The partly ice covered volcanic system has been highly active in the Holocene with at least 21 eruption in the last 1100 years. The last eruption to break through the ice took place in 1918 CE.
Geological setting and textonic context
The Katla volcanic system is about 80 km long, located in the EVZ, with 1500 m high, 30 km diameter central volcano (Katla) lying near its southern end. Rifting occurs on the northern part of the system while it fades out towards SW with no signs of rifting south of Katla. Faint signs of NV-SE tensional tectonics appear on the NE-part. A short, now inactive part of the fissure swarm lies towards the south from the Katla central volcano.
The dominant magma type is trans-alkali basalt, but within the central volcano silicic magma, dacite to rhyolite is occaionally erupted. Geothermal activity associated with the caldera implies a presence of a shallow magma chamber.
Morphology and topography
The Katla central volcano is a hylaoclastite massif with the 100 km2 caldera at its center. The upper parts are covered by the 600 km2 Mýrdalsjökull ice cap. Within the caldera the ice is 400-700 m thick, north of the caldera it is 100-400 m thick and mostly less than 100 m on the ice covered slopes outside the caldera to the west, south and east. Bedrock in nunataks on the caldera rims rise to 1300-1380 m above sea level. The central volcano shows signs of radial tectonics and E-W trending fissures connect Katla and the neighbouring Eyjafjallajökull central volcano.
The fissure swarm is ice-free but cuts through areas of surface water and high ground water. It narrows to a few km in the extreme NE where it is defined by a single volcanic fissure. Geothermal fields within the caldera are expressed as cauldrons in the surface of the ice.
Eruption history and patterns
Volcanism at Mýrdalsjökull began >800,000 years ago but no details are known before the final stages of the Last Glacial (~last 10,000 years). Eruption history during the last 8500 years is however well known. In terms of frequency, most eruptions occur within the caldera and rarely on radial fissures on its slopes. Only a few eruptions have occurred on the fissure swarm. Eruptions in the ice filled caldera have lead to catastrophic floods (jökulhlaups) with discharge in range 30,000 – 300,000 m3/s, when ice is rapidly melted during the early part of the eruption.
Phreatomagmatic (hydrovolcanic) basaltic eruptions:
Dominant eruption type for Katla. Records of tephra in soils verify about 200 eruptions in the last 8500 years, while the total estimated number for this period is over 300. Tephra volumes (as freshly fallen) range from ≤0.02 to ≥2 km3. All eruptions during the last 1000 years took place within the caldera. During the last 500 years ash from three basaltic Katla eruptions has been reported overseas (Faroes, Scandinavia, British Isles).
Explosive silicic eruptions:
Some 14 eruptions have been verified and estimated number during the last 8500 years is ~20. Tephra volumes range from 0.02 to >0.27 km3, 1-2 orders of magnitude smaller than the major silicic eruption that formed the pre-Holocene Vedde ash 10,200 BCE. All eruptions of this type took place within or at the caldera rim. No silicic eruptions are known from the last 1000 years.
Effusive and explosive basaltic eruptions at margins of the central volcano:
Less than 10 eruptions are known to have occurred in last 8500 years, dominantly effusive, producing lava, sometimes with explosive component. Volumes range from ~0.01 to ~1 km3 of lava. Length of fissures ranges from several hundred meters to several km.