Geomorphological evidence for jökulhlaups from Kverkfjöll volcano, Iceland

Jokulhlaups (glacial outburst floods) are known to have drained along the Jokulsa a Fjollum river in Iceland during the Holocene. However, little is known about their number, age, source, and flow characteristics. This paper provides detailed geomorphological evidence for jokulhlaups that have routed from the Kverkfjoll ice margin and hence into the Jokulsa a Fjollum. Erosional evidence of jokulhlaups from Kverkfjoll includes gorges, cataracts, spillways, subaerial lava steps, and valley-wide scoured surfaces. Depositional evidence includes wash limits, boulder bars, cataract-fill mounds, terraces, slackwater deposits, and outwash fans. Some of these landforms have been documented previously in association with jokulhlaups. However, subaerial lava surfaces that have been scoured of the upper clinker, gorges within pillow–hyaloclastite ridges, gorges between pillow–hyaloclastite ridges and subaerial lava flows, subaerial lava lobe steps, cataract-fill mounds, and boulder run-ups are previously undocumented in the literature. These landforms may therefore be diagnostic of jokulhlaups within an active volcanic rifting landscape. The nature and spatial distribution of these landforms and their stratigraphic association with other landforms suggest that there have been at least two jokulhlaups through Kverkfjallarani. The Biskupsfell eruption occurred between these two jokulhlaups. Kverkfjallarani jokulhlaups were very strongly influenced by topography, geology, and interevent processes that together determined the quantity and nature of sediment availability. Such controls have resulted in jokulhlaups that were probably fluidal, turbulent, and supercritical over large areas of the anastomosing channel bed. Kverkfjallarani jokulhlaups would have had highly variable hydraulic properties, both spatially and temporally. The knowledge of flow characteristics that can be gained from jokulhlaup impacts has implications for recognising jokulhlaups in the rock record and for hazard analysis and mitigation within similar landscapes and upon other glaciated volcanoes.