The volcanic system of Krýsuvík (or Krísuvík, both pronounced [ˈkʰriːsʏˌviːk] in Icelandic, also Trölladyngja-Krýsuvík or Krýsuvík-Trölladyngja [ˈtʰrœtlaˌtiɲca] volcanic system), is situated in the south–west of Iceland on the Reykjanes peninsula. It is located in the middle of Reykjanes and on the divergent plate boundary of the Mid-Atlantic Ridge which traverses Iceland. It was named after the Krýsuvík area which is part of it and consists of a fissure system without a central volcano. However, there are some indications—namely, the discovery by geophysical methods of what scientists interpret as a buried caldera, combined with the well-known, vigorous hydrothermal system above it—that an embryonic central magma chamber may already exist or be actively developing. The volcanic system has a length of 55 km (34 mi), a width of around 13 km (8.1 mi), covers an area of 350 km2 (140 sq mi), and its highest elevation is 393 m (1,289 ft). It is one of 4 (or up to 7, depending on the source) volcanic systems situated within the Reykjanes Volcanic Belt. The volcanic systems are arranged en echelon and at an angle that varies from 20 to 45° to the direction of the rift zone on the divergent plate boundary traversing Reykjanes.
General characteristics The volcanic system of Krýsuvík has no central volcanic edifice, but rather a fissure swarm that is 50 km (31 mi) long, is composed of a mixture of volcanic and tectonic fissures and faults, of which 30 km (19 mi) are volcanic fissures. Recent geophysical work on the system, employing magnetotelluric resistivity sampling and modeling, indicates the presence of a buried caldera with possibly an embryonic central magma chamber beneath the system. This suggests that it could eventually develop into a central volcano like the similar, but more mature Hengill volcanic system farther to the east on the peninsula. There are no known submarine fissures of the system which nevertheless reaches from the south coast in direction south–west to north–east over the Reykjanes Peninsula. The northernmost fissures are thought to reach Lake Rauðavatn [ˈrœyːðaˌvahtn̥] on the outskirts of Reykjavík. There are no ice-covered volcanoes connected to the Krýsuvík system, but Lake Kleifarvatn lies within the system and geothermal activity is found at the lake bottom. The Krýsuvík volcanic system has a tendency to effusive basaltic fissure eruptions; the last eruption took place in the 14th century. The Fagradalsfjall fissure swarm that erupted in 2021 was initially considered potentially a branch or a secondary part of the Krýsuvík volcanic system, but it is now usually considered a separate volcanic system. The eruption products of the Krýsuvík system consist exclusively of basalt.
Eruptions The volcanic system is centered on the divergent plate boundary on Reykjanes peninsula. It is easier there for magma to reach the surface, because of the multitude of tectonic and volcanic faults and fissures in such regions. There have been at least 10 volcanic episodes within the volcanic system in the last 8,000 years. These episodes each comprised many single eruptions and were most probably connected to rifting. Some Holocene eruptions have been dated specifically, especially the eruption that produced Búrfellshraun (ca. 5290 BP). Since the time of settlement in Iceland, which is thought to have been in the 9th century, more eruptions have taken place within the system, all of them in the Middle Ages. The Krýsuvík fires were a period of volcanic activity which started in the middle of the 12th century, probably in 1151 and written sources indicate that they ended in 1188. The activity of Pleistocene shield volcanoes such as Þráinskjöldur [ˈθrauːɪnˌscœltʏr̥] and Hrútagjá [ˈr̥uːtaˌcauː], as well as of tuyas like Fagradalsfjall within the volcanic system, are seen as separate from the fissure system, although the bigger volcanoes control parts of the topography. The Krýsuvík system has a tendency to phreatic explosions, often within rifting episodes or during eruption series. The underground of Reykjanes peninsula is soaked with water as it has a high groundwater level as well as saline sea water in its cave systems. There is a prehistoric maar complex around Grænavatn at Krýsuvík which has its origin in phreatic explosions connected to a period of effusive eruptions. There an explosion connected to geothermal activity of an old borehole in 1999 at Seltún. Starting on 27 September 2021, an intense earthquake swarm began that was concentrated around the Keilir region with over 1000 earthquakes with a magnitude 4.2 event on 2 October. The earthquakes sparked concern that a second eruption could begin in the area but it was not known definitely what was causing the swarm. During the overnight hours of 10 October 2021, a strong M3.2 earthquake occurred 2 km (1.2 mi) SSW of Keilier. From about July 2020 until autumn 2023 there was upward vertical ground motion in the Krýsuvík area and after this the ground started subsiding. This may be associated with magma movement underground.
List of lava fields These are some lava fields which originated in eruptions of the Krýsuvík volcanic system since the end of the last glacial spell 13,000 years ago.
Búrfellshraun Around 8,000 years ago, the Búrfell crater near Hafnarfjörður produced a 18 km2 (6.9 sq mi) lava field called Búrfellshraun [ˈpurˌfɛlsˌr̥œyːn]. Today, a big part of midtown Hafnarfjörður is built onto and around Búrfellshraun. The crater contains a lava channel called Búrfellsgjá [ˈpurˌfɛlsˌcauː].
Óbrinnishólabruni The Óbrinnishólabruni [ˈouːˌprɪnːɪsˌhouːlaˌprʏːnɪ] lavas came 2,000 years ago from some craters near Bláfjallavegur [ˈplauːˌfjatlaˌvɛːɣʏr̥] (Road 407) which have since been destroyed by quarrying. The name Óbrinnishólar [ˈouːˌprɪnːɪsˌhouːlar̥] means that there was no “fire” in them during further eruptions in the region in historical time. Parts of Hafnarfjörður (midtown and Vallahverfi [ˈvatlaˌkʰvɛrvɪ]) are located on top of this lava field.
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