Fagradalsfjall

Vorlage:Short description Vorlage:Use American English Vorlage:Use dmy dates Vorlage:Infobox mountain Fagradalsfjall (Vorlage:IPA-is) is a tuya volcano formed in the Last Glacial Period on the Reykjanes Peninsula,[1][2] around Vorlage:Convert from Reykjavík, Iceland.[3] Fagradalsfjall is also the name for the wider volcanic system covering an area Vorlage:Convert wide and Vorlage:Convert long between the Svartsengi and Krýsuvík systems.[4] The highest summit in this area is Langhóll Vorlage:IPA-is (Vorlage:Convert).[5] No volcanic eruption had occurred for 800 years on the Reykjanes Peninsula until 19 March 2021 when a fissure vent appeared in Geldingadalir to the south of Fagradalsfjall mountain.[6][7] The 2021 eruption was effusive and last emitted fresh lava on 18 September 2021.[8] The eruption was unique among the volcanoes monitored in Iceland so far and is expected to develop into a shield volcano.[9][10] Due to its relative ease of access from Reykjavík, the volcano has become an attraction for local people and foreign tourists.[11][12]

Etymology

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 (Vorlage:IPA-is, "fair dale" or "beautiful valley") which is at its northwest.[5] The 2021 lava field is named Fagradalshraun Vorlage:IPA-is.[13]

Tectonic setting

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.[1][14] Other scientists propose that Fagradalsfjall could represent a separate volcanic system from Krýsuvík and it is regarded as such in some publications.[15][1] 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.[16] 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.[17] The Fagradalsfjall mountain was formed from an eruption under the ice sheet in the Pleistocene period,[1] and it had lain dormant for 6,000 years until an eruption fissure appeared in the area in March 2021.[18]

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.[19] However, eruptions at this location were unexpected as other nearby systems on the Reykjanes Peninsula had been more active.[20]

The 2021 eruption is the first to be observed on this branch of the plate boundary in Reykjanes.[20] 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.[21][22]

2021 eruption series

Precursors

Beginning December 2019 and into March 2021, a swarm of earthquakes, two of which reached magnitude Vorlage:M, rocked the Reykjanes peninsula, sparking concerns that an eruption was imminent,[23][24][25] because the earthquakes were thought to have been triggered by dyke intrusions and magma movements under the peninsula.[26] Minor damage to homes from a 4 February 2021 magnitude 5.7 earthquake was reported.[27] In the three weeks before the eruption, more than 40,000 tremors were recorded by seismographs.[28]

Eruption fissures in Geldingadalir

On 19 March 2021, an effusive eruption started at approximately 8:45 PM local time in Geldingadalir (Vorlage:IPA-is;Die Verwendung dieser Vorlage ist in der deutschsprachigen Wikipedia unerwünscht. the singular "Geldingadalur" Vorlage:IPA-is is also often used)[29] to the south of Fagradalsfjall,[6] the first known eruption on the peninsula in about 800 years. Fagradalsfjall had been dormant for 6,000 years.[30][31] The eruptive activity was first announced by the Icelandic Meteorological Office at 9:40 PM.[32] Reports stated a Vorlage:Convert fissure vent began ejecting lava,[33] which covered an area of less than Vorlage:Convert. As of the March eruptions, the lava flows posed no threat to residents, as the area is mostly uninhabited.[3]

The eruption has been called Geldingadalsgos (Vorlage:IPA-is "Geldingadalur eruption").[34] As of 26 March, the main eruptive vent was at 63.8889 N, 22.2704 W, on the site of a previous eruptive mound. The eruption may be a shield volcano eruption,[35] which may last for several years.[35] It can be seen from the suburbs of the capital city of Reykjavík[36] and has attracted a large number of visitors.[37] However, high levels of volcanic gases such as carbon dioxide and carbon monoxide make parts of the area inaccessible.[38]

Geldingadalir eruption near Fagradalsfjall, 24 March 2021.
People on the slopes of Fagradalsfjall, watching the Geldingadalir eruption.
Satellite image from 29 April 2021

On 13 April 2021, 4 new craters formed in Geldingadalir within the lava flows. The lava output which had been somewhat reduced over the last days, increased again.[39]

Eruption fissures on Fagradalsfjall

Around noon on 5 April a new fissure, variously estimated to be between about Vorlage:Convert long, opened a distance of about Vorlage:Convert 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.[40][41][42]

Some time later, another eruption fissure opened parallel to the first on the slopes of Fagradalsfjall.[43]

The lava production of all open eruption fissures in the whole was estimated on 5 April 2021, being around Vorlage:Convert [44][45] and is flowing into the valley Meradalir (Vorlage:IPA-is, "mare dale") via a steep gully.[46]

The new eruption fissures.
The new eruption fissures to the left, the older ones to the right, seen from a helicopter, view to the east.

About 36 hours later, around midnight on 6–7 April, another eruption fissure opened up. It is about Vorlage:Convert long and about Vorlage:Convert to the north-east of the first fissure, between the Geldingadalur fissures and the ones on the slope of the mountain.[47][48][49] Search and rescue crews observed a new depression, about Vorlage:Convert deep there the previous day. The lava from this fissure flowed into Geldingadalur valley.[50]

Another fissure opened during the night of 10–11 April 2021 between the two open fissures on the slopes of Fagradalsfjall.[51] 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.[52]

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.[53] The rim of the volcano itself had risen to a height of Vorlage:Convert above sea level by September 2021.[21] The lava flowed into the Meradalir valley,[54] and later the Nátthagi Vorlage:IPA-is valley.[55]

A number of smaller openings appeared temporarily, one small vent was reported to have erupted near the main crater on 1 July.[56] 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.[57] Cracks appeared on Gónhóll that was once popular with spectators in August but no lava flowed at the site.[58] 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.[59]

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 Vorlage:Convert 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 Vorlage:Convert . 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 Vorlage:Convert 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.[60][61]

On 12 April, scientists from the University of Iceland measured the lava field's area to be Vorlage:Convert and its volume to be Vorlage:Convert. The flow rate of the lava was Vorlage:Convert, and sulfur dioxide, carbon dioxide and hydrogen fluoride were being emitted at Vorlage:Convert respectively.[62]

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 Vorlage:Convert at the Moho under Reykjanes.[63][64][65]

Examples of basaltic lava collected in late March

Results from measurements published by 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.[66] The eruption itself also changed in character at the same time, and was producing lava fountains up to Vorlage:Convert in height on Sunday, 25 April 2021.[67] On 28 April 2021, the lava fountains from the main crater reached a height of Vorlage:Convert.[68]

Lava fountains of the Fagradalsfjall eruption, seen from Reykjavík on 9 May 2021

The eruption pattern changed on 2 May from a continuous eruption and lava flow to a pulsating one, where periods of eruptions alternated with periods of inactivity, with each cycle lasting 10 minutes to half an hour.[69][70] The magma jets became stronger, producing lava fountains of Vorlage:Convert in height, visible from Reykjavík,[71][72] with the highest one measured at Vorlage:Convert.[70] The lava jets have been explained as explosive release of ancient trapped water or magma coming in contact with groundwater.[73][74] The lava flow rate in the following weeks was also double that of the average for the first six weeks,[75] with an average lava flow rate of Vorlage:Convert from 18 May to 2 June.[76]

The increase in lava flow is unusual, as eruption outputs typically decrease with time. Scientists from the University of Iceland hypothesize that there is a large magma reservoir deep under the volcano, not the typical smaller magma chamber associated with these kinds of eruptions that empty over a short time.[77] From the composition of the magma sampled, they also believe that there is a discrete vent feeding the main lava flow from a depth of Vorlage:Convert from the Earth's mantle, and may be of a more primitive kind than those previously observed.[78] The channel widened in the first six weeks leading to increased lava flow.[21] The eruption may create a new shield volcano if it continues for long enough.[79] The formation of such volcano has not been studied before in real time, and this eruption can offer insights into the working of the magmatic systems.[10]

Two defensive barriers were created starting 14 May as an experiment to stop lava flowing into the Nátthagi valley where telecommunication cables are buried, and further on to the southern coastal road Suðurlandsvegur.[80] However, the lava soon flowed over the top of eastern barrier 22 May, and cascaded down to the Nátthagi.[81][82][83] Lava flowed over the western barrier on 5 June.[84] Lava flow blocked the main trail that provide access to the main viewing area on Gónhóll, first on 4 June,[85] then again early in the morning of 13 June at another location.[86] A further wall five meters high and 200 meters long was then created on 15 June in an attempt to divert lava flow away from Nátthagakriki Vorlage:IPA-is with important infrastructure to its west and north.[87] A barrier of 3 to 5 m high started to be constructed on 25 June at the mouth of Nátthagi to delay the flow of the lava over the southern coastal road and properties on Ísólfsskáli Vorlage:IPA-is, although it was expected that the lava would eventually flow over the area into the sea.[88][89] A proposal to build a bridge over the road to allow the lava flow underneath was rejected.[90]

Around three months after the volcano first erupted, the lava flow was a steady Vorlage:Convert, and the lava now covered an area of more than Vorlage:Convert increasing by around Vorlage:Convert.[91][92] Lava had accumulated Vorlage:Convert deep around the volcano.[93] The lava flow became continuous, which can be either above or below ground, although the eruptions also became calmer with the occasional increase in activity.[94][95] There appeared to be no direct connection between the activity at the crater and lava flow.[96] The lava flow can be tracked by helicopter or satellite, for example via radar imaging that can penetrate through the clouds and volcanic smog that had become more frequent in the area by July.[97][98]

The eruptions stayed unusually constant until 23 June, and the activity then reduced significantly on 28 June, becoming inactive for many hours,[99][100] and resuming on 29 June.[101][102] It shifted to a pattern of many hours of inactivity, for example on 1 July and 4 July,[103][104] with the eruptions resuming later.[105] Lava flow from the crater ceased for 4 days from 5 July until 9 July,[106][107] when eruptions resumed, initially with a periodicity of around 10 to 15 minutes,[108] then lengthening to 3 to 4 an hour by 13 July.[109] Lava has also been observed emerging from the bottom of the volcano on 10 July with considerable amount of lava flowing into the Meradalir valley,[108][110][111] and a section of the volcano on the northeastern side also broke off on the 14 July.[112] Lava flow was estimated to be around Vorlage:Convert but averaged to Vorlage:Convert due to the periods of inactivity from late June to mid-July, half of the flow rate in May and June.[113] The periodic lull in activity continued,[114][115] with 7 to 13 hours of inactivity and similar period of eruption by late July,[116] which lengthened to a pattern of mostly around 15 hours of inactivity alternating with around 20 hours of continuous eruption in August.[117] It has been speculated that there are blockages at the top hundred metres of the eruption channel.[115] By July, this eruption had become larger than most eruptions that have ever occurred on the Reykjanes peninsula.[118] Measurement taken on 27 July indicated that the lava flow had increased again, returned to and possibly exceeding the peak level last seen in June.[119] The measurement indicated an average flow of Vorlage:Convert over 8–10 days, the highest observed thus far, but with a large margin of error.[120] After a couple of months where the lava flowed mainly into the Meradalir valley, the lava started to flow down the Nátthagi valley again on 21 August.[121][122] The eruption by now had become the second longest in Iceland of the 21st century.[123]

The volcano stopped erupting on 2 September,[124] but lava flow resumed on 11 September, with the magma breaking through the lava field surface in several places.[125] However, the main crater channel appeared to have been blocked, and the crater was filled with lava from a source underneath the northwestern wall through a crack on the wall,[126] and lava also flowed outside the volcano through the wall.[59] The average lava flow over the past 32 days had returned to Vorlage:Convert, and the lava field of Vorlage:Convert now covered an area of Vorlage:Convert.[21][127] After a period of continuous eruption, a pulsing pattern of activity last seen in April/May started on 13 September,[128] a pattern believed to be similar to what is observed in geysers where the frequency of eruption may be determined by the size of the reservoir below and how quickly it is filled up. The volcano was pulsing at a rate of around 8 eruptions per hour on 14 September.[129] No lava flowed out directly from the crater, instead lava began to emerge in significant amount from outside the volcano on 15 September.[130] On 16 September 2021, after 181 days of eruption, it became the longest eruption of the 21st century in Iceland.[131] Average lava flow was Vorlage:Convert from 11 to 17 September when flow resumed, with the lava field increasing to Vorlage:Convert covering an area of Vorlage:Convert.[132] Eruption stopped again on 18 September but the activity decreased unusually slowly.[133] On October 18, the alert level was lower from "Orange" to Yellow" due to no lava having erupted since September 18. The Icelandic meteorological office also stated that "it is assessed that Krýsuvík volcano is currently in a non-eruptive state. The activity might escalate again so the situation is monitored closely".[134]

Supposed burial site

The area where the volcano first erupted is thought to be the ancient burial site of an early Norse settler Ísólfur frá Ísólfsstöðum Vorlage:IPA-is.[135] However, a quick archaeological survey of Geldingadalur after the eruption started found no evidence of human remains in the area.[136]

1943 accident

Recovery operation at the wreckage of Hot Stuff after the accident

On 3 May 1943, LTG Frank Maxwell Andrews, a U.S. Army senior officer and founder of the United States Army Air Forces, was killed along with fourteen others when their B-24 aircraft Hot Stuff crashed into the side of the mountain.[137][138]

Notes

Vorlage:Reflist

References