Scientists say they are stunned by what they have learned about the intensity of Tonga’s January volcanic eruption.
When the underwater mountain blew its top, it sent ash and water vapor halfway into space, creating 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 sea floor has been eroded and shaped by violent debris flows for a distance of over 80 km (50 miles).
The Hunga-Tonga Hunga-Ha’apai seamount mapping exercise was led by New Zealand’s National Institute of Water and Atmospheric Research (Niwa).
The data collected shows that at least 9.5 cubic kilometers, perhaps as much as 10 cubic kilometers, of material were displaced during the catastrophic event. This is a volume equivalent to approximately 4,000 Egyptian pyramids.
Two-thirds of this was ash and rock ejected through the volcano’s caldera, or vent.
“You can think of it like a shotgun blast straight into the sky,” said marine geologist and Niwa project leader Dr. Kevin Mackay. “Some of this material even went beyond the stratosphere into the mesosphere (57km altitude) – the tallest recorded eruptive column in human history,” he told BBC News.
The other third consisted of material scraped from the top and sides of Hunga-Tonga as debris fell back and swept across the sea floor.
This transport took the form of pyroclastic density currents, which are avalanches of falling, scorching rock. In the water, their searing heat would have enveloped them in a frictionless cushion of steam that they could just walk and walk on at very high speeds.
The surveying work tracked currents that even managed to overcome heights of several hundred meters.
This explains, for example, the failure of the undersea cable connecting Tonga to the global internet. Much of that data link was cut off, despite being 50km south of Hunga-Tonga and behind a large hill on the seabed.
“Where you had those rivers, there’s nothing living today. It’s like a desert 70km from the volcano,” said Dr. Mackay. “And yet, amazingly, just below the volcano’s rim, in places that have avoided these dense currents, life is found. You can find sponges. You dodged a bullet.”
The pyroclastic flows also play a role in the tsunami history of Hunga-Tonga.
Waves have been recorded across the Pacific, but also in other ocean basins—in the Atlantic and even the Mediterranean.
The Niwa team says there were four main ways water was displaced to create these tsunamis: by the density currents, which pushed the water out of the way; due to the explosive power of the eruption also pressure on the water; as a result of the dramatic collapse of the caldera floor (it has sunk 700 m); and by pressure waves from the atmospheric explosion impacting the sea surface.
At certain stages during the eruption, these mechanisms likely worked together.
A good example is the largest wave that hit Tonga’s main island, Tongatapu, 40 miles (65 km) south of Hunga-Tonga. This happened just over 45 minutes after the first major eruptive explosion. A wall of water several meters high swept over the Kanokupolu Peninsula, destroying beach resorts in the process.
dr Emily Lane, a natural hazards expert in Niwa, believes an anomaly in atmospheric pressure increased the height of the tsunami waves.
“For the large local waves — to get them right, I think you also have to have that atmospheric coupling,” she explained. “We had a huge pressure anomaly that would have produced a tsunami on its own. So if you already have waves, just add energy to them.”
The Niwa survey, officially known as the Tonga Eruption Seabed Mapping Project (TESMaP), was conducted in two parts.
The first stage, mapping and sampling the seafloor around the volcano, was carried out from the New Zealand research vessel (RV) Tangaroa.
The British robot boat USV Maxlimer took over the second stage directly above the mountain. This unmanned vehicle, operated by Sea-Kit International from a control room 10,000 miles away in Tollesbury, UK, was able to identify sustained, albeit relatively muted, volcanic activity. The boat did this by tracing a stubborn layer of glassy ash in the caldera back to its source – a new vent cone about 200m underwater.
It is noteworthy that only six people died in the January 15 event, and two of them in Peru. It could have been a lot worse.
All of the results from TESMaP will ultimately feed into hazard mitigation by preparing Pacific nations located near the volcanic zone that stretches from New Zealand’s North Island to Samoa. They will now know better where to build infrastructure and how to protect it; and most importantly, to assess the level of risk they are exposed to.
“We’ve always underestimated submarine volcanoes,” said Taaniela Kula of Tonga Geological Services. “There are five others near Tongatapu. That means we need more planning and more urgent planning.”
TESMaP was funded by the Nippon Foundation of Japan and organized with help from Seabed2030, an international initiative to accurately map the Earth’s seabed.