VOLCANICALLY GENERATED JÖKULHLAUPS FROM THE NORTHERN MARGIN OF VATNAJÖKULL ICE CAP, ICELAND

The Gjálp subglacial volcanic eruption beneath Vatnajökull in 1996 resulted in the release of nearly 4 km³ of meltwater in a southerly direction to Skeidarársandur as a jökulhlaup. Had the eruption been slightly further north, this flood would have drained the northern margin of Vatnajökull. We present evidence of giant Holocene and historic jökulhlaups draining the northern margin of Vatnajökull.

Erosional evidence of jökulhlaups consists of: extensively washed high gradient valleys up to 2 km wide; streamlined erosional hills; truncated moraine ridges; washed and eroded lava surfaces supporting localised cataract and scabland development; and localised, well-developed fluvially sculpted bedrock bedforms. Depositional jökulhlaup landforms include: pendant and eddy bars; widespread terraced sandur surfaces containing boulders of up to 12 m in diameter and 'boulder-ring' structures, produced by the meltout of large ice blocks. Large-scale erosional and depositional evidence is inconsistent with present 'normal' ablation-controlled discharges. Jökulhlaup deposits range from well-imbricated and stratified boulder-rich units reflecting fluidal sedimentation, to heterogeneous matrix-supported units, interpreted as the product of non-Newtonian flows.

Jökulhlaup evidence is found along 60 km of the northern margin of Vatnajökull from Dyngujökull, Bárdarbunga subglacial volcano to Kverkfjöll glaciated volcano. Jökulhlaups drained from Kverkfjöll and from the northern flanks of Bárdarbunga or nearby fissure system. The presence of jökulhlaup deposits below and above recent lava flows suggests the passage of between 2 and 3 large prehistoric floods. Jökulhlaup deposits present both within and outside recent moraines suggest the passage of several additional floods of historic age.

Large jökulhlaups from the northern margin of Vatnajökull were much more frequent than previously assumed and originated from a variety of sources. The jökulhlaup landscape of the Jökulsá á Fjöllum testifies to the tremendous productivity of Iceland's northern volcanic zone for sediment and freshwater influx to the north Atlantic and to large scale glacier-volcano interaction.