Thermally buffered remnant melts sustain explosivity after caldera collapse: Cryptotephra evidence from Aira, Japan

• Four early post-collapse crystal-poor, highly explosive eruptions identified at Aira Caldera. • Aira’s evolutionary path redefined: rapid fermentation & residual melt eruption stages. • Residual melts drive >10-kyr explosivity post-collapse decoupled from magmatic resurgence. • Thermally buffered remnant explosivity established as a distinct caldera recovery pathway. • Distal cryptotephra reveals hidden eruptions vital for early post-collapse hazard assessment. Silicic caldera systems exhibit complex post-collapse behaviors. While early post-caldera volcanism is common, its geological record is often obscured by proximal preservation biases. Such activity is conventionally attributed to deep magmatic recharge, which typically produces chemically less-evolved, lower-intensity eruptions. To overcome these proximal biases, we integrate high-resolution distal marine cryptotephra stratigraphy to reconstruct magmatic evolution at Japan’s Aira caldera. Melt compositions reveal a system maturation phase from > 90 to 31 ka, followed by rapid volatile accumulation (fermentation) from 31 to 29.6 ka, culminating in the 29.6 ka caldera-forming eruption (CFE). Crucially, we identify four previously unrecognized early post-collapse eruptions (28.1–16.8 ka) from Aira in marine sediment cores. These events reveal that highly explosive eruptions resumed within ∼1.5 kyr of caldera formation and persisted cyclically for over 10 kyr. Because these events share identical melt compositions with the CFE, they were fueled by trapped residual syn-caldera melts rather than deep magmatic rejuvenation. We propose a mechanism of geometric isolation and second boiling: rapid post-collapse re-sealing prevented extensive devolatilization, buffering the remnant melts against rheological lock-up. Subsequent minor crystallization concentrated volatiles, generating the overpressures required to trigger explosive eruptions. Ultimately, our findings establish thermally buffered remnant explosivity as a distinct caldera recovery pathway. This demonstrates that silicic systems can sustain high explosivity via isolated residual melts, fundamentally decoupling early post-caldera hazards from magmatic resurgence. The Aira case study illustrates how distal tephrostratigraphy provides critical insights for more precise volcanic hazard assessments.