They rank among the most catastrophic natural disasters on Earth, second only in destructive power to an asteroid impact that could ensure humanity meets the same fate as the dinosaurs.
Super eruptions make those from normal volcanoes look like sputters of dust. They blast enough material into the air to bury large cities beneath kilometres of ash, and the particles they send into the sky can cool the planet for years.
But the most alarming aspect of these rare and violent events, which have left deep scars on the planet's surface, is that the forces that drive them have never been understood.
Until now. In research published in the journal Nature Geoscience on Sunday, two research groups have homed in on an answer. Through separate approaches, they found a single process that can trigger the devastating eruptions that spew out more than 450 cubic kilometres of lava and can leave craters 100km wide.
The findings will help scientists spot super-eruptions in the making, perhaps in time to prepare against their worst effects. "This is a step in the chain that will get us to a point where we can predict these eruptions when the need arises," said Wim Malfait, a geologist and leader of one of the groups at the university ETH Zurich.
Super eruptions are extremely rare, happening every few hundred thousand to a million years, and have never been documented by witnesses. But they have left vast craters, or calderas, around the world. The largest, at La Garita in Colorado, ejected 5,000km3 of material around 28m years ago. Another, at Yellowstone in Wyoming, has erupted three times in the past 2m years.
Normal volcanic activity is meagre in comparison. The spectacular eruptions of the Eyjafjallajökull volcano in Iceland in 2010, and Mount Pinatubo in 1991, ejected around 0.1% of the volume of material produced by a super eruption. The Pinatubo eruption threw so much sulphur dioxide into the atmosphere that it cooled the world by 0.4C for a few months. Super eruptions could dwarf that, sending up enough of the particles to cool the planet by 10C for a decade.
Scientists had long suspected that the triggers for common volcanic eruptions and super eruptions differed. More common eruptions happen when molten rock flows into "magma chambers" underneath volcanoes and produces enough pressure to blast through the surface layer of rock that lies on top. But the same process fails to explain super eruptions.
To investigate what drives much larger eruptions, the Swiss researchers created various compositions of magma and subjected them to the kinds of temperatures and pressures they would experience in a supervolcano. They found that differences in buoyancy forced magma up to the roof of the magma chamber, causing a gradual build-up of pressure that could eventually blast through the rocky roof.
"The magma pushes up at the top of magma chamber, just like if you try to hold a football underwater," said Malfait. "This overpressure at the roof of the chamber is enough to break the rock on top and start an eruption."
The same process was found by Luca Caricchi, a volcanologist at Bristol University who, with others, used computer models to show that early on in their formation, small magma chambers are surrounded by relatively cool walls of rock. When magma flows into these chambers, the pressure rises inside until the magma breaks through the surface.
The run-up to a super-eruption is different. Magma builds up so slowly that the rocky walls of the chamber heat up and expand, allowing the chamber to grow to a huge size. This relieves pressure inside the magma chamber. But as the chamber grows, more buoyant magma is forced upwards, and can eventually fracture the surface rock, causing a massive eruption.
In an accompanying article in the journal, Mark Jellinek, a volcanologist at the University of British Columbia, writes that the magma's buoyancy alone can trigger the eruptions.
The work will be crucial for helping scientists predict when super eruptions are nigh, Jellinek told the Guardian. "Understanding the underlying mechanisms for both super eruptions and Pinatubo-sized events is key for eruption forecasting. Individual super eruptions can potentially influence global climate for a sufficiently long period to change our place in the world," he said.
"It is always intriguing to see independent teams of researchers converge on the same scientific conclusion – in this case that the buoyancy of a magma chamber is likely the key factor in initiating super-eruptions," said Clive Oppenheimer, professor of volcanology at Cambridge University.
The process might also explain why super-eruptions are so much rarer than geologists would predict from the frequency of smaller eruptions, he added.