Ice cauldrons and their relationship with volcanic and geothermal activity in ice-covered volcanoes in Iceland

About 10% of Iceland is covered by glaciers, including three of the four most active central volcanoes, Grímsvötn, Katla and Bárðarbunga, where the ice-filled calderas have glacier thickness of several hundred meters.  Ice cauldrons, defined as circular or elongated depressions in the surface of glaciers which form due to enhanced basal melting, are very common in the ice-covered volcanic areas in Iceland.  Many have concentric crevasses.  Over 100 cauldrons have been identified in the glaciers of Iceland, with over half being located within or adjacent to the calderas of Grímsvötn, Katla and Bárðarbunga.  Ice cauldrons are usually a sign of subglacial geothermal activity, while occasionally cauldrons are formed during subglacial volcanic eruptions.  Some cauldrons are short term features, existing for some years, indicating transient geothermal activity or recent occurrence of a volcanic eruption.  Others are semi-stable features that have existed for several decades, possibly centuries.  Stable cauldrons are always associated with geothermal activity and the same applies to many of the intermittently active ones as well.  Cauldrons formed in volcanic eruptions tend to be deep and with larger depth/width ratios than geothermal cauldrons.  The widest cauldrons observed in Iceland reach 4-5 km in diameter while diameters of 0.5-1.0 km are the most common.   The size and form of geothermal ice cauldrons depends to a considerable degree on the ice thickness.  Where thickness is small (<100 m), cauldrons can be holes with steep slopes or vertical ice walls and may in some cases reach the base of the glacier.  Where ice thickness exceeds 200-300 m, geothermal cauldrons rarely form vertical ice walls.  In most cases, it is clear which cauldrons are formed by sustained geothermal activity. Volcanic cauldrons tend to reach their maximum size rapidly, followed by gradual decline as the erupted volcanic material loses its heat quickly and ice flow gradually fills the depression.  However, despite gradual infill by ice flow and reduction in cauldron depth, volcanically formed depressions may persist for decades.  An example of this is the tens-of-meters deep and 2-3 km wide depression at the site of the 1996 Gjálp eruption, which is still visible.  Ice thickness at this site was ~600 m prior the eruption. Beneath some ice cauldrons meltwater accumulates at the base of the glacier.  This may result in semi-periodic release of meltwater in jökulhlaups.  Other cauldrons may drain more or less continuously.