The Japan Academy

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Vol. 94 No. 5 (2018)

Vol. 94 No. 5 (2018)

Why Is Hot Magma Generated in Subduction Zones Where A Cold Plate Subducts?

  In subduction zones like Japanese islands, earthquakes occur and volcanoes are formed. It has long been puzzled for why hot magma is generated and volcanoes are formed in island arcs where a cold oceanic plate subducts. A key process likely to be involved is the corner flow induced by the subducting plate that drags the mantle wedge at its base. Although this is a generally accepted idea, seismological imaging of the corner flow is far behind seismological imaging of the subducting plate.
  Hasegawa (this issue, pp. 217-234) reviews seismic structures of the mantle wedge typically in NE Japan and those in other subduction zones along the Pacific Rim. In most of the regions with good data coverages, the ascending flow of subduction-induced convection has been imaged as a seismic low-velocity layer sub-parallel to the subducting plate. In the top illustration, the cold subducting plate beneath NE Japan is imaged as an inclined high velocity layer (colored in blue). The double seismic zone discovered by the author’s group in 1978 is shown to be two parallel layers of earthquake hypocenters within the cold subducting plate. Hot ascending flow of the subduction-induced convection is imaged as an inclined low velocity layer (colored in red) in the mantle wedge. It reaches the Moho (boundary between the crust and mantle) and resides there, where the partially molten component (magma) accumulates, ascends through the crust and reaches the Earth’s surface as volcanoes. Images of the ascending flow exhibit an along-arc variation with a large amount of melt periodically occurring every ~80 km along the arc above which volcanoes are densely clustered. The bottom illustration schematically summarizes this three-dimensional aspect of arc volcanism.

Yoshio Fukao
Member of the Japan Academy