Mantle melting deep below NZ
Scientists have built a fascinating picture of how mantle rock beneath the Central North Island is being melted by a driving force deep in the Earth.
The findings, just published in the international journal Nature, examine movements of the Earth’s crust and mantle in the region extending from Lake Taupo to the Bay of Plenty, based on data from GPS systems that have tracked shifts in the horizontal and vertical positions of the region over the past decade.
“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,” said study lead author Associ- ate Professor Simon Lamb, of Victoria University’s School of Geography, Environment and Earth Sciences.
“In general, there is volcanic activity whenever there is molten rock at depth. So the volcanism seen in the North Island indicates a vast volume of molten rock beneath the surface, but we don’t really understand what is causing the melting, and how its presence controls movements of the Earth’s surface.
“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 deep movements gave the scientists a clue that the melting process itself was in the deep flow of the Earth’s mantle, Lamb said.
“Hot mantle rises up beneath regions of volcanic activity to reach levels where the pressure is much lower. It’s this drop in pressure that causes the melting.”
As this material flowed, it created a suction force that pulled down the overlying crust, but the strength of the force depended on how stiff or sticky the mantle rocks were.
“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,” he said. “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.
“Using the GPS technology we are now able to see directly where, when and by how much the underlying mantle is melting.
“This melt will eventually rise up through the crust to feed the overlying volcanoes.”
Lamb said it was “amazing” that satellites could now be used to monitor the process of melting itself deep inside the Earth.
“This is something that might help us with assessing the likelihood of volcanic eruptions in the future.”
The study was co-authored by Victoria professors Tim Stern and Euan Smith and Dr James Moore of Singapore’s Nanyang Technological University.