The Toba eruption

Here is a Google Earth image showing part of Sumatra with its capital Medan. The Malaysian coast is at the right with the harbour of Port Klang. Lake Toba, about 100 kilometres long, 30 kilometres wide, and up to 505 metres deep is the largest volcanic lake in the world. Volcanic? Yes, about 74.000 year ago, there was a volcanic eruption, the largest-known explosive eruption on Earth in the last million years. The eruption left a caldera, which is now lake Toba.

There exists a classification for explosive volcanic eruptions, similar to the Richter scale for earthquakes. It is called the Volcanic Explosivity Index (VEI). During an explosive volcanic eruption lots of (molten) rocks and ash (called tephra) are expelled into the air and it is the (estimated) volume of this tephra that is used to classify the eruption. Here is the VEI scale. Like the Richter scale it is logarithmic, each following step means a ten-fold increase in ejected volume. The “How often?” row gives an estimate of the frequency of the eruption, not surprisingly huge eruptions are very rare. There is an older classification of volcanic eruptions, Strombolian, Plinian etc, see Eruption Classifications. The last row gives an estimate of the plume height.

Here are a few examples

The Etna is Europe’s largest active volcano located in Sicily in southern Italy. On average it has a VEI index of 2. I visited this volcano twice, in July 1971 and July 1979. In 1971 it had erupted in April. Lava flows had caused a lot of damage and it was a special sensation to walk over solidified lava that was still hot under your feet. In 1979 it erupted in August but during my visit I still could climb up to the rim of the crater.

The Vesuvius volcano is also located in Italy, near Naples. It is not very active at the moment but erupted in 79 AD. spewing 3.25 km³ of tephra, destroying the cities of Pompeii and Herculaneum. VEI index 5. A catastrophic event.

And that is only VEI-5. Here is the island Santorini in Greece, Or rather what is left of it after a volcano erupted on the island circa 1600 BCE . The eruption volume is an estimated 34.5 km³, so the VEI index is 6. In the center of the caldera two small volcanic islands have been formed. There is still a lot of discussion about the exact date of the eruption, it may have caused the downfall of the Minoan culture.

VEI-6 eruptions are not rare, they occur globally with a frequency of 50-100 years. In 1883 the Krakatoa erupted (8–25 km³ ) . Similar to Santorini, in the resulting caldera a volcanic island, Anak Krakatoa has formed. In this photo seen in the foreground, with Krakatoa in the background.

The most recent VEI-6 eruption was Mount Pinatubo in 1991 with an erupted volume of 12.5 km³ Two photos, one taken about one month after the eruption, the other one taken in 2012, where a lake has formed in the caldera. What a contrast 😉

In recorded history there is only one VEI-7 eruption, Mount Tambora in 1815 with an eruption volume of 144–213 km³ Located on the Indonesian island of Sumbawa, it is now a tourist attraction. The caldera is 6-7 km wide and 600-700 meter deep.

VEI-7 eruptions are so powerful that the plume reaches the stratosphere. This can lead to a volcanic winter event. The lighter particles and ash fall/rain down, but the plume also contains massive amounts of gases like SO2 and H2S which in the stratosphere  react with OH and H2O to form H2SO4 (sulphuric acid) aerosols. They reflect the sunlight, remain in the stratosphere for months or even years and spread globally. The effect is a temporary climate change, affecting crops etc. The year 1816, one year after the Tambora eruption was a Year Without a Summer ! Average global temperature was 0.4–0.7 °C lower, causing major food shortages in many countries.

After this long introduction, it’s time to come back to the Toba eruption. Because it happened so long ago, the estimates of the ejected tephra are of course less accurate, they vary between 2000-13200 km³ A recent one gives 8600 km³. Meaning that the Toba eruption was at least a VEI-8 (more than 1000 km³) and possibly even a VEI-9 (more than 10.000 km³).

How much is 8600 km³? Well, if all that tephra would be deposited over Peninsular Malaysia, it would result in a layer of about 65 meter thick!

When you compare the Tambora eruption with the Toba eruption, Toba should have resulted in a much stronger volcanic winter, possibly lasting many years if not decades.

Could such a volcanic winter have affected our species, Homo Sapiens ? The Toba eruption happened during the Last Glacial Period . It was in that same period that waves of Homo Sapiens migrated out of Africa. This graph shows global temperatures during the last 150.000 years. The Eemian and the Holocene (our present era) are warmer Interglacials. During the Last Glacial Period temperature dropped as did the sea levels. See my blog posts Ice Ages and Sundaland. As you see in the graph, the temperature variations are rather irregular. There are markers for the Out of Africa waves and the Toba eruption

In this map the spreading of Homo Sapiens is shown (in red) together with the distribution of earlier human ancestors, the Neanderthals and the Homo Erectus. In 2012 I did a DNA-test to find out when my ancestors left Africa. For my paternal ancestor that was around 50.000 years ago, my maternal ancestor left earlier, ~70.000 years ago. Both after the Toba eruption. Here is my blog: My ancestors .

The Homo Sapiens population in Africa was small and the migrating groups even smaller, think about numbers in the thousands. They were hunter-gatherers. Sudden climate change might threaten their existence. The Toba Catastrophe Theory, developed a few decades ago, holds that the Toba eruption caused a global volcanic winter, leading to a near extinction of Homo Sapiens, causing what is called a (human) genetic bottleneck. Here is an illustration of a genetic bottleneck. Because only a small number of individuals survives the bottleneck, their genetic diversity is limited.

It is generally accepted that there have been many bottlenecks in the human evolution, Click here for an article about it. And a few months ago Scientific American published an article Human Ancestors Nearly Went Extinct 900,000 Years Ago That was the time of Homo Erectus, long before Homo Sapiens evolved.

The Toba Catastrophe is controversial. Did the Toba eruption produce a bottleneck? Was the Toba Volcanic winter so severe and long lasting that the global population of Homo Sapiens was reduced to about 1000 breeding pairs?

One vocal proponent of the Toba Catastrophe is Donald Prothero, an American geologist , paleontologist and prolific writer In 2018 he published When Humans Nearly Vanished about the Toba Catastrophe.

But here is an equally vocal opponent, John Hawks, an American paleoanthropologist with a popular blog, who wrote in the same year 2018: The so-called Toba bottleneck simply didn’t happen. He mentions research that humans thrived in South Africa during the Toba eruption.

I am not an expert, so don’t expect a verdict from me 😉

Nowadays Lake Toba and Samosir are tourist attractions of Sumatra. Here is a more detailed Google Earth image. Samosir was originally a peninsula until in 1907 a canal was opened through the isthmus.

I have visited Samosir in 1994, thirty years ago, I don’t remember much about it. Peaceful, interesting Batak culture. You don’t realise that you are staying on top of a dormant volcano. Deep below Lake Toba is a huge magma chamber (50.000 km³ ) that is filling up slowly with magma. This has lifted Samosir already around 450 m. Will there be another supervolcano eruption? Yes, but no need to worry, that may take another 600.000 years.

4 thoughts on “The Toba eruption

  1. Thanks for this most interesting blog Jan
    As usual , so well researched and full of detail
    And leaves lots of food for thought !!

  2. Hi Jan, what a well researched blog. When I listen too many of the world’s top scientists all predicting the world is doomed because of global warming caused by us humans using carbon based fuels. Very few will mention the earth based processes, Volcanoes, Earthquakes and Tectonic plate activities which all discharge huge amounts of toxic gas into the atmosphere. For an example Etna pumps 16,00 tonne of C02 daily on a good day. In southern India ash from Toba was found to be 75m deep.
    Regarding human migration some recent research has shown that another route out of Africa was via the Horn of Africa as the crossing would have been very easy some 100,000 years ago. As I understand it people first arrived in Malaysia about 85.000 Years ago.
    Well that’s my two pennyworth.

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