Fagradalsfjall (Icelandic pronunciation: [ˈfaɣraˌtalsˌfjatl̥]) is a tuya volcano formed in the Last Glacial Period on the Reykjanes Peninsula,[4][5] around 40 kilometres (25 mi) from Reykjavík, Iceland.[6] Fagradalsfjall is also the name for the wider volcanic system covering an area 5 kilometres (3 mi) wide and 16 kilometres (10 mi) long between the Svartsengi and Krýsuvík systems.[7] The highest summit in this area is Langhóll [ˈlauŋkˌhoutl̥] (385 m (1,263 ft)).[1] No volcanic eruption had occurred for 815 years on the Reykjanes Peninsula until 19 March 2021 when a fissure vent appeared in Geldingadalir to the south of Fagradalsfjall mountain.[8][9] The 2021 eruption was effusive and last emitted fresh lava on 18 September 2021.[10] The eruption was unique among the volcanoes monitored in Iceland so far and is expected to develop into a shield volcano.[11][12] Due to its relative ease of access from Reykjavík, the volcano has become an attraction for local people and foreign tourists.[13][14]
The name is a compound of the Icelandic words 'fagur' ("fair", "beautiful"), 'dalur' ("dale", "valley") and 'fjall' ("fell", "mountain"). The mountain massif is named after Fagridalur ([ˈfaɣrɪˌtaːlʏr̥], "fair dale" or "beautiful valley") which is at its northwest.[1] The 2021 lava field is named Fagradalshraun [ˈfaɣraˌtalsˌr̥œiːn].[15]
The mountain Fagradalsfjall is a volcano in an area of eruptive fissures, cones and lava fields also named Fagradalsfjall. The Fagradalsfjall fissure swarm is considered in some publications to be a branch or a secondary part of the Krýsuvík-Trölladyngja volcanic system on the Reykjanes Peninsula in southwest Iceland.[4][16] Other scientists propose that Fagradalsfjall could represent a separate volcanic system from Krýsuvík and it is regarded as such in some publications.[17][4] It is in a zone of active rifting at the divergent boundary between the Eurasian and North American plates. Plate spreading at the Reykjanes peninsula is highly oblique and is characterized by a superposition on left-lateral shear and extension.[18] The Krýsuvík volcanic system has been moderately active in the Holocene, with the most recent eruptive episode before the 21st century having occurred in the 12th-century CE.[19] The Fagradalsfjall mountain was formed from an eruption under the ice sheet in the Pleistocene period,[4] and it had lain dormant for 6,342 years until an eruption fissure appeared in the area in the March of 2021.[20]
The unrest and eruption in Fagradalsfjall are part of a larger unrest period on Reykjanes Peninsula including unrest within several volcanic systems and among others also the unrest at Þorbjörn volcano next to Svartsengi and the Blue Lagoon during the spring of 2020.[21] However, eruptions at this location were unexpected as other nearby systems on the Reykjanes Peninsula had been more active.[22]
The 2021 eruption is the first to be observed on this branch of the plate boundary in Reykjanes.[22] It appears to be different from most eruptions observed where the main volcanoes are fed by a magma chamber underneath, whose size and pressure on it determine the size and length of eruption. This eruption may be fed by a relatively narrow and long channel (~ 17 km) that is linked to the Earth's mantle, and the lava flow may be determined by the properties of the eruption channel.[23][24]
Beginning December 2019 and into March 2021, a swarm of earthquakes, two of which reached magnitude Mw5.6, rocked the Reykjanes peninsula, sparking concerns that an eruption was imminent,[25][26][27] because the earthquakes were thought to have been triggered by dyke intrusions and magma movements under the peninsula.[28] Minor damage to homes from a 4 February 2021 magnitude 5.7 earthquake was reported.[29] In the three weeks before the eruption, more than 40,000 tremors were recorded by seismographs.[30]