The volcanoes are fed by molten rock rising within a wedge of the Earth's mantle above the subducting plate. Although geologists have a pretty good picture of the processes that produce volcanic arcs, a new study finds that the structure of the mantle wedge may be far more complex than anyone had imagined.
"Geology textbooks show simple cartoons of the processes happening in these mantle wedges--a sinking slab and some melting that comes up in volcanoes--but our results suggest that those cartoons are grossly inadequate," said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz.
Lay is a coauthor of a paper describing the new findings published this week in the Online Early Edition of the journal Science. The first author of the paper is Yingcai Zheng, a UCSC graduate student working with Lay, and the other coauthors are Megan Flanagan of Lawrence Livermore National Laboratory and Quentin Williams, professor of Earth and planetary sciences at UCSC.
The researchers used a seismic imaging technique (reflection seismology) to detect layered structures within the mantle wedge of the Tonga subduction zone in the southwestern Pacific. Analyzing data from deep earthquakes that occurred beneath the subduction zone, they looked for seismic waves that traveled upward into the mantle wedge, reflected from the underside of layers within the mantle, and were recorded by seismic sensors in distant locations.
"We were stunned to find many reflecting boundaries in the mantle wedge above the sinking slab, and these are laterally extensive throughout the wedge region," Lay said. "This is surprising because the textbook version of mantle wedges suggests that there would be little structure."
The researchers attributed these unexpected features to the effects of water and other materials that are squeezed out of the subducting slab and rise up through the mantle wedge. When water is added to hot mantle rock, one of the main effects is to lower the melting temperature of the rocks. The resulting pockets of molten rock rise up through the mantle and feed volcanoes at the surface.
"That all happens in the upper 100 kilometers or so and has been well understood for at least a couple of decades. But we're seeing structure much deeper down, at depths as much as 450 kilometers," Lay said. "We think the fluids don't come out all at once, but are released progressively as the pressure increases with depth and then have to percolate up through the overlying wedge."
Whereas the added water causes mantle rock to melt in the upper layers, the researchers said different effects are likely to occur at greater depths. Under the intense pressures found at depth, added fluids would cause changes in the composition of the mantle rock, and structures composed of these altered minerals within the mantle wedge would be detectable by their altered seismic reflectivity. The detection of layered structures was attributed to the mantle wedge being progressively flushed with fluids expelled from the sinking slab, producing not only melts that rise in volcanoes but also mineralogical structures with seismic reflectivity.
"This has many implications for how volcanic arcs evolve and how they produce the thick piles of remelted rocks that eventually add to the continents," Lay said.
Zheng noted that some of the reflecting structures he detected extend far to the west, away from the trench where the subducting slab currently dives down beneath the overriding plate.
"If those reflectors to the west were created in the subduction process by infiltration of fluids from the slab, they might represent the historical past of the Tonga subduction zone," Zheng said.
The study used methods similar to those used in oil exploration, in which sensors record reflections of seismic waves from explosions or vibrations with shallow manmade sources at or near the surface. To study the mantle wedge, the researchers essentially turned this approach upside down, using deep earthquakes as the energy sources and looking for reflections of seismic waves traveling up through the wedge. The Tonga subduction zone is a good place to use this technique because of the frequency of deep earthquakes in this dynamic region.
"An earthquake is like a flash of lightning in the dark Earth interior. We used multiple earthquakes of different mechanisms, each illuminating in a different angle, to achieve a clear three-dimensional picture of the mantle structures," Zheng said.
Tim Stephens | EurekAlert!
Oasis of life in the ice-covered central Arctic
24.10.2016 | Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung
Receding glaciers in Bolivia leave communities at risk
20.10.2016 | European Geosciences Union
Terahertz excitation of selected crystal vibrations leads to an effective magnetic field that drives coherent spin motion
Controlling functional properties by light is one of the grand goals in modern condensed matter physics and materials science. A new study now demonstrates how...
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.
"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...
COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.
In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
24.10.2016 | Earth Sciences
24.10.2016 | Life Sciences
24.10.2016 | Physics and Astronomy