Scientists say they are stunned by what they have learned about the ferocity of Tonga’s volcanic eruption in January.
When the seamount exploded, it sent ash and water vapour halfway through space, generating tsunami waves worldwide.
A survey by New Zealand and British vessels have fully mapped the area around the Pacific volcano.
It shows that the seabed has been eroded and carved by violent debris flows over more than 80 km (50 miles).
The Hunga-Tonga Hunga-Ha’apai seamount mapping exercise was led by New Zealand’s National Institute for Water and Atmospheric Research (Niwa).
Data collected indicates that at least 9.5 km3, possibly as much as 10 km3, of material was moved during the cataclysmic event. This is a volume equivalent to something approaching 4,000 Egyptian pyramids.
Two-thirds were ash and rock ejected from the volcano’s caldera or opening.
“You can think of it as ‘a shotgun blast’ straight into the sky,” said marine geologist and Niwa project director Dr Kevin Mackay.
“Some of this material even went beyond the stratosphere into the mesosphere (57 km altitude) – the highest recorded eruption column in human history,” he said. told BBC News.
The other third was made up of material discarded on the top and sides of Hunga-Tonga as the debris fell to sweep the ocean floor.
This transport took the form of pyroclastic density currents, and avalanches of hot, tumultuous rock. In the water, their scorching heat would have enveloped them in a frictionless cushion of steam on which they could run and run at very high speeds.
Survey work has tracked flows that have even managed to ascend and cross altitudes of several hundred meters.
This explains, for example, the loss of the submarine cable linking Tonga to the global internet. Much of it has been cut off from this data link despite being 50 km south of Hunga-Tonga and beyond a large hill on the seabed.
“Where you had these flows, there’s nothing alive now. It’s like a desert 70km from the volcano,” Dr Mackay said. “And yet, surprisingly, you find life just below the volcano’s edge, in places that avoided those density currents. You find sponges. They dodged a bullet.”
Pyroclastic flows also have a role in the history of the Hunga-Tonga tsunami.
Waves have been recorded across the Pacific and other ocean basins in the Atlantic and even in the Mediterranean Sea.
Niwa’s team says there were four ways the water was moved to generate these tsunamis: by density fluxes pushing the water out of the way, by the explosive force of the eruption also pushing on the water, following the dramatic collapse of the caldera floor (it fell 700 m), and by atmospheric blast pressure waves acting on the sea surface
