Skip to content

Ice-magma interactions in a shallow subglacial fissure eruption at Northern Laki (1783 CE); deposition and post-eruptive evolution in a dynamic environment.

Catherine R. Gallagher1,Magnús Tumi Guðmundsson1, Birgir V. Óskarsson2, Robert A. Askew2, and Rosie Cole1

  • Affiliations: 1The Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland; 2The Natural Science Institute of Iceland, Garðabær, Iceland 

  • Presentation type: Talk [Invited]

  • Presentation time: Tuesday 15:45 - 16:00, Room R280

  • Programme No: 3.5.9

  • Theme 3 > Session 5

Abstract

Substantial parts of Iceland's active volcanic zones are presently ice-covered, and fissure eruptions beneath glaciers are common. It has been hypothesised that fissure eruptions at or within ice-marginal settings would be predominantly phreatomagmatic and generate jökulhlaups. However, globally, few historical examples have been directly observed. Analysis of rare landforms like these is important for interpreting paleo ice-extents and understanding how they are preserved in this dynamic erosive environment. The final phase of the 1783--84 CE Laki flood basalt event in the Síða highlands of South Iceland was characterised by fissure propagation under Síðujökull, an outlet glacier from the Vatnajökull ice-cap. Dry, magmatic eruptive activity transitioned laterally to a phreatomagmatic eruption. 2.5 km-long shallow sub-glacially erupted formations offer anatural laboratory to understand the dynamics of fissure eruptions in a shallow subglacial or intraglacial setting from a rare known historical example. Field mapping and drone photogrammetry reveal a sequence dominated by phreatomagmatic tuff deposits intercalated with hackly jointed lobate lava flows, hackly jointed intrusions, and debris flows capped by spatter fed lavas suggestive of fluctuating water levels. The morphology of the formation displays increasing degrees of lateral confinement by the glacier towards to NE. A thin layer of glacial till caps the top of the formation indicating Síðujökull re-advanced over the area after the eruption. The glacier has since receded, removing lateral support from the fragile formation and triggering multiple landslides., Historic photogrammetry datasets show past glacial surges (1964-1994 CE), which may have contributed to weakening and failure.