Mýrdalsjökull

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. Ice cauldrons are found within the caldera. The glacier's recent peak ice cover has reached 1493 m in height and in the year 1980 it covered an area of approximately 595 km2. 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. Björnsson originally described 12 geothermal ice cauldrons and three eruption sites since 1755 associated with Mýrdalsjökull. Later work clarified that these newly formed and could become inactive. 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. There are regular surveys now, with by 2019 twenty mapped ice cauldrons. Smaller and more frequent jökulhlaups tend to be associated with the geothermal ice caldron process. There is a warning system which uses in the case of geothermal events the increased conductivity of the river waters and can detect seismic flood tremor.

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.