Earth's mantle deep beneath the North Island volcano zone is melting
Scientists have a new understanding of what's going on under the Taupo zone - one of the most volcanically active regions in the world.
Researchers from the Victoria University of Wellington used GPS data to examine vertical and horizontal movements of the Earth's crust around Lake Taupo and the Bay of Plenty.
Tracking GPS data allowed the team from the School of Geography, Environment and Earth Sciences to map horizontal and vertical movements in the region over a decade.
They've been able to get a sense of the processes in the mantle - about 20km beneath the surface - and where the semi-solid mantle melts and eventually rises to the surface to feed volcanoes.
Lead author associate professor Simon Lamb said the volcanism of the North Island indicated a vast volume of molten rock at depth.
However, geologists and volcanologists do not know how melting is driven or how the movements of the mantle - the semi-solid layer of the Earth descending for 3000km under the surface - affects movement at the surface such as earthquakes and volcanic eruptions.
A chain of calderas runs under the zone from Mt Ruapehu to White Island.
Under Lake Taupo there is a chain of chambers containing a vast volume of molten rock, essentially a lake of molten rock about 50km wide and 170km long.
"The region has some of the most prolific volcanic activity anywhere in the world, but understanding what is causing this activity has proved to be tricky.
"Our study found a remarkably symmetrical and widespread pattern of movement extending for up to 150km laterally, which we would expect from a driving force deep in the Earth nearly 20km underground, near the base of the crust."
The movements provided a clue to the action of the melting process deep inside the semi-solid flow of the mantle.
The Taupo volcano zone is active and the lake itself is a flooded supervolcano caldera forming part of a chain of volcanoes and geothermal features running from Mt Ruapehu to White Island and beyond.
Lamb said hot mantle rises towards the surface to areas beneath volcanic fields where the pressure is lower and this pressure drop leads to melting.
"As this material flows, it creates a suction force that pulls down the overlying crust, but the strength of this force depends on how stiff or sticky the mantle rocks are.
"Our research showed that if the mantle's rocks don't contain much melt, they will be much stickier, causing the overlying crust to be pulled down rapidly. But if the rocks have just melted, then this makes the flow of the rocks runnier, effectively reducing the pull down force and allowing the overlying crust to spring back up again."
GPS data allowed the team to track corresponding movements on the surface and beneath the surface to gauge how much the mantle was melting.
This melted mantle eventually rises through the crust to feed magma chambers and volcanoes.
Lamb said the study could help assess the likelihood of volcanic eruptions in the future, although not in the short term.
"I think this work basically provides better understanding of where there are large reservoirs of mantle melt that could potentially rise up to feed the volcanoes, and also some feel for how long that melt has been accumulating, based on how long a region has been uplifting or subsiding.
"I don't think it can be used in short term predictions of individual volcanic eruptions, which will be dependent on what happens at shallow levels beneath the volcano. But the work gives an overall context to the volcanic activity that could feed into some sort of hazard assessment."
The paper - co-authored by Professors Tim Stern and Euan Smith from Victoria University and Dr James Moore from the Earth Observatory of Singapore, Nanyang Technological University - was published in the journal Nature.
GeoNet provided the GPS data.
The study revealed the Taupo zone is extending at a rate of 6mm to 15mm each year and, at the same time, a 70km long segment of the rift zone - where the Australian and Pacific tectonic plates meet - is contracting and subsiding in places.
This 70km rift zone is flanked by areas of uplift and subsidence. These features are formed when the upper crust is pulled down or forced up along the length of the rift axis. Depending on the viscosity, the mantle can force the crust up or the crust subsides.
Taupo volcano last erupted around 1800 years ago and its remnants filled with what is now the largest lake in New Zealand.
The eruption was the largest volcanic eruption of the last 5000 years.