Jökulhlaup
Encyclopedia
A jökulhlaup is a glacial outburst flood. It is an Icelandic
term that has been adopted by the English language. 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 after the last ice age (e.g. Lake Agassiz
), and presumably at earlier times, although the geological record is not well preserved.
(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 north-western Germany catchment of 642x106 m3 during the last Weichselian glaciation.
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/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 no pre-existing 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.
Piotrowski has developed a detailed analytic model of the process, which predicts a cycle as follows:
n Coast Mountains
, resulting in a jökulhlaup. The flood surge of from 100 to 300 m3/second flowed 11 km through Farrow Creek to terminate in Chilko Lake, causing significant erosion. The ice dam has not reformed. Similar British Columbian jökulhlaups are summarized in the table below.
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 geologist as to where these events occurred, they likely took place when the ice sheet receded from the Adirondack Mountains
and the St. Lawrence Lowlands.
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.
Icelandic language
Icelandic is a North Germanic language, the main language of Iceland. Its closest relative is Faroese.Icelandic is an Indo-European language belonging to the North Germanic or Nordic branch of the Germanic languages. Historically, it was the westernmost of the Indo-European languages prior to the...
term that has been adopted by the English language. It originally referred to the well-known subglacial outburst floods from Vatnajökull
Vatnajökull
Vatnajökull is the largest glacier in Iceland. It is located in the south-east of the island, covering more than 8% of the country.-Size:With an area of 8,100 km², Vatnajökull is the largest ice cap in Europe by volume and the second largest in area Vatnajökull is the largest glacier in...
, Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption
Subglacial eruption
A subglacial eruption is a volcanic eruption that has occurred under ice, or under a glacier. Subglacial eruptions can cause dangerous floods, lahars and create hyaloclastite and pillow lava. Subglacial eruptions sometimes form a subglacial volcano called a tuya. Tuyas in Iceland are called table...
, but it is now used to describe any large and abrupt release of water from a subglacial
Subglacial lake
A subglacial lake is a lake under a glacier, typically an ice cap or ice sheet. There are many such lakes, with Lake Vostok in Antarctica being by far the largest known at present.-Characteristics:...
or proglacial lake/reservoir
Proglacial lake
In geology, a proglacial lake is a lake formed either by the damming action of a moraine or ice dam during the retreat of a melting glacier, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around the ice...
.
Since jökulhlaups emerge from hydrostatically-sealed lakes with floating levels far above the threshold, their peak discharge
Discharge (hydrology)
In hydrology, discharge is the volume rate of water flow, including any suspended solids , dissolved chemical species and/or biologic material , which is transported through a given cross-sectional area...
can be much larger than that of a marginal or extra-marginal lake burst. The hydrograph
Hydrograph
A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, or other channel or conduit carrying flow...
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 after the last ice age (e.g. Lake Agassiz
Lake Agassiz
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.-Conception:First...
), and presumably at earlier times, although the geological record is not well preserved.
Subglacial water generation
Subglacial meltwater generation is one key to understanding of subglacial meltwater flow. Meltwater may be produced on the glacier surface (supraglacially), below the glacier (basally) or in both locations. AblationAblation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
(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 north-western Germany catchment of 642x106 m3 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 aquiferAquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
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/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
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
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.
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 pre-existing 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
Tunnel valley
A tunnel valley is a large, long, U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages....
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.
Piotrowski has developed a detailed analytic model of the process, which predicts a cycle as follows:
- Meltwater is produced as a result of geothermal heating from below. Surface ablation water is not considered as it would be minimal at the glacial maximum and evidence indicates that surface water does not penetrate more than 100 meters into a glacier.
- Meltwater initially drains through subglacial aquifers.
- When the hydraulic transmissivity of the substratum is exceeded, subglacial meltwater accumulates in basins.
- Water accumulates sufficiently to open the ice blockage in the tunnel valley which accumulated after the last discharge.
- The tunnel valley discharges the meltwater excess – turbulent flow melts out or erodes the excess ice as well as eroding the valley floor.
- As the water level drops, the pressure decreases until the tunnel valleys again close with ice and water flow ceases.
Jökulhlaup examples
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 ice age.Iceland
- MýrdalsjökullMýrdalsjökullMýrdalsjökull mire dale glacier" or " mire valley glacier") is a glacier in the south of Iceland. It is situated to the north of Vík í Mýrdal and to the east of the smaller glacier Eyjafjallajökull. Between these two glaciers is Fimmvörðuháls pass. Its peak reaches in height and in 1980 it covered...
is subject to large jökulhaups when the subglacial volcano Katla erupts, roughly every 40–80 years. The 1755 eruption is estimated to have had a peak discharge of 200,000–400,000 m³/sCubic metres per secondA cubic metre per second is a derived SI unit of flow rate equal to that of a stere or cube with sides of one metre in length exchanged or moving each second...
. Katla has not erupted since 1918 and is overdue. - The GrímsvötnGrímsvötnThe Grímsvötn sub-glacial lakes and the volcano of the same name are in South-East Iceland. They are in the highlands of Iceland at the northwestern side of the Vatnajökull ice-cap. The lakes are at , at an elevation of...
volcano frequently causes large jökulhaups from VatnajökullVatnajökullVatnajökull is the largest glacier in Iceland. It is located in the south-east of the island, covering more than 8% of the country.-Size:With an area of 8,100 km², Vatnajökull is the largest ice cap in Europe by volume and the second largest in area Vatnajökull is the largest glacier in...
. The 1996 eruption caused a peak flow of 50,000 m³/sCubic metres per secondA cubic metre per second is a derived SI unit of flow rate equal to that of a stere or cube with sides of one metre in length exchanged or moving each second...
and lasted for several days. - EyjafjallajokullEyjafjallajökullEyjafjallajökull is one of the smaller ice caps of Iceland, situated to the north of Skógar and to the west of Mýrdalsjökull. The ice cap covers the caldera of a volcano with a summit elevation of . The volcano has erupted relatively frequently since the last glacial period, most recently in...
volcano can cause jökulhlaups. The 2010 eruption caused a jökulhlaup with a peak flow of about 2000-3000 m³/sCubic metres per secondA cubic metre per second is a derived SI unit of flow rate equal to that of a stere or cube with sides of one metre in length exchanged or moving each second...
North America
In July 1994, an ice-dammed surface lake drained via a subglacial tunnel through Goddard Glacier, in the British ColumbiaBritish Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
n Coast Mountains
Coast Mountains
The Coast Mountains are a major mountain range, in the Pacific Coast Ranges, of western North America, extending from southwestern Yukon through the Alaska Panhandle and virtually all of the Coast of British Columbia. They are so-named because of their proximity to the sea coast, and are often...
, resulting in a jökulhlaup. The flood surge of from 100 to 300 m3/second flowed 11 km through Farrow Creek to terminate in Chilko Lake, causing significant erosion. The ice dam has not reformed. Similar British Columbian jökulhlaups are summarized in the table below.
| Lake name | Year | Peak discharge (m3/sec) | Volume (km3) |
|---|---|---|---|
| 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
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. Though there is still much debate among geologist as to where these events occurred, they likely took place when the ice sheet receded from the Adirondack Mountains
Adirondack Mountains
The Adirondack Mountains are a mountain range located in the northeastern part of New York, that runs through Clinton, Essex, Franklin, Fulton, Hamilton, Herkimer, Lewis, Saint Lawrence, Saratoga, Warren, and Washington counties....
and the St. Lawrence Lowlands.
- First, Glacial Lake IroquoisGlacial Lake IroquoisGlacial Lake Iroquois was a prehistoric proglacial lake that existed at the end of the last ice age approximately 13,000 years ago.The lake was essentially an enlargement of the present Lake Ontario that formed because the St. Lawrence River downstream from the lake was blocked by the ice sheet...
drained to the Atlantic in catastrophic Hudson ValleyHudson ValleyThe Hudson Valley comprises the valley of the Hudson River and its adjacent communities in New York State, United States, from northern Westchester County northward to the cities of Albany and Troy.-History:...
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 erraticGlacial erraticA glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometres...
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 SeaChamplain SeaThe Champlain Sea was a temporary inlet of the Atlantic Ocean, a paratropical subsea or epeiric sea created by the retreating glaciers during the close of the last ice age...
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 circulationThermohaline circulationThe term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
and the short-lived Northern Hemisphere Intra-Allerød cold periodAllerød OscillationThe Allerød period was a warm and moist global interstadial that occurred at the end of the last glacial period. The Allerød oscillation raised temperatures , before they declined again in the succeeding Younger Dryas period, which was followed by the present interglacial period.In some regions,...
.
Lake Agassiz
Lake Agassiz
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.-Conception:First...
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
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
and 8,400 BP.
External links
Sources
- Beaney, C. L. and Shaw, J., (2000). The subglacial geomorphology of southeast Alberta: evidence for subglacial meltwater erosion. Can. J. Earth Sci., 37:51-61.
- Björnsson, H., (2002). Subglacial lakes and jökulhlaups in Iceland. Global and Planetary Change, 35:255–271.
- Alley, R.B., Dupont, T.K., Parizek, B.R., Anandakrishnan, S., Lawson, D.E., Larson, G.J. and Evenson, E.B., (2005). Outburst flooding and the initiation of ice-stream surges in response to climatic cooling: A hypothesis. Geomorphology, 75: 76-89.
- Erlingsson, U., (1994). The ‘Captured Ice Shelf’ hypothesis and its applicability to the Weichselian glaciation. Geogr. Ann., 76A (1–2): 1–12.
- Erlingsson, U., (2008). A jökulhlaup from a Laurentian captured ice shelf to the Gulf of Mexico could have caused the Bølling warming. Geogr. Ann., 90 A (2): 125-140.
- Shaw, J., (1983). Drumlin formation related to inverted melt-water erosional marks. J. Glaciol., 29:461–479.