The Big One: Toba explodes

Prior to the eruption of 73,000 years ago, Toba had produced at least two earlier major eruptions 800,000 and 500,000 years ago. The earlier outbreaks were not as large as the colossal eruption of 73,000 years ago which is the only eruption of class VEI 8 to have taken place since primates have appeared on planet earth. The famous Krakatoa eruption (also near Sumatra) of 1883 was a minor event by comparison. The following charts and drawing try to illustrate the sequence of events of the eruption 73,000 years ago.

Fig 3-1. The monsoonal and trade wind systems (shown below for January and July) at the time of the YTT event 73,000 years ago are thought to have been fairly similar to those of today. The cone of the YTT fall-out zone is oriented in a west-north-westerly direction, in line with the high level wind systems of the troposphere. Only the Summer monsoon could have caused Toba ash to fall in the South China Sea, implying that the eruption took place sometime during the northern Summer (ref. Bühring C. et al., 2000). It has also been estimated that the eruption lasted around 9-14 days (ref. Ledbetter M. et al, 1979).

=== Likely fall-out area for ash, as affected by the seasonal wind systems
<–– High-level wind system (largely unaffected by the seasons)
<–– Surface level wind system (strongly affected by the seasons)
x Location of Toba ash found in the South China Sea
1 Andaman islands 2 Nicobar islands 3 Mentawei island

January (Northern winter)••••••••••••••••••••••••••••••••••••••July (Northern summer)


The Toba YTT event and the Andaman and Nicobar islands

The Andaman and Nicobar islands were deep within the main fallout cone of the Summer-time YTT event . Life on them must have been largely destroyed in the course of that event and its aftermath. No evidence of a Toba ash layer on the islands is known (Indian Geological Survey please note and publish). However, it can be taken for granted that an event that covered much of India with 3-6 m of ash (ref. ¬ÝAcharya, S.A. and Basu P.K., 1993) and parts of Malaysia with 9 m (ref. Scrivenor J.B. 1931) will have dumped at least similar quantities of ash on the islands, wiping out all plant and animal life there. This means that Toba gives us an “earliest date” from which the present unusual environment on the islands could have developed ( environmentalists, botanists, zoologists, please note).

Homo sapiens , of course, arrived in the Andaman islands much later than 73,000 years ago. Recent genetic work (Endicott et al. 2003) has dated the coalescent process of the mtDNA lineage M2 of haplogroup M of the Andamanese Negrito people to 63,000 +/- 6,000 years ago and the Andamanese M4 lineage to 32,000+/-7,500 years. These dates do not tell us when the ancestral Andamanese actually reached the islands. They must have survived the YTT event somewhere on the mainland of Asia, Sundaland upwind from Toba, or Africa. The Negrito are thought to have been the earliest modern Homo sapiens to reach (or survive Toba in) Asia. They settled the Andaman islands either after the coalescence of the genetic M4 lineage or the coalescence took place somewhere on the Asian mainland with colonization of the islands following later.

Whether there have ever been Negritos in the Nicobars is unknown but is now thought unlikely on genetic grounds. The islands were settled relatively recently (some few thousand years ago) by Mon-Khmer speaking Mongolid people, mostly from mainland Asia with some lesser components from Indonesian groups.

The Toba YTT event and Mentawei island

In the controversy over the magnitude of the Toba YTT event, the remarkable and ancient biodiversity of Mentawei island has been used as an argument to show that the YTT event could not have been as large as claimed (Gathorne-Hardy F.J. et al., 2003). The argument is that a large Toba eruption would have destroyed that diversity. Mentawei island is only 350 km southwest of Toba. Indeed, the island must have had a very close shave – but it was saved by the northern Summer monsoon (see Fig. 3-1 above) (ref. Ambrose S.H., 2003). Had Toba erupted during the northern Winter, Mentawei biodiversity would probably have been destroyed.

Fig. 3-2. Effect of wind direction and height on fallout pattern. The eruption column would certainly have gone right through the troposphere and high into the stratosphere. Some of the erupting ash would have been boosted by the thermal buoyancy of superheated gases, hurtling its ash load nearly straight up to the upper stratosphere (ref. Rose W.I. et al., 1990).

January (Northern winter)•••••••••••••••••••••••••••••••••••••••••••••July (Northern summer)

It has been estimated that the YTT event (at the very least) has produced ejecta of 2,800 cu. km. Some researchers consider this as an under-estimate. The total is thought to be made up of the following components (ref. Rose W.I. et al., 1990):

Lava flows  Lava flows of 1,000 cu. km, covering an area on Sumatra reaching from coast to coast of 20,000 to 30,000 sq. km and between 50 to 150 m (sometimes up to 400 m) thick near the caldera and approximately 50 m thick on average. The temperature of the lava before eruption was around 750oC. The temperature of the emerging material at the time it came to rest is thought to have been around 550oC and within a few days cooled to around 100oC – but below the surface it remained hot much longer.

Fig. 3-3. The lava fields produced by the YTT event on Sumatra. Adapted from ref. Rose W.I et al, 1990.

Ignimbrite in the caldera – Ignimbrite in the caldera of 1,000 cu. km, covering an area of 2,500 sq. km and on average 400 m thick.

Pyroclastic ash fall – Pyroclastic ash fall of 800 cu. km at a thickness of 10 cm averaged over the entire earth. It has been noted that where located, the fall deposits show little decrease in thickness and grain size with distance from Toba so that the deposit can perhaps be traced much further than the current limit at 3,100 km in central India. If so, the 800 cu. km estimate may be much less than the actual value.

Sulphuric acid – In addition, it is thought that the staggering amount of 1010 metric tons of H2SO4 (sulphuric acid) was blown into the atmosphere by the YTT event (Huang et al. 2001).

Using an eruption time of 9-14 days (ref. Rose W.I., 1990) with an eruption volume of 2,800 cu. km, an average eruption rate of 8 x 1012 g/s has been calculated – 8 million metric tons of material per second (ref. Rose W.I., 1990).

The areas directly affected by ash fall from the Toba explosion must be speculative but one site (red 6 in the map below) in central India, for example, today has a thickness of no less than 6 m (20 ft) (ref. Acharya S.K. et al., 1993). It is possible that this thickest of deposits found so far represents an accumulation of wind-blown or water-driven ash, but even if it does, most of Southeast Asia, parts of Sunda, the Andaman and Nicobar islands, along with all of the the Indian subcontinent and Sri Lanka were covered in deep ash. Such a heavy fall would have exterminated most plant and animal life in the affected areas. The deepness of the ash reduces gradually towards the west but can still be expected to have been substantial in the Middle East and parts of East Africa. That ash layers attributable to the Toba eruption of 73,000 years ago have not been located so far west of the Indo-Pakistani coast but this is probably because few have been specifically looking for it. The ash layers are also likely to thin out west of the Middle East and are consequently are harder to locate and identify.

Ash from the YTT can be unambiguously assigned to Toba through its characteristic chemical composition. There is no question of confusing Toba YTT ash layers with layers laid down by eruptions from other volcanoes (of which there is no shortage in the area).

Toba ash has been dated using the K-Ar, the 40Ar/39Ar and oxygen isotope/biostratigraphic methods. The abrupt drop in sea water temperature was determined with the oxygen isotope (O18/O16) method. The diverse methods used on a variety of Toba materials are in rough agreement as to the dating of the event: it definitively took place sometime between 77,000 and 69,000 years ago.

Fig. 3-4. Traces of the last Toba eruption and predominant sea surface currents today. With the possible exception of the seasonally reversible currents on both sides of the Indian subcontinent, these currents were the same 73,000 years ago as they are today. Archaeological sites with tools and bones lying below Toba ash (and therefore older than 73,000 years) are red numbers 1 (in Malaysia) as well as 5, 7, 8 and 10 (in India) in the map below. Also shown as thick red lines are the shores that (based on the predominant sea currents) are likely to have received major amounts of floating pumice material if the volcano did indeed expel such material into the sea. Pumice floats are an idea contributed to the ongoing discussion by Mr. Tim Gillin of Australia. No such finds have yet been made – possibly because no one has been looking for them on in the right places yet. The beaches of 73,000 years ago are now below sea level and difficult to locate and even more difficult to excavate. Pumice is a very light, prorous, froth-like volcanic glass that floats on water. In a major eruption it could have covered large areas of water and drifted considerable distances, carrying possible animal and plant life to distant shores. The YTT event was big enough for lava streams to reach the sea at both the east and west coasts of Sumatra (ref. Rose W.I. et al., 1990). There is no reason, therefore, why pumice in large quantities could not also have been produced and reached the sea – through direct evidence for this has not been found yet.



limits the area in which Toba fallout (YTT) has been found or where substantial  amounts of fallout are likely to have been deposited.
|| indicates likely locations of Toba pumice floats washed up on beaches
1 red numbers: traces of Toba ash found on land (for details see list below)
1 blue numbers: traces of Toba ash found at the bottom of the sea (for details see list below)

Land sites with archaeological finds1 Tampan, in Malaysia, Toba ash layer 1.55-3.0 m (5-10 ft.) thick, mid-palaeolithic, acheuleen-type tools found below the YTT layer (further downriver from the Tampon site, at the junction of the Pelus and Perak Rivers 9 m of Toba ash has been reported by Scrivenor J.B. 1931)

5 Vansadhara, in India, Toba ash layer 0.5-1.5 m (4.1-16.4 ft.) thick,
mid-palaeolithic, acheuleen-type tools found below the YTT layer

7 Son, in India, Toba ash layer 1.5-3.0 m (1.6-5 ft.) thick,
fossil bones and stone tools found below the YTT layer

8 Narmada, in India, Toba ash layer 0.4-0.8 m (1.3-2.6 ft.) thick,
fossil bones and stone tools found below the YTT layer

10 Kukdi, in India, Toba ash layer 0.2-1.0 m (0.6-2.3 ft.) thick,
mid-palaeolithic, acheuleen-type tools found below the YTT layer

Land sites without archaeological finds:

Bogra, India, 0.5 m (1.6 ft.)

3 Barakar, India, 2 m (6.5 ft.)

Mahanadi, India, 1.5-3.0 m (5-10 ft.)

6 Vansadhara, 1.5-6.0 m (5-20 ft.)

9 Sagileru, 0.3-2.0 m (1.0-6.5 ft.)

10 Kukdi, 0.2-1.0 m (0.6-2.3 ft.)

The substantial difference between the measured thicknesses of Toba layers on land (red, above) and on the sea floor (blue, below) is largely due to bioturbation of the sea floor. Bottom-dwelling sea animals disturb and re-distribute the ash layers far more efficiently than land animals. Moreover, sea animals were far less likely to have been killed by the ash fall itself. Bioturbation re-distributes the ash upwards and downwards, mixing it with adjacent matter. This makes measuring the thickness of ash layers at the bottom of the sea more difficult and less precise.

Sea sites:

1 2.5 cm (1 in.)

3.5 cm (1.3 in.)

40 cm (15 in.)

3 cm (1.1 in.)

12 cm (4.7 in.)

11 cm (4.3 in.)

8 cm (3.1 in.)

15 cm (5.9 in.)

7 cm (2.7 in.)

10 2 cm (0.8 in.)

11 9 cm (3.5 in.)

12 3 cm (1.1 in.)

13 11 cm (4.3 in.)

14 9 cm (3.5 in.)

15 10 cm (3.9 in.)

16 2 cm (0.8 in.)

Fig 3-5. Ocean currents in the Indian Ocean (dotted lines are complex currents in the Bay of Bengal and the Arabian Sea that reverse seasonally, basically clockwise in Winter and anticlockwise in Summer)

What are the chances that Toba will explode again? The slow movement of geological plates that create volcanoes like Toba continues to build up pressure. A new explosion is sure to come but it will not happen any time soon and none of us will be around to witness it. Based on Toba’s past, we can expect the next major event in 300,000 to 500,000 years from now.

While Toba is no threat to human survival for the foreseeable future, there are other volcanoes that have not produced major eruptions for a long time and that may well have a similar potential for destruction. Life has always been a dangerous business on this beautiful and unpredictable planet of ours. While humanity as a whole is not doing too badly at the moment, there are a lot of dangers that with all of our technology we do not even come close to controlling. Volcanism is one of them, and the Toba YTT event provides the best illustration of what a truly major eruption can do to mankind and its property values.

To finish this chapter on a note of optimism rather than doom: Homo sapiens has come though the Toba YTT event, Homo sapiens will also make it through the next VEI8 event. Somehow.

Next: Aftermath: Climate and Environment