The Toba supervolcano eruption 75,000 years ago was one of the largest volcanic events since the evolution of our species, posing a severe threat to humanity's survival. New evidence shows this was not a one-and-done event as previously imagined but was followed by smaller eruptions for thousands of years.

Supervolcanoes are defined as those that release more than 1,000 cubic kilometers (240 cubic miles) of material, scoring eight on the logarithmic volcanic explosivity index (VEI) scale. Such eruptions are separated by periods of many thousands of years, leading volcanologists to discount the dangers in between.

However, in Nature – Earth and Environmental Sciences Toba, in Indonesia, is revealed to have erupted many times after its big event in explosions that had little impact worldwide but would have been very significant for anyone nearby.

“Learning how supervolcanoes work is important for understanding the future threat of an inevitable super-eruption, which happen about once every 17,000 years.” said lead author Associate Professor Martin Danišík of Australia's Curtin University in a statement.

This is important, Danišík noted, because, rare as they are; “Super-eruptions are among the most catastrophic events in Earth’s history, venting tremendous amounts of magma almost instantaneously. They can impact global climate to the point of tipping the Earth into a ‘volcanic winter’, which is an abnormally cold period that may result in widespread famine and population disruption.”

Danišík told IFLScience an additional essential feature for supervolcanoes is to create a caldera, as a result of the surface collapsing, rather than leaving the familiar conical volcano with a small crater at its peak.

In the centuries or millennia after an eruption, people may repopulate the area around a supervolcano. If the next explosion is thought to be thousands of years away, without even the looming volcanic peak to remind residents of the threat, they may not look out for the warning signs of something smaller.

Danišík and co-authors found this would be unwise because, contrary to previous assumptions, eruptions can occur even in the absence of liquid magma under the caldera. In Toba’s case, 5,000 to 13,000 years after the main eruption remnant magma in its vast chamber continued to ooze out, despite being thought to be “uneruptible”.

Toba's last really big eruption had a VEI of eight, producing approximately 10 times the material required to make it a supervolcano. Consequently, even something 100 times smaller would be a VEI seven, as large as anything in recorded history.

“Supervolcano eruptions in the quaternary are very diverse,” Danišík told IFLScience. “Each system is unique so we can only make general assumptions.” As a result, he is uncertain whether other supervolcanoes, let alone ordinary volcanoes, remain similarly prone to relatively small eruptions long after their main event.

Prior warning of ordinary volcanic eruptions offers an opportunity to evacuate people living nearby. It's less clear what could be done about a future Toba-scale event, which might plunge the entire planet into a volcanic winter. Danišík told IFLScience there is little support for the idea of drilling into the magma chamber to allow it to let off some pressure before becoming really dangerous. “Temperatures are close to 1,000 degrees,” he said. “At this temperature any hole will seal almost immediately.”