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Scientists found two merging magma chambers underneath HTHH massive eruption

Nuku'alofa, Tonga


Two merging magma chambers caused the massive eruption of the Hunga-Tonga Hunga Hunga-Ha’apai underwater volcano in 2022, scientists have found in a recent study. Researchers discovered three magma chambers, two of which fed the record-shattering eruption, they say.

Almost two years after the eruption rocked Tonga's HTHH volcano, scientists have finally mapped the huge magma plumbing system that gave birth to the record-breaking blast.

The Hunga Tonga-Hunga Ha'apai explosion was such a force that it triggered the most intense lightning storm ever recorded and the first documented mega tsunami since antiquity, Live Science reported.

The eruption was felt worldwide, but the volcano's underwater setting posed a challenge for scientists trying to understand how such a violent blast occurred.

In a study published on 15 December in the journal Science Advances, researchers have mapped slight variations in the pull of gravity in waters around the island before and after the eruption and found the explosion was likely fed by two magma chambers that merged.

The magma reservoirs sit at different depths between 6,600 and 33,000 feet (2 and 10 kilometres) under the volcano and, likely, stored a high proportion of liquid magma before the 2022 eruption, according to the study.

The researchers found that the blast ejected roughly 30% of the magma, more than 2.1 cubic miles (9 cubic kilometres) from a shallow central chamber, which caused the roof of the volcano to cave in and form a 2,800-foot-deep (850 meters) bowl-like depression called a caldera.

As the pressure in the central reservoir dropped following the explosion, magma stored in a deeper reservoir to the north may have burst through the crust and replenished the central reservoir, opening up a channel between the two chambers. It's also possible that magma from a gas-rich source deeper within Earth's crust rose to the central chamber, which "may also explain the violence of the 2022 eruption," according to the study.

A third pocket of magma, located to the northwest of the central chamber, seems disconnected from the system and might represent "an older, solidifying mush zone," the authors wrote.

Up to 6.2 cubic miles (26 cubic km) of eruptible magma could still lurk in the two main reservoirs beneath the Hunga volcano, according to the study, enough to fill 10 million Olympic-size swimming pools.

While the study revealed what fuelled the volcano, it couldn't show what triggered the massive eruption.

"By themselves the gravity results wouldn't directly allow us to conclude on the eruption trigger," Le Mével said, but they give researchers "an idea of where and how much magma could be stored under the volcano."

The findings may also be limited because they used satellite data that may have been affected by ocean waves and changes in gravity from the seafloor to the sea surface, the researchers noted in the study.

"We only have information about what changed over a one year interval so we cannot say specifically what happened during the eruption," Le Mével said. But the data suggest "new pathways between reservoirs were created," she added.

It's been 900 years since an eruption of this scale last shook the island, Le Mével said, but smaller eruptions occur more frequently.

A previous eruption in 2015 birthed a cone of land connecting two islands, Hunga Tonga and Hunga Ha'apai, which form the only visible part of the 6,600-foot-tall (2,000 m) volcano.

The giant 2022 eruption destroyed the cone and reduced the subaerial Hunga Tonga-Hunga Ha'apai edifice to two thin slivers of land.

Multireservoir magmatic system could reactivate rapidly

The study provides geophysical evidence for a complex, multireservoir system at Hunga volcano.

“Preliminary isotopic measurements and magmatic volatiles analysis from the 2022 eruptive products indicate that different andesitic magma sources mingled both before and during the eruption. The large-scale caldera-forming January 2022 eruption, therefore, would have tapped several parts of the magmatic system. Our finding of multiple, connected magma reservoirs underneath Hunga is consistent with this hypothesis,” the report stated.

The research said that the magma pathways between the two main reservoirs could reactivate rapidly.

“In our interpretive model of the Hunga magma plumbing system, magma pathways between the two reservoirs could form shortly before or during an eruption or may exploit a preexisting feature. On the basis of the short timescales of magma mingling, in the order of minutes, derived from volatile diffusion studies, the formation or reactivation of these pathways could occur rapidly. One of the main changes observed in our 3D density model results after the eruption is the new and increased connection between the central and northern storage zones.”

“This study provides a snapshot of the state of a multireservoir magma system before and after one of Earth’s largest witnessed eruptions. We show that the magmatic system under Hunga volcano is spatially complex with a large amount of magma with high melt content stored in two reservoirs. Even with as much as 66% of the stored magma evacuated or removed during the 2022 eruptive sequence, we estimate that two crustal fluid regions containing up to 85% volume fraction of melt remain. This corresponds to up to 9  and 17 (95% CI [6, 29]) km3, respectively, of remaining eruptible magma still stored in two reservoirs.

“Understanding the conditions and configuration of melt storage under Hunga—depth and volume—is crucial to better assess volcanic hazards in the Tonga archipelago and will eventually help provide a more accurate forecast of the size and likelihood of future eruptions. On the basis of the location and extent of magma reservoirs mapped in this study, future shipborne surveys to collect additional geophysical data should not solely focus over the central, summit caldera but extend on the flanks. This eruption also illustrates the potential risks posed by other unstudied submarine volcanoes,” the report stated.

The work and research was supported by the New Zealand Government Ministry of Business, Innovation and Employment, Strategic Science Investment Fund; The University of Auckland, Faculty of Science Development Research Fund; and The Ministry of Lands and Natural Resources, The Kingdom of Tonga.