Was the Toba Eruption Not the Volcanic Catastrophe We Thought It Was?

By Erik Klemetti | February 9, 2018 12:41 pm
The Toba caldera in Indonesia seen from the Space Shuttle in 1985. NASA

The Toba caldera in Indonesia seen from the Space Shuttle in 1985. NASA

The largest eruption in the past million years was at Toba in Indonesia. How big was it? It erupted over 3000 cubic kilometers of volcanic debris. 3000 cubic kilometers! That would pave the entirety of Rhode Island in a kilometer deep of ash, rocks, and pumice. It would take roughly 300 Pinatubo-scale eruptions to match that. The eruption itself created a caldera formed when the land collapsed when all that material erupted that spans 100 kilometers from end to end and 30 kilometers from side to side. Thick deposits of ash can be found in India, thousands of kilometers away. This is truly a “super-eruption”.

You’d expect that an eruption of this enormous magnitude would have a big impact on the Earth’s climate. The eruption occurred ~73,000 years ago, which is well after the arrival of modern humans on the scene, so how did humans survive what must have been a global catastrophe? Geologists like Michael Rampino have suggested that the eruption created a “genetic bottleneck” for humans, meaning perhaps only 10,000 survived the Toba eruption at all and all modern humans since them came from this population. This idea has been pretty resoundingly discredited by geologists and anthropologists alike thanks to better dating of the eruption and better understanding of modern humans’ genetic ancestry. Yet, the question remains: how did humans survive such a massive eruption and its aftermath?

Maybe they survived because the aftermath of the eruption didn’t end up causing the sort of many-years-long volcanic winter that many geologists have postulated. A new study in the Journal of Human Evolution by Chad Yost and others took a look at what actual evidence exist for climate change related to the eruption in Eastern Africa. They looked at plant remains and charcoal (that suggest fires from drier conditions) above and below the ash layers from Toba found in sediment in Lake Malawi.

Now, the ash layer in eastern Africa wouldn’t even be noticed by most people. It is what we call a “cryptotephra“, where you need to carefully examine each layer of sediment looking for infrequent glass shards (ash) that fell after being blown in the stratospheric winds. However, ash from most eruptions can be correlated by looking at the composition and shape of the ash, so these few fragments can be tied to Toba.

However, looking above and below the layer with ash from Toba, Yost and others found no real evidence of dramatic changes in the plant diversity or fires related to climate shifts due to a prolonged volcanic winter. There is a brief increase in charcoal and certain grass species that suggest that just after the eruption there may have been slight drying in Eastern Africa, but no evidence for years of cooling from a volcanic winter caused by Toba.

Instead, Yost and others suggest that maybe we’re just wildly overestimating the amount of climate change that Toba might have caused. Although the eruption was much larger that the 1815 eruption of Tambora, also in Indonesia, the amount of sulfur found in the volcanic deposits from Toba suggest that maybe it didn’t really emit much more sulfur than that Tambora eruption. Of course, the Tambora eruption did famously cause the “Year without a Summer”, but definitely not nearly a decade of global “volcanic winter”. In fact, the climate impact on Eastern Africa might have only been about three times that of the Pinatubo eruption even though the Toba eruption was 100 times larger by volume.

So, it might have been cooler and drier for a year or two in Eastern Africa, but definitely not an extreme enough change to explain the shrinking numbers of modern humans that have been estimated by looking at DNA. This is a prime example of correlation likely not leading to causation: Toba happened to occur during this drop in modern human populations, but it does not appear to be the reason for the drop.

It also shows that when we talk about the climate influence of big volcanic eruptions, we can’t just scale the impact based just on the amount of stuff that was erupted. I’ve always wondered why few people talk about finding the massive climate signal of the most recent Yellowstone eruption. Part of the issue is that we don’t have complete climate records that go back that far. The other might be that these massive eruptions might not be the endtimes event they are sometimes made out to be. Instead, we need a better understanding of how much sulfur might be in the magma during a specific eruption before we can compare them to others.

Toba was definitely a monster eruption, but maybe it wasn’t as bad as it seems to the planet as a whole.

CATEGORIZED UNDER: Rocky Planet, Science, Science Blogs
  • OWilson

    The Earth appears to be far more resilient and adaptive, than does it’s self appointed “protectors” :)

  • GuillermoChile

    It’s the same in the opposite direction: small eruptions that emitted a lot of sulphur triggered worldwide catastrophes, like Laki eruption

  • Dany Ouellet

    I disagree with your explanation here. Neither one of the papers cited in reference is bringing evidence strong enough to dismiss the potential impact of this catastrophic event on climate or human population. From one unique sample location (Lake Malawi), you can’t extrapolate to the whole world. Perhaps the equatorial zone was spared from the worst of these impacts, but what about higher latitudes. 75 000 thousands years ago, mankind was already out of Africa, including Neanderthals. Therefore, the impact out of Africa could have been different on human populations and climate. Moreover, the authors supplied this warnings statement to conclude their paper; “Whilst from this we can hypothesize that the global climatic impact was not as dramatic as some have suggested, we will need to find similarly high-resolution records of past climate from other regions that also contain Youngest Toba Tuff in order to definitively test this,” 

    The Toba eruption was as catastrophic as it was thought since it was discover as the culprit for a worldwide found cryptotephra layer dated back to 74 000 years roughly. When the volcano blew up, the planet was already in a ice age stage and it is said that this eruption had a chilling affect on the climate visible on climate date for a thousand years following the event.

  • Bruce Fenton

    It is a shame that many scientists still cling to this East African Garden of Eden model despite it having been completely debunked in recent years. OK we get it, humans in Sub-Equatorial East Africa were fine, they barely noticed that the volcano had even exploded. But, what about all the hominins living from North Africa to Australasia? How did they fare after the eruption? Why is there an assumption here that if East Africans are alright than we need not concern our models with the fate of humans living dispersed across the globe?

    Toba almost certainly did devastate hominin populations in the Northern Hemisphere, and that does have implications for our origins narrative, we should not just skip over it because ‘Eden’ was spared. There is very clear evidence that modern humans migrated away from the climate change and entered the refuge areas of Sub-Equatorial Africa and the Sunda and Sahul regions. Surely this matters?

  • Alan_McIntire

    I’ve had my doubts about this “bottleneck” hypothesis ever since I first read about it. If humans suffered a dramatic loss 70,000 years ago, why doesn’t this same “bottleneck” show in other animals living in the same area as humans?

    • question?

      Not a scientist, so I may be off base, but has research ruled out a plague?

      • Alan_McIntire

        i suppose a volcanic eruption could affect climate locally, but I DON’T see how one could cause a PLAGUE.

        • Bryan Schmick

          I’m not saying that an eruption caused a plague, but it is well known that a healthy diet strengthens the immune system. Particularly access to fruits (vitamin C). Cold weather can reduce fruit crops. So what happens if fruits in a region are devastated?

          • Alan_McIntire

            I ought to be more clear in my objection. I doubted the volcano/ climate hypotheses- a large volcanic eruption affecting climate a quarter of the world away and leading to a bottleneck in humans would ALSO lead to a bottleneck in many other animals.

            As you pointed out, there could be a bottleneck for reasons unrelated to the Toba eruption, like a devastating disease drastically reducing the human population. I think geneticists DID detect such a bottleneck- the question is , what was the cause?

        • question?

          I am not suggesting the eruption caused the plague. I am asking if a plague could have happened at that moment in time. Any densly (by prehistoric standards) populated areas would have transmitted a pathogen amoung themselves leaving the remote population to survive. The populations were likely weakened by hunger so the spread a disease such as cholera is not hard to imagine.

          • Alan_McIntire

            As I said before, if the catastrophe were the result of the TUBA volcano eruption, affecting climate a quarter of the world away, OTHER creatures living at that time should have been similarly affected and ALSO suffered bottlenecks.

  • Karl Kopp

    There is an other aspect: Why did the Neanderthals in Europe survied the eruption by more than 100.000 years? Even the Tambora eruption dosen’t effect all the world. Russia and eastern Europe was abel to export grains to the western parts of Europe, which was sold at hight prices. We must examine the real effects of major volcanic eruptions.

  • Barb1953

    First, congratulations, Erik, on your new (to me) venue. I’ve been working on a series of general-interest ebooks on cat evolution (first one due out soon) and not closely following some topics I used to.

    Second, I was surprised to come across the Toba eruption while looking up the evolutionary history of clouded leopards on Sumatra (the diardi subspecies). I don’t know if URLs are allowed here, but according to these highly cited researchers https://www.sciencedirect.com/science/article/pii/S1055790310004306 there may have been an effect, but nobody really knows for sure; and apparently no large-mammal extinctions have been linked to the eruption, either. That *was* a surprise.


Rocky Planet

Rocky Planet covers all the geologic events that made and will continue to shape our planet. From volcanoes to earthquakes to gold to oceans to other solar systems, I discuss what is intriguing and illuminating about the rocks beneath our feet and above our heads. Ever wonder what volcanoes are erupting? How tsunamis form and where? What rocks can tell us about ancient environments? How the Earth might change in the future? You'll find these answers and more on Rocky Planet.

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