What would be the precursors to a repeated Lake Toba super eruption?

Question

Lake Toba, Indonesia is a supervolcano which was the site of

a massive supervolcanic eruption estimated at VEI 8 that occurred 69,000 to 77,000 years ago, representing a climate-changing event. It is the largest known explosive eruption on Earth in the last 25 million years.

It is believed by some scientist to have caused the Toba Catastrophe Theory, resulting in a 'bottleneck' in early human populations.

The region is most definitely still very active with continuing seismic and volcanic activity.

What would be the precursors to a repeat of a Lake Toba super-eruption?

Answer 1

Massive, caldera-forming eruptions like the one at Toba usually happen at large silicic systems. At such systems, we expect a surface uplift as the pressure builds in the magma reservoir. This can be measured by radar interferometry (InSAR) and/or GPS. It is the case at Laguna del Maule volcano (Chile), were uplit up to 28 cm per year has been measured by InSAR (Feigl et al. 2014). A similar pattern would be observed at Toba if a magma reservoir was being replenished. However:

• We don't know the timescale needed for overpressure, thus eruption, of the reservoir. It depends on many parameters, such as reservoir size and depth, strength of host rock, rate of fresh magma supply...
• We don't even know if such uplift will lead to an eruption. The magma could very well be intruded into the crust (in the form of dikes and sills), releasing the pressure in the reservoir. The uplift would then cease without an eruptive crisis.

Edit $-$ Two new studies about the Laguna del Maule volcanic field:

• Bai et al. (2020) used seismic data to image the crust below, finding a superficial zone with an estimated 70 km$^3$ of magma.

• Le Mével et al. (2021) used GPS and InSAR data to model surface uplift, finding an acceleration of uplift rate in recent years.

Again, this does not mean that the system is going to erupt anytime soon. But if the Toba magmatic system was being replenished, we would expect to measure similar ground displacement.

Answer 2

If I understand right, you want to know if we would notice signs of an eruption before the actual eruption? As is written in the article that you cite, the region is still very active, but being active doesn't mean that anything will happen any time soon. A volcano can be active for millions of years, without much happening. It just means that there is a magma chamber and some heat transfer to the surface.

If Toba would erupt we might notice some sings in advance. Probably there would be some increased geomthermal heatflux. Hot springs would get more active, the earth surface would uplift, etc... And then after a few days/weeks/months the actual eruption might happen. But the eruption may also happen quite surprising.

I think this question is not specific to Toba, but rather to volcanic eruptions in general. there is a lot of literature about that. You may want to start with the wikipedia article on that topic.

Answer 3

The question is not if there would be signs before a VEI8 eruption but if they would be recognized as precursors of a super eruption at all or early enough to be prepared when it happens.

Unrest is such an usual thing at big explosive Calderas, and there is no Data suggesting at what level of exacerbation the Volcano becomes really dangerous (=Vei 7-8 imminent).

Another point is the fact that a warning would have to be given pretty early in advance. If the unrest significantly increases just a few months, weeks or even days before the eruption it is too late, and warnings that just suggest that a super eruption might be possible within the next ten Years are probably too vague to make so many efforts for getting prepared.