Scientists say they are baffled by what they have learned about the ferocity of the Tonga volcano eruption in January.
When the underwater mountain blew its top, it sent ash and water vapor halfway into space, sending tsunami waves around the world.
A survey by New Zealand and British ships has now fully mapped the area around the Pacific volcano.
It shows that the seabed was scoured and sculpted by violent debris flows over a distance of more than 80 km (50 mi).
The Hunga-Tonga Hunga-Ha’apai seamount mapping exercise was led by NZ’s National Institute of Water and Atmospheric Research (Niwa).
The data collected indicates that at least 9.5 cubic kilometers, perhaps as much as 10 cubic kilometers, of material was moved during the cataclysmic event. This is a volume equivalent to about 4,000 Egyptian pyramids.
Two-thirds of that was the ash and rock thrown out through the volcano’s caldera, or vent.
“You can think of it as ‘a shotgun blast’ straight up into the sky,” says marine geologist and Niwa project director Dr Kevin Mackay. “Some of that material even went beyond the stratosphere to the mesosphere (57 km altitude) – the highest recorded eruption column in human history,” he told BBC News.
The other third was material scraped off the top and sides of Hunga-Tonga as the debris fell back and swept the ocean floor.
This transport took the form of pyroclastic density currents, which are avalanches of tumbling, scorching rocks. In the water, their scorching heat would have enveloped them in a frictionless cushion of steam on which they could simply run and run at very high speed.
The research work followed streams that even managed to reach heights of several hundred meters.
This explains, for example, the disappearance of the submarine cable that connects Tonga to the global internet. Much was cut from this data link despite being 50 km south of Hunga-Tonga and beyond a large mound on the sea floor.
“Where you had these streams, nothing lives there today. It’s like a desert 70 km from the volcano,” said Dr. Mackay. “And yet, amazingly, just below the rim of the volcano, in places that avoided these density currents, you find life. You find sponges. They dodged a bullet.”
The pyroclastic flows also play a role in the Hunga-Tonga tsunami story.
Waves were recorded across the Pacific, as well as in other ocean basins – in the Atlantic and even the Mediterranean.
The Niwa team says there were essentially four ways water was moved to generate these tsunamis: by the density currents pushing the water out of the way; by the explosive force of the eruption also pressing on the water; due to the dramatic collapse of the caldera floor (it dropped by 700 meters); and by pressure waves from the atmospheric explosion acting on the sea surface.
At certain stages during the eruption, these mechanisms probably worked together.
A good example is the largest wave that hit the main island of Tonga, Tongatapu, 65 km south of Hunga-Tonga. This happened just over 45 minutes after the first major eruption. A meter-high wall of water washed over the Kanokupolu peninsula, destroying beach resorts in the process.
Dr. Niwa natural hazards specialist Emily Lane believes an anomaly in atmospheric pressure has increased the height of the tsunami waves.
“For the big local waves — to understand them correctly, I believe you have to have this atmospheric coupling as well,” she explained. “We had a huge pressure anomaly that in itself would have caused a tsunami. So if you already have waves, you just add energy to them.”
The Niwa survey, formally named the Tonga Eruption Seabed Mapping Project (TESMaP), was conducted in two parts.
The first phase, which mapped and sampled the seafloor around the volcano, was conducted from the New Zealand research vessel (RV) Tangaroa.
The second leg, directly above the mountain, was entrusted to the British robotic boat USV Maxlimer. Operated by Sea-Kit International from a control room 10,000 miles away in Tollesbury, UK, this unmanned vehicle was able to identify ongoing, if relatively subdued, volcanic activity. The boat did this by tracing a persistent layer of glassy ash in the caldera back to its source – a new vent cone some 200 meters underwater.
Remarkably, only six people died during the January 15 event, and two of those were in Peru. It could have been much worse.
All TESMaP results will eventually be used for risk mitigation, preparing Pacific countries close to the volcanic zone that runs from New Zealand’s North Island to Samoa. They now know better where to build infrastructure and how to protect it; and, most importantly, to appreciate the magnitude of the risk they face.
“We have always underestimated submarine volcanoes,” said Taaniela Kula of Tonga Geological Services. “There are five more near Tongatapu. It means we need more planning and urgent planning.”
TESMaP was funded by the Nippon Foundation of Japan and organized with the help of Seabed2030, an international effort to properly map the Earth’s ocean floor.