Continental rift exposes mantle melting at work

Mid-ocean ridges form a vast chain of undersea mountains that encircle Earth. At those ridges, melted rock from the planet’s mantle rises to fill the gap between spreading tectonic plates (see Physics Today, January 2005, page 21 ). But researchers still have much to learn about the process of melting and when it might drive volcanic eruptions. In a new study, researchers have used GPS measurements to build a model that explains the interplay between the rise and fall of the surface crust and the state of melting rock below. Simon Lamb and colleagues at Victoria University of Wellington studied New Zealand’s Taupo Volcanic Zone (TVZ), where the underwater mid-ocean ridge transitions onshore into a zone of continental rifting. Although the 200-km-long rift zone is widening overall, the researchers were surprised to find that a 70-km-long segment is locally narrowing and subsiding, even though it is flanked by regions that are expanding and uplifting.

The model by Lamb and colleagues accounts for the TVZ’s structure via the bending of an elastic upper crust. Rocks from the mantle flow upward, but on reaching the more rigid lower crust they are forced to turn and migrate horizontally. The change in flow direction causes a downward suction-like force on the upper crust, effectively loading it from below. The strength of the force is determined by mantle viscosity. Mantle with a high proportion of melted material is less viscous and reduces the downward force, which allows the crust to relax upward. Conversely, mantle with less melted material will be more viscous and thus create a stronger downward force that drives subsidence.

Using the GPS data, the researchers think they can trace regions of molten rock nearly 20 km beneath the TVZ. The work demonstrates the potential of using remote sensing to explore below Earth’s surface and monitor where and for how long the mantle is melting. Future measurements could lead to improved predictions of volcanic activity. (S. Lamb et al., Nature 547, 84, 2017, doi:10.1038/nature22962 .)