Can we predict the next supervolcano eruption?

Beneath the calm waters of Lake Toba on the Indonesian island of Sumatra, a massive supervolcano is lurking. This geological monster is one of only a handful of volcanoes capable of producing a super-eruption that could catastrophically change the global climate system for decades. So how can we predict the next supervolcano eruption?

An international team of geologists has investigated the potential early warning signs of such a world-shaking eruption, focusing on the volcano beneath Lake Toba.

This volcano has produced two of the most massive eruptions the Earth has ever seen, occurring 840,000 years ago and 75,000 years ago. Each produced enough ash to blanket the whole of Switzerland with seven centimetres of ash.

There has been no equivalent-sized eruption in recorded human history, so this research is crucial to informing how we might be affected – and how we might respond.

The new study, published in PNAS, measures how long it took for the Toba volcano to build up its magma reservoir in the lead up to these two past super-eruptions. Magma is what makes super-volcanoes so dangerous: the molten rock builds up and up in the mantle below the crust, unable to break through, until the pressures become so great that the whole system explodes.

The good news? The researchers think that Toba’s next major super-eruption is still 600,000 years away.

The bad news? The eruption won’t be heralded by weird geological signs. In fact, we won’t know when it’s coming: the build-up will be slow and silent.

To come to these conclusions, the team analysed the levels of uranium and lead in zircons, a tiny crystal found in volcanic rocks. Previous research has shown that zircons can be used to estimate the volume of magma stored below volcanoes.

“One of [zircon’s] characteristics is that it takes uranium within its structure,” says lead author Ping-Ping Liu, from Peking University in China.

She explains that over time the uranium decays into lead, “so by measuring the amount of uranium and lead in zircon with a mass spectrometer, we can determine its age”.

The team dated zircons from a number of different Toba eruptions, where the youngest zircon indicates the date of the eruption and older ones indicate how magma has built up.

“The first super-eruption occurred around 840,000 years ago, after 1.4 million years of magma input, whereas magma fed the second super-eruption at 75,000 years accumulated only in 600,000 years,” says Luca Caricchi, co-author from the University of Geneva (UNIGE), Switzerland.

The eruptions were the same size; the researchers say that the second eruption built up its magma more quickly because over time, magma heats up the continental crust around the volcano, which keeps the magma hot for longer.

“This is a ‘vicious circle’ of eruptions: the more the magma heats the crust, the slower the magma cools and the faster the rate of magma accumulation becomes,” says Ping-Ping Liu – meaning that super-eruptions can become more frequent as time goes on.

This zircon-dating technique allowed the team to estimate how much magma is simmering below the surface of the Toba volcano.

“Today, we estimate that about 320 cubic kilometres of magma could be ready to erupt within the reservoir of Toba volcano,” says Caricchi.

This wouldn’t be classed as a super-eruption (which needs to be greater than 1000 cubic kilometres) and would definitely be smaller than the 2800 cubic kilometres spewed out in Toba’s two big eruptions, but it’s still enough to create global disturbance.

The researchers say that an extra four cubic kilometres of magma builds up in Toba every thousand years. This means that next equivalent super-eruption would occur in 600,000 years – though smaller ones could happen in the meantime.

This method could also be applied to other supervolcanoes around the globe, like Yellowstone in the US.

“Our study also shows that no extreme events occur before a super-eruption,” adds Caricchi. “This suggests that signs of an impending super-eruption, such as a significant increase in earthquakes or rapid ground uplift, might not be as obvious as pictured in disaster movies by the film industry.

“At Toba volcano, everything is happening silently underground, and the analysis of the zircons now gives us an idea of what is to come.”