Abstract
The 15 January 2022 eruption of Hunga caldera volcano was the most explosive volcanic event in the last 140 years, yet the erupted composition was similar to historical smaller-scale eruption episodes at this center. By analyzing the petrology and geochemistry of the full pyroclastic sequence, we infer the presence of two distinct magma storage zones. Early Surtseyan eruptions (29 December 2021 to 14 January 2022) tapped a resident shallow magma (A2) similar to the 2009 and 2014–2015 episodes and were of similar magnitude. A sudden increase in eruption magnitude over 16 h on 13–14 January 2022 was accompanied by the first arrival of a slightly more primitive and gas-rich magma (A1), along with sudden vent-subsidence below sea-level. Rapid magma withdrawal, subsidence and deep-fracture formation destabilized the upper magmatic system and allowed vesiculation and seawater penetration before the climactic eruption began ∼6 h later. There is no evidence of deep mafic recharge and instead plagioclase rims record a sudden decompression of A1 magma as it moved into the emptying shallow A2 reservoir. The catastrophic 15 January 2022 eruption was thus triggered by the high eruption-rates of depressurizing volatile-rich resident A1 magma, coupled with runaway phreatomagmatism. An overall narrow range of compositions suggests that the Hunga magmatic system is simple, without prolonged magma storage or segregation to produce evolved compositions. More primitive crystals and less-evolved magma appeared for the first time at Hunga in 2022, contrasting with the previous ∼1040–1180 CE caldera-formation cycle, which terminated with more-evolved magmas. This may suggest that repeated caldera collapses have reduced the lithostatic load of the edifice and possibly weakened the upper crust, allowing more primitive magmas to erupt.