A team of geologists from University of Oregon and Bristol say the more magma crystallizes, the hotter it gets and the more likely a volcano will erupt. Their results will aid monitoring volcanic hot spots around the world.

The team showed that rapid crystallization of magma within one to two kilometers bellow the surface causes magma to heat up to as much as 100 degrees Celsius (212 degrees Fahrenheit). "While this sort of heating has been expected in theory, we are the first to show that we can measure it," said Katharine Cashman of University of Oregon.

"These results have important consequences for models of magma ascent beneath volcanoes, as increasing the melt temperatures causes the melt viscosity to decrease so that it can flow more easily, like heating up a jar of honey to allow the honey to flow out of the jar."

Explosive volcanic eruptions are determined by the escape of volcanic gases from magma stored several kilometers below the surface. The prediction of such disastrous eruptions needs knowledge of where the magma is at every moment and what it is happening to it.

"This work is now being used to gauge the direction of the volcanic activity currently happening at Mount St. Helens and could be applied to any active volcano for which monitoring and petrological records are available," said Jon Blundy, professor of earth sciences at the University of Bristol (United Kingdom).

In an experiment, the team used small pockets of melt that get trapped in crystals, as they expand to demonstrate that crystals grow by decompression, as magma rises toward the surface. They also showed that these crystals grow in a manner of months, rather than years. "This may sound counter-intuitive, but think about the need to add heat to something to melt it," Cashman said.

The researchers reconstructed changes in pressure, temperature and crystallization that occur in a volcano's magma before an eruption. As pressure decreases, crystallinity increases; the more magma crystallizes, the hotter it gets.

The finding states that a decrease in pressure rather than a loss of heat to surrounding rocks signals an eminent volcano eruption.

If ascending magma can heat itself up simply by magma crystallizing bellow, a volcano will erupt without a new heat source such as a shot of hotter magma from deep below the surface.

All these processes happen on a short timescale of several years, a more effectively monitored period of time for a volcano.