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Unexpected offshore hazards and sediment fluxes resulting from ocean-entering lahars

Michael Clare1, Isobel Yeo1, Jeremy Phillips2, Jenni Barclay2, Richard Robertson3, Sebastian Watt4, Andrew Hogg2, Tom Sheldrake5, Lewis Bailey6, Matthew West7, Jennifer Cruickshank-Howard8

  • Affiliations: 1National Oceanography Centre, UK; 2School of Earth Sciences, University of Bristol, Bristol, UK; 3The University of West Indies Seismic Research Centre, Trinidad and Tobago; 4Birmingham University; 5University of Geneva, Switzerland; 6University of Calgary, Canada; 7OceanIQ, UK; 8Ministry of Agriculture, Foresty, Fisheries and Rural Transformation

  • Presentation type: Talk

  • Presentation time: Tuesday 11:15 - 11:30, Room R280

  • Programme No: 3.5.4

  • Theme 3 > Session 5

Abstract

Lahars are fast-moving flows of volcanic sediment mixed with water, representing some of the deadliest volcanic hazards and largest volume sediment flows of any type on Earth. Long runouts (up to 100 kms) and proximity of many volcanic centres to coastlines mean that lahars regularly reach the ocean, coincident with, and up to years after explosive volcanic eruptions. While many studies have characterised the behaviour and impacts of lahars on land, an absence of offshore observations means we lack an understanding of what happens when lahars enter the ocean. The existing (conceptual) model assumes a sediment-laden flow immediately and directly plunges beneath the sea surface as a hyperpycnal flow where it reaches the coast. New observations of ocean-entering lahars following the 2020/21 eruption of La Soufriere, St Vincent challenge this model, revealing more complex behaviour and hazards. Following the 2021 eruption, seafloor sediment flows destroyed five important fisheries and carried boulders and fresh vegetation tens of kilometres offshore, damaging >50 km length of subsea telecommunications cables. Lahars frequently and concurrently entered the ocean along multiple catchments forming buoyant surface plumes. The nature of offshore sediment delivery is often staged in time, wherein progradational deltas develop at river outflows that subsequently collapse into the ocean days to weeks later. Drawing on these observations we propose a new process-based model for ocean-entering lahars and discuss the implications for critical offshore infrastructure, fisheries, and fluxes of volcanic ash that play an important role in biogeochemical cycling at this and other coastal volcanoes worldwide.