Jökulhlaup

thumb|300x300px|A jökulhlaup

thumb|300x300px|The impounded lake a month earlier, before the same jökulhlaup

A jökulhlaup ( ) (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.

Formation process

upright=2|thumb|Jökulhlaup landforms in the world

Subglacial water generation

Subglacial 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. In 1997 analyses concluded that, based on basal meltwater production rates, the annual production of subglacial water from one typical northwestern Germany catchment was during the last Weichselian glaciation.

Supraglacial and subglacial water flow

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.

Episodic releases

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.

Examples

thumb|A former bridge at [[Skaftafell, Iceland, twisted by the jökulhlaup from Grímsvötn's 1996 eruption of Gjálp.]]

Whilst jökulhlaups were originally associated with Vatnajökull, they have been reported in the literature over a broad range of locations including the present day Antarctic, and there is evidence that they also occurred in the Laurentian ice sheet and the Scandinavian ice sheet during the Last Glacial Maximum.

Iceland

Mýrdalsjökull is subject to large jökulhlaups when the subglacial volcano Katla erupts, roughly every 40 to 80 years. The eruption in 1755 is estimated to have had a peak discharge of .

The Grímsvötn volcano frequently causes large jökulhlaups from Vatnajökull. The 1996 eruption of Gjálp sent melt water southwards into Grímsvötn, that caused a jökulhlaup with a peak flow of and lasted for several days. This is the largest Grímsvötn glacial flood ever recorded.

The Eyjafjallajökull volcano can cause jökulhlaups. The 2010 eruption caused a jökulhlaup with a peak flow of about .

Work in Iceland has categorised jökulhlaups by origin and size. The categories of origin are:

From an ice-dammed marginal lakes such as when an outlet glacier closes off an ice-free side valley.
Grænalón was a former example of such a lake.
From a supraglacial lake formed by accumulation of melt water in a depression on the surface of the glacier.
Such jökulhlaups tend to be small in Iceland
From a subglacial lake.
Tend to be formed by geothermal activity as found in the eastern and western Skaftá ice cauldrons and Grímsvötn.
From a volcanic eruption underneath a glacier (subglacial eruption).
While melt water might flow immediately away from the eruption site, it might also accumulate near the site creating an unstable reservoir as has happened in the jökulhlaup from Eyjafjallajökull in 2010 and Katla eruptions underneath Mýrdalsjökull .
From melting after a pyroclastic flow onto snow and ice in explosive eruptions in stratovolcanoes.
This type of flood may be a lahar and has happened at Hekla.
From a glacier surge into a proglacial lake or at the end of the surge when the subglacial drainage system re-establishes itself.
The 1999 surge of Langjökull at its outlet glacier of Eystri-Hagafellsjökull was into a lake which then flooded downstream.
From a landslide. Substantial potential energy is released in such slides, which can cause melting of ice and creation of flood water either if the slide falls onto a glacier or displaces water in a glacier dammed lake.
The 1967 rockslide onto the Steinsholtsjökull outlet glacier of Eyjafjallajökull caused such a flood.

{| class="wikitable"

|+ University of Iceland jökulhlaup scale

{| class="wikitable sortable" style="border-collapse:collapse"

|+ British Colombian jökulhlaups

!align="right"|Lake name

!align="left" |Year

!align="left" |Peak discharge (m<sup>3</sup>/s)

!align="left" |Volume (km<sup>3</sup>)

|Alsek

|1850

|30

|4.5

|Ape

|1984

|1600

|0.084

|Tide

|1800

|5,000-10,000

|1.1

|Donjek

|1810

|4000-6000

|0.234

|Summit

|1967

|2560

|0.251

|Tulsequah

|1958

|1556

|0.229

As the Laurentide Ice Sheet receded from its maximum extent from around 21,000 to 13,000 years ago, two significant meltwater rerouting events occurred in eastern North America. Though there is still much debate among geologists as to where these events occurred, they likely took place when the ice sheet receded from the Adirondack Mountains and the St. Lawrence Lowlands.

First, Glacial Lake Iroquois drained to the Atlantic in catastrophic Hudson Valley releases, as the receding ice sheet dam failed and re-established itself in three jökulhlaups. Evidence of the scale of the meltwater discharge down the Hudson Valley includes deeply incised sediments in the valley, large sediment deposit lobes on the continental shelf, and glacial erratic boulders greater than 2 metres in diameter on the outer shelf.
Later, when the St. Lawrence Valley was deglaciated, Glacial Lake Candona drained to the North Atlantic, with subsequent drainage events routed through the Champlain Sea and St. Lawrence Valley. This surge of meltwater to the North Atlantic by jökulhlaup about 13,350 years ago is believed to have triggered the reduction in thermohaline circulation and the short-lived Northern Hemisphere Intra-Allerød cold period.
Finally, Lake Agassiz was an immense glacial lake located in the center of North America. Fed by glacial runoff at the end of the last glacial period, its area was larger than all of the modern Great Lakes combined, and it held more water than contained by all lakes in the world today. It drained in a series of events between 13,000 BP and 8,400 BP.
Also, into the Pacific Ocean, large drainage events took place through the Columbia River Gorge, dubbed the Missoula or Bretz Floods.
Since 2011, periodic glacial floods have occurred from the Suicide Basin through the Mendenhall Glacier in Juneau, Alaska multiple times each year, with major such floods occurring in the summer of 2023 and 2024.

Sweden

Around 9500 BC the Baltic Ice Lake was tapped on water as the ice front retreated north of mount Billingen.

References

External links

Video of a jökulhlaup at Eyjafjallajokull volcano, Iceland 14 April 2010, Blog of Professor Dave Petley, Durham University
NOAA. Jokulhlaup Brochure. Jökulhlaup in the United States.