landsat 8 image of Galán caldera in the early 1980s. Look for the oval-shaped depression in this vast landscape. It’s got a hill (resurgent dome) on the right and a little lake — deadly but inhabited Laguna Diamente — on the left side. (Image: USGS via Wikimedia, public domain).

This is what boffins now call a “Valles-type” caldera. That is, it’s your basic giant ring of hills surrounding an inner hill (resurgent dome).

Another reason why Cerro Galán became the go-to field example of a supervolcano (Cashman and Cas), rather than Valles Caldera in New Mexico, might be because Galán — the older of the two — looks almost brand new.

The dry Puna climate has preserved geological details here that disappeared from Valles as it weathered over a million years into the scenic wonder that it is today.

Also, since there are some towns nearby, Galán is much more accessible to both tourists and scientists than Altiplano “rock stars” like La Pacana, which hosts the largest caldera in the Central Andes, and other supervolcanoes.

Seriously, you can take a Teddy Roosevelt-style 21st-century luxury trip through this Mars-like landscape today, complete with epicurean picnic!

These are not geologists. Union rules require field workers to drink beer, not famous Argentinian wines. Also, the vehicles they use to get to volcanoes and other interesting places generally don’t exhibit the Mercedes-Benz symbol. And they carry LOTS more equipment.

So, Cerro Galán is now the world’s “typical supervolcano” and the focus of many different studies.

This post therefore could go in a variety of directions. I’ll follow the 2011 paper by Cas et al. (see source list) that describes events during Galán’s last supereruption, roughly 2 million years ago.

Why? Because these earth scientists report that supervolcanoes don’t necessarily erupt the way we expect them to.

The Cerro Galán Supereruption

In some ways, Roland Emmerich may have been fairly close to real events at a supervolcano when he had Yellowstone erupt in the movie “2012.”

We’ve seen this clip in one of the other South American supervolcano posts, but it’s far enough over the top to enjoy again (also, check out the start-off, the way an RV bumping over dirt road escapes pyroclastic flows, and the nuclear-type imagery of the actual supereruption):

Supersized from the start

Emmerich’s movie shows a few preliminary little spurts as Yellowstone starts to go off. Then the landscape begins collapsing. Then, BOOM!

That indeed may be how real-life supereruptions begin (though triggering mechanisms and collapse processes are still under discussion).

The geologic record shows no evidence of plinian ash deposits at the base of supereruption deposits at Galán, Toba, and some other sites. That suggests that those supereruptions started off big, rather than escalating from a more “normal”-sized eruption.

Knowing about this possibility can help us make plans, as a global civilization, to deal with the next supereruption.

It’s important to remember, though, that every volcano, regardless of size, is unique.

Geologists do report supereruptions where the main event apparently began with plinian or ultraplinian eruptions that gradually intensified as the caldera “unzipped.”

But still, point to Emmerich.

Slow pyroclastic flows

“Whoosh!” (Image: C. Newhall/USGS via Wikimedia, public domain)

After seeing videos of pyroclastic flows zipping down Mount St. Helens’ flank in 1980, as well as that lateral blast shooting outwards at almost 160 miles per hour, I thought it was ridculous to have an RV outrun a supereruption ash flow, especially on back country dirt roads.

But Cas et al. painstakingly pieced together Galán’s supereruption from its remaining exposed ignimbrite sheets and found that those pyroclastic flows did travel slowly.

In the very early stages, you could have outrun them — well, if not for the accompanying heat, poisonous gases, hydrothermal and other explosions, darkness, and huge volumes of ash.

Just. So. Much. Ash.

“I think this was Bramble Town.” What a normal-sized pyroclastic flow can do. (Image: Leonora Enking, CC BY-SA 2.0)

Cas et al. suspect that Galán’s flows might have moved across the land — which for the most part had no more slope than it does today — at an astonishingly slow 7 miles per hour.

That was fast enough to pick up rocks on the ground but not strong enough to separate the pieces after the flow’s heat had shattered said rocks.

Another point to Emmerich.

If anything, he had the flows coming out of Yellowstone’s eruptive column too quickly.

About that column, though . . .

Boiling-over eruptions

How can movie makers get the audience to experience an event they’ve never seen before?

By making it look like another kind of world-destroying catastrophe.

I think the Yellowstone initial eruption in “2012” resembles a nuclear explosion. The actors even dodge (and in one case fail to dodge) missile-like lava bombs.

Of course, there are also natural catastrophes. Just recently, as of the time of writing, we saw one strike the Bahamas.

And “normal”-sized volcanic eruptions can ruin lives, too.

Perhaps this is why the BBC went with “ash hurricane” in their 2005 depiction of a Yellowstone supereruption.

Also unzipping the caldera with separate plinian/ultraplinian eruptions.

But Cas et al. have collected enough information, while probing the site of a real supereruption 2 million years ago at Cerro Galán, to suggest a very different eruption style.

Earth wins this point for doing the unexpected.

We must use our imagination here, because what these scientists describe is unlike anything that has ever happened during recorded history.

Ordinarily, volcanism in Iceland, as shown in the following video, is about as different from Andes-type explosive volcanism as you can get.

However, the lava fountaining shown here is the closest real-world process to show what I understand the researchers are talking about: a “boiling over” silicic supereruption.

No evidence of such pretty red lava streams has been reported for the Cerro Galán Ignimbrite eruption. Instead, try to imagine bigger fountains that are fiery gray, boiling up from a roughly 17 x 10-mile-wide crater rather than a line of elegant fissures. And they’re pumping out ground-hugging ash clouds at incredibly high rates and volumes. Just. So. Much. Ash.

Imagine an “ash tsunami” that keeps coming, for days, weeks, months, perhaps even years; pouring out of the vent in every direction.

Today, the Galán Ignimbrite covers about a thousand square miles. Cas et al. estimate that the original coverage was at least twice that.

No high-velocity surges were necessary to fill in valleys and bury ridges located at least 80 miles from the vent under hundreds of feet of ignimbrite.

The sheer volume of material from Galán was enough to do the job.

Game over?

Why so sluggish? Things get technical and complex very quickly, but my understanding is that it’s because the ignimbrite contains lots of crystals. For example, here’s a look at crystals in lava from Mount St. Helens in 2004 (left, USGS via Wikimedia, public domain) and from an igneous rock type called granodiorite, ? source, that’s in the same ‘family’ as ignimbrites (right, via Wikimedia, CC BY-SA 3.0)

You’re probably wondering what the climate effects of such a natural catastrophe would be.

So are the scientists.

Is it really the end of the world when a supervolcano lets loose?

Of course not, since things are perking along quite well these days despite all the supereruptions that have occurred in the past.

However, such an event would challenge our ability to keep intact the present interconnected global society we enjoy together.

We do need to think about how to survive a supereruption and, wherever possible, make emergency plans for one.

Direct climate effects are hard to predict.

Location certainly matters. Central Andes supervolcanoes are fairly close to the Intertropical Convergence Zone. That could mean that their eruptions would affect both hemisphere.

Sulfur and carbon dioxide content is important, too.

Volcanic effects on climate depend on how much of these two materials, particularly sulfur, an eruption burps out into the atmosphere.

Long-distance impacts appear if the sulfur is blown high enough to get into the stratosphere. (See the in-depth discussion of this in Oppenheimer’s book, referenced in source list, about eruptions that “shook the world”).

National Park Service/Jessica Ferracane, public domain.

No one knows what the original volatile content of the Cerro Galán Ignimbrite was. Cas et al. speculate that there was enough gas in the pyroclastic currents to cushion their flow along the ground but not enough to overcome the density of that crystal-rich ash and push it up to the stratosphere.

This might mean global climate wouldn’t take the severe hit that might otherwise be expected, but it’s very iffy.

Another uncertainty is whether any unusual physics and/or chemistry happens during a supereruption.

A lot more needs to be learned about supervolcanoes and their processes. Besides data, more computing power is necessary to reliably model all the complexities of such a “super” situation.

Is another supereruption on the way?

Opinions vary, but nothing seems imminent (over at least the next few decades). We’ll look at this in a little more detail next time.

That’s a proper Hollywood-style cliffhanger to end this post on, but it doesn’t hurt to point out that there reportedly are a few monitors on Galán and that its aviation code at present is Green/Normal.

It’s also reassuring to know that many of the world’s volcanologists have their eye on this place.

Supervolcanoes are scary, and we do need to learn everything we can about them in order to prepare for the inevitable next one (hopefully in the very distant future).

But there’s no need to panic. Think how blissfully unaware of supervolcanoes our ancestors were and how stunned they would have been if one had gone off during historical times.

Forewarned is forearmed. We know some of our vulnerabilities — infrastructure, communications, and so forth — and we can take steps to protect ourselves before the disaster arrives.

Whether we succeed or not, tomorrow will take care of itself. Somehow.

Just as it always has.

*This is based on the magnitude 8.4 given to the Cerro Galán Ignimbrite eruption by Mason et al.

Featured image: Rodoluca88, CC BY 2.0.


Allmendinger, R. W.; Jordan, T. E.; Kay, S. M.; and Isacks, B. L. 1997. The evolution of the Altiplano-Puna plateau of the Central Andes. Annual Review of Earth and Planetary Sciences, 25(1): 139-174.

Bianchi, M.; Heit, B.; Jakovlev, A.; Yuan, X.; and others. 2013. Teleseismic tomography of the southern Puna plateau in Argentina and adjacent regions. Tectonophysics, 586: 65-83.

Cas, R. A.; Wright, H. M.; Folkes, C. B.; Lesti, C.; and others. 2011. The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galán Ignimbrite, NW Argentina, and comparison with other flow types. Bulletin of Volcanology, 73(10): 1583-1609.

Cashman, K., and Cas, R. 2011. Introduction to the special issue of Bulletin of Volcanology,“The Cerro Galan ignimbrite and caldera: Characteristics and origins of a very large volume ignimbrite and its magma system,” 73 (10): 1425–1426.

Delph, J. R.; Ward, K. M.; Zandt, G.; Ducea, M. N.; and Beck, S. L. 2017. Imaging a magma plumbing system from MASH zone to magma reservoir. Earth and Planetary Science Letters, 457, 313-324.

Donovan, A., and Oppenheimer, C. 2016. Imagining the unimaginable: communicating extreme volcanic risk, in Observing the Volcano World (pp. 149-163). Springer, Cham.

Folkes, C. B.; de Silva, S. L.; Wright, H. M.; and Cas, R. A. 2011. Geochemical homogeneity of a long-lived, large silicic system; evidence from the Cerro Galán caldera, NW Argentina. Bulletin of Volcanology, 73(10): 1455-1486.

Guzmán, S.; Grosse, P.; Martí, J.; Petrinovic, I. A.; and Seggiaro, R. 2017. Calderas cenozoicas argentinas de la Zona Volcánica Central de los Andes–procesos eruptivos y dinámica: una revisión. in Ciencias de la Tierra y Recursos Naturales del NOA. Relatorio del XX Congreso Geológico Argentino, San Miguel de Tucumán (pp. 518-547).

Klemetti, E. 2012. Dr. Shanaka de Silva answers your questions about supervolcanoes, Uturuncu, and more.

Lindsay, J. M.; De Silva, S.; Trumbull, R.; Emmermann, R.; and Wemmer, K. 2001. La Pacana caldera, N. Chile: a re-evaluation of the stratigraphy and volcanology of one of the world’s largest resurgent calderas. Journal of Volcanology and Geothermal Research, 106(1-2): 145-173.

Mason, B. G.; Pyle, D. M.; and Oppenheimer, C. 2004. The size and frequency of the largest explosive eruptions on Earth. Bulletin of Volcanology, 66(8): 735-748.

Oppenheimer, C. 2011. Eruptions That Shook the World. Cambridge: Cambridge University Press. Retrieved from

de Silva, S. L. 1989. Altiplano-Puna volcanic complex of the central Andes. Geology, 17(12): 1102-1106.

de Silva, S.; Zandt, G.; Trumbull, R.; Viramonte, J. G.; and others. 2006. Large ignimbrite eruptions and volcano-tectonic depressions in the Central Andes: a thermomechanical perspective. Geological society, London, special publications, 269(1): 47-63.

de Silva, S. L., and Gosnold, W. D. 2007. Episodic construction of batholiths: Insights from the spatiotemporal development of an ignimbrite flare-up. Journal of Volcanology and Geothermal Research, 167(1-4): 320-335.

Salisbury, M. J.; Jicha, B. R.; de Silva, S. L.; Singer, B. S.; and others. 2011. 40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province. Bulletin, 123(5-6): 821-840.

Schnurr, W. B. W.; Trumbull, R. B.; Clavero, J.; Hahne, K.; and others. 2007. Twenty-five million years of silicic volcanism in the southern central volcanic zone of the Andes: geochemistry and magma genesis of ignimbrites from 25 to 27 S, 67 to 72 W. Journal of Volcanology and Geothermal Research, 166(1): 17-46.

Ward, K. M.; Delph, J. R.; Zandt, G.; Beck, S. L.; and Ducea, M. N. 2017. Magmatic evolution of a Cordilleran flare-up and its role in the creation of silicic crust. Scientific Reports, 7(1): 9047.

Wikipedia. 2019. Cerro Galán. Last accessed September 18, 2019.

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