Why supervolcanoes erupt with cataclysmic explosions

Lake Toba in North Sumatra today is a calm 100-kilometre stretch of water, flanked by green hills and rocky outcrops.

But a mere 74,000 years ago, the region couldn’t have been more different: the supervolcano that forms the lake’s bowl blasted up to 5,300 square kilometres of hot rock and dust into the atmosphere and surface, leading to a volcanic winter which dropped global temperatures by 3 to 5 ºC for the best part of a decade. 

There’s no doubt these supereruptions are capable of widespread destruction.

Some scientists believe the “Toba catastrophe” killed off most humans. And Toba today is one of many supervolcanoes scattered across the globe. 

To better understand why supervolcanoes erupt majestically then sit quiet for hundreds of thousands of years, rather than burble along at a moderate rate, a team from Egypt, France, Russia and Saudi Arabia delved into Toba’s plumbing and found it’s fed by a massive magma reservoir that gradually builds up over millennia before exploding.

Russian Academy of Sciences Ivan Koulakov and colleagues examined how waves from earthquakes shook the region in 1995 and 2008.

In particular, they looked at P-waves – which can course through liquid as well as rock and are the first waves of an earthquake to show up on a seismograph – and S-waves, which can only move through solids.

From these they determined the complex, multi-level structure of the Earth 150 kilometres beneath the Toba caldera and movements of rock and volatiles such as water.

The “magma factory” starts with the Indo-Australian plate, which is sliding underneath Indonesia at around 56 millimetres each year. It’s torn, too – one end is denser, and sinking faster, than the other end. The tear is known as the Investigator Fracture Zone and runs underneath the Toba caldera.

At around 150 kilometres below the surface on the frature zone, the plate melts. Volatiles escape the rock to burble up through the mantle and fill a magma reservoir of around 50,000 square kilometres, which sits 30 to 50 kilometres below Lake Toba. 

The magma in the chamber is too dense to make its way any further and the overlying crust hems it in. But as more volatiles rise and join the reservoir, bringing heat with them, they cause more rocks to melt.

When the molten upper crust is saturated with these volatiles, they cause what the researchers call an “avalanche-style process”. Overheated fluids turn to gases, increasing pressure in the reservoir and bam! The contents empty in a massive eruption.

The process, then, begins again. It will repeat until the fracture zone is completely subducted.

So is there going to be another Toba super-explosion soon? Koulakov thinks not – the next will happen in the next tens or hundreds of thousands of years. They found S-waves managed to move through the magma reservoir, signifying a large chunk of solid rock. If it was liquid, though, the next explosion would happen much sooner.

The work was published in Nature Communications.