A new type of volcano has been discovered in the western Pacific Ocean. The findings may reduce the strength of a popular theory of "hotspot" volcanism, researchers say.
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.
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.
"I was unbelievably excited to discover this volcanism," Hirano says. "The possibility had been proposed in the past, but had never been adequately documented."
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.
When Hirano's team analysed the trace element geochemistry and compositions of noble gas isotopes in their volcanic flows, they concluded that the material originated in the asthenosphere - a layer of the mantle directly under Earth's crust. Samples of the flows were collected by submersibles.
"These findings are probably the best evidence to date that not all chains of mid-plate volcanoes are formed by plumes," says Marcia McNutt of the Monterey Bay Aquarium Research Institute in Moss Landing, California, US, who wrote a commentary on the paper published in Science. "These findings do not mean that plumes don't exist at all - there may be some volcanic chains that are caused by plumes, but they are no longer a universal explanation."
The question remains whether this new type of volcanism could explain other volcanic hotspots. There are lava fields near Samoa and in Hawaii for which a flexing of the tectonic plate had been suggested as a cause. But these suggestions had not been documented well, and the studies were largely discounted by other researchers, according to Hirano.
Now, in the light of the new research, scientists will go back to observations for other volcano chains with a more critical eye, McNutt believes. "They'll look to see what observations were ignored or swept under the rug because they couldn't explain the finding in the context of plume theory."
