Jökulhlaup

A jökulhlaup (Icelandic pronunciation: ​) (literally 'glacial run') is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst floods from Vatnajökull, Iceland, which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption, but it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir. Since jökulhlaups emerge from hydrostatically-sealed lakes with floating levels far above the threshold, their peak discharge can be much larger than that of a marginal or extra-marginal lake burst. The hydrograph of a jökulhlaup from Vatnajökull typically either climbs over a period of weeks with the largest flow near the end, or it climbs much faster during the course of some hours. These patterns are suggested to reflect channel melting, and sheet flow under the front, respectively. Similar processes on a very large scale occurred during the deglaciation of North America and Europe after the last ice age (e.g., Lake Agassiz and the English Channel), and presumably at earlier times, although the geological record is not well preserved. Subglacial meltwater generation is one key to the understanding of subglacial meltwater flow. Meltwater may be produced on the glacier surface (supraglacially), below the glacier (basally) or in both locations. Ablation (surface melting) tends to result in surface pooling. Basal melting results from geothermal heat flux out of the earth, which varies with location, as well as from friction heating which results from the ice moving over the surface below it. Analyses by Piotrowski concluded that, based on basal meltwater production rates, the annual production of subglacial water from one typical northwestern Germany catchment was 642x106 m3 during the last Weichselian glaciation. Meltwater may flow either above the glacier (supraglacially), below the glacier (subglacially/basally) or as groundwater in an aquifer below the glacier as a result of the hydraulic transmissivity of the subsoil under the glacier. If the rate of production exceeds the rate of loss through the aquifer, then water will collect in surface or subglacial ponds or lakes. The signatures of supraglacial and basal water flow differ with the passage zone. Supraglacial flow is similar to stream flow in all surface environments—water flows from higher areas to lower areas under the influence of gravity. Basal flow under the glacier exhibits significant differences. In basal flow the water, either produced by melting at the base or drawn downward from the surface by gravity, collects at the base of the glacier in ponds and lakes in a pocket overlain by hundreds of metres of ice. If there is no surface drainage path, water from surface melting will flow downward and collect in crevices in the ice, while water from basal melting collects under the glacier; either source can form a subglacial lake. The hydraulic head of the water collected in a basal lake will increase as water drains through the ice until the pressure grows high enough either to force a path through the ice or to float the ice above it. If meltwater accumulates, the discharges are episodic under continental ice sheets as well as under Alpine glaciers. The discharge results when water collects, the overlying ice is lifted, and the water moves outward in a pressurized layer or a growing under-ice lake. Areas where the ice is most easily lifted (i.e. areas with thinner overlying ice sheets) are lifted first. Hence the water may move up the terrain underlying the glacier if it moves toward areas of lower overlying ice. As water collects, additional ice is lifted until a release path is created. If no preexisting channel is present, the water is initially released in a broad-front jökulhlaup which can have a flow front that is tens of kilometres wide, spreading out in a thin front. As the flow continues, it tends to erode the underlying materials and the overlying ice, creating a tunnel valley channel even as the reduced pressure allows most of the glacial ice to settle back to the underlying surface, sealing off the broad front release and channelizing the flow. The direction of the channel is defined primarily by the overlying ice thickness and second by the gradient of the underlying earth, and may be observed to 'run uphill' as the pressure of the ice forces the water to areas of lower ice coverage until it emerges at a glacial face. Hence the configuration of the various tunnel valleys formed by a specific glaciation provides a general mapping of the glacier thickness when the tunnel valleys were formed, particularly if the original surface relief under the glacier was limited. The rapid, high-volume discharge is highly erosive, as evidenced by the debris found in tunnels and at the mouth of tunnels, which tends to be coarse rocks and boulders. This erosive environment is consistent with creation of tunnels over 400 m deep and 2.5 km wide, as have been observed in the Antarctic.