The Katla volcanic system, in southern Iceland, comprises a central volcano and a fissure system. It is partly covered by the Mýrdalsjökull ice cap, which infills the c. 650 m-deep summit caldera. Katla is one of Iceland's more active volcanoes.
Katla has had about 20 eruptions in the last 1100 years, with major glacial floods (jökulhlaups) accompanying observed eruptions. The durations of documented eruptions range from two weeks to four months.
The last eruption to break through the ice cap occurred in 1918, although subglacial flood events in 1955 and 1999 may have been caused by small eruptions. The 1918 ash plume was documented to have reached heights of 14 km.
Eruptions from Katla are mainly basaltic although there have been some silicic eruptions. They typically produce plumes of volcanic ash, formed by the interaction of the erupting magma with the ice. However, the Katla volcanic system was the source of one of the largest known flood basalt eruptions in Iceland, the Eldgjá eruption of AD 934–940.
Since June 2011, Katla has been showing signs of unrest, with many hundreds of small earthquakes. A short-lived subglacial flood occurred in July 2011, and a particularly intense swarm of earthquakes was recorded on 5 October 2011. The volcano is being closely monitored by the Icelandic Met Office
Katla lies some 25 km to the east of Eyjafjallajökull, which erupted in the spring of 2010. Previous eruptions of Eyjafjallajökull, in 1612 and 1821–1823, were followed within a few months by eruptions at Katla, but linkages between the two volcanoes are not well understood.
Katla is known to be seismically active — small, shallow earthquakes are commonly recorded at the volcano, with earthquake activity peaking in the autumn.
This seasonality may be due to reduction in pressure on the magma chamber as the ice cap melts during the summer.
Earthquakes at Katla occur in two main areas — Goðabunga in the west, and the central caldera.
Earthquake monitoring at Katla is carried out by the Icelandic Met Office (IMO), in collaboration with the BGS. The IMO publish a regularly updated earthquake map that shows data for earthquakes in the area over the previous 48 hours.
Seismic activity such as that seen during the autumn of 2011 doesn't always lead to a major eruption.
Periods of increased earthquake activity were seen at Katla in 1967, 1976 to 1977, 1999, and 2002 to 2004. The July 1999 activity was associated with a subglacial flood or jökulhlaup.