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Volcanoes slide silently to their death

On the slide One of the largest trenches in the Earth's crust is slowly consuming undersea volcanoes, and to the surprise of geologists, it appears to lessen the risk of earthquakes.

The Tonga Trench is a feisty, earthquake-prone fault zone running between Tonga and Samoa. Earthquakes and resulting tsunamis are a concern here, just as they are along the Japan Trench and the even deeper Mariana Trench to the south near Guam.

At a whopping 10.9 kilometres deep in some areas, the Tonga Trench is the second-deepest submarine canyon in the world. It marks the boundary where a westward-moving chunk of the earth's outer crust, the Pacific plate, is forced downward, beneath the adjacent Indo-Australian plate, along a tectonic subduction zone.

Geologists long assumed that the destruction of giant volcanoes along subduction zones might add to the earthquake risk. But Professor Tony Watts of the University of Oxford and his colleagues noticed recently that the opposite might be true.

Right at the spot where giant volcanoes are sliding to their doom, the Tonga Trench is surprisingly, seismically quiet. This past summer the researchers scanned the seafloor with sonar to find out why.

At a recent meeting of the American Geophysical Union, they shared with journalists a stunning visual analysis of volcanoes now on the verge of entering the trench.

The images show dozens of giant, flat-topped old volcanoes, called the Louisville Ridge Seamounts, riding westward atop the Pacific tectonic plate at up to six centimetres per year. When they reach the trench, they are dragged down into the chasm.

Previously it was thought that the volcanoes would add friction to the movement of the two plates, leading to a greater build-up of strain and, consequently, to more violent earthquakes in that spot.

But Watts and his team could see that the doomed seamounts are already highly fractured. This may offer a possible reason why, contrary to expectations, this region along the trench is virtually earthquake free.

The team's new hypothesis is that by breaking up early on, the volcanoes probably provide a kind of buffer that eases the subduction process, and actually reduces the risk of large, tsunami-generating earthquakes.

The ultimate fate of the volcanoes is still unclear. Watts and his colleagues still don't know whether the fractured volcanoes will be scraped off onto the Indo-Australian plate or chewed up and carried down deep into the mantle.