Ice cauldrons are ice formations within

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...

s that cover some

subglacial volcano A subglacial volcano, also known as a glaciovolcano, is a volcanic form produced by subglacial eruptions or eruptions beneath the surface of a glacier or ice sheet which is then melted into a lake by the rising lava. Today they are most ...

es. They can have circular to oblong forms. Their surface areas reach from some meters (as indentations or holes in the ice) to up to 1 or more kilometers (as bowl shaped depressions). Their existence is connected to ice-volcano interaction in two possible ways: They can be formed in the course of a

subglacial eruption Subglacial eruptions, those of ice-covered volcanoes, result in the interaction of magma with ice and snow, leading to meltwater formation, jökulhlaups, and lahars. Flooding associated with meltwater is a significant hazard in some volcan ...

or on top of a continuously active subglacial high temperature geothermal area. In both cases, a

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 flood ...

may be produced in connection with them.

Formation and continued existence of ice cauldrons

Ice cauldrons and subglacial eruptions

When an eruption takes place under a bigger

, e.g. an

ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical features ...

, it normally begins with an

effusive In physics and chemistry, effusion is the process in which a gas escapes from a container through a hole of diameter considerably smaller than the mean free path of the molecules. Such a hole is often described as a ''pinhole'' and the escape ...

stage. The heat forms an ice cave and

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of disconti ...

is produced. After some time, the eruption has reached a stage where the pressure drops within the ice vault and the eruption style changes to become

explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...

Hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margin ...

is produced and the heat is transferred to the meltwater. „At this stage, the surface ice begins to act brittle and creates concentric fractures that cave in towards the meltwater reservoir. This is referred to as the ice cauldron."S. E. Ackiss: Investing the Mineralogy and Morphology of Subglacial Volcanoes on Earth and Mars. Dissertation. Department of Earth, Atmospheric, & Planetary Sciences West Lafayette, Indiana May 2019. 28 August 2020. When the eruption continues, “the meltwater reservoir becomes so large that the ice cauldron collapses inward towards the edifice, exposing the meltwater reservoir and allowing the breach of both the reservoir and the explosive lava, releasing plumes of gasses and jets of hyaloclastites “. The ice cauldron can develop further into an ice canyon, as was the case during the 1996 Gjálp eruption. It can continue to exist after the meltwater has left the eruption site and the eruption is terminated. But in most cases, ice flow will fill up the ice cauldron again and make it disappear as soon as the eruption products have cooled down enough.

Ice cauldrons on top of subglacial geothermal areas

Another case are ice cauldrons situated on top of geothermal areas. "(…) hydrothermal systems are created that bring heat up from a magma body, continuously melting ice into water that may be stored at the glacier bed until it breaks out in jökulhlaups."Helgi Björnsson: Subglacial lakes and jökulhlaups in Iceland. Global and Planetary Change 35 (2002) 255–271
Retrieved 31 August 2020. Many examples for a decades long existence of such ice cauldrons are to be found in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

Ice cauldrons around the world

Examples from Iceland

Skaftárkatlar (Skaftá cauldrons)

These are two depressions in the ice cover above two subglacial lakes in the southwestern part of

Vatnajökull Vatnajökull ( Icelandic pronunciation: , literally "Glacier of Lakes"; sometimes translated as Vatna Glacier in English) is the largest and most voluminous ice cap in Iceland, and the second largest in area in Europe after the Severny Island ice ...

. In the whole, many cauldrons are to be found within Vatnajökull glacier (8,100 km in 2015), the largest of which in the western part of the ice cap are the Skaftá cauldrons.I. Galeczka, etal.: The effect of the 2002 glacial flood on dissolved and suspended chemical fluxes in the Skaftá river, Iceland. Journal of Volcanology and Geothermal Research 301 (2015) 253–276.
Retrieved 31 August 2020. These ice cauldrons "are created by melting at subglacial geothermal areas". The meltwater accumulates in lakes "under the cauldrons until it drains every 2–3 years in a jökulhlaup" of normally up to 2,000 m/sec. An unusually big outburst flood (jökulhlaup) was recorded in 2015. The eastern Skaftá cauldron had accumulated meltwater in this case during around 5 years. It was discharged down the

Skaftá The Skaftá () is a river in South Iceland. It is primarily glacial in origin and has had its course modified by volcanic activity; as a result of both, it often floods because of glacial melting. Course The river's primary source is two subglac ...

river in September 2015 with a peak of 3,000 m/sec or even more. The cauldron then partially collapsed and formed a depression of up to 110 m deep in its center and a max. width of 2.7 km.

Katla

Famous examples from

are the ice cauldrons within the Katla caldera. Katla is an important

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

and

central volcano Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...

situated under the

Mýrdalsjökull Mýrdalsjökull (pronounced , Icelandic for "(the) mire dale glacier" or "(the) mire valley glacier") is an ice cap in the south of Iceland. It is to the north of Vík í Mýrdal and to the east of the smaller ice cap Eyjafjallajökull. Between ...

glacier cap in the southern part of Iceland's

East Volcanic Zone East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth. Etymology As in other languages, the word is formed from the f ...

. 150–200 eruptions during

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

have been attributed to her, and 17 of these happened since

Settlement of Iceland The settlement of Iceland ( is, landnámsöld ) is generally believed to have begun in the second half of the ninth century, when Norse settlers migrated across the North Atlantic. The reasons for the migration are uncertain: later in the Middle ...

in the 8th century. Most of the eruptions had their origin in the ice covered caldera. The last contested big eruption took place in 1918 and initiated a jökulhlaup with an estimated peak discharge of 300,000 m/sec.McCluskey, O (2019) Constraining the characteristics of a future

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

genic Jökulhlaup from Katla, Iceland, through seismic analysis and probabilistic hydraulic modelling, Master's thesis, School of Earth and Environmental Sciences, University of Portsmouth Within the caldera 12–17 ice cauldrons are supra- and inglacial manifestations of a near-surface magmatic storage system. K. Scharrer even explains that "twenty permanent and 4 semi-permanent ice cauldrons could be identified on the surface of Mýrdalsjökull indicating geothermally active areas in the underlying caldera." They have a depth of 10–40 m and a width of 0.6–1.6 km. In 1955, 1999 and 2011 small to medium-sized jökulhlaup originated from some new ice cauldrons. It is still subject of discussion if they were eruption caused or initiated by heating up of the geothermal areas under these cauldrons."The geothermal heat output is in the order of a few hundred megawatt."Magnús T. Guðmundsson, etal.: Geothermal activity in the subglacial Katla caldera, Iceland, 1999–2005, studied with radar altimetry. Annals of Glaciology 45 2007.
Retrieved 30 August 2020.

Ice cauldrons in other environments

Fourpeaked - Fumarole on NW side (2007-02-22)

Ice cauldrons of course do not form only in Iceland, but also at many other places where there is subglacial volcanic activity, e.g. in Alaska (

Mount Redoubt Redoubt Volcano, or Mount Redoubt (Denaʼina language, Dena'ina: ''Bentuggezh K’enulgheli''), is an active volcano, active stratovolcano in the largely volcanic Aleutian Range of the U.S. state of Alaska. Located at the head of the Chigmit ...

Mount Spurr Mount Spurr ( Dena'ina: ''K'idazq'eni'') is a stratovolcano in the Aleutian Arc of Alaska, named after United States Geological Survey geologist and explorer Josiah Edward Spurr, who led an expedition to the area in 1898. The Alaska Volcano Obser ...

).J. Barr. Volcanic impacts on modern glaciers: a global synthesis. Preprint. Manchester University. (2018)
/ref>

Ice cauldrons and volcano monitoring

As deepening and widening of the ice cauldrons e.g. at Katla volcano, and esp. in combination with increased

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

activity at the sites, are interpreted as signs of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

inflow, the cauldrons are closely monitored.

References

{{reflist Volcanism Glaciers Volcanism of Iceland Glaciovolcanism Subglacial eruptions Volcano monitoring