The volcanic eruption in Tonga that triggered a tsunami was hundreds of times more powerful than the atomic bomb the US dropped on Hiroshima during World War II, Nasa has said.
The eruption "obliterated" a volcanic island north of the Tongan capital Nuku'alofa, the US space agency said.
The new land had risen above the water in 2015 and joined two islands, and Garvin and the team have been monitoring changes. The team used satellite observations and surface-based geophysical surveys to track the evolution of the piece of Earth.
The maps show dramatic changes at Hunga Tonga-Hunga Ha‘apai which is the upper part of an underwater volcano.
According to the map, it rises 1.8 kilometres from the seafloor and stretches 20 kilometres across. It is topped by a submarine caldera 5 kilometres in diameter. The island is part of Hunga Caldera’s rim and was the only part of the edifice above water. All new lands now are gone with large chunks of the two older islands.
“This is a preliminary estimate, but we think the amount of energy released by the eruption was equivalent to somewhere between 4 to 18 megatons of TNT,” said Garvin.
He added, “That number is based on how much was removed, how resistant the rock was, and how high the eruption cloud was blown into the atmosphere at a range of velocities.”
Garvin worked with researchers to develop maps of Hunga Tonga-Hunga Ha‘apai above and below the water.
They used high-resolution radar, optical observations, and altimetry. They also used sonar-based bathymetry data.
Nasa researchers, Columbia, the Tongan Geological Service, and the Sea Education Association worked together for six years to determine how the terrain was eroding due to waves and cyclones.
They also noted how wildlife had moved from the ecosystems of Hunga Tonga and Hunga Ha‘apai and colonised the landscapes of the newer land.
Things changed dramatically in January.
The volcanic activity seemed typical enough for the first few weeks of this year. The ongoing eruptions were reshaping the landscape and enlarging the island.
“By early January, our data showed the island had expanded by about 60 per cent compared to before the December activity started,” said Garvin.
He added that “The whole island had been completely covered by a tenth of a cubic kilometre of new ash. All of this was pretty normal, expected behaviour, and very exciting to our team.”
A powerful set of blasts sent ash surging into the stratosphere on January 13-14. On January 15, they launched material as high as 40 kilometres in altitude and possibly as high as 50 kilometres, covering nearby islands with ash and causing tsunami waves.
Most Surtseyan eruptions involve a small amount of water coming into contact with magma.
“If there’s just a little water trickling into the magma, it’s like water hitting a hot frying pan. You get a flash of steam and the water burns burn off quickly,” explained Garvin.
“What happened on the 15th was really different. We don’t know why — because we don’t have any seismometers on Hunga Tonga-Hunga Ha‘apai — but something must have weakened the hard rock in the foundation and caused a partial collapse of the caldera’s northern rim. Think of that as the bottom of the pan dropping out, allowing huge amounts of water to rush into an underground magma chamber at very high temperature,” he added.
Magma’s temperature is usually more than 1,000 degrees Celsius while seawater temperature is close to 20 degrees Celsius. The mixing can be incredibly explosive, especially in the confined space of a magma chamber, he said.
“This was not your standard Surtseyan eruption because of the large amount of water that had to be involved. In fact, some of my colleagues in volcanology think this type of event deserves its own designation. For now, we’re unofficially calling it an ‘ultra Surtseyan’ eruption'," Gavin said.
Watching the birth and evolution of a “Surtseyan island” is "fascinating" for Gavin because there have not been many examples in the modern era. Most new Surtseyan islands get eroded away within a few months or years.
He was interested because it may teach us about Mars. “Small volcanic islands, freshly made, evolving rapidly, are windows in the role of surface waters on Mars and how they may have affected similar small volcanic landforms,” he said.
“We actually see fields of similar-looking features on Mars in several regions.”
Published : May 29, 2022
Published : January 26, 2022
By : THE NATION