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Mýrdalsjökull

Glacier in Iceland

Field	Value
name	Mýrdalsjökull
photo	Myrdalsjökull 01.jpg
photo_caption	Aerial view of Mýrdalsjökull.
type	Ice cap
area	520 sqkm
elevation_max	1450 m
thickness	Average 230 m
terminus	Sléttjökull, Öldufellsjökull, Sandfellsjökull, Kötlujökull, Huldujökull, Mosakambsökull, Klifurárjökull, Sólheimajökull, Jökulsárgilsjökull, Hrunajökull, Tungnakvíslarjökull, Goðalandsjökull, Hrútárökull, and Entujökull
status	Retreating
location	Southwestern Iceland
mapframe	no
map_image
map_caption	Map of Mýrdalsjökull glacier showing its named outlet glaciers and surface glacial catchments (light grey shading with white outline). Clicking on the map to enlarge it enables mouse over that allows identification of individual glaciers.

Mýrdalsjökull (pronounced , Icelandic for "(the) mire dale glacier" or "(the) mire valley glacier") is an ice cap on the top of the Katla volcano in the south of Iceland. It is to the north of the town of Vík í Mýrdal and to the east of the smaller ice cap Eyjafjallajökull. Between these two glaciers is the Fimmvörðuháls pass.

The glacier contributes to the most serious natural hazard area of Iceland.

Setting

The icecap of the glacier covers an active volcano. The caldera of Katla has a diameter of 10 km and the volcano erupts usually every 40–80 years. The last eruption took place in 1918. Scientists are actively monitoring the volcano, particularly after the eruption of nearby Eyjafjallajökull began in April 2010. There is a further historic relationship with Eyjafjallajökull as the two glaciers were continuous as a single ice cap at the end of the 19th century and only separated into the larger Mýrdalsjökull and smaller Eyjafjallajökull in the middle of the twentieth century. Since the year 930, 16 eruptions have been documented and in the last 8400 years about 300 explosive basalt eruptions are known to have originated from Katla.

While Katla mountain is 30 – in diameter at its base, more relevant to the present ice cover is its diameter closer to 20 km at 700 m elevation. The lowest pass out of the caldera is at 740 m. This caldera is between 650 – deep and is surrounded by a rim of 1300 – high mountains. In 2016 the covered area of Mýrdalsjökull was believed to be 540 km2. The area was 520 sqkm in 2019. The volume of ice is about 140 km3.

The Eldgjá, a volcanic eruption fissure about 30 km long, which erupted in the year 939, is part of the same volcanic system.

Before the Hringvegur (the main ring road round the island) was built, people feared traversing the plains in front of the volcano because of the frequent jökulhlaups (glacial floods) and the deep rivers to be crossed, although the road is still vulnerable to major events. Especially dangerous was the glacial flood after the eruption of 1918 when the coastline was extended by 5 km by laharic flood deposits.

Mýrdalsjökull is an exceedingly high precipitation location, with models suggesting parts of it receive more than 10 metres of rainfall equivalent annually.

Glaciation History

The last glacial period came to an end 11,500 years ago and was followed by the Holocene. Mýrdalsjökull is likely to have been present throughout the Holocene and definitely the last 8400 years as there is no evidence of degassed erupted tephra from Katla. Jökulhlaup's 8000 to 6000 years ago took a path over the highest pass in the caldera wall which does not support a crater lake source but would happen with an ice cap.

Until the development of remote satellite imaging it was impossible to monitor to annual accuracy the mass balance of Mýrdalsjökull. Such techniques allowed it to be determined for Mýrdalsjökull that it is the summer temperatures that are the predominant driving factor in the recent recession of the glacier. However past proxy means of measurement such as outlet length of the Sólheimajökull outlet glacier were also consistent with this. Any such studies related to climate change have to be qualified as past volcanic processes have resulted in the loss of up to 5% of the ice cap on more than one occasion.

Jökulhlaups

There is complexity with assigning the outlet glacier catchments, but from the point of view of hazard assessment related to the glaciers this is simplified considerably, as the hazard risk of jökulhlaups relates to water drainage sectors based upon the ice caps surface topography rather than ice flow. Such an approach creates three drainage sectors which do not reflect intracaldera bedrock topography or many of the outlet glaciers.

The biggest Mýrdalsjökull jökulhlaups, as in the 1775 eruption, are massive floods by any standard with maximum discharges of the order of several 100000 m3/s ten to a hundred times other recent jökulhlaup's in Iceland. Most recent jökulhlaup (17 out of the 20 since the year 874) have descended to the east through Kotlujökull onto the Mýrdalssandur plain with about two large floods a century (i.e. presently overdue). This Ko sector has about 60 km2 within the caldera rim but underlies about 323 km2 of the ice cap itself. However, due to location factors of the original melt water, the Katla 934 and 1860 jökulhlaups also descended through Sólheimajökull to the Sólheimasandu flood plain. This So sector has about 19 km2 of its water drainage catchment within the caldera rim and a total ice cap catchment of 108 km2. There have been 5 jökulhlaups since 874. It is perhaps not so significant from a hazard point of view as the 23 km2 water drainage catchment within the caldera rim northwest through Entujökull where a large jökulhlaup occurred in 874. This En sector with total ice cap catchment of about 167 km2, has rare but very large jökulhlaups every 500 to 800 years down the fairly well populated and developed Markarfljótsaurar flood plain.

Jökulhlaup triggering can be complex (there are at least seven potential causes), and at Mýrdalsjökull results from geothermal processes, ice dams and their sudden removal by floating as well as eruptions which can be primary or secondary to the removal of overpressure. Further work mapped at least 17 ice cauldrons, all associated with the caldera of Katla and three areas of increased seismic activity, that have been postulated to be likely jökulhlaup sources, being Austmannsbunga in the north-east of the caldera and the 1755 and 1823/1918 eruption sites to the west and south as noted by Björnsson.

The most likely next very disruptive volcanogenic jökulhlaup will almost certainly flow from Kotlujökull, over Mýrdalssandur to the sea. This has been assigned a 89% probability within 30 years, affecting possibly the town of Vík, likely associated with eruption of 1.5 km3 of tephra (so disruptive to air travel) and likely occurring in the months June to September. There is a lower probability of an Entujökull jökulhlaup of the same size which is expected to have far graver environmental, social and geomorphological consequences affecting the town of Hvolsvöllur and surrounds. 3:from Kötlujökull to Blautakvísl river area of Mýrdalssandur in (1625), 1660, 1755, 1823, 1918 4:from Kötlujökull to Múlakvísl river area of Mýrdalssandur towards Kötlutangi and Höfðabrekkufjara beaches in 1612, 1625, 1660, 1721, 1755, 1823, 1860, 1918 5:from Kötlujökull to Múlakvísl river area of Mýrdalssandur towards Höfðabrekkufjara beach in 1612, 1625, 1660, 1721, 1755, 1823, 1860, 1918, 1955, 1999, 2011, 2014, 2017 6:from Múlakvísl river area towards Kerlingardalsfjara beach in 1660, 1721, 1755, 1823, 1860 7:from Sólheimajökull towards Sólheimasandur in 1860, 1999 8:from Jökulsárgilsjökull towards Jökulsárgil in 1936 | arc-coordA =
| arc-textA = Mýrdalsjökull | arc-text-sizeA = 14 |arc-text-colorA = #d0d0ff | arc-angleA = 5 | arc-gapA = 1 | arc-radiusA = 1.1 | arc-coordC = | arc-textC = Kötlujökull | arc-text-sizeC = 12 |arc-text-colorC = #d0d0ff | arc-angleC = 5 | arc-gapC = 1.8 | arc-radiusC = 0.4 | arc-coordB = | arc-textB = Sólheimajökull | arc-text-sizeB = 12 |arc-text-colorB = #d0d0ff | arc-angleB = -40 | arc-gapB = 1.8 | arc-radiusB = 0.3

Year	Drainage	Geography of flow	Comment
2025	Ko	Area of Mýrdalssandur along rivers Leirá Syðri and Skálm	url =https://en.vedur.is/about-imo/news/a-jokulhlaup-from-myrdalsjokull-into-the-rivers-leira-sydri-and-skalm	access-date=11 July 2025	title =A jökulhlaup from Mýrdalsjökull into the rivers Leirá Syðri and Skálm	publisher=Icelandic Weather Office}}
2024	Ko	Area of Mýrdalssandur along river Skálm	On 27th July an unusual in origin geothermal jökulhlaup via mainly the Sandfellsjökull outlet area damaged the ring road. Peak flow was about 1000 m3/s
2017	Ko	Area of Mýrdalssandur along Múlakvísl River	On 29th July a small geothermal jökulhlaup occurred.
2014	Ko/So	Area of Mýrdalssandur along Múlakvísl and Jökulsá á Sólheimasandi rivers	Small geothermal floods in July
2011	Ko	Area of Mýrdalssandur along Múlakvísl River	url=https://www.vedur.is/um-vi/frettir/nr/2281	access-date=15 June 2024	title =Hlaupórói í Mýrdalsjökli	date=6 September 2011	publisher=Icelandic Weather Office}}
1999	So	Sólheimajökull (Sólheimasandur)	On 17th July one day small jökulhlaup. Peak flow was 4400 ±
1955	Ko	Area of Mýrdalssandur along Múlakvísl River	Small 2500 m3/s jökulhlaup of geothermal origin destroyed the Ring Road bridge
1936	So	Jökulsárgilsjökull (Jökulsárgil ravine)	5}}
1918	Ko	Kötlujökull (Leirá, from foot of glacier to Austastikælir stream, area of Mýrdalssandur including Múlakvísl and Blautakvísl rivers, towards Kötlutangi and Höfðabrekkufjara beaches)	17}}
1860	Ko/So	Kötlujökull (from foot of glacier to Austastikælir stream, area of Mýrdalssandur including Múlakvísl towards Kötlutangi, Höfðabrekkufjara and Kerlingardalsfjara beaches but excluding Blautakvísl river) and Sólheimajökull (Sólheimasandur)	17}}
1823	Ko	Kötlujökull (from foot of glacier to Austastikælir stream, area of Mýrdalssandur including Múlakvísl and Blautakvísl rivers, towards Höfðabrekkufjara and Kerlingardalsfjara beaches)	17}}
1755	Ko	Kötlujökull (Leirá, from foot of glacier to Austastikælir stream, area of Mýrdalssandur including Múlakvísl and Blautakvísl rivers, towards Kötlutangi, Höfðabrekkufjara and Kerlingardalsfjara beaches)	17}}
1721	Ko	Kötlujökull (Leirá, from foot of glacier to Austastikælir stream, Mýrdalssandur including Múlakvísl towards Höfðabrekkufjara and Kerlingardalsfjara beaches but excluded Blautakvísl river and Kötlutangi beach)	17}}
1660	Ko	Kötlujökull (from foot of glacier to Austastikælir stream, Mýrdalssandur including Múlakvísl and Blautakvísl rivers and definitely toward Höfðabrekkufjara and Kerlingardalsfjara beaches)	17}}
1625	Ko	Kötlujökull (from foot of glacier to Austastikælir stream, Mýrdalssandur including Múlakvísl (and possibly Blautakvísl river), towards Höfðabrekkufjara beach)	17}}
1612	Ko	Kötlujökull (from foot of glacier to Austastikælir stream, Mýrdalssandur including Múlakvísl river, towards Höfðabrekkufjara beach)	17}}

Image:Myrdalsjökull13.JPG|Mýrdalsjökull ice cap concealing Katla volcano Image:Mýrdalsjökull.jpg|Mýrdalsjökull Image:"ice castle" formation on Mýrdalsjökull.jpg|An "ice castle" formation on Mýrdalsjökull File:Glacier Iceland.JPG|Mýrdalsjökull glacier covered volcanic ash Image:Mýrdalsjökull glacier, Iceland.jpg|Outlet glacier Sólheimajökull

Notes

References

(2020). "A national glacier inventory and variations in glacier extent in Iceland from the Little Ice Age maximum to 2019". Jökull.
(2007). "Survival of the Mýrdalsjökull ice cap through the Holocene thermal maximum: evidence from sulphur contents in Katla tephra layers (Iceland) from the last∽ 8400 years". Annals of Glaciology.
(2008). "Satellite Image Atlas of Glaciers of the World series (U.S. Geological Survey Professional Paper 1386-D".
(2010). "5 Variations of Mýrdalsjökull during postglacial and historical times". Developments in Quaternary Sciences.
(2011). "Late Holocene Glacial History of Sólheimajökull, Southern Iceland". University of Iceland, Reykjavík.
Humlum, Ole. (1985). "Changes in Texture and Fabric of Particles in Glacial Traction with Distance from Source, Mỳrdalsjökull, Iceland". Journal of Glaciology.
Björnsson, Helgi. (2017). "Glaciers of Southern Iceland". Atlantis Press.
(28 August 2019). "Driving mechanisms of subaerial and subglacial explosive episodes during the 10th century Eldgjá fissure eruption, southern Iceland". Volcanica.
(2006). "The use of remote-sensing data for mass-balance studies at Mýrdalsjökull ice cap, Iceland". Journal of Glaciology.
(2003). "Subglacial lakes and jökulhlaups in Iceland". Global and Planetary Change.
(2019). "Constraining the characteristics of a future volcanogenic Jökulhlaup from Katla, Iceland, through seismic analysis and probabilistic hydraulic modelling, Master’s thesis". School of Earth and Environmental Sciences, University of Portsmouth.
(2015). "Glacial outburst floods".
(2007). "Geothermal activity in the subglacial Katla caldera, Iceland, 1999–2005, studied with radar altimetry". Annals of Glaciology.
"Katla - Monitoring of ice cauldrons". Institute of Earth Sciences, University of Iceland.
(28 July 2024). "Hlaup úr Mýrdalsjökli. Hlaupið hefur ekki náð hámarki við þjóðveg 1". Icelandic Weather Office.
"Search:Múlakvísl & Mýrdalssandur". Icelandic Met Office.
"A jökulhlaup from Mýrdalsjökull into the rivers Leirá Syðri and Skálm". Icelandic Weather Office.
(29 July 2017). "Glacial outburst flood in Múlakvísl". Icelandic Weather Office.
(10 July 2014). "Small glacial outburst floods around Mýrdalsjökull". Icelandic Weather Office.
(6 September 2011). "Hlaupórói í Mýrdalsjökli". Icelandic Weather Office.
(2010). "An unusual jökulhlaup resulting from subglacial volcanism, Sólheimajökull, Iceland". Quaternary Science Reviews.
(1989). "Íslandshandbókin. Náttúra, saga og sérkenni". Örn og Örlygur.

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