jokulhlaup

Home > Earth and the Environment > Ecology and Environmentalism > Environmental Studies

jökulhlaup Jökulhlaup, the Icelandic term for a glacier-burst, is used worldwide to describe sudden flood-releases of melt water from glaciers and ice sheets. The phenomenon is relatively common in actively glaciated environments, where it can pose a considerable hazard.

Jökulhlaups are generated by a variety of mechanisms, including volcanic activity and the sudden drainage of stored glacial melt water. Volcanic eruptions underneath glaciers cause the rapid melting of glacier ice, releasing large volumes of melt water and erupted material. Volcanic jökulhlaups are best documented in Iceland. The 1918 eruption of the volcano Katla underneath the Myrdalsjökull, Iceland, generated a peak discharge of 1.6 × 10⁶ m³ s⁻¹ within 4–5 hours with depths of 70 m. This jökulhlaup deposited enough sediment to extend the nearby coastline by 4 km.

Storage of melt water either in glacier marginal lakes or as water bodies on, within, or underneath the glacier is relatively common. Glacier margin collapse may also result in the storage and sudden release of melt water. Glacial moraine ridges impounding lakes often fail rapidly, generating jökulhlaups. Many ice-dammed lakes drain and refill regularly with a periodicity ranging from several months to decades. The frequency of jökulhlaups is controlled by the size of the lake basin, the refill rate, and the mechanism of dam failure.

Ice-dammed lake drainage can be triggered by several mechanisms:(1) flotation of the ice dam once the lake reaches nine-tenths of the ice dam thickness;(2) over-topping of the glacier dam accompanied by sudden downcutting;(3) plastic deformation of the ice dam when ice-dammed lake depths exceed 200 m;(4) pressure variations within the glacier-drainage network; and(5) catastrophic ice dam failure.Once drainage is initiated, tunnels conveying lake water through the glacier enlarge exponentially through melting, allowing progressively more water to drain. This sequence produces the distinctively steady exponential rising limb of many jökulhlaup hydrographs.

Jökulhlaups vary in duration from several hours to several weeks or even months. Volcanic jökulhlaups can reach peak flows exceeding 10⁶ m³ s⁻¹ within as little as 4 hours. In contrast, a jökulhlaup draining from an ice-dammed lake may take months to reach a similar discharge and would involve a much greater volume of water. It is thought that over 2000 km³ of water drained from the Late Pleistocene glacial Lake Missoula in less than 4 days, producing flow velocities up to 30 m s⁻¹ and a peak discharge of 17 × 10⁶ m³ s⁻¹. Volcanic jökulhlaups are commonly ‘hyperconcentrated’ or heavily laden with erupted debris, giving the flow the consistency of wet cement. As jökulhlaups have higher discharges than flows generated by other mechanisms, sedimentary structures are usually well preserved. Large-scale jökulhlaup bars can extend for tens of kilometres, and can have relief amplitudes of up to 100 m; they commonly support large areas of gravel and boulder dunes reaching heights of up to 15 m.

Jökulhlaups play an important role in transporting sediment away from glaciers and ice sheets as well as creating distinctive fluvial landforms and deposits. The impact of jökulhlaups from vast Late Pleistocene ice-dammed lakes in North America and Siberia extended well beyond the immediate glacier margin into marine and climatic systems.

Andrew J. Russell

Bibliography

Maizels, J. K. and and Russell, A. J. (1992) Quaternary perspectives on jökulhlaup prediction. In Gray, J. M. (ed.) Applications of Quaternary research, Quaternary Proceedings, pp. 133–53.

jökulhlaup A sudden, violent, but short-lived increase in the discharge of a melt-water stream issuing from a glacier or ice cap, sometimes owing to volcanic activity beneath. A lake may develop above the heat source; this is subsequently breached to produce a torrent of melt-water (e.g. Lake Grimsvotn on Vatnajökull, Iceland). Flow velocity may reach 7–8 m/s and the discharge may attain 100 000 m³/s (e.g. the Katlahlaups from Myrdalsjökull, Iceland), comparable to rates of flow of the Amazon river.

jökulhlaup Sudden, violent, but shortlived increase in the discharge of a meltwater stream issuing from a glacier or ice-cap, sometimes due to volcanic activity beneath. A lake may develop above the heat source; this is subsequently breached to produce a torrent of meltwater, e.g. Lake Grimsvotn on Vatnajökull, Iceland. Flow velocity may reach 7–8 m/s and the discharge may attain 100 000 m³/s (e.g. the Katlahlaups from Myrdalsjökull, Iceland), comparable to rates of flow of the Amazon river.