Katla () is an active subglacial volcano in southern Iceland. This volcano has been very active historically with at least twenty documented major eruptions since 2920 BC. In its recent history though, Katla has been less active as the last major eruption occurred in 1918. These eruptions have had a Volcanic Explosivity Index (VEI) of between 4 and 5 on a scale of 0 to 8. In comparison, the 2010 Eyjafjallajökull eruption had a VEI of 4. Larger VEI-5 eruptions are comparable to Mount St. Helens 1980 eruption. These eruptions have produced very large glacial outburst floods. Several smaller (minor) events measuring VEI-1 and below have occurred since.
Geography and physical appearances
Katla is one of the largest volcanoes in Iceland. It is situated to the north of Vík í Mýrdal and to the east of the smaller glacier Eyjafjallajökull. Its peak reaches and is partially covered by the Mýrdalsjökull glacier. The system has an area of . The Eldgjá canyon is part of the same volcanic system, and extends as a fissure swarm to the north-east. The lavas from this eruption in 939 to 940 almost reach the south eastern coast, although are partially buried.
The caldera of the Katla volcano has a diameter of and is covered with of ice. The volcano normally erupts every 40–80 years.
Historic activity
thumb|300px|Locations of [[caldera rim and previous eruptions from 1755 to 1999. The surface depressions shown are updated elsewhere on this page.]]
There have been up to twenty three eruptions recorded for Katla since 920 CE. Only two people are known to have died because of events directly associated with one of these eruptions in 1755. The 1918 eruption of Katla started in the late afternoon of 12 October 1918 and lasted for 24 days, with the first day having the most dangerous events, It has been graded a VEI 4 level eruption. which is ten to a hundred times the size of most jökulhlaups. Apart from the 1755 and 1918 eruptions already mentioned in this regard, both of which like the majority of large floods drained onto the Mýrdalssandur plain to the volcanoes south-east, an eruption in 822 drained from the north-east aspect of the caldera down the Markarfljót river flood plain to the mountain's south-west covering an area of . This is a convoluted route that outputs on to a large flood plain, the Markarfljótsaurar. The volcano's present dormancy is among the longest in its known history. Particularly, monitoring has been intensified following the March 2010 eruptions of a smaller neighbouring volcano, the Eyjafjallajökull glacier. The eruption of this nearby long-dormant volcano in March and April 2010 prompted fears among some geophysicists that it might trigger an eruption at the larger and more dangerous Katla. In the past 1,000 years, all three known eruptions of Eyjafjallajökull have triggered subsequent Katla eruptions.
Increased earthquake activity had been noticed on Katla since the eruption at Eyjafjallajökull, but no other signs of an imminent eruption were observed. These quakes are located mainly on the northwestern rim of the caldera. On 9 October 2010, a sudden rise in harmonic tremor was observed in the stations around Katla, but although a sign of a possible impending eruption, none occurred.
2011 activity
In 2011, geologic activity led many to speculate that a very small subglacial eruption had taken place.
On 8 and 9 July 2011, another spike in harmonic tremors occurred, as a small eruption of Katla took place. Cracks formed on the glacier, as well as a cauldron. Icelandic media reported a small subglacial eruption might have started. On the morning of 9 July 2011, a glacier flood was reported in the river Múlakvísl, and also later in the river Skálm. The bridge across Múlakvísl was destroyed as well as other parts of the road, Route 1, on the Icelandic ring road. Helicopter pilots flying over the glacier also reported cracks in the glacier. Scientists monitoring the activity said speculation that it was caused by a "very small" subglacial eruption lacked confirmation by visual or seismic evidence.
2016 and 2017 activity
Tremors were detected under Katla in late August 2016.
A "Specialist Description" describing the activity on 29 August 2016 noted that there was:<blockquote>... a seismic swarm in Mýrdalsjökull on the 29th of August with two events in the northern Katla caldera rim measured at magnitude 4.5. These are the biggest earthquakes in Katla volcano since 1977. Earthquake eruption checks confirmed that most earthquakes occurred between 0:40 and 1:50 PM. The big earthquakes were about thirty seconds apart at 1:47. They were followed by more than 50 aftershocks until 15:12 PM, when there was an earthquake of magnitude 3.3 and then the cycle and seismic activity in the region reduced again. No unrest was measured along these earthquakes. The geothermal has decreased in the following week and is now negligible.</blockquote>
An update written at 11 Sep 16:38 GMT reported:<blockquote>Today, shortly before 14:00, a small earthquake swarm began in Mýrdalsjökull. The largest earthquakes of the swarm were of magnitude 3.3 and 3.0 at 16:12 and 15:57. More than 10 smaller earthquakes were detected in the swarm. All of the earthquakes were shallow and located in the caldera of Katla volcano. It is not unusual for seismic swarms of this type to occur in this area.</blockquote>
In February 2017, seismic activity at the volcano continued.
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External links
- Katla in the "Catalogue of Icelandic Volcanoes"]
- Volcanism 1
- University of Iceland's Institute of Earth Sciences page about Katla
- Webcams from Iceland, including Katla
- Katla: preparedness for tourists
- Icelandic Meteorological Office: Katla status
- Constantly updating map of earthquakes in Mýrdalsjökull
- (with narration audio [mp3 format])