Ice cauldrons are ice formations within

glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

s that cover some subglacial volcanoes. 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 or on top of a continuously active subglacial high temperature geothermal area. In both cases, a jökulhlaup 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 By definition, ice caps are not constrained by topogra ...

, it normally begins with an effusive 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 discontinu ...

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 ex ...

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 marg ...

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''". 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 a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

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 south-western 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 i ...

. In the whole, many cauldrons are to be found within Vatnajökull glacier ( in 2015), the largest of which in the western part of the ice cap are the Skaftá cauldrons. 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 . 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 or even more. The cauldron then partially collapsed and formed a depression of up to deep in its center and a maximum width of

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 volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

and

central volcano A central volcano is a type of volcano formed by basalts and silica-rich volcanic rocks. They contain very few or no volcanic rocks of intermediate composition, such that they are chemically bimodal volcanism, bimodal. Large silicic eruptions at c ...

situated under the

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 ...

glacier cap in the southern part of Iceland's East Volcanic Zone. 150–200 eruptions during

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

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

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

in the 8th century. Most of the eruptions had their origin in the ice covered caldera. The last large eruption took place in 1918 and was associated with a jökulhlaup with an estimated peak discharge of about . Within the caldera 12–17 ice cauldrons are supra- and inglacial manifestations of a near-surface magmatic storage system. K. Scharrer 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''". Others have also documented the change with time of ice cauldrons at Katla. They have a depth of and a width of . 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''".

Ice cauldrons in other environments

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 ( Dena'ina: ''Bentuggezh K’enulgheli''), is an active stratovolcano in the largely volcanic Aleutian Range of the U.S. state of Alaska. Located at the head of the Chigmit Mountains subrange in Lake C ...

, Mount Spurr).

Ice cauldrons and volcano monitoring

As deepening and widening of the ice cauldrons at Katla volcano, and especially 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 (or generally, quakes) and the generation and propagation of elastic ...

activity, are interpreted as signs of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

inflow, the cauldrons are closely monitored.

Notes

References

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