New "Petit Spot" Volcanoes

Naoto Hirano at the Tokyo Institute of Technology, Japan, and colleagues have discovered miniature volcanoes - between 0.005 cubic kilometres and 1 km3 in size - near the underwater Japan Trench. These volcanoes, dubbed "petit spot" because of their size, cannot be accounted for by any of the conventional theories of volcanism.

Volcanoes are thought to form in three settings: where tectonic plates are diverging (for instance at mid-ocean ridges); where tectonic plates are converging (in island arcs, for example); and in "hotpots" (a generic term for volcanic activity that cannot be attributed to plate tectonic movements. Hotspots are generally thought to be formed by hot, buoyant plumes rising rapidly from the boundary between Earth's core and the mantle.

The "new" volcanoes, which are actually between one and eight million years old, are not at plate boundaries. But neither were they formed by deep plumes. The team thinks the mini-volcanoes were created when cracks formed in the Earth's crust during the elastic bending of the northwestern Pacific plate, which is diving under the Kuril and Japan trenches. They think partially melted material from the upper mantle squeezed out of the cracks, to form the volcanoes.

(Story credit: Emma Young, New Scientist Online,
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(Journal reference: Science (DOI: 10.1126/science.1128235 and 1131298)