Zircon crystals from Mt Tarawera are helping scientists understand magma and how it behaves in the lead-up to an eruption.

Analysis of crystals from a massive central North Island eruption 700 years ago could help make it easier to know when a volcano is at increasing risk of erupting.

A team of researchers from Canterbury University and universities in the US and Singapore analysed zircon crystals extracted from volcanic rocks around the slopes of Mt Tarawera, deposited during the Kaharoa eruption of around 1314.

That eruption was roughly five times larger than the 1980 eruption of Mt St Helens in the northwestern US.

SUPPLIED An illustration of what the later 1886 eruption of Tarawera looked like.

By analysing uranium and thorium isotopes in the seven crystals studied, researchers were able to determine they had been under the ground for tens to hundreds of thousands of years. Then by analysing the diffusion of lithium within the crystals, they were able to understand their exposure to rock-melting over time.

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The results suggest the magma below the Taupo Volcanic Zone was only subjected to high temperatures of 650-750C for a period of years to a few centuries.

SUPPLIED Mount St Helens, pictured here in 2004, erupted catastrophically in 1980, killing 57 people.

Study co-author Kari Cooper, professor of earth and physical sciences at the University of California, Davis said the picture of the magma that emerged was less a seething mass of molten rock, than something that was mostly solid and crystalline, with a little liquid seeping through it.

To create an eruption, a certain amount of that solid, crystalline magma had to melt and start to move, she said. It possibly did that by interacting with hotter liquid stored elsewhere in the reservoir.

The pre-eruption magma was likely to draw material from different parts of the reservoir over decades to centuries. That implied that it may be possible to identify volcanoes at highest risk of eruption by looking for those where magma was most mobile.

"Mobility in magma is a function of temperature and most of the time when it's sitting there in the Earth's crust under the volcano it's cool," another co-author, Adam Kent, an Oregon State University geologist, said.

"But to erupt onto the Earth's surface magma needs to heat up so it can be runny enough to be squeezed along cracks in the Earth and pushed up to the surface. At lower temperatures, the magma is too crystal-rich and viscous to move." he said.

"It takes higher temperatures to get things moving - and then our data show it's only a period of years or decades before it erupts."

The same technique used with the Kaharoa crystals would have wide applications to volcanoes in many parts of the world.

A paper on the research, just published in Science, said there had been a longstanding debate about whether or not magma bodies were largely liquid for long periods of time.