The Toba Eruption

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Podcast Transcript

74,000 years ago, the Earth suffered the greatest natural disaster that it has seen in the last several million years. 

A supervolcano erupted in what is today the country of Indonesia. 

In addition to completely changing the Earth’s climate for years, it may have pushed a species known as Homo Sapiens to the brink of extinction. 

Learn more about the Toba Eruption and how it might have almost spelled the end for humanity on this episode of Everything Everywhere Daily.

I’ve talked about several different volcanoes and volcanic eruptions in previous episodes. 

However, none of them have come close, not even an order of magnitude, to the eruption that I’ll be talking about in this episode. 

To put things into perspective, there is a scale known as the Volcanic explosivity index, or VEI, which measures the power of volcanic explosions. The VEI is a logarithmic scale, so each unit on the scale is 10x greater than what came before it.

The current eruption of Mout Etna in Sicily or the 2019 eruption on White Island in New Zealand ranks a 2 on the VEI. 

The 1997 Soufrière Hills eruption on the island of Montserrat that caused two-thirds of the island to be permanently evacuated ranks a 3.

The 2010 Eyjafjallajökull (AY-uh-fyat-luh-YOE-kuutl )eruption in Iceland, which caused a shutdown in international air travel, ranks a 4.

The Mount Vesuvius eruption in the year 79 that destroyed Pompeii, the 1980 Mount Saint Helens eruption, and the 2022 eruption of the Hunga Tonga–Hunga Ha?apai volcano all rank only a 5 on the VEI.

The 1883 eruption of Mount Krakatoa and the 1991 eruption of Mount Pinatubo in the Philippines both rank a 6. 

Finally, the 1815 eruption of Mount Tambora in Indonesia, on which I’ve done a previous episode, and which was the greatest volcanic eruption in recorded human history, ranks a seven.

There are, however, eruptions that are even more powerful than what was seen at Mount Tambora. 

These eruptions are known as supervolcanoes. They are not only the most powerful of all volcanic eruptions, but they are also correspondingly rare. 

Geologists have estimated that over the last 130 million years, there have only been 40 supervolcano eruptions that have ranked an eight on the Volcano Explosivity Index.

The last time such an explosion took place was 26,500 years ago on the North Island of New Zealand. The Taup? Volcano exploded, which resulted in the creation of Lake Taup?.

However, a much larger explosion occurred approximately 50,000 years before that, Mount Toba. 

The eruption of Mount Toba wasn’t just bigger, it is believed to have been the largest volcanic eruption in the last 25 million years. 

Unlike the other volcanic eruptions I’ve covered in previous episodes, we have no eyewitness testimony of the eruption. If there were any eyewitnesses, they probably didn’t survive to tell the tale. 

Toba was the biggest eruption in human history, and by that, I mean that the eruption took place when there were human beings on the planet, not that they were documenting history.

Toba is located in the northern part of the Indonesian island of Sumatra. Today, Lake Toba is a 100-kilometer-long lake that also happens to be the largest volcanic lake in the world. Larger than Lake Taupo in New Zealand and Lake Yellowstone in Wyoming, both of which were also formed by supervolcanoes.

The eruption took place approximately 74,000 years ago. Based on the distribution of ash from the eruption, it is believed that it probably took place during the summer in the northern hemisphere, as monsoon winds during that time would explain ash distribution. 

The eruption probably didn’t occur in one single explosion. Like the Mount Tambora eruption, it most likely took place over a two-week period. 

The amount of debris that was spewed out of Toba was incredible. The total amount ejected was at least 2,800 cubic kilometers or 670 cubic miles and may have been as much as 50% greater. 

To put that into perspective, the total amount ejected from the Mount Saint Helens eruption in 1980 was only about one-quarter of a cubic kilometer. This eruption would have been about 10,000 Mount Saint Helens eruptions.

It was enough to fill all of Lake Victoria with volcanic matter and, if the upper estimates are correct, enough to fill Lake Huron.  It would have been three to five times the volume of Mount Everest. 

In previous episodes, I’ve spoken about how dangerous and deadly pyroclastic flows are. These are superheated clouds of gas and ash that travel at incredible speeds from volcanoes. There is nothing you can do if you find yourself facing one.  

You can’t outrun it. You can’t hide from it or take shelter. 

The area hit by pyroclastic flows was estimated to be over 20,000 square kilometers. An area the size of Belize. 

Most of the ash which was ejected was sent to the north and to the west. Most of the area north and west of Sumatra is the Indian Ocean, which would have made the sea in that area a toxic pool.

However, there was enough debris piled on land that indicated just how incredible the eruption was. 

In the area of Sumatra closest to the volcano, the ash deposits were 600 meters or 2,000 feet thick. 

Over in Malaysia, across the Strait of Malacca, there were areas with nine feet or three meters of ash. 

India and South Asia had it just as bad over a much larger area. Almost all of South Asia had a layer of ash at least 15 centimeters or 6 inches deep. In one part of central India, the amount of ash was 6 meters or 20 feet deep. 

The ashfall destroyed is believed to have destroyed entire forests in India. 

There were even ash deposits discovered as far away as Lake Malawi in Africa. 

Researchers studying ice cores taken in Greenland and Antarctica have found a large spike in sulfate particles during this period.

This incredible amount of ash that was sent into the atmosphere had enormous consequences for the Earth’s climate. 

If you remember back to my episode on the 1815 Mount Tambora eruption, it resulted in the year without a summer in 1816. There were freezing temperatures in July and August across much of the Northern Hemisphere as well as famine due to crop failures.

The impact of the Toba eruption would have been even more severe. 

Estimates are that global temperatures dropped by 3.0 to 3.5 °C or 5 to 6 °F for several years, with a short-term drop in temperatures by as much as 15°C or 59 degrees Fahrenheit. 

Some researchers think that the Toba eruption may have reduced global temperatures for as long as 100 to 1000 years. 

Everything I’ve just outlined isn’t very controversial. There is ample evidence of the Toba eruption all over South and Southeast Asia, as well as a giant caldera in Sumatra which can still be seen today. 

The controversy has to do with another theory that comes from the seemingly distant world of genetics. 

About roughly the same time as the Toba eruption, humanity seems to have gone through some sort of genetic bottleneck. It can be explained if the population of humans around this point shrunk to incredibly low numbers, possibly as few as only 1,000 people. 

This theory became known as the Toba Catastrophe Theory. It was an elegant theory that linked together two known events. The Toba eruption, which had a huge impact on the planet, and the genetic bottleneck that humanity seemed to have gone through. 

The implication is that the Toba eruption and its aftermath would have killed off almost all of humanity. After all, if the much smaller Tambora eruption caused the problems it did, then certainly the much larger Toba eruption would have been even worse.

The theory, mainly due to its simplicity and elegance, gained popularity in the 1990s and 2000s. 

However, research afterward challenged the theory and indicated that that wasn’t just incorrect, but it might have been the total opposite of what really happened.

Not all volcanoes are the same. The type of magma that they emit has different chemical compositions. In particular, the magma has different abilities to carry sulfur. 

More recent studies on the magma emitted from the Toba eruption show that it was much lower in sulfur than the magma from the Tambora eruption in 1815.

This is important because it is sulfur particulate matter in the atmosphere, which is responsible for blocking and reflecting sunlight which lowers the temperature. 

This means that while the Toba eruption undoubtedly influenced the Earth’s climate for years, it might not have been as bad as the sheer quantity of material ejected would indicate. 

The other damage to the theory comes from archeological evidence found in Africa. 

The Toba eruption left a layer of debris across Africa that left a very unique signature in the stratigraphic record. This is handy for archeologists because it serves as a marker. Anything below this level occurred before the Toba eruption, and anything after that layer occurred after the Toba eruption.

If the Toba Catastrophe theory is true and the eruption did result in the near extinction of humanity, then we should expect to see a gap of at least several thousand years of little to no human activity after the eruption, as it would take that long for humanity to repopulate. 

However, that isn’t what the archeological evidence shows. In fact, it shows just the opposite. There was an increase in human activity after the eruption.

The same was found in India, which was hit even harder than Africa. After the Toba eruption, there were more animal remains and more evidence of toolmaking. 

Moreover, there is no evidence of a sudden drop in populations in Neaderthals or any human populations across most of Eurasia and Africa. 

So, if the Toba eruption didn’t result in a massive decrease in humans, what explains the bottleneck that geneticists find in human genetic history? 

A genetic bottleneck is basically a decrease in genetic diversity within a gene pool. It can result from a decrease in population, so linking the Toba eruption to this wasn’t a ridiculous idea.

However, genetic bottlenecks can occur in other ways. One is through a phenomenon known as the founder effect. 

As humanity spread out from Africa, they separated into different groups that became isolated from each other. These small groups would have limited genetic diversity amongst their group as they were distanced from the larger human genetic pool found in Africa.

Thus, the genetic bottleneck is more a reflection of the fragmentation of human groups after they left Africa rather than the result of a massive reduction in the population of humans which almost resulted in their extinction. 

The Toba Catastrophe Theory is still being debated, and there are still proponents of it, but it holds much less sway than it did even 10 years ago. 

Nonetheless, regardless of how the Toba eruption impacted human genetics, it was still one of the largest volcanic eruptions ever. While there is no indication that there are any supervolcano eruptions on the horizon, you also can’t say that it is impossible. 

The odds of an eruption of this magnitude occurring in our lifetimes is very small, but it also isn’t zero. 

The eruption of Mount Toba 74,000 years ago shows that no matter how devastating and destructive a volcano can be, there is always the potential for something worse.