Immense crater hole created in Tonga volcano

Researchers have just finished mapping the mouth of the underwater Tongan volcano that, on 15 January, produced Earth's biggest atmospheric explosion in over a century.

The caldera of Hunga-Tonga Hunga-Ha'apai is now 4km (2.5 miles) wide and drops to a base 850m below sea level.

Before the catastrophic eruption, the base was at a depth of about 150m.

It drives home the scale of the volume of material ejected by the volcano - at least 6.5 cubic km of ash and rock.

"If all of Tongatapu, the main island of Tonga, was scraped to sea level, it would fill only two-thirds of the caldera," Prof Shane Cronin from the University of Auckland, New Zealand, said.

Prof Cronin has spent the past two and a half months in the Pacific kingdom, seconded to its geological services department.

Their report, issued on Tuesday, assesses the eruption and makes recommendations for future resilience.

Although Hunga-Tonga Hunga-Ha'apai (HTHH) is unlikely to give a repeat performance for many hundreds of years, there are at least 10 volcanic seamounts in the wider region of the south-west Pacific that could produce something similar on a shorter timescale.

New Zealand's National Institute for Water and Atmospheric (NIWA) Research released its bathymetry (depth) map for the area immediately around the volcano, on Monday.

But the agency has yet to take soundings directly over the top of HTHH.

So Prof Cronin and colleagues' data literally fills a hole in the NIWA survey.

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