South American Supervolcanoes: La Pacana


BBC/”Supervolcano”

Want to experience a supereruption?

The BBC has you covered.

But in the real world, believe it or not, people can walk around in a supervolcano and never know it.

For example, here is a hauntingly beautiful video posted by someone who visited Láscar, the northern Andes’ most active volcano, but missed nearby La Pacana — one of the world’s biggest calderas and site of the world’s fifth largest known supereruption, per Mason et al. (see source list).

To do these adventurous travelers justice, not even the pros could see La Pacana until the 1980s, after satellite imagery had become available.

It’s that big.



Yes, space scientists do use this part of the world for research. And those young, healthy adults are trudging along because there isn’t much oxygen in the air at 15,000 (trailhead/plain) and 18,000 feet (Láscar’s summit)!


Láscar’s steaming, sulfurous crater is enormous, but this is still a “normal” volcano. So are the nearby mountains.

Where’s the supervolcano? Well, as I say, they mostly missed it.

But there are some hints.

La Pacana’s western caldera rim is just east of Láscar. Going by shadows (and assuming that they spent the morning climbing and reached the top shortly after noon), I think at least some of the flat land shown, particularly towards the end of the video, might be part of this 37 x 22-mile-wide caldera.

For comparison, by the way, Yellowstone’s caldera is 35 x 50 miles wide, while Toba’s is 18 x 60 miles. We are truly walking with giants here.

A map to identify the sights in that video would be nice, but this is the Andes. Multiple supereruptions have happened here — and not just from La Pacana! — as well as lots of “normal” volcanism that gorgeously clutters up the landscape.

All of that makes for a very busy geologic map:


Figure 2, Delgado and Pavez, CC BY-SA 4.0


Gah!


You could remove the text and have a good Halloween mask with that.

Still, we laypeople can find a surprising amount of information in those colors and lines. For instance:

  • Solid heavy black lines: National borders of Chile, Argentina, and Bolivia. Supervolcanoes are always big enough to have transnational effects even when they don’t sit so close to a “triple junction” like this.
  • Láscar is just left of center. Remember how big it looked in the video?
  • Long black dashes curve around La Pacana’s caldera. That rim isn’t easy to see, even on a map, because of all of the other geological violence that has happened there since the supereruption 4 million years ago. (Note: Here’s how a caldera forms in a nonexplosive, relatively cool lab situation.)
  • Stuff that’s an intense yellow color, including the “Salar de Quisquiro” and “Cordon La Pacana” sections, as well as that unlabeled jigsaw puzzle piece on the far left by Toconao, is part of La Pacana’s huge blowout, when an estimated 1,600 km3 of magma erupted in vast pyroclastic flows to form what’s now called the Atana Ignimbrite. (For comparison, Toba produced 2,700 km3 74,000 years ago; Yellowstone’s Huckleberry Ridge eruption 2 million years ago had a dense rock volume of 2,200 km3; the world’s last known supereruption, at Taupo in New Zealand some 27,000 years ago, was 530 km3; a-a-a-and Mount St. Helen produced about 1 km3 in 1980).


    “I think I can, I think I can . . . “. Fortunately for all forms of life within about 70 miles (not to mention the rest of the world), it couldn’t. There isn’t any vast silicic magma reservoir in modern Cascadia for it to tap. This is a very different tectonic setting from the Miocene and Pliocene high-heat-flow situation in the Central Andes that led to so many supereruptions.


    Some experts suspect that Atana Ignimbite might fill parts of La Pacana’s collapsed caldera to a depth of almost 2 miles!

  • Speaking of Toconao, that little blob of intense orange near it on the map is the work of La Pacana, too, though this Toconao Tuff was “only” about 130 km3 in volume.
  • Other ignimbrites: The lighter orange pieces, up in the far right of the map came from a different volcano — Guacha Caldera, a little to the north of La Pacana. The lighter yellow marks a large but not super-sized ignimbrite from the Purico complex (upper left).

As well, except for the blue salt pans, most of the colorful smaller blobs on this geological map were produced by “normal” volcanism that started after the Atana Ignimbrite blast (the landscape before that supereruption, of course, is buried).

This activity still goes on today, especially at Láscar, but there doesn’t seem to be much heat left in La Pacana.

What few geothermal springs remain are generally at around 80 degrees Fahrenheit. The boiler here has cooled off quite a bit.

During a recent science blog Q&A, an expert on La Pacana and other regional supervolcanoes explained why “normal” volcanism dominates the region today (I added some images and a link):

Wind, not volcanism, shaped this “Rock Tree.” (Image: Justin Vidamo, CC BY 2.0)

The Central Andean plateau is one of the most mindblowing landscapes on the planet. You feel like you are on another planet (we use it as analog for Mars) and everywhere you look are volcanoes and volcanic deposits! Millions of years of volcanic history are beautifully preserved waiting to be admired and deciphered. It is a high altitude desert and logistically quite challenging. It is relatively poorly studied, so it is something of an open book, few other groups work there, which has allowed us to really pick and choose some great projects out there. Every place you go to triggers enough questions to fill a lifetime of work for a volcanologist. It is rare to have such a wonderful natural laboratory available.

. . .

Between 10 and 1 Ma the region was characterized by many large caldera for “supervolcanic” eruptions and since then activity had settled down to steady state composite cone building activity along the chain of volcanoes that forms the crest of the Central Andes. This change signals the rate at which heat was being introduced into the crust changed about a million years ago.

The beauty of the landscape is woven into the artwork of local people, like those who made this Bolivian tapestry. (Image: Jose Luis Hidalgo R, CC BY 2.0)

What controls the heat influx into the crust? This is very complex, but I will give you the essence without delving too deeply into details – just the facts. Remember that the Andes and it’s magmatism is part of the Ring of Fire produced by convergence of plates. At convergent margins there is a downgoing (subducting) plate and an overriding plate – in this case the downgoing Pacific or Nazca plate and the continent of South America. Heat is advected into the South American plate crust from the mantle in the form of magma primarily and the rate of magma (and therefore heat) being produced is controlled by the rate at which the mantle can melt. The rate at which the mantle melts is related to a variety of factors . . .

. . .

The Earth does some complex interweaving in a subduction zone, using intense pressure and a very hot “needle.” (Image: Michael Zuo via Wikimedia, CC BY-SA 4.0)</p)

These factors are controlled by the geometry of the plates – the angular relationship between the downgoing (subducting) plate and the overriding continental plate. We believe between 10 and 1 Ma years ago the angle of the downgoing plate changed from shallow to steep and this resulted in a rapid increase in the rate at which the mantle was melted and heat was introduced into the crust – think of it as mantle moving upwards into the area opening up between the downgoing and the crust. As it moves upwards the mantle melts and so more heat is being introduced into the crust. Once the downgoing plate settled into its current configuration (steep) about 1 Ma, the flow of mantle ceased and no extra melting occured and a normal heat flux returned – the volcanism settled down to the steady state “dribble” or “sputter” seen today.

Dr. Shanaka de Silva

Only a volcanologist could call something like this magnificent and terrible VEI 4 eruption a “dribble”:



Here’s more information about this very active south Chilean volcano. Of note, the experts keep looking for but, as far as I know, have not yet found evidence of a Plinian column like this before a supereruption — it’s possible that those . . . just . . . happen.


But such perspective must come to those who can walk through a supervolcano and really see it, unravelling the debris of geologic time and patiently documenting each moment of a real-life VEI 8+ supereruption while standing at what was, 4 million years ago, Ground Zero.

There is practical value in this difficult work, as well as scientific interest.

To protect ourselves and as much of civilization and our infrastructure as possible, we need to learn everything we can about supereruptions before the inevitable next one comes along, most probably in the remote future.

In the meantime, there’s always the movies to help us indulge in both the fascination and the terror the mere thought of these powerful events brings to us:



Next week: Bolivia’s Pastos Grandes.



Featured image: “The Monk of La Pacana,” wind-carved ignimbrite in the caldera; image by Diego Delso via Wikimedia, CC BY-SA 4.0.



Sources:

Delgado, F., and Pavez, A. 2015. New insights into La Pacana caldera inner structure based on a gravimetric study (central Andes, Chile). Andean Geology, 42(3): 313-328.

Gardeweg, M., and Ramírez, C. F. 1987. La Pacana caldera and the Atana ignimbrite—a major ash-flow and resurgent caldera complex in the Andes of northern Chile. (Abstract only) Bulletin of Volcanology, 49(3): 547-566.

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.

Lindsay, J. M.; Schmitt, A. K.; Trumbull, R. B.; De Silva, S. L.; and others. 2001. Magmatic evolution of the La Pacana caldera system, Central Andes, Chile: Compositional variation of two cogenetic, large-volume felsic ignimbrites. Journal of Petrology, 42(3): 459-486.

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.

Oregon State University. n. d. La Pacana Caldera. http://volcano.oregonstate.edu/oldroot/CVZ/lapacana/index.html Last accessed September 6, 2019.

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.

Wikipedia. 2018. La Pacana. https://en.wikipedia.org/wiki/La_Pacana Last accessed September 6, 2019.




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