Scientists can't predict the exact time when a volcano will erupt but clues from an ancient super-volcano explosion could reveal ways to calculate when the next massive eruption.

Earth is littered with roughly one dozen super-volcanoes that can cause more havoc than ordinary volcanoes. For example, Mount Toba on the Indonesian island of Sumatra released 700 cubic miles of magma and a thick layer of ash over South Asia when it erupted 74,000 years ago. Compared to the largest eruption in recorded history, the explosion of Krakatoa in 1883 which only released 3 cubic miles of volcanic material, super-volcano eruptions are more devastating. When a super-volcano erupts it has the potential to trigger global catastrophic events so predicting when one is about to erupt is vital. Unfortunately, super-volcanoes can lie dormant for thousands of years before blowing so scientists haven't been able to monitor the pre-eruption events as closely as they have with volcanoes.

"When volcanoes awaken and when the magma starts to ascend to the surface, cracking rock as it does, it sends out signals," Professor Tim Druitt, a volcanologist at France's Blaise Pascal University and lead researcher told BBC News.

"You get seismic signals, you get deformation of the surface, increasing gas emission at the surface - and this can be detected.

"The question we're addressing here is what's going on at depth prior to these big eruptions. The classical view was that during long repose periods over thousands of years, magma slowly accumulates a few kilometers below the volcano and finally it blows."

To learn more about super-volcanoes and the signs leading up to an eruption, the researchers studied the remnants of another cataclysmic volcanic explosion, the Santorini eruption 3,600 years ago that may have inspired the legend of the lost city of Atlantis.

The researchers analyzed 300 crystals of the mineral feldspar at the Santorini site which indicates that something was going on in the magma right before the explosion. Druitt and his colleagues found that large changes occur in magma composition right before a large eruption.

"Before these big eruptions, these explosive volcanoes have long periods of inactivity, of dormancy - they're just stagnant. We found that they can reawaken after thousands of years very quickly, on a time scale of several decades," he said.

The researchers found out that there was an acceleration of build-up in the magma reservoir under Santorini within 100 years prior to the eruption. These findings could be used to detect changes in magma reservoirs and forecast when the next big eruption will occur.

"If you're not looking at these volcanoes with the right instruments, you might not detect their reawakening until maybe a few months before the eruptions," Druitt said. "But if you do have the right instruments, and if we can learn to interpret what signals these volcanoes give off, maybe we can get years more warning as to what might happen."

Predicting such events could prove to be life-saving especially with a spike in recent volcanic activities like the Iceland volcano eruption. Druitt and his team believe that there should be monitoring equipment near all the caldera volcanoes to observe magma build-up. An early warning system could give governments enough time to work out long term plans to minimize the impact of such super-volcanoes, like the one under the U.S. Yellowstone Park.

The researchers' findings are published in the Thursday issue of the scientific journal, Nature.