Predicting when a volcano will erupt, and how badly it will erupt, is the inarguable Holy Grail for volcanologists. Although scientists are able to predict volcanic eruptions marginally better than they are able to determine when an earthquake will occur, the precision of these forecasts is notoriously poor. A new study, published in Scientific Reports, attempts to address this controversial, longstanding question, revealing exactly what is required to happen within a magma chamber to cause it to rupture, using Santorini in Greece as an example.

Recent studies have focused on the speed at which large magma chambers hiding beneath supervolcanoes – such as the Yellowstone caldera in Wyoming – “recharge”, with one particular piece of research suggesting that it only takes 500 years for a supervolcanic magma chamber to cataclysmically rupture from the time the recharge process begins. This novel study ramps things up a gear: using a combination of meticulous fieldwork and careful observation of the Santorini volcanic complex in Greece, the researchers have conceived a unique model that precisely constrains the magma volume and pressure increases necessary for an explosive volcanic eruption to occur.

Santorini is long thought to be the most likely inspiration for the Atlantis myth. Roughly 3,650 years ago, the Minoan civilization of Thera – the ancient word for the island – was destroyed when a powerful volcanic blast rocked Santorini, rapidly releasing 60 cubic kilometers (14 cubic miles) of magmatic material during a violent depressurization event. The core of the island collapsed into the cavernous magma chamber after it had emptied, allowing the Aegean Sea to flood in.

Another volcano has since grown out of the waters of Santorini, producing minor but damaging volcanic activity every few decades. The magma chamber beneath the island is recharging, there’s no doubt about it, but volcanologists are unsure as to when it will erupt again.

Two PhD researchers from the Royal Holloway, University of London traveled to the island, carefully measuring the size of a series of ancient magma-filled fractures (“dykes”) under the surface. When magma initially causes these fractures to form, specific types of earthquakes are produced, and the ground begins to swell as the magma chamber begins refilling. This was observed in 2011 to 2012 – volcanologists wondered if the volcano was about to erupt, but the activity died down.

For a magma chamber to erupt, the pressure of the thick, gassy magma within the chamber has to overcome that of the overburden pressure of the surrounding rock, which clearly did not happen in 2012. But how much magma is required for this to happen?

Each magma chamber recharge event increases the overall pressure of the cache of magma, so knowing the amount of magma the 2011 to 2012 dyke surge added to the total volume of the magma chamber allowed the researchers to calculate how much pressure is added each time. By comparing this rate of pressure increase to the mechanical properties of the rock, the researchers determined just how much of an increase in the magma chamber pressure is required to cause a significant rupture and induce a major eruption.

A rupture at Santorini, then, is likely to occur if just 0.04 cubic kilometers (0.01 cubic miles) of magma reaches the chamber. The restless period in 2011 to 2012 added half of this in under a single year, meaning that a rupture isn’t likely to be too far off in the future.