Undersea mountains march into the abyss

By David Shukman Environment & science correspondent, BBC News

Sonar images of the Tonga Trench. Video courtesy of the Universities of Oxford and Durham and NERC

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Startling new images from the depths of the Pacific Ocean reveal one of Earth's most violent processes: the destruction of massive underwater mountains.

The pictures were created by sonar in waters up to 6km (4mi) deep.
They expose how tectonic action is dragging giant volcanoes into a chasm in the seabed.
The volcanoes are strung across several thousand kilometres of ocean floor and are moving westward on the Pacific tectonic plate at up to 6cm per year.
The extraordinary scene was captured along the Tonga Trench during a research expedition last summer.
The trench is a highly active fault line running north from New Zealand towards Tonga and Samoa.
The first images have been released to BBC News as the findings are presented to the annual conference of the American Geophysical Union.
They are the result of a joint project by the universities of Oxford and Durham, funded by the Natural Environment Research Council.
Into the abyss Where the Pacific plate collides with the Indo-Australian plate, it is forced downwards into the trench, a subduction zone, and the volcanoes are carried with it.
The trench, reaching a depth of 10.9km, forms the second deepest stretch of seabed anywhere in the world - easily large enough to hold Mount Everest.

One image shows the volcano nearest the edge of the abyss - the next to be destroyed - already starting to collapse.
With frequent earthquakes, the region is vulnerable to tsunamis and one aim of the research is to understand whether the destruction of the volcanoes adds to the risk.
One theory is that the volcanoes add friction to the movement of the two plates which leads to a greater build-up of tension and consequently to a more explosive quake.
Another is that by shearing into blocks as they collapse, the volcanoes provide a kind of buffer easing the subduction process.
Earthquake puzzle Professor Tony Watts of Oxford University, joint leader of the project, says that earthquakes are less frequent at the precise point where the volcanoes enter the trench.
"When you see size of these features you'd think they'd cause massive earthquakes and disruption - and that was our starting hypothesis."
"But we found that the volcanoes were highly fractured before they entered the trench - which is very important for what happens after they enter the system.
Analysis so far has not determined the precise impact of this process.

Professor Watts says the key question still isn't settled: "Are they added to the Australian plate or are they carried down in fragments into the deep earth mantle?"