How the Earth's Crust and Mantle Move Underneath New Zealand

Earth Observatory Blog

How the Earth's Crust and Mantle Move Underneath New Zealand

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Dr James Moore, Research Fellow at the Earth Observatory of Singapore and one of the co-authors on this recent paper on mantle melt (Source: Rachel Siao)

New research from the Earth Observatory of Singapore and Victoria University of Wellington has provided in-depth information into how the Earth’s mantle deep beneath the central North Island of New Zealand is melting.

Our paper, published today on 6 July 2017 in Nature, examines the movements of the Earth’s crust and mantle in the region extending from Lake Taupo to the Bay of Plenty (the Taupo Volcanic Zone).

The research is based on data from Global Positioning System (GPS) measurements, which gave information on shifts in the horizontal and vertical positions of the region over the past decade.

The data revealed a remarkably symmetric and widespread pattern of movement extending up to 150 kilometres (km) laterally. In particular the study focussed on the large vertical movements, with central subsidence and flanking uplift.

Diagram illustrating the upwelling mantle flow beneath the Taupo Volcanic Zone, which creates a vertical force on an overlying crust (Source: Simon Lamb)

The Taupo Volcanic Zone is one of the most prolific volcanic regions in the world, but understanding the link between these widespread surface movements and the volcanic systems was a difficult question to address.

The scale of the volcanism we see in the North Island of New Zealand reflects the vast quantities of molten rock at depth but the causes of melting and effect it has on the movements at the Earth’s surface are still not well known.

Our study revealed that the widespread movements are due to a driving force near the base of the crust, nearly 20km below the surface.

The depth of the movements provides a clue that the melting process itself is in the flow of the Earth’s mantle. As the hot mantle rises beneath regions of volcanic activity the pressure drops. It is this drop in pressure that causes the melting. Once the flow reaches the top of the mantle and the base of the crust, it has to bend round. The divergence in the mantle flow creates a suction force that pulls down the overlying crust. 

Figure B: If the mantle viscosity undergoes an increase, due to melt extraction, then this will result in a broad symmetrical pattern of subsidence, horizontal contraction and flanking extension. Figure C: However, if melt accumulation near the base of the crust has reduced the viscosity of the underlying mantle, then this will drive uplift. (Source: Simon Lamb)

The key to understanding the surface movements lies in the fact that the strength of the suction force depends on how stiff or sticky the mantle rocks are. 

When the mantle rocks contain less melt then they are stickier and can more easily pull down the crust, as we see today. Conversely, if there were to be an increase in melting then the mantle rocks would be weaker, the suction force would reduce, and the subsidence would reverse as the crust is allowed to spring back up again.

Of course, this raises the question as to where the melt goes, as eventually it rises up through the crust to feed the overlying volcanoes. Using the GPS network we are now able to monitor melting processes in the underlying mantle.

Eventually we may well be able to use this in monitoring volcanic hazards to help assess the likelihood of volcanic eruptions in the future.

The paper was co-authored by Dr James Moore from the Earth Observatory of Singapore, and Associate Professor Simon Lamb, Professors Tim Stern and Euan Smith from Victoria University.

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