Tonga's strange volcanic eruption was even more massive than we knew

The ferocious 2021 explosion blew out 2.3 cubic miles of rock, unleashing a 35-mile-high plume and a global tsunami that sent scientists racing to understand the blast. Now they're finally putting together the pieces.

By Maya Wei-Haas
Published 21 Nov 2022, 10:36 GMT
In December 2021, a volcano in the Kingdom of Tonga, known as Hunga Tonga-Hunga Ha'apai, began ...
In December 2021, a volcano in the Kingdom of Tonga, known as Hunga Tonga-Hunga Ha'apai, began erupting in an event that culminated in an explosion so powerful it sent atmospheric ripples circling the planet multiple times. The eruption destroyed much of the volcanic peak that poked above the waves, shown here.
Photograph by Maxar via Getty Images

Crimson hues flushed across the early morning skies over the Kingdom of Tonga as Grace Frontin-Rollet spotted a pair of small rocky islands from the bow of the RV Tangaroa. Though the scene was picturesque, a tinge of sulphur in the air reminded the marine geologist what she and a team of scientists had travelled for six days over rough waters to see. In the expansive gap between the two bits of land, hidden on the ocean floor, lay the crater of a massive volcano that erupted just months before in one of the largest and strangest blasts ever seen.

Photograph by Maxar via Getty Images

"I don't think the scale of what had happened hit us until we reached the site," says Frontin-Rollet, who is from New Zealand’s National Institute for Water and Atmospheric Research (NIWA).

In December 2021, the volcano—called Hunga Tonga-Hunga Ha'apai after the two islands that sit on its rim—awoke in a series of tantrums that turned into outright turmoil on January 15, 2022. The peak unleashed a blast so loud it was heard in Alaska, some 6,000 miles away. But much of what happened that day has remained a mystery, until now. Scientists, including the team aboard the RV Tangaroa, are finally putting together the pieces, and the picture that has emerged is mind-boggling.

As the team announced today, recent surveys of the seafloor suggest that the blast excavated about 2.3 cubic miles of rock. If confirmed, the eruption would be the largest recorded in the last century, surpassing the 1991 blast at Mount Pinatubo.

Other recent analyses reveal even more record-breaking measures. The blast jettisoned a plume of searing hot gas and ash 35.4 miles into the sky, higher than ever seen before. It injected an unprecedented 146 teragrams of vaporised water into the atmosphere, which some speculate might result in a slight, temporary warming of the climate. And it sparked a tsunami that surprised scientists when it travelled around the world.

"It's just a massive event," says Kevin MacKay, a marine geologist at NIWA who was also on the RV Tangaroa. "The more we study it, the bigger the event becomes."

Understanding the explosive peak's many effects is far from just scientific curiosity. Many similar submarine volcanoes lurk offshore coastlines around the world. Most that have been identified are not monitored—and even more are yet to be discovered.

"We along with other nations of the Pacific Ring of Fire know only too well how at mercy we are to nature," says Taaniela Kula, Tonga's Deputy Secretary for Natural Resources, in a May press conference. The eruption, he says, "is a reminder that there's always more to learn about the giants of our home planet Earth."

Window into the deep

An ancient tectonic battle between the Pacific and Indo-Australian plates birthed a line of volcanoes in the South Pacific Ocean, including the mighty Hunga Tonga-Hunga Ha'apai. Today, the only parts of the volcano that poke above the sea are the two little islands that signalled to Fontin-Rollet that the team had arrived. She describes the sobering realisation that their nearly 230-foot ship was tiny compared to the geologic beast that lay hidden under the water before them.

This shroud of water has made it challenging for scientists to learn what happened during the tumultuous eruption last January. Whenever magma and water mix, billowing steam forms and thus big eruptions ensue. But what sparked the surprising cascade of events at Hunga Tonga-Hunga Ha'apai? To find answers scientists had to get a closer look.

This photograph, taken by an astronaut on board the International Space Station, shows clouds of ash lingering in the atmosphere a day after the intense explosion of Hunga Tonga-Hunga Ha'apai.
Photograph by NASA

In April, the RV Tangaroa arrived on site in the first leg of a two-part survey conducted as a collaboration between NIWA and The Nippon Foundation called Tonga Eruption Seabed Mapping Project (TESMaP). The expedition was followed in August by SEA-KIT International's uncrewed vessel Maxlimer, which was remotely operated by a team in the U.K.

The research efforts scrutinised the region, mapping the seafloor, snapping pictures and taking video, analysing the water column, and collecting samples of volcanic rock and ash.

In total, the team mapped almost 8,500 square miles around the volcano, discovering much of the area devoid of life, blanketed in a ghostly white layer of fine sediments. Rocky cores collected in these zones reveal the source of devastation: a deadly fast-moving avalanche of hot ash and volcanic rubble called a pyroclastic flow that forms from the collapse of the rising plume of ash and gas.

Scientists can only speculate about the dynamics of pyroclastic flows under the water, MacKay cautions, because none have been witnessed in person. It’s theorised that as the debris plunges into the sea, the hot ash may vaporise the water to form a gas layer that helps propel the tongue of material across the ocean floor.

Evidence of multiple pulses of pyroclastic flows radiated around the caldera's rim. The volcanic avalanches travelled up over rises and into valleys, extending to the edge of the team's survey area some 50 miles away, hinting debris perhaps travelled even further.

The pyroclastic flows likely were also behind the severing of both domestic and international communication lines for Tonga, which hindered initial recovery efforts. Modelling conducted by NIWA's Emily Lane suggests that volcanic material poured into a valley that housed one of the cables, ricocheting off the walls of the depression. Such behaviour could help explain how a broken fragment of the cable was dragged northward back toward the peak.

The latest research also documented that the volcano’s blast excavated down 2,300 feet of rock in the central crater. That confirms results from an earlier survey in May by volcanologist Shane Cronin from the University of Auckland in New Zealand and a team from Tonga Geological Services. "There's a huge hole in the ground where it wasn't before," says Richard Wysoczanski, a marine geologist with NIWA. "It's quite spectacular."

Three quarters of the excavated and erupted rock appears to have landed within 12 miles of the volcano. Much of the remaining material likely circulated in the atmosphere as dust for months, intensifying the colours of sunrises and sunsets, MacKay says. The same phenomenon happened after the 1883 eruption of Krakatoa, and likely inspired the red skies in Edvard Munch’s painting “The Scream.”

While the eruption has quieted, the volcano hasn't fully returned to its slumber: Now it’s piping out hot water laced with fragments of volcanic glass. "It's not completely dead," Wysoczanski says, but adds that another big eruption in the near future is unlikely.

View from the sky

Scientists were surprised to find that the blast left the volcanic slopes intact. All the eruptive energy seems to have been directed straight up to the sky, says MacKay. This oddity may help explain another remarkable feature of the eruption: plume height.

When the volcano let loose, gas and ash billowed skyward, captured by Earth-orbiting satellites. Just how high did the plume travel? When Simon Proud, an expert in satellite remote sensing at the UK's National Centre for Earth Observation, worked through the calculations, he initially stared at his numbers in disbelief. A double check of his work confirmed the improbable figure: The plume shot 35.4 miles high into a layer of atmosphere called the mesosphere, a zone where most planes can't fly and shooting stars light night skies.

"We've never seen anything get up anywhere near this high before," says Proud, lead author of the paper on the plume height. "It really was quite breathtaking."

As the terrifying plume spread over the island nation, tsunami waves began crashing on nearby shores; by some estimates, swells were more than 50 feet high. To scientists' surprise, the disturbance spread to oceans around the globe, causing sea levels to rise by a foot in the Mediterranean Sea on the opposite side of the world.

A multitude of events conspired to produce such an unusual tsunami. Close to the volcano, factors like the collapse of the caldera floor and pyroclastic flows stirred the turbulent sloshing of the sea. The blast also caused sharp drops in the air pressure, "pumping energy into the tsunami," says NIWA's Lane, who is an expert in the violent phenomena.

Far from the volcano, a slightly different process was at work. The enormous volcanic plume bursting into the sky shoved aside the atmosphere, sending ripples racing around the world four times over six days. "The explosion was so large that it started making the atmosphere oscillate," says Quentin Brissaud, a geophysicist with the Norwegian Seismic Array and an author on a paper describing the eruption's atmospheric effects.

As they zipped around the globe, these ripples disturbed the ocean surface in what's known as a meteo-tsunami. The only other time this has been recorded was during the 1883 explosion of Krakatau, one of the most powerful and deadly volcanic eruptions in recorded history.

Still, many questions remain unanswered about Hunga Tonga-Hunga Ha'apai, including exactly what triggered the volcano's ferocious bellow. 

As Wysoczanski says, "The science is just starting."


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