BioEd Online

A cluster of tiny underwater volcanoes off the northeastern coast of Japan has demonstrated that the Earth's inner mantle may not be as solid as was once thought. In a study published online this week by Science¹, a team of researchers provides evidence that regions of the mantle contain molten material that can leak out on to the surface through cracks in the plate above.

Most volcanoes on the planet arise where tectonic plates either part or collide as around the 'Pacific ring of fire', for example, where volcanoes sprout up on the edges of the Pacific Ocean plate.

Other volcanoes, such as Hawaii, poke through the middle of a plate thanks to hot material seeping up from deep below. The prevailing theory is that these volcanoes are fed by a 'plume' of heat coming from so deep within the planet that the hot spot stays relatively still while the tectonic plate slides past above it, creating a chain of volcanic islands on the surface.

Now researchers have investigated, in depth, a class of tiny volcanoes that pepper the seafloor, which they suspect may be created a different way.

Clues of the volcanoes' existence came a few years ago when a robotic submarine recovered young basalt rock, which is characteristic of volcanoes, in an area some 600 kilometres from where the Pacific plate dips below Japan not close enough to be characterized as a volcano fed by a plate boundary².

To learn more, Naoto Hirano of the Tokyo Institute of Technology, Japan, and his colleagues deployed the 26-ton, three-person submersible, SHINKAI (Japanese for 'deep sea'), to the region. After descending nearly 6 kilometres below sea level, the crew of the SHINKAI traversed lava fields littered with large black lava rocks to travel up and down the sides of the volcanoes, which were about 1 kilometre in diameter, taking samples of rock as they went.

Cracked up

Analysis of the rock composition and the structural features of the nearby crust suggested that the lava source was molten material from the inner-mantle region called the asthenosphere the 'plastic' part of the mantle, some 90-400 metres below the ground, upon which tectonic plates move about.

The researchers point out that the Pacific plate bends sharply near where the mini-volcanoes are found, as it dips down under Japan. Perhaps this stress caused nearby areas of the plate to crack, they suggest, allowing hot asthenosphere material to seep up.

"If our hypothesis is correct," says Stephanie Ingle, a researcher at the University of Hawaii in Honolulu and co-author of the study, "then we should see this type of volcano in lots of places on the Earth, particularly near subduction zones with similar plate flexing." They note that lavas from some Hawaiian volcanoes, as well as some western Samoan islands, carry a similar chemical composition to the mini Japanese volcanoes.

Godfrey Fitton, a professor of geosciences at the University of Edinburgh, UK, adds that there are tens of thousands of small volcanoes on the Pacific seafloor, some along the spreading zone where new plate is being formed and volcanism would be expected, but many others elsewhere. "Sometimes you see these little pimples," he says. It seems obvious, he adds, that these aren't on a plate boundary and are too small to be made by 'plumes', so something else must be responsible.

Empty pot

Scientists proposed decades ago that parts of the asthenosphere may be molten, but they ran into trouble explaining how such a region could sustain itself. "We started to make calculations and found that if it was really molten, the melts would creep upwards and it would dry up very quickly," says Guust Nolet, professor of geosciences at Princeton University, New Jersey. "If it leaks, at some point you reach the stage at which the low-melting-point rock is depleted, and there's nothing left that can melt."

So scientists largely turned away from the molten-asthenosphere hypothesis, says Nolet. But the new data from Hirano and his colleagues, he adds, cause the old question to resurface: how does the asthenosphere survive if it is constantly leaking?

One possible explanation is that these areas are fed by molten material created by a deep-sourced plume of heat, says Nolet. A supply of material from deeper in the mantle could prevent the asthenosphere from slowly wasting away.

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