Fagradalsfjall

Fagradalsfjall (Icelandic: [ˈfaɣraˌtalsˌfjatl̥] ^ⓘ) is an active tuya volcano formed in the Last Glacial Period on the Reykjanes Peninsula, around 40 kilometres (25 mi) from Reykjavík, Iceland. 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 Eldvörp–Svartsengi [ˈɛltvœr̥p–ˈsvar̥(t)sˌeiɲcɪ] and Krýsuvík systems. The highest summit in this area is Langhóll [ˈlauŋkˌhoutl̥] (385 m (1,263 ft)). 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. The 2021 eruption was effusive and continued emitting fresh lava sporadically until 18 September 2021.

The eruption was unique among the volcanoes monitored in Iceland so far and it has been suggested that it could develop into a shield volcano. Due to its relative ease of access from Reykjavík, the volcano has become an attraction for local people and foreign tourists. Another eruption, very similar to the 2021 eruption, began on 3 August 2022, and ceased on 21 August 2022. A third eruption appeared to the north of Fagradalsfjall near Litli-Hrútur [ˈlɪhtlɪ-ˌr̥uːtʏr̥] on 10 July 2023, and ended on 5 August 2023.

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. The 2021 lava field is named Fagradalshraun [ˈfaɣraˌtalsˌr̥œyːn].

The mountain Fagradalsfjall is a volcano in areas of eruptive fissures, cones and lava fields also named Fagradalsfjall. The Fagradalsfjall fissure swarm was 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, but scientists now consider Fagradalsfjall to be a separate volcanic system from Krýsuvík and it is regarded as such in some publications. 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 of left-lateral shear and extension. 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. The Fagradalsfjall mountain was formed from an eruption under the ice sheet in the Pleistocene period, and it had lain dormant for 6,000 years until an eruption fissure appeared in the area in March 2021. The wider peninsula had been dormant for nearly 800 years, since the end of the Reykjanes Fires in 1240.

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. However, eruptions at this location were unexpected as other nearby systems on the Reykjanes Peninsula had been more active.

The 2021 eruption is the first to be observed on this branch of the plate boundary in Reykjanes. 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 (11 mi)) that is linked to the Earth's mantle, and the lava flow may be determined by the properties of the eruption channel. However, the channel may also be linked to a deep magma reservoir located near the boundary between the crust and the mantle. Some scientists believed that volcanic activities in the area may last for decades.

Beginning December 2019 and into March 2021, a swarm of earthquakes, two of which reached magnitude M_w5.6, rocked the Reykjanes peninsula, sparking concerns that an eruption was imminent, because the earthquakes were thought to have been triggered by dyke intrusions and magma movements under the peninsula. Minor damage to homes from a 4 February 2021 magnitude 5.7 earthquake was reported. In the three weeks before the eruption, more than 40,000 tremors were recorded by seismographs.

On 19 March 2021, an effusive eruption started at approximately 20:45 local time in Geldingadalir ([ˈcɛltiŋkaˌtaːlɪr̥]; the singular "Geldingadalur" [ˈcɛltiŋkaˌtaːlʏr̥] is also often used) to the south of Fagradalsfjall, the first known eruption on the peninsula in about 800 years. Fagradalsfjall had been dormant for 6,000 years. The eruptive activity was first announced by the Icelandic Meteorological Office at 21:40. Reports stated a 600–700-metre-long (2,000–2,300 ft) fissure vent began ejecting lava, which covered an area of less than 1 square kilometre (0.39 sq mi). As of the March eruptions, the lava flows posed no threat to residents, as the area is mostly uninhabited.