Toba


For all we know, the YTT supereruption at Toba did happen on June 16, 70,000-and-something, B.C.

Let’s say, then, that today is the 74th millennial anniversary of the moment when a Sumatran volcano began blasting out what would total seven thousand billion metric tons of magma in “one of the largest single eruptions ever documented, along with the 27.8-million-year-old Fish Canyon Tuff associated with La Garita caldera in the San Juan Mountains of Colorado . . . and the two-million-year-old Huckleberry Ridge Tuff of Yellowstone.” (Oppenheimer, links added)

Tambora, shown above, is much smaller than Toba, but its 1815 eruption caused “The Year Without A Summer” in the Northern Hemisphere. (Image: Jialiang Gao via Wikimedia Commons, CC BY-SA 3.0)

This Top Three Me 8.8 event was two whole orders of magnitude greater than the largest eruption during historic times, when Tambora volcano, a few Indonesian islands east of Sumatra’s Toba, killed at least 60,000 people in 1815.

And it was seven times as powerful as the world’s last known supereruption, some 26,000 years ago, at Taupo in New Zealand.

The 74,000-year-old Youngest Toba Tuff (YTT) eruption commands a lot of respect from some of the best minds on the planet.

Yes, there’s also an Oldest Toba Tuff (840,000 years old) and a Middle Toba Tuff (500,000 years old). But the first evidence that Toba was going to break the typical Indonesian-volcano mold showed up a little over a million years ago, when there might have been a stratovolcano here rather than the vast hole in the ground that exists today.


Toba’s 62- x 19-mile-wide crater is fully appreciated only from space. It currently holds Earth’s largest volcanic lake. (Image: NASA via Wikimedia)

Toba Volcano

Nowadays, YTT deposits form a plateau that slopes away in all directions from the great lake and covers over 11,000 square miles of northern Sumatra.

Most of the caldera formed in this eruption, but not all of it. Volcanologists can see evidence there of three earlier caldera-forming eruptions.

Dr. Craig Chesner, a widely respected authority on Toba, writes, in the paper listed at the end of this post in the sources, that there may also have been a typical “pointy” volcano here, just north of the present caldera, at one time.

In other words, the Toba area probably looked like the rest of Sumatra at that point.



And in Indonesia, despite the presence of well over a hundred active “non-super” volcanoes today, life is possible, though challenging.


What follows is a hypothetical scenario, per Dr. Chesner, of how that long-vanished stratovolcano morphed into the gigantic caldera that sits on northern Sumatra today.

Toba’s caldera-forming eruptions

A little over a million years ago, the stratovolcano at Toba got more explosive with an eruption similar to (but a little bit smaller than) the 50 km3 event in North America that formed Oregon’s Crater Lake.

Such blasts do occur sometimes at “normal” subduction-zone volcanoes.



Toba is only a hundred miles or so north of the Equator, but that 1.2-million-year-old stratovolcano, if it existed, might have had snow on it — as Oregon’s Mount Mazama does in this simulation of the Crater Lake eruption — depending on its altitude and what point Earth was at in the ice ages.


But Toba was just getting started.

At that point, the truncated ancient cone in Indonesia probably had a crater lake, too, but all the evidence of its eruption that remains is an igneous rock layer at Toba called the Haranggaol Dacite Tuff (HDT).

Why is that all that’s left?

Because, 340,000 years after the HDT eruption, Toba had its first known supereruption — the Oldest Toba Tuff (OTT), with a volume of at least 500 km3.

Dr. Chesner thinks it possible that both the original stratovolcano and the small HDT caldera survived the OTT event because, after another 340,000 years, another “Crater-Lake”-sized eruption — the Middle Toba Tuff — occurred here.

That brings us up to 501 Ka (501,000 years ago). And, for a change, things remained calm at Toba for the next 430,000 years or so.

However, trouble was brewing.

Ever since the OTT supereruption, much of northern Sumatra had been slowly bulging up in what geologists call the “Batak Tumor.”

Mount St. Helens on April 27, 1980. This is hardly even a pimple compared to the swelling of northern Sumatra — called the Batak Tumor — before the megacolossal eruption there 74,000 years ago. (Image: Peter Lipman, CVO, via Wikimedia)

It’s difficult to believe that solid rock can balloon out, but that’s exactly what the north slope of Mount St. Helens did in the spring of 1980 as magma domed up inside the mountain.

Even balloons must burst at some point.

At Mount St. Helens, an earthquake on the morning of May 18th triggered a debris avalanche that suddenly removed tons of rock that had been holding down the magma. The result was exactly the same as removing a cork from a bottle of champagne.

Since volcanologists haven’t yet found evidence of preliminary “normal-sized” eruptions, it’s possible that something sudden like the Mount St. Helens directed blast in 1980 might have happened 74,000 years ago today at the Batak Tumor, only four orders of magnitude bigger (and perhaps vertically).

As the Youngest Toba Tuff eruption began, a 60- x 20-mile-wide parcel of North Sumatran real estate collapsed, releasing thousands upon thousands of cubic kilometers of pressurized molten rock.

The huge hole that we now call Toba began to form as the magma chamber roof collapsed, taking down the ancient stratovolcano and most of the earlier three calderas. At least 1000 km3 of ash, in the form of pyroclastic flows, surged down all flanks of the Batak Tumor, out across the coastline, and over the neighboring water.

An additional 800 km3 or more rained down upon more distant regions, including the South China Sea, peninsular India, and the Arabian Sea.

All told, about 1% of Earth’s surface was covered by Toba’s ash to a depth of 4 or more inches. (Oppenheimer)

And that’s not counting any of the 1000 km3 that stayed inside the new 1.3-mile-deep caldera.

No one knows for sure how long this ~2800-plus km3 eruption lasted. A minimum estimate of 9 to 14 days has been made, though that would have required an incredibly intense eruption rate at the vent(s). (Oppenheimer)

The aftermath

A few millennia after the Youngest Toba Tuff eruption ended, per Mucek et al. (see source list as well as the following video), the magma chamber began to refill, uplifting the caldera floor almost a mile to form Samosir Island. This process, known as resurgence, has been noted at most large active calderas.



Such resurgence is typical at active calderas.

Today, the experts report, geophysical studies and small-scale local signs of underground heat, such as fumaroles, all show that there is more magma underneath Toba. However, they disagree on whether its volume is “supersized” yet.

The good news is that Samosir Island is not rising now. Neither is the ground in northern Sumatra. And Toba hasn’t erupted since the last ice age ended.

Toba’s effect on global climate

Afterglow from Mount Pinatubo’s sulfur aerosols, still present nine years after the eruption! (Image: CSIRO via Wikimedia, CC BY-SA 3.0)

You might have heard that this Youngest Toba Tuff eruption almost wiped out our ancestors, and as we’ll see this coming Friday, it may have had an effect on tigers and a few other species, too.

However, there’s no unequivocal proof of any such effects on life from the supereruption.

It is clear that volcanic sulfur in the stratosphere can really mess up global climate.

Why? There are too many details to go into here, but if you are really curious, check out Oppenheimer’s book on world-shaking eruptions (source list) or a helpful website for a thorough discussion of it.

Very generally, volcanoes can cause warmer winters, cooler summers, a drop in global rainfall, and/or many regional weather disturbances.

Since nothing like the YTT eruption has ever happened in recorded history, experts use computer modelling to recreate its potential effects on world climate.

The first few studies were scary. It seemed likely that such an enormous event injected huge quantities of sulfur into the atmosphere, causing a multi-year “volcanic winter” that almost wiped out Homo sapiens.

Genetic evidence of a bottleneck in our past seemed to corroborate this. However, such bottlenecks can happen for other reasons and further research is showing that the amount of sulfur that reaches the stratosphere and stays there long enough to matter doesn’t simply scale up with eruption size.

Also, each magma batch brews up in a unique way. Some eruptions, like that of Pinatubo in 1991, contain a lot of sulfur; others don’t. Petrological studies currently indicate that the Youngest Toba Tuff had surprisingly little sulfur. (Oppenheimer)

That’s lucky for life on Earth. The YTT eruption’s effects might have “only” been as bad as those that followed the 1815 Tambora eruption — the Year Without A Summer, or as Oppenheimer describes it, “the last great subsistence crisis in the Western world.”

Will it happen again?

There will be another supereruption, at Toba or some other known supervolcano like Yellowstone or Campi Flegrei. That’s why supervolcanoes are monitored carefully.

A supereruption could also happen at a new and completely unsuspected source.

Fortunately, no signs of impending trouble have been detected anywhere. A supereruption would be the worst natural challenge modern society has ever faced.

Now that we do know about Toba’s YTT eruption, we can use technological tools and other resources to thoroughly review this past event and learn from it. That will help us prepare for the future.

Archaeological records at Jwalapuram, India, show that Middle Paleolithic human activities there ceased when Toba ash began to fall — not a surprise. But those people returned afterward and at the same technological level. Somehow, they had maintained through the biggest known eruption on this planet during the last two million years.

Of what lasting value is our own civilization, highly networked and interdependent as it is, if we can’t do the same thing when our turn comes?

. . . [P]erhaps the experiences of our ancestors, the physical and psychological ways they found to cope with the trauma, really did shape the cognitive skills that we enjoy today. When I worked at the Jwalapuram excavations, exhuming the ancient land surface beneath the ash, it certainly made me wonder about the community that dwelt there, and what people must have thought when the Sun failed to appear one morning, and the land had turned to lifeless, grey powder.

— Clive Oppenheimer, in “Eruptions That Shook The World”


Edited June 16, 2019



Featured image: Samosir Island, Lake Toba, by John Hill, via Wikimedia, CC BY-SA 3.0.


Sources:

Chesner, C. A. 2012. The Toba caldera complex. Quaternary International, 258: 5-18.

Donovan, A., and Oppenheimer, C. 2016. Imagining the Unimaginable: Communicating Extreme Volcanic Risk, in Observing the Volcano World, eds. Fearnley, C. J.; Bird, D. K.; Haynes, K; McGuire, W. J.; and Jolly, G., 149-163. Springer, Cham.

Global Volcanism Program. n. d. Toba. https://volcano.si.edu/volcano.cfm?vn=261090 Last accessed June 5, 2019.

Koulakov, I.; Kasatkina, E.; Shapiro, N. M.; Jaupart, C.; and others. 2016. The feeder system of the Toba supervolcano from the slab to the shallow reservoir. Nature Communications, 7: 12228.

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

Mucek, A. E.; Danišík, M.; de Silva, S. L.; Schmitt, A. K.; and others. 2017. Post-supereruption recovery at Toba Caldera. Nature Communications, 8: 15248.

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

Petraglia, M.; Korisettar, R.; Boivin, N.; Clarkson, C.; and others. 2007. Middle Paleolithic assemblages from the Indian subcontinent before and after the Toba super-eruption. science, 317(5834): 114-116.

Wikipedia. 2019. Sunda Shelf. https://en.wikipedia.org/wiki/Sunda_Shelf Last accessed June 5, 2019.

___. 2019. Sundaland. https://en.wikipedia.org/wiki/Sundaland Last accessed June 5, 2019.



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