For years, the Hunga Tonga-Hunga Ha’apai volcano protruded from the waves, forming a duo of narrow rocky isles – one called Hunga Tong and the other Hunga Ha’apai. From late 2014 to early 2015, an eruption built up a third island that later married the pair of islands, forming a single giant landmass. And when the volcano awoke from its deep slumber in December 2021, volcanic activity seemed typical enough, according to NASA scientists. Throughout the weeks heading into 2022, intermittent and small explosions gently rocked the island, spewing volcanic rock and ash that added 45% more land.
But on January 14, the beast was truly awoken. Extraordinarily powerful blasts sent ash billowing into the stratosphere. Then came the cataclysmic eruption on January 15. NASA estimated the explosion to amount to somewhere between 4 to 18 million tons of TNT – hundreds of times the equivalent mechanical energy of the Hiroshima nuclear explosion. The blast sent a tsunami darting across the Pacific Ocean. A sonic boom zipped around the world, twice. A plume of ash and gas shot into the stratosphere at an altitude of almost 31 kilometres (19 miles).
A global group that monitors atomic testing called Tonga’s blast the “biggest thing that we’ve ever seen”. Now, according to satellite images, the two once-conjoined but now separated islands of Hunga Tonga and Hunga Ha’apai betray the beast lurking beneath the waves. And scientists are racing to fathom the fury of Tonga’s ferocious volcano, which is still shrouded in a thick blanket of mystery.
Plumbing the blast’s origins.
In a new study published in Lithos, scientists uncovered some clues to what primed the mechanisms for such a remarkable eruption, by studying Tonga’s volcanic past.
The chemistry of rocks that cooled and solidified from lava in previous eruptions may give away pieces of the puzzle as to what made this eruption so powerful. As a magma system cools, crystals of different minerals form at different rates, which gives rise to a panoply of different rock chemistries. And like tree rings, the solidified crystals safeguard these variations.
By analysing the rings of crystals in the rocks, volcanologists from the study found that the volcano’s magma reservoir takes many centuries to refill. But when the volcano unleashes a catastrophic but rare eruption, just like the recent outburst, it’s the result of an intense and rapid emptying of that magma chamber. Like a ticking timebomb, the explosion had to have a trigger. And the researchers have a hypothesis for that.
As time goes by, more and more water dissolves in the magma and bubbles out as steam, and the pressure within the volcano system gets increasingly intense. The structure of the volcano slowly expands under pressure, generating fractures in the rocky cap. Situated at a “Goldilocks depth” of about 200 metres (650 feet), here’s when plenty of seawater is allowed to penetrate the volcano and encounter the magma, cooking up a recipe for disaster. What ensues is the instant vaporisation of an enormous volume of water into gas, blasting the magma apart. And like a violent chain reaction, the first explosion clears the way for even more magma to interact with seawater, all while the expansive reservoir of magma dramatically depressurises and gushes into the sea.
Decoding the blast with chemistry.
To validate the hypothesis, researchers will need to get their hands on the ashes produced before and after the calamitous event. The ash particles’ chemical and textural signatures serve as a window into the explosion’s trigger. If found to be minute and decorated with many tiny fractures, then the ashes are likely to originate from a supercharged volcanic explosion.
For now, Hunga Tonga-Hunga Ha’apai has returned to quiescence and restorative efforts are heavily underway. Experts agree that decoding the science behind this eruption’s devastating features could greatly enhance monitoring activities and save many lives in the future – and international collaboration is crucial to achieving this scientific goal.
