Delayed, long-lived and globally-significant impacts of ocean-entering lahars

Abstract Hazardous, volcanic sediment-laden flows called lahars are among the largest sediment flows on Earth, yet their impacts beyond the coastline remain essentially unknown. Although lahars frequently reach the ocean, a lack of offshore observations limits understanding of their contribution to marine sediment budgets, biogeochemical cycling, ecosystem impacts, and hazards to coastal communities and critical infrastructure. Based on the first offshore observations of ocean-entering lahars, we show that the prevailing conceptual model of direct, short-lived sediment delivery is fundamentally-incomplete. Instead, lahar-derived sediment is transferred offshore through complex, delayed and long-lived process cascades, conveying substantially greater volumes of material to the deep sea than previously-recognized, producing widespread damage. A single lahar in a small, normally dry river catchment delivered >1,600 tonnes of sediment to the ocean in ~20 minutes, while others damaged subsea telecommunications cables and economically-important fisheries. Lahars from a single volcanic island contribute 7% of the annual sediment flux from all ocean islands worldwide, revealing lahars are a dominant and previously-overlooked agent of volcanic sediment delivery to the ocean. As climate change intensifies rainfall, lahar-driven ocean sediment fluxes and associated hazards are likely to increase, with far-reaching implications for marine systems and global infrastructure at hundreds of coastal volcanoes worldwide.