South American Supervolcanoes: Pastos Grandes


Say “supervolcano” and most of us think of Yellowstone — a smoldering but scenic giant.

Is there a supervolcano here?
(Image: Allan Grey, CC BY-SA 2.0)

Geologists, however, look to the Central Andes, where these great beasts can be seen in all their well-preserved, terrifying glory.

That is, if you can find any amid the debris of their past supereruptions.

Much of the Central Andes landscape looks so flat and unearthly because it’s blanketed by the remains of countless supereruptions. And the supervolcanoes are calderas (holes in the ground, more or less).

They’re really hard to recognize.

This is a high-altitude region for tectonic reasons, but in addition to that there’s also enough ignimbrite here to make a volcanic plateau!

Why?

Yellowstone, however supersized, is still just a single volcano, but the Central Andes is part of a subduction zone.

And it has had an ignimbrite flareup, with well over 10,000 km3 (Best et al) of thick pyroclastic flow deposits erupted from a number of different Andean supervolcanoes over the last 10 million years.

This number is beyond comprehension for us laypeople — let’s not even try to do the swimming-pool or dump-truck conversion here.

It’s only counting the best-studied section, too: the Altiplano-Puna Volcanic Complex (APVC) of Chile, Argentina, and Bolivia. There’s lots more large-scale volcanism outside the APVC still awaiting in-depth research.

This excellent preservation is good news for geologists, of course. But they face the daunting task of identifying all those individual ignimbrite sheets (sort of like this) and mapping each one back to its source.

It’s rough on tourists — imagine also having to haul a lot of scientific equipment up here and keep it functioning! (Image: Leonora Enking, CC BY-SA 2.0)

It’s a monumental work in progress, but researchers have one thing going for them. There isn’t much erosion here, so most of the geological evidence of the past 10 million years is still in place.

Of course, erosion also helps earth scientists by exposing cross-sections that otherwise would remain buried underground, but there have been some successes in the Altiplano-Puna complex, particularly since satellite imagery and remote sensing became available.

Experts found enormous La Pacana, for example, by looking at the ground from space.

That’s also how they recognized another super-sized caldera — Pastos Grandes (“Big Grass”), about 125 miles north of La Pacana.

In the early 2000s, Pastos Grandes made the list of “largest explosive eruptions” for its presumed association with the 590-km3 Sifon ignimbrite (Mason et al.), but according to later workers, the source of this particular 8-million-year-old supereruption is not so clear.

However, Pastos Grandes may have been involved, and it also deserves its “super” reputation thanks to two other, more recently identified eruptions:

  1. The Chuhuilla Ignimbrite eruption of about 1,200 km3 of magma some 5-1/2 million years ago. (You can see a little bit of it sitting atop an ignimbrite sheet from another volcano in one of the images here.)
  2. The Pastos Grande Ignimbrite about 3 million years ago, with an estimated magma volume of 1,500 km3.

Just for comparison, Yellowstone’s last supereruption, the Lava Creek tuff, some 600,000 years ago, had a volume of 900 km3, while its big Huckleberry Ridge blast 2 million years ago was 2,200 km3.

Both of Pastos Grande supereruptions formed calderas. Oddly enough, though the Pastos Grande Ignimbrite (PGI) had a larger volume, its caldera is smaller, “only” about 25 x 16 miles in size. It’s partly filled with a tiny lake and a lot of evaporite sediments today.


An astronaut on the space station zoomed in on the PGI caldera, which includes white salt pans (I’m using Kaiser et al., 2017, as a reference). That’s a resurgent dome in the middle of the caldera. More recent volcanic activity hides much of the southern and western portions. (NASA)


All that can be seen of the older Chuhuilla caldera is its eastern rim.

On this much closer satellite view, that rim looks like a curving cliff just to the right of the lake/salt pan inside the northern part of the PGI caldera.


This is a Landsat image via Wikimedia.

Here is a very good virtual tour of the Pastos Grande caldera and some of its features.

I’m going to stop for now. We’ll return to this region in a couple of weeks when we look at areas of the Altiplano-Puna Volcano Complex that might still be active.

After all, unlike La Pacana, the boiler is still going. As one sign of this, Pastos Grandes does host a geothermal area or two.

You can see Sol de Mañana fumaroles in the following video (one that’s delightful to watch even though the rest of it doesn’t come anywhere near Pastos Grandes).

Those are often called geysers, but they’re really fumaroles.




Next week: Cerro Galan.


Featured image: Amazing Places On Our Planet (the video embedded above).


Sources:

Best, M. G.; Christiansen, E. H.; de Silva, S.; and Lipman, P. W. 2016. Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of arc volcanism. Geosphere, 12(4): 1097-1135.

Kaiser, J. F. 2014. Understanding large resurgent calderas and associated magma systems: the Pastos Grandes Caldera Complex, southwest Bolivia. (Abstract only) https://ir.library.oregonstate.edu/downloads/gx41mm66t

Kaiser, J. F.; de Silva, S.; Schmitt, A. K.; Economos, R.; and Sunagua, M. 2017. Million-year melt–presence in monotonous intermediate magma for a volcanic–plutonic assemblage in the Central Andes: contrasting histories of crystal-rich and crystal-poor super-sized silicic magmas. Earth and Planetary Science Letters, 457: 73-86.

Salisbury, M. J.; Jicha, B. R.; de Silva, S. L.; Singer, B. S.; Jiménez, 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.

de Silva, S. L.; Self, S.; Francis, P. W.; Drake, R. E.; and Carlos, R. R. 1994. Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano‐Puna Volcanic Complex. Journal of Geophysical Research: Solid Earth, 99(B9): 17805-17825.

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.

Watts, R. B.; de Silva, S. L.; de Rios, G. J.; and Croudace, I. 1999. Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia. Bulletin of Volcanology, 61(4): 241-264.




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