Volcanoes dragged into chasm in seabed by tectonic action

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New sonar images deep in the Pacific Ocean reveal huge volcanoes being slowly dragged one-by-one into a great ravine in the seabed.

The extraordinary images were captured along the Tonga Trench, which runs north from New Zealand towards Tonga and Samoa, during a research expedition last summer.

One aim of the research is to understand if the destruction of these volcanoes adds to the risks of tsunamis in the region.

The BBC's David Shukman says a better understanding of these activities could act as an early warning signal.

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