Fagradalsfjall () is an active tuya volcano formed in the Last Glacial Period on the Reykjanes Peninsula, around from Reykjavík, Iceland. Fagradalsfjall is also the name for the wider volcanic system covering an area wide and long between the Eldvörp–Svartsengi and Krýsuvík systems. The highest summit in this area is Langhóll (). 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 on 10 July 2023, and ended on 5 August 2023.
Etymology The name is a compound of the Icelandic words ("fair", "beautiful"), ("dale", "valley") and ("fell", "mountain"). The mountain massif is named after (, "fair dale" or "beautiful valley") which is at its northwest. The 2021 lava field is named Fagradalshraun .
Tectonic setting 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.See eg.: Geirsson, H., Parks, M., Vogfjörd, K., Einarsson, P., Sigmundsson, F., Jónsdóttir, K., Drouin, V., Ófeigsson, B. G., Hreinsdóttir, S., and Ducrocq, C.: The 2020 volcano-tectonic unrest at Reykjanes Peninsula, Iceland: stress triggering and reactivation of several volcanic systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7534, https://doi.org/10.5194/egusphere-egu21-7534 , 2021. https://meetingorganizer.copernicus.org/EGU21/EGU21-7534.html Retrieved: 6 April 2021 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.See eg.: Geirsson, H., Parks, M., Vogfjörd, K., Einarsson, P., Sigmundsson, F., Jónsdóttir, K., Drouin, V., Ófeigsson, B. G., Hreinsdóttir, S., and Ducrocq, C.: The 2020 volcano-tectonic unrest at Reykjanes Peninsula, Iceland: stress triggering and reactivation of several volcanic systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7534, https://doi.org/10.5194/egusphere-egu21-7534 , 2021. https://meetingorganizer.copernicus.org/EGU21/EGU21-7534.html Retrieved: 6 April 2021 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 (~ ) 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.
2019 to 2021 activity and eruptions
Precursors Beginning December 2019 and into March 2021, a swarm of earthquakes, two of which reached magnitude , 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.
Eruption fissures in Geldingadalir On 19 March 2021, an effusive eruption started at approximately 20:45 local time in Geldingadalir (; the singular "Geldingadalur" 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 fissure vent began ejecting lava, which covered an area of less than . As of the March eruptions, the lava flows posed no threat to residents, as the area is mostly uninhabited.
The eruption has been called ( "Geldingadalur eruption"). On 26 March, the main eruptive vent was at , on the site of a previous eruptive mound. The eruption may be a shield volcano eruption, which may last for several years. It could be seen from the suburbs of the capital city of Reykjavík and had attracted a large number of visitors. However, high levels of volcanic gases such as carbon dioxide and carbon monoxide made parts of the area inaccessible.
On 13 April 2021, four new craters formed in Geldingadalir within the lava flows. The lava output which had been somewhat reduced over the last days, increased again.
Eruption fissures on Fagradalsfjall Around noon on 5 April 2021, a new fissure, variously estimated to be between about long, opened a distance of about to the north/north-east of the still-active vents at the center of the March eruption. As a precaution the area was evacuated by the coast guard.
Some time later, another eruption fissure opened parallel to the first on the slopes of Fagradalsfjall."Tvær nýjar sprungur og hraunið rennur í Meradali". RÚV. 5 April 2021. Retrieved 6 April 2021
The lava production of all open eruption fissures in the whole was estimated on 5 April 2021, being around "Gosið hefur vaxið - 10 rúmmetrar af kviku á sekúndu". . RÚV. Retrieved 6 April 2021."Eldgos í geldingadölum". . Háskóli Íslands. Jarðvínsindastofnun. Retrieved: 6 April 2021 and is flowing into the Meradalir valleys (, "mare dales") via a steep gully.
About 36 hours later, around midnight on 6–7 April, another eruption fissure opened up. It is about long and about to the north-east of the first fissure, between the Geldingadalur fissures and the ones on the slope of the mountain.https://en.vedur.is/#tab=quakes "Specialist remark"]. . Earthquake page of Icelandic Met Office. Retrieved 7 April 2021."Ný sprunga búin að opnast". RÚV. Retrieved 7 April 2021 Search and rescue crews observed a new depression, about deep there the previous day. The lava from this fissure flowed into Geldingadalur valley.
Another fissure opened during the night of 10–11 April 2021 between the two open fissures on the slopes of Fagradalsfjall."Ný sprunga opnaðist í Geldingadölum í nótt". . RÚV. Retrieved 13 April 2021 In total, 6 fissures had opened until the 13 April and at each fissure, activity concentrated and formed individual vents. Towards the end of April, activity at most vents, apart from Vent 5, started to decrease.
By 2 May 2021, only one fissure, Vent 5 that appeared near the initial eruption site on Geldingadalir, remained active. It developed into a volcano with the occasional explosive eruptions within its crater that sometimes reached heights of hundreds of meters. The rim of the volcano itself had risen to a height of above sea level by September 2021. The lava flowed into the Meradalir valleys, and later the Nátthagi valley.
A number of smaller openings appeared temporarily, one small vent was reported to have erupted near the main crater on 1 July. On 14 August, lava spurted from what appeared to be a hole on the crater wall, and this turned out to be an independent eruption. Cracks appeared on Gónhóll that was once popular with spectators in August but no lava flowed at the site. After eight and a half days of inactivity at the main volcano, lava broke through the surface in the lava field to the north of the crater in a number of places.
Lava and gas output: Development of the eruption The eruption showed distinct phases in its eruption pattern. The first phase lasted for about two weeks with continuous lava flow of around from its first crater, the second phase also lasted around two weeks with new eruptions to the north of the first crater with variable lava flow of . This is followed by a period of two and a half months of eruption at a single crater with largely continuous and sometimes pulsating eruption and lava flow of around lasting until the end of June. From then on until early September was a phase of fluctuating eruption with periodic strong lava flow interrupted by periods of inactivity.
On 12 April, scientists from the University of Iceland measured the lava field's area to be and its volume to be . The flow rate of the lava was , and sulfur dioxide, carbon dioxide and hydrogen fluoride were being emitted at respectively."Eldgos í Fagradalsfjalli". . Jarðvísíndastofnun Háskóla Íslands. Retrieved 13 April 2021.
The lava produced by the eruption shows a composition differing from historical Reykjanes lavas. This could be caused by a new batch of magma arriving from a large magma reservoir at a depth of about at the Moho under Reykjanes.
Results from measurements published by the University of Iceland on 26 April 2021 showed that the composition of eruption products had changed, to more closely resemble the typical Holocene basalts of Reykjanes peninsula."Eldgos í Fagradalsfjalli". . Jarðvísindastofnun. Háskóli Íslands. Retrieved 28 April 2021. The full study of the lava sample sequence confirmed that at the start of the eruption shallowest mantle sources dominated the melt but over the next three weeks magma was sourced from a greater depth from a near-Moho reservoir and rapidly rose to the surface. While it was known that most mid-ocean ridge (MOR) basalts result from crustal reservoir melts this proved the suspicion that more rarely, mid-ocean ridge eruptions may be supplied from levels deeper than , as the near real time lava sampling prevented later lavas confusing the time sequence of the change in composition. Mixing in the magma reservoir happened in a timescale of weeks, and it was replenished with new deeper melt within 20 days.
The eruption itself also changed in character at the same time, and was producing lava fountains up to in height on Sunday, 25 April 2021."Eldgosið síðasta sólarhringinn – aukin sprengivirkni". . RúV. 27 April 2021. Retrieved 28 April 2021. On 28 April 2021, the lava fountains from the main crater reached a height of .