It was the most powerful volcanic eruption ever experienced by humankind.
Around 73,000 years ago, the Toba supervolcano blew, ejecting more than 2,800 cubic kilometres of volcanic ash into the skies above India and Indonesia.
“[It] pushed humanity close to extinction,” said Professor Valentin Troll from the University of Uppsala in Sweden. “It will hopefully take many thousands of years, but the fact is it is only a matter of time before the next super eruption.”
Such eruptions might be rare, but they have a long-lasting impact on the planet, cooling the global climate and devastating swathes of land.
And they’re poorly understood.
But now scientists led by Professor Troll may have discovered the cause of Toba’s historic eruption.
The researchers have been analysing quartz crystals that grow in magma to identify chemical changes in the molten rock before Toba erupted.
It’s a similar process to using tree-rings to determine climate variations, said Uppsala University’s Dr David Budd.
“The problem is that each ‘tree ring’-analogue is only a few micrometers across, which is why they are extremely challenging to analyse in detail,” Budd added.
When the researchers began looking at the quartz, they noticed that there was a distinct shift in the number of neutrons towards the outer rim of the crystals.
Uppsala University’s Dr. Frances Deegan explained: “The low ratio of 18O to 16O contents in the crystal rims indicate that something in the magmatic system changed drastically just before the big eruption.
“The explanation behind these chemical signatures is that the magma melted and assimilated a large volume of a local rock that itself is characterised by a relatively low ratio of 18O to 16O.
“This rock type also often contains a lot of water, which may be released into the magma, producing steam, and thereby an increased gas pressure inside the magma chamber.
“This rapidly increased gas pressure eventually allowed the magma to rupture the overlying crust, and send thousands of cubic kilometres of magma into the atmosphere.”
Professor Troll said the next explosion could take place at Toba, Yellowstone or another site, but that he was hopeful we’ll be better prepared next time round.