Central North Island sitting on magma film

Scientists have discovered a film of molten magma under the central North Island, dispelling old ideas about the workings of the fiery region.

As a result of the find, monitoring of the potentially explosive Taupo Volcanic Zone, the scene of enormous eruptions in the past, will be improved.

It was thought magma lurked in unconnected pockets under volcanoes and geothermal zones, said Hugh Bibby of GNS Science.

But new measurements have revealed the molten rock lies across a zone 50 kilometres wide and 160km long, northeast of Taupo.

"The key is that it is like a continuous film wetting the surface. The wet surface is right across the Taupo Volcanic Zone, not just little blobs under each geothermal system or volcano," Dr Bibby said.

Scientists used magnetotellurics, which measure differences in the earth's magnetic field, to make the discovery. The research will lead to a better understanding of how the volcanic zone works, Dr Bibby said.

When the system is stable, a layer of hot rock with its small amount of magma provides the heat necessary to fuel more than 20 geothermal systems in the region. But if there is too much magma it can build up at shallow depths, eventually leading to a giant eruption. About 26,000 years ago such an explosion formed what is now Lake Taupo.

Until now there had been little evidence about the extent of the magma system under the central North Island. The Taupo Volcanic Zone was a perfect place for the study, Dr Bibby said.

"Eruptions from the Taupo area are enormous, the scale is difficult to imagine. Ruapehu erupted about 0.1 cubic kilometres of material in the 1995-96 eruption. Imagine an eruption of 400 cubic kilometres of material."

The Dominion Post