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Hekla (Icelandic pronunciation: [ˈhɛhkla] ⓘ), or Hecla,[2][3] is an active stratovolcano in the south of Iceland with a height of 1,491 m (4,892 ft). Hekla is one of Iceland's most active volcanoes; over 20 eruptions have occurred in and around the volcano since the year 1210.[4] During the Middle Ages, the Icelandic Norse called the volcano the "Gateway to Hell" and the idea spread over much of Europe.

Key Information

The volcano's frequent large and often initially explosive eruptions have covered much of Iceland with tephra, and these layers can be used to date eruptions of Iceland's other volcanoes. Approximately 10% of the tephra created in Iceland in the last thousand years has come from Hekla, amounting to 5 km3 (1.2 cu mi). Cumulatively, the volcano has produced one of the largest volumes of lava of any in the world in the last millennium, around 8 km3 (1.9 cu mi).

Etymology

[edit]

In Icelandic Hekla is the word for a short hooded cloak, which may relate to the frequent cloud cover on the summit. An early Latin source refers to the mountain as Mons Casule.[5]

Reputation

[edit]
Detail of Abraham Ortelius' 1585 map of Iceland showing Hekla in eruption. The Latin text translates as "The Hekla, perpetually condemned to storms and snow, vomits stones under terrible noise".
Illustration from Olaus Magnus's Historia de gentibus septentrionalibus, book 2, 1555

After the eruption of 1104, stories, probably spread deliberately through Europe by Cistercian monks, told that Hekla was the gateway to Hell.[6] The Cistercian monk Herbert of Clairvaux wrote in his De Miraculis (without naming Hekla):

The renowned fiery cauldron of Sicily, which men call Hell's chimney ... that cauldron is affirmed to be like a small furnace compared to this enormous inferno.[7]

— Herbert of Clairvaux, Liber De Miraculis, 1180

A poem by the monk Benedeit from c. 1120 about the voyages of Saint Brendan mentions Hekla as the prison of Judas.

In the Flatey Book Annal it was recorded that during the 1341 eruption, people saw large and small birds flying in the mountain's fire which were taken to be souls.[8] In the 16th century Caspar Peucer wrote that the Gates of Hell could be found in "the bottomless abyss of Hekla Fell". The belief that Hekla was the gate to Hell persisted until the 19th century.[7] There is still a legend that witches gather on Hekla during Easter.[9]

Geography

[edit]

Hekla is part of a volcanic ridge, 40 km (25 mi) long. The most active part of this ridge, a fissure about 5.5 km (3.4 mi) long named Heklugjá [ˈhɛhklʏˌcauː], is considered to be within Hekla proper. Hekla looks rather like an overturned boat, with its keel being a series of craters, two of which are generally the most active.[10][11]

Geology

[edit]
A map of the volcanic systems of Iceland

Hekla has a morphological type between that of a fissure vent and stratovolcano (built from mixed lava and tephra eruptions) sited at a rift-transform junction in the area where the south Iceland seismic zone and eastern volcanic zone meet. The unusual form of Hekla is found on very few volcanoes around the world, notably Callaqui in Chile.[12] The 5.5 km (3.4 mi) Heklugjá fissure opens along its entire length during major eruptions and is fed by a magma reservoir estimated to have a top 4 km (2.5 mi) below the surface with centroid 2.5 km (1.6 mi) lower. The chamber extends to an unusual depth of more than 10 km (6.2 mi),[13]: 36  and the more silicic lavas have matured at more than 9 km (5.6 mi).[14]: 81 

Many of the eruptions commence with thicker more explosive rhyolite, dacite or andesite eruptives which create tephra and have the potential for pyroclastic flows.[13]: 36–38 [14]: 81  Other or the later part of eruptions come from thinner basalt tending magma which forms lava fields.[13]: 36–38 

The tephra produced by its eruptions is high in fluorine, which is poisonous to animals. Hekla's basaltic andesite lava generally has a SiO2 content of over 54%, compared to the 45–50% of other nearby transitional alkaline basalt eruptions (see TAS classification).[15][16][17][18] It is the only Icelandic volcano to produce calc-alkaline lavas.[19] Phenocrysts in Hekla's lava can contain plagioclase, pyroxene, titanomagnetite, olivine, and apatite.[20]

When not erupting Hekla is often covered with snow and small glaciers; it is also unusually aseismic with activity only starting 30–80 minutes before an eruption.[21] Hekla is located on the mid-ocean ridge, a diverging plate boundary. Hekla is closely studied today for parameters such as strain, tilt, deformation and other movement and seismic activity.[16] Earthquakes in the volcano's vicinity are generally below magnitude 2 while it is dormant and magnitude 3 when erupting.[21]

Eruption history

[edit]
Tephra horizons in south-central Iceland. The thick and light coloured layer at center of the photo is rhyolitic tephra from Hekla.

The earliest recorded eruption of Hekla took place in 1104. Since then there have been between twenty and thirty considerable eruptions, with the mountain sometimes remaining active for periods of six years with little pause. Eruptions in Hekla are varied and difficult to predict. Precursor seismic activity may only be for a couple of hours or less.[14]: 80  Some are very short (a week to ten days) whereas others can stretch into months and years (the 1947 eruption started 29 March 1947 and ended April 1948). But there is a general correlation: the longer Hekla goes dormant, the larger and more catastrophic its opening eruption will be.[22] The most recent eruption was on 26 February 2000.

Prehistoric eruptions

[edit]
Hekla beyond a snowy field of volcanic ash

One of the largest Holocene eruptions in Iceland was the Hekla 3 (or H3) eruption c. 1000 BC,[4][23] which threw about 7.3 km3 (1.8 cu mi)[17] of volcanic rock into the atmosphere, placing its Volcanic Explosivity Index (VEI) at 5. This would have cooled temperatures in the northern parts of the globe for a few years afterwards. Traces of this eruption have been identified in Scottish peat bogs, and in Ireland a study of tree rings dating from this period has shown negligible tree ring growth for a decade.[23] The dates were recently recalibrated of the major eruptions and a table is given below as the difference in dates could cause confusion.

Dates of major eruptions in prehistoric times:[A]
Eruption Year (2019 values) Year (2024 values)
H-5 5050 BCE 5103 ± 260 BCE[24]
H-Sv 3900 BCE[25][26] 3900 BCE
H-4 2310 ± 20 BCE 2375 ± 8 BCE[24]
H-3 950 BCE 1063 ± 140 BCE[24]

A Unless otherwise stated eruption dates in Year (2019 values) column are from Global Volcanism Program and Catalogue of Icelandic Volcanoes,[4][27] As other sources can disagree,[23][25] there has been a recent literature update. The values and range given in the Year (2024 values) column used IntCal20 for H-5 and H-3 and ice core data for H-4 which were not available in 2019.[24]

Hekla 3, 4, and 5 produced huge amounts of rhyolitic ash and tephra, covering 80% of Iceland[28] and providing useful date markers in soil profiles in other parts of Europe such as Orkney,[29] Scandinavia,[9] and elsewhere.[30] H3 and H4 produced the largest layers of tephra in Iceland since the last ice age.[31] During the last 7,000 years, one third of the volcanic ash deposited in Scandinavia, Germany, Ireland, and the United Kingdom originated from Hekla.[32]

1104 to 1878

[edit]
1104 (H1)
Main building of the replica of Stöng, which was buried under volcanic ash from the 1104 eruption

Hekla had been dormant for at least 250 years when it erupted explosively in 1104 (probably in the autumn), covering 55,000 km2 (21,000 sq mi) which is over half of Iceland with 1.2 km3[31] / 2.5 km3[33] of rhyodacitic tephra. This was the second largest tephra eruption in the country in historical times with a VEI of 5. Farms upwind of the volcano 15 km (9.3 mi) in Þjórsárdalur valley, 50 km (31 mi) at Hrunamannaafréttur and 70 km (43 mi) at Lake Hvítárvatn were abandoned because of the damage. The eruption caused Hekla to become famous throughout Europe.[4][31]

1158

A VEI-4 eruption began on 19 January 1158 producing over 0.15 km3 (0.036 cu mi) of lava and 0.2 km3 (0.048 cu mi) of tephra. It is likely to be the source of the Efrahvolshraun lava on Hekla's west.[17][31]

1206

The VEI-3 eruption began on 4 December.

1222

The VEI-2 eruption and the 1206 eruption distributed around 0.24 km3 (0.058 cu mi) of tephra mainly to the northeast.[17][34]

1300–1301
Hekla in 2006 and an Icelandic horse

This VEI-4 eruption, which started on 11 July and lasted for a year, was the second largest tephra eruption of Hekla since Iceland was settled, covering 30,000 km2 (12,000 sq mi) of land with 0.31 km3 (0.074 cu mi) of tephra. Over 0.5 km3 (0.12 cu mi) of lava was also expelled. The tephra caused significant damage to the settlements of Skagafjörður and Fljót, leading to over 500 deaths that winter.[17][35] The material output from this eruption had SiO2 levels of between 56% and 64%, and apart from a slight abundance of olivine the lava, was typical of Hekla eruptions.[36]

1341

A small eruption (VEI-3) started on 19 May and deposited around 5×107 km3 (1.2×107 cu mi) of tephra over the areas west and southwest of Hekla, leading to many cattle deaths, probably mainly from fluorosis.[17][35]

1389

In late 1389 Hekla erupted again (VEI-3), starting with a large ejection of tephra to the southeast. Later "the eruption fissure moved itself out of the mountain proper and into the woods a little above Skard". Skard and another nearby farm were destroyed by a large lava flow that now forms the 12.5 km2 (4.8 sq mi) Nordurhraun. In total around 0.3 km3 (0.072 cu mi) of lava and 5×107 m3 (1.8×109 cu ft) of tephra were produced.[17][35]

1440

An eruption may have occurred around 1440 at Raudölder; despite being close to Hekla this is not classed as an eruption of Hekla based on the SiO2 content of the lava.[15][17][37]

1510
17 cm long volcanic bomb found in the lava-fields of Hekla

Details of the 1510 eruption were not recorded until a century later. It started on 25 July and was particularly violent (VEI 4), firing volcanic bombs as far as Vördufell, 40 km (25 mi) west. Tephra was deposited over Rangárvellir, Holt and Landeyjar, 0.2 km3 (0.048 cu mi) in total. A man in Landsveit was killed.[17][35]

1597

A VEI-4 eruption began on 3 January and lasted for over 6 months, with 0.15 km3 (0.036 cu mi) of tephra being deposited to the south-southeast, damaging Mýrdalur.[17][35]

1636–1637

A small (VEI-3) eruption began on 8 May 1636 and lasted for over a year. The 5×107 m3 (1.8×109 cu ft) of tephra from the eruption damaged pasture to the northeast causing death of livestock.[17][38]

1693

Starting 13 February and lasting for over 7 months the eruption was one of Hekla's most destructive (VEI-4). Initially tephra was produced at 60,000 m3·s−1, 0.18 km3 (0.043 cu mi) during the entire eruption, which also caused lahars and tsunami. The tephra was deposited to the northwest, destroying and damaging farms and woodland in Þjórsárdalur, Land, Hreppar and Biskupstungur. Fine ash from the eruption reached Norway. There was damage to wildlife with significant numbers of trout, salmon, ptarmigan and farm animals dying.[17][38]

1725
The flanks of Hekla

A very small eruption, possibly only VEI-1, took place on 2 April 1725, producing flows of lava from locations around Hekla which have since been covered by later lava flows. These eruptions are not classed as of Hekla itself based on the SiO2 content of the lava.[15][17]

1766–1768

The eruption of 1766 was large (VEI-4) and produced the second largest lava flow, 1.3 km3 (0.31 cu mi) covering 65 km2 (25 sq mi), and third largest tephra volume, 0.24 km3 (0.058 cu mi), of any Icelandic volcano during the inhabited era. The eruption started at around 3:30 am on 5 April 1766 and ceased in May 1768. Initially a 2–4 cm layer of tephra was deposited over Austur-Húnavatnssýsla and Skagafjördur, resulting in the deaths of both fish and livestock. Rangárvellir, Land and Hreppar also suffered damage. During the eruption up to 0.5 m (1 ft 8 in) lava bombs were thrown 15–20 km (9.3–12.4 mi) away, and flooding was caused by the sudden melting of snow and ice on Hekla's slopes.[17][38]

1845–1846
Hekla c. 1851

Hekla was dormant for more than sixty years before 1845, when it suddenly burst forth on 2 September at 9 am:

After a violent storm on the night of the 2nd of September in that year, the surface of the ground in the Orkney Islands was found strown with volcanic dust. There was thus conveyed to the inhabitants of Great Britain an intimation that Hecla had been again at work. Accordingly, tidings soon after arrived of a great eruption of the mountain. On the night of 1 September, the dwellers in its neighbourhood were terrified by a fearful underground groaning, which continued till mid-day on the 2nd. Then, with a tremendous crash, there were formed in the sides of the cone two large openings, whence there gushed torrents of lava, which flowed down two gorges on the flanks of the mountain. The whole summit was enveloped in clouds of vapour and volcanic dust. The neighbouring rivers became so hot as to kill the fish, and the sheep fled in terror from the adjoining heaths, some being burnt before they could escape. On the night of 15 September, two new openings were formed — one on the eastern, and the other on the southern slope — from both of which lava was discharged for twenty-two hours. It flowed to a distance of upwards of twenty miles, killing many cattle and destroying a large tract of pasturage. Twelve miles from the crater, the lava-stream was between forty and fifty feet deep and nearly a mile in width. On 12 October a fresh torrent of lava burst forth, and heaped up another similar mass. The mountain continued in a state of activity up to April 1846; then it rested for a while, and began again in the following month of October. Since then, however, it has enjoyed repose. The effects of these eruptions were disastrous. The whole island was strewn with volcanic ash, which, where it did not smother the grass outright, gave it a poisonous taint. The cattle that ate of it were attacked by a murrain, of which great numbers died. The ice and snow, which had gathered about the mountain for a long period of time, were wholly melted by the heat. Masses of pumice weighing nearly half a ton were thrown to a distance of between four and five miles.[39]

— Anonymous, 1872

The eruption ceased around 5 April 1846. Initially in this VEI-4 eruption tephra was produced at 20,000 m3·s−1. The tephra deposition of a total amount of 0.17 km3 (0.041 cu mi) was mainly to the east-southeast; immediately to the east of Hekla the layer was 20–40 cm (7.9–15.7 in) deep. Fine ash was carried to the Faroes, Shetland and Orkney. Lava flows to the west and north-west covered an area of 25 km2 (9.7 sq mi) with a volume of 0.63 km3 (0.15 cu mi) of lava. Large quantities of dark ash were deposited over pasture in the same directions leading to many livestock deaths through fluorosis for the next two years.[17][40]

1878
Hekla c. 1893

A small eruption (VEI-2) occurred between 27 February 1878 and April 1878, around 10 km (6.2 mi) east of Hekla, and produced 0.2 km3 (0.048 cu mi) of lava from two parallel fissures covering 15.5 km2 (6.0 sq mi).

1913 to 1948

[edit]
1913
Hekla c. 1904

A small eruption (VEI-2) occurred between 25 April 1913 and 18 May 1913, around 10 km (6.2 mi) east of Hekla, and caused large fissures at Mundafell and Lambafit which produced 3.8 and 6.3 km2 (1.5 and 2.4 sq mi) of lava respectively.[17][40]

1947–1948

The VEI-4 eruption started on 29 March 1947 and ended on 21 April 1948. It is likely that this was both the second greatest lava eruption of Hekla whilst Iceland was inhabited and the second greatest lava eruption in the world in the period 1900–1970. A total lava volume 0.8 km3 (0.19 cu mi) was produced with 0.21 km3 (0.050 cu mi) of tephra. The height of Hekla was 1,447 m (4,747 ft) before the eruption, increasing to a maximum of 1,503 m (4,931 ft), before dropping to 1,491 m (4,892 ft) subsequently.

The eruption occurred over a century after the last eruption of Hekla proper, the longest dormant period since 1104. Before the eruption the volcano had been visible from the surrounding area but nothing remarkable was noticed. The eruption occurred at 6:41 am ± 3 min with a loud roar; later eruptions could be heard throughout Iceland. An earthquake at 6:50 am measured 6 on the Mercalli intensity scale and increased the eruption intensity until it covered a 4 km (2.5 mi) fissure on the ridge. The cloud from the eruption had ascended to a height of 30 km by 7:08 am, the wind then carried it southwards towards Eyjafjallajökull, turning it black. Pumice first landed on Fljótshlíð at around 7:10 am, and tephra and ash continued falling until it formed a 3–10 cm (1.2–3.9 in) layer. A lava bomb that landed 32 km (20 mi) from Hekla was 0.5 m (1 ft 8 in) across and weighed 20 kg (44 lb). Between Vatnafjöll and Hekla, a layer of tephra up to 1 m (3 ft 3 in) thick was deposited, and this included bombs with a diameter larger than 0.5 m (1 ft 8 in). Bombs with surface areas of 50 m2 (540 sq ft) were dropped onto the slopes of Hekla, for up to 1 km (0.62 mi). 51 hours after the eruption had started, ash fell on Helsinki, Finland, having covered 2,860 km (1,780 mi) in this time.

The initial tephra production rate in the first 30 minutes of the eruption was 75,000 m3·s−1, dropping to 22,000 m3·s−1 for the next half-hour. The initial phase produced 0.18 km3 (0.043 cu mi) of tephra, equating to 4.5×107 km3 (1.1×107 cu mi) of Dense-rock equivalent, covering 3,130 km2 (1,210 sq mi) of land and sea. 98 farms were damaged by the eruption, but only 2 were no longer farmed in 1970. A large volunteer effort was mobilized to clear the tephra – around 1000 man-days by the end of July. The eruption produced around 3 ML (110×10^3 cu ft) of water (snowmelt and directly from the fissure) which caused flooding of the Ytri Rangá river.

In the first 20 hours of the eruption approximately 3,500 m3·s−1 of lava was produced from the fissure, dividing into various branches and covering 12–15 km2 (4.6–5.8 sq mi). On the second day, 8 distinct eruption columns were discernible. A crater formed at 860 m (2,820 ft) called the Lava Crater (Hraungígur), producing a constant flow of lava. Another crater named the Shoulder Crater (Axlargígur) produced a column of smoke every 10 seconds together with loud explosions that created visible compression waves in the smoke. By the fourth, fifth, and sixth days, the eruption was greatly diminished, and only the shoulder and summit craters were erupting explosively.

The Pæla lava field in 2009 with a lava river from the 1947 eruption

The explosive eruption increased in strength from 9–12 April and then from 28 April it reduced again. On 3 May, the volcano stopped throwing out lava in sudden explosions from its craters and changed to continuously ejecting tephra and ash for long periods, until early June when this reduced. On 2 September, the Shoulder Crater had a 960 m (3,150 ft) circumference at its top and the Summit Crater a 700 m (2,300 ft)circumference at its highest point, 90 m (300 ft) above the ridge. Sandy tephra and ash fell over Iceland in May and June, sometimes making it dark in the daytime near Hekla. The tephra caused fluorine poisoning of grazing sheep, making them unable to walk. That winter more craters formed, building up cones. Explosive activity had ceased six months after the first eruption. Lava flowed from the Lava Crater continuously during the eruption, starting at a rate of over 100 m3·s−1, dropping to 5–10 m3·s−1 in April and early May at a speed of around 20 cm·s−1 before increasing, eventually reaching 150 m3·s−1 at the end of June and at similar levels until mid-July with a peak flow speed of 2–2.5 m·s−1. From there it gradually decreased to under 10 m3·s−1 in November. Initially the lava comprised 57–58% SiO2 and 11% Fe2O3, from the time of peak flow onwards this changed to 54% SiO2 and 13.5% Fe2O3.

The lava river sometimes ran through lava tubes before emerging again. The lava front had a height of up to 15 m (49 ft). On 15 and 16 June, a branch of lava flow to the south of Melfell traveled over 1 km (0.62 mi) in 30 hours before slowing and stopping by 21 June, 7.8 km (4.8 mi) from the Lava Crater. The longest lava stream produced was 8 km (5.0 mi) long and stopped in Stóraskógsbotnar. A scientist filming one of the lava streams on 2 November was hit by a block of lava and was killed.[41]

The lava flow stopped after 13 months on 21 April, having covered 40 km2 (15 sq mi) and with a maximum depth of 100 m (330 ft). The lava beds produced were mainly the ʻAʻā lava type with Pāhoehoe and lava a budella (lava tubes) areas. In April and May 1948 CO2 emitted from cracks in the ground pooled in hollows near to Hekla, killing 15 sheep and some wild animals and birds. In total 24,000 tonnes (26,000 tons) of CO2 was emitted. Ditches were dug by farmers to drain these hollows, and the CO2 emission had stopped by the end of the year.[17][42]

1970 to 1991

[edit]
1970
Lava flow from the 1970 eruption, seen in 1971

The 1970 eruption of Hekla started at 9:23 pm on 5 May 1970 and lasted until 5 July. It had a VEI of 3 and produced 0.2 km3 (0.048 cu mi) of lava covering an area of 18.5 km2 (7.1 sq mi) and 6.6×10 7 m³ of tephra, deposited over an area of 40,000 km2 (15,000 sq mi), mainly to the northwest of the volcano.[17]

The main Hekla fissure only erupted at its far southwest end, most of the eruption was from other fissures nearby. The eruption stopped in the south-southwest on 10 May and in Hlídargígar on 20 May, but a new fissure opened on the same day and lava flowed from this until 5 July. The lava was andesite containing olivine, similar to the lava produced later in the eruption of 1947.[20]

Before the eruption, a greater than normal amount of snow melting had occurred, indicating the volcano was heating up. Earth tremors began at 8:48 pm on the evening of the eruption; the largest had a magnitude of 4. The eruption started weakly at 9:23 pm IMT ± 2 min before increasing in power. The first pumice fell on Búrfell power station, 15 km (9.3 mi) away, at 9:35 pm causing people to evacuate. The eruption seems to have started in two locations at the same time – to the Shoulder crater's south-southwest and below the Lava Crater. At 10:30 pm a crater at 780 m (2,560 ft) was producing a lava column which reached an altitude of around 1 km (0.62 mi). During the night a 700 m (2,300 ft) high lava fountain was thrown up from the main crater. A 500 m (1,600 ft) long fissure starting below the Lava Crater opened, and lava fountains and other lava flows emanated from it. One hour into the eruption, a new 400 m (1,300 ft) fissure opened to the northeast, producing two main lava fountains, and shortly after another adjoining fissure opened producing lava fountains to a height of 500 m (1,600 ft). At around midnight, another fissure opened northwest of the Lava Crater, later hurling an over 300 m (980 ft) long lava fountain, 200–300 m (660–980 ft) into the air. By midnight lava had already covered over 1 km2 (0.39 sq mi) and this extended to 7.5 km2 (2.9 sq mi) by next morning implying a flow rate of around 1500 m³/s.

For the first two hours, tephra was produced at the rate of 10000 m³/s. The cloud from the eruption, which had reached 53,000 feet (16,154 m) by 10:10 pm, caused a lightning storm. The tephra was transported northwards by the wind, causing the sky to turn black in places – 190 km (120 mi) away at Blönduós tephra fell from midnight until 2 am, and ash fell on a trawler 330 km (210 mi) away at 2 am. Icelanders sampled the tephra fall in their locality by putting a plate outside to capture everything that fell onto it. This, and other measurements, showed the area covered was long and narrow with the 1 mm contour (an equivalent of 8 tonnes per hectare) extending to the north coast.[17][20]

Hekla in winter 2010

By 5:30 on 6 May, the lava flow measured 4 km (2.5 mi) long. Many lava bombs were found near the main crater, one had an area of 6 m2 (65 sq ft) and a likely weight of 12 tons.[ambiguous][citation needed] Xenoliths formed around 2% of the material produced by the craters. These were of rock types including basalt, andesite, ignimbrite and sedimentary rock.

The eruption became stronger at Skjólkvíar on 12 May, with columns of steam attaining a height of 2,500 m (8,200 ft). The eruption intensity then gradually reduced until it stopped on 20 May. The lava field then had an area of 5.8 km2 (2.2 sq mi). Later that day a 900 m (3,000 ft) long fissure opened 1 m2 (11 sq ft)1 km north of the main Hlídargígar crater. That night it contained 17 lava fountains, each 20–50 m (66–164 ft) in height. By the evening of the next day, 10–12 craters had formed, each throwing pieces of lava 50–100 m (160–330 ft) in the air. This row of craters was named Öldugígar. Gradually the number of active craters decreased, the most active of these built a cone 100 m (330 ft) higher than the level of the ridge. Lava flowed from its base until mid-June when the lava cut through the north crater wall. The larger cones produced more tephra, occasionally with lightning within the tephra cloud. By 5 July, the eruption had stopped.[20]

During eruptions of Hekla, fluorine is produced and sticks to the surfaces of the grains of tephra. Fine grains can have a fluorine content of 350 ppm, and fluorine poisoning can start in sheep at a diet with fluorine content of 25 ppm. At 250 ppm, death can occur within a few days. In 1783, 79% of the Icelandic sheep stock were killed, probably as a result of fluorosis caused by the eruption of Lakagígar. Some of the ash produced in this eruption had a fluorine content of 0.2%, and two days after the eruption contaminated grass had a dry weight content of up to 0.4% fluorine. 450 farms and 95,000 sheep were affected by the eruption. Some sheep were kept inside and fed on hay or moved, but other farmers were forced to graze their flocks outside.[20]

1980 and 1981
Hekla erupting in 1980. Seen from 4 km NE of the summit.

This VEI-3 eruption started at 13:28 on 17 August 1980 and lasted until 20 August 1980. It was a mixed eruption producing a lava volume of 0.12 km3 (0.029 cu mi) and a tephra volume of 5.8×107 m3 (2.0×109 cu ft). The fissure opened along a 7 km (4.3 mi) length. Shortly before the eruption started a steam column was produced; eventually the eruption column reached a height of 15 km (9.3 mi). The main tephra deposits were to the north-northeast and lasted for around 2 hours. Deposits were 20 cm (7.9 in) thick 10 km (6.2 mi) from the summit, decreasing to less than 1 mm (0.039 in) at the coast 230 km (140 mi) away. Lava was initially produced from close to the summit, spreading to other parts of the fissure and covering an area of 22 km2 (8.5 sq mi) in around 24 hours. The last scoria were seen on the morning of 20 August. This was an unusual eruption both in the short time since the previous eruption – the shortest since 1104, and the length – previous eruptions had lasted from 2 months to 2 years rather than just 3 days.[17][43]

The 1981 eruption, which is regarded as being a continuation of the previous year's eruption, began at 3 am on 9 April 1981, had a VEI of 2 and produced 3×107 m3 (1.1×109 cu ft) of lava, lasting until 16 April 1981. The eruption threw ash to a height of 6.6 km (4.1 mi), and a new crater formed at the summit from which 3 lava flows originated. These extended to a maximum of 4.5 km (2.8 mi) from the volcano, covering 5–6 km2 (1.9–2.3 sq mi).[17][37]

1991
A summer 2009 view of Hekla from the side.

A VEI-3 eruption occurred from 17 January 1991 to 11 March 1991, producing 0.15 km3 (0.036 cu mi) of lava and 2×107 m3 (7.1×108 cu ft) of tephra. The eruption, which was preceded by sulphurous smells and earthquakes, started as a Plinian eruption, producing an ash cloud reaching an altitude of 11.5 km (7.1 mi) within 10 minutes which had travelled over 200 km (120 mi) north-northeast to the coast within 3 hours. The eruption then began producing andesitic lava, the flows eventually covering an area of 23 km2 (8.9 sq mi) to an average depth of 6–7 m (20–23 ft). Initially, part of the Heklugjá fissure and other fissures erupted with lava fountains reaching 300 m (980 ft) in height. By the second day, the activity stopped in all but one fissure where the main crater formed. During these 2 days, 800 m³/s of lava were produced, slowing to between 1 m³/s and 14 m³/s for most of the eruption. This low viscosity lava had a SiO2 content of approximately 54%.[17][44]

2000

[edit]
A lava field on Hekla in July 2000

The most recent eruption was relatively short; it started at 18:18 on 26 February 2000 and lasted until 8 March. It was a VEI-3 eruption producing a lava volume of 0.189 km3 (0.045 cu mi), DRE[45] 0.29 km3 (0.070 cu mi) [17] and 1×107 m3 (3.5×108 cu ft)m3 of tephra.[17] The eruption went through four phases:

  1. initial explosive stage
  2. fire fountains
  3. bursts of Strombolian eruption
  4. effusion of lava[45]

Eruption activity was at a maximum in the first hour, and by the first night the fissure on Hekla had opened to a length of 6–7 km (3.7–4.3 mi). The steam column rose to a height of almost 15 km (9.3 mi), and ash was transported to Grímsey on the North coast of Iceland.[46] During this eruption, a NASA DC-8 aeroplane accidentally flew through the plume with all instruments switched on, resulting in unprecedented measurement of a young volcanic plume.[47]

Up until this eruption, it had been assumed that Hekla was incapable of producing the most dangerous of volcanic phenomena, the pyroclastic flow. In January 2003, however, a team from the Norvol Institute in Reykjavík, under the leadership of Dr. Ármann Höskuldsson, reported that they found traces of a pyroclastic flow, roughly 5 km long, stretching down the side of the mountain. This will call for a reappraisal of volcanic eruptions of the basic rock type, which up to now were generally thought not to produce large pyroclastic flows. It will also require that the public and curious spectators who always rush to the scene at the start of a new outbreak, be kept much further away from the volcanic activity than was thought necessary during previous outbreaks.

Steam at the summit of Hekla

Eruption Summary

[edit]
Summary of known eruptions
Start Date[4] Years before 1950 (BP) VEI[4] Lava volume
(km3)[27] 		Tephra volume
(km3)[27] 		Comment
2000 February 26, 2000 - 3 0.19 0.01 Eruption ended March 8, 2000[4]
1991 January 17, 1991 - 3 0.15 0.02 Eruption ended March 11, 1991[4]
1981 April 9, 1981 - 2 0.03[48] - Eruption ended April 16, 1981[4]
1980 August 17, 1980 - 3 0.12[48] 0.06 Eruption ended August 20, 1980[4]
1970 May 5, 1970 3 0.2 0.07 Eruption ended July 5, 1970[4]
1947 March 29, 1947 3 4 0.8 0.18 Eruption ended April 21, 1948[4]
1913 April 25, 1913 37 2 0.05 - Eruption ended May 18, 1913[4]
1878 February 27, 1878 72 2 0.2 - Eruption ended April 1878[4]
1845 September 2, 1845 96 4 0.63 0.23 Eruption ended about April 5, 1846[4]
1766 April 5, 1766 184 4 1.3 0.4 Eruption ended in May 1768.[4] Tephra composition is rhyolite to dacite (Icelandite).[49]
1725 April 2, 1725 225 1 - -
1693 February 13, 1693 257 4 - 0.3 Eruption ended about September 14, 1693[4]
1636 May 8, 1636 314 3 - 0.18 Eruption ended in June 1637[4]
1597 January 3, 1597 353 4 0.29 Eruption ended in or after June 1597[4]
1554 May 1554 396 2 0.1 - Eruption ended about June 1554
1510July 25, 1510 440 4 - 0.32
1389 December 1389 561 3 0.2 0.15 Eruption ended in 1390
1341 May 19, 1341 609 3 - 0.18
1300 July 11, 1300 650 4 0.5 0.5 Eruption ended in July 1301. Tephra composition is rhyolite to dacite (Icelandite).[49]
1222 1222 728 2 - 0.04
1206 December 4, 1206 744 3 - 0.4 Dated using historical records.
1158 January 19, 1158 792 4 0.1 0.33
1104 October 15, 1104 846 5 0 2.0 H 1 tephra, composition is rhyolite.[49] Erupted within 45 days of date.
1050 1050 ± 500 900 ± 500 - - -
781 781 ± 2 1169 ± 2 - - - Greenland ice core[50]
753 753 ± 2 1197 ± 2 - - - Greenland ice core[50]
650650 ± 500 1300 ± 500 - - -
550 550 ± 1500 1400 ± 1500 - - -
350 350 ± 500 1600 ± 500 - - -
250 250 ± 1500 1700 ± 1500 - - -
-150 150 ± 2500 BCE 2100 ± 2500 - - -
-250 250 ± 500 BCE 2200 ± 500 - - -
-650 650 ± 2500 BCE 2600 ± 2500 - - -
-750 750 ± 500 BCE 2700 ± 500 - - -
-850 850 ± 80 BCE 2800 ± 80 - - - H C dacite (Icelandite) tephra.[49]
-1063 1063 ± 140 BCE 3013 ± 140 5 - 10 H 3 dacite, rhyolite tephra.[49] Cal. BP[24]
-1150 1150 ± 1500 BCE 3100 ± 1500 - - -
-1250 1250 ± 1500 BCE 3200 ± 1500 - - -
-1350 1350 ± 2500 BCE 3300 ± 2500 - - -
-1550 1550 BCE 3500 4 - -
-1650 1650 ± 2500 BCE 3600 ± 2500 - - -
-1750 1750 ± 500 BCE 3700 ± 500 - - -
-1850 1850 ± 2500 BCE 3800 ± 2500 - - 2.0
-2375 2375 ± 8 BCE 4325 ± 8 5 - 10.0 H 4 rhyolite tephra.[49]Ice core age used.[24]
-2335 2335 ± 180 BCE 4285 ± 180 - - - H 4270 alkali basalt tephra.[49] Age corrected for H4 ice core age to maintain mean 10 year separation.[24]
-2450 2450 ± 1500 BCE 4400 ± 1500 - - -
-2750 2750 ± 2500 BCE 4700 ± 2500 - - -
-2950 2950 ± 500 BCE 4900 ± 500 - - -
-3350 3350 ± 2500 BCE 5300 ± 2500 - - -
-3450 3450 ± 1500 BCE 5400 ± 1500 - - -
-3750 3750 ± 1500 BCE 5700 ± 1500 - - -
-3950 3950 ± 500 BCE 5900 ± 500 - - -
-4050 4050 ± 500 BCE 6000 ± 500 - - -
-4110 4110 ± 100 BCE 6060 ± 100 5 0 1
-4150 4150 ± 2500 BCE 6100 ± 2500 - - -
-4250 4250 ± 500 BCE 6200 ± 500 - - -
-4650 4650 ± 500 BCE 6600 ± 500 - - -
-4700 4700 BCE 6650 4 - - Date only constrained by before and after eruptions
-4750 4750 ± 2500 BCE 6700 ± 2500 - - -
-4950 4950 ± 2500 BCE 6900 ± 2500 - - -
-5050 5050 BCE 7000 - - - Date only constrained by before and after eruptions
-5103 5103 ± 260 BCE 7053 ± 260 5 - 3 H 5 basaltic to rhyolite tephra. Cal BP age used.[24]
-5850 5850 ± 2500 BCE 7800 ± 2500 - - -
-8600 8600 ± 150 BCE 10550 ± 150 - - - H 10550 alkali basalt tephra.[49]

Flora and fauna

[edit]
Sea Campion colonising ground close to Hekla
The lichens Stereocaulon vesuvianum and moss Racomitrium ericoides on lava originating from Hekla.

The Hekla area was once forested. Forest and some grasses are much more resilient to ash and pumice fall than low vegetation, but the combined effect of human habitation and the volcanic activity has left an unstable surface very susceptible to erosion. Hekluskógar, a 90,000 ha (220,000 acres) reforesting project is attempting to restore the birch and willow woodland to the slopes of Hekla, starting with soil fertilisation and grass sowing. This would stabilize the large areas of volcanic ash, help to reduce wind erosion of the frost heaved surface, slow drainage rates and hence water erosion, and ultimately increase biodiversity. It is the largest reforestation of its type in Europe.[51][52] After an eruption, almost all of the 'safe sites' on new lava flows are colonised by mosses within 20 years[53] expanding to a homogeneous layer up to 20 cm (7.9 in) thick typically within 50 years.[54]

Past eruptions have been associated with death of birds and live-stock related to either the high fluoride content of the tephra, carbon dioxide suffocation or toxic carbon monoxide gas release,[14]: 81  and must have had local die back in the ecosystems. Local plant succession on the lava fields after eruptions has been better studied and there are four broad stages of natural succession with importantly the potential for regression:[55]

  1. In first 70 years colonization and cover coalescence of moss Racomitrium lanuginosum and the Stereocaulon mosses
  2. Moss secondary colonization to Racomitrium lanuginosum dominance which can take between 170 and 700 years
  3. After 600 years vascular plant dominance evolving towards the birch wood land climax ecosystem in Iceland if no disturbance
  4. Highland conditions/retrogression after tephra deposition which had occurred up to 860 years after the initial lava flow

Local factors and other disturbances influence these rates but the 1991 laval flow first stage was completed in 24 years.[55] The basaltic volcanic soil development is typical for Iceland.[55] Vegetation height prior to a tephra fall is the single most important factor for vascular plant survival so the presence of a woodland before a further large tephra deposit enhances regrowth.[55]

Sport and recreation

[edit]
Walkers on the slopes of Hekla (June 2005)
The Hekla Centre

Hekla is a popular destination for hiking. Following the most recent eruption the path goes most of the way to the summit;[56] the walk takes 3 to 4 hours.[9] In spring, skiing is possible on short routes around the rim of the crater. In summer, there are easy (F) mountaineering routes also around the crater rim,[57] and it is possible to snowcat to the top in winter. The volcano can be reached using the buses to Landmannalaugar 30 km (19 mi) further east, and it is possible to stay or camp at farms in the area.[58] A visitor centre, The Hekla Center at Leirubakki Farm, opened in 2007.[59]

[edit]

Hekla has featured in artistic works since the time of its medieval infamy.

Architecture

[edit]

The Tour Hekla, a 220-metre-tall (720 ft) skyscraper in La Défense, Paris, France, built in 2022, is named after the volcano.

Films

[edit]

In the Spanish apocalyptic science fiction film, Los Últimos Días (2013), some news reporters speculate that three recent eruptions of Hekla could have caused the spreading form of agoraphobia that kills affected people who go outside.

The climax of Robert Eggers's 2022 film The Northman takes place on the slopes of Hekla.

Food

[edit]

In the Boston, Massachusetts area, Hekla pastries can be found – large, upside-down cinnamon rolls with white sugar icing spooned over the top to look like the snow-topped volcano.[60]

Literature

[edit]

The British poet William Blake showed Winter being banished to Hekla in To Winter, one of the works from his Poetical Sketches.[61]

In To Lie With Lions, by Dorothy Dunnett, a party of merchants visiting Iceland in the year 1471 witnesses the spectacular (fictional) eruption of both Hekla and Katla.

Mt. Hekla is referenced in the third chapter of Herman Melville's novel Moby Dick, in EE Ryan's The Odd Saga of the American and a Curious Icelandic Flock, and in the final chapters of Joan Aiken's novel Is.

The Hekla 3 eruption and the ensuing volcanic winter play a large role in Stephen Baxter's alternate-history novel Bronze Winter.

Music

[edit]

The piece Hekla, Op 52 (1964) by Icelandic composer Jón Leifs, has been called the "loudest classical music of all time". The requirements for a performance of Hekla include four sets of rocks hit with hammers, steel plates, anvils, sirens, cannons, metal chains, choir, a large orchestra, and organ.[62]

Icelandair's Hekla Aurora in 2014

Transportation

[edit]

A small Danish cruiser launched in 1890 was named Hekla; it was scrapped in 1955.

A Danish steamer called Hekla also saw combat in the First Schleswig War.

Icelandair named one of their aircraft after Hekla.

There have been several ships of the Royal Navy named HMS Hecla

Performing Arts

[edit]

Heklina, the stage name for prominent Icelandic-American drag performer Stefan Grygelko, was inspired by Hekla.[63] Grygelko, whose mother was Icelandic, spent part of his childhood in Iceland and helped to open a gay bar in Reykjavik before relocating to San Francisco.[63][64][65] Heklina's mysterious and unsolved death in London in April 2023, and the ensuing homophobia demonstrated by London police, caused international headlines.[66]

Organizations

[edit]

In October 2011 a German left-wing militant group called the Hekla-Empfangskomitee (Hekla Reception Committee) set at least 17 incendiary devices on railways in the Berlin area, with 2 of them going off.[67]

The DBU Copenhagen football club Boldklubben Hekla play at Hekla Park.

See also

[edit]

References

[edit]

Bibliography

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hekla

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from Grokipedia

Hekla is a ridge-shaped situated in southern at the junction of the Eastern Volcanic Zone and the South Iceland Seismic Zone, with a of 1,490 meters.
It forms the central of the Hekla-Vatnafjöll system and features a prominent 5.5-km-long ENE-WSW trending fissure that has driven its elongated profile through repeated eruptions of lavas and .
Hekla has erupted approximately 23 times since Iceland's settlement around 874 AD, with historical activity commencing in 1104 CE via a highly explosive VEI 5 that produced widespread fallout.
Its eruptions characteristically initiate with intense explosive phases generating fluorine-enriched ash hazardous to grazing , transitioning to prolonged effusive activity along fissures that bury landscapes in lava.
This pattern has earned Hekla the medieval moniker "Gateway to Hell" for the ominous dark plumes and devastating impacts on southern Iceland's farms and settlements during events like the 1300, 1693, and 1766 eruptions.
In the alone, five significant eruptions occurred—1947–1948 (VEI 4), 1970, 1980–1981, 1991, and 2000 (VEI 3)—collectively producing substantial lava volumes exceeding 1 km³ and underscoring its status as one of Iceland's most active volcanic systems.

Physical Geography

Location and Topography


Hekla is situated in the southern highlands of Iceland, approximately 110 kilometers east-southeast of Reykjavík, within the Eastern Volcanic Zone at its intersection with the South Iceland Seismic Zone, a transform fault. The volcano's summit coordinates are 63°59′N 19°42′W. It lies between the Þjórsárver wetland to the east and the Þjórsárdalur valley to the west, forming part of a 40-kilometer-long volcanic ridge that extends across the region. The topography of Hekla features an elongated morphology, manifesting as a approximately 9 kilometers long and 3.5 kilometers wide at 800 meters , oriented in a WSW-ENE direction. The reaches a height of 1,491 meters above , with steep, symmetrical slopes characteristic of its cone-shaped formation, though repeated eruptions have produced an overturned boat-like profile with a of aligned craters along the . Lava flows from eruptions have further elongated the , contributing to rough, uneven surrounding the main edifice.

Regional Context

Hekla lies in southern at the western margin of the Eastern Volcanic Zone (EVZ), where it intersects the South Iceland Seismic Zone (SISZ), a right-lateral system. This rift-transform junction contributes to the volcano's frequent activity, as divergent plate motions along the propagate inland through the EVZ, while dominate the SISZ. The region experiences high and volcanic productivity, with Hekla forming part of a 60 km long central -fissure swarm system oriented northeast-southwest. The surrounding landscape consists of undulating basaltic plateaus, extensive lava fields from past Hekla eruptions, and glaciated highlands to the north, such as the Veiðivötn area. Southward, the terrain transitions to lowlands and coastal plains, including the Þjórsárver wetland preserve, which have been periodically impacted by fallout and jökulhlaups from subglacial activity elsewhere in the zone. The EVZ itself stretches from central southward, accommodating oblique spreading between the North American and Eurasian plates at rates of 1.8-2 cm per year. Proximity to Reykjavik, approximately 110 km to the west, underscores the regional significance of Hekla's eruptions for infrastructure and agriculture in south-central , though the immediate vicinity remains sparsely populated due to harsh volcanic terrain. The area's reflects repeated construction and erosion of volcanic edifices, with ridges and tindar cones dotting the flanks, indicative of interactions between and past glaciations during the Pleistocene.

Geological Characteristics

Tectonic and Structural Features

Hekla occupies a tectonically complex position at the intersection of the Eastern Volcanic Zone (EVZ), a propagating segment of the where the North American and Eurasian plates diverge, and the South Iceland Seismic Zone (SISZ), a right-lateral that accommodates oblique spreading. This juncture exposes the volcano to combined extensional stresses from rifting and shear deformation from transform motion, influencing its eruptive behavior and seismicity. The overall setting aligns with 's neovolcanic zones, where magmatism is driven by mantle upwelling beneath the plate boundary. Structurally, Hekla manifests as a central stratovolcano with hybrid characteristics intermediate between a fissure vent row and a conical edifice, elongated along a WSW-ENE trend (N70°E) at the plate boundary. The summit forms a prominent ridge approximately 9 km long and 3.5 km wide, bisected by the 5.5-km-long Heklugjá fissure, which serves as the primary conduit for historical eruptions. Unlike central volcanoes with pronounced calderas, Hekla lacks a large collapse structure, instead featuring a linear summit depression aligned with the fissure swarm that dominates its morphology. The broader Hekla volcanic system encompasses a 60 km-long fissure swarm extending southward from the central edifice, linking it to subsidiary vents like Vatnafjöll, which contribute to the region's basaltic to rhyolitic activity. This swarm reflects the rift-transform interaction, with fissures oriented subparallel to the EVZ and facilitating dyke propagation during recharge events. Deformation studies indicate ongoing shallow and microseismicity tied to accumulation beneath the ridge, underscoring the dynamic interplay of tectonic extension and volcanic loading.

Magma Composition and Chamber Dynamics

Hekla's erupted s form a calc-alkaline series ranging from to rhyolite, with whole-rock SiO₂ contents spanning 54–70 wt%. This compositional spectrum reflects differentiation from primitive parents through fractional crystallization, yielding more silicic melts enriched in volatiles such as H₂O, which increases from ~0.8 wt% in end-members to ~5.7 wt% in rhyolitic compositions. Unlike the tholeiitic basalts dominant in Iceland's zones, Hekla's suite exhibits higher degrees of evolution, attributed to prolonged crustal interaction and mixing within a stratified . Eruptive sequences typically initiate with volatile-undersaturated, water-rich rhyolitic pumice (SiO₂ >70 wt%), transitioning to andesitic and lavas as recharge dilutes the upper chamber. patterns, including elevated incompatible elements like Zr and REE fractionation, support polybaric involving , , and oxides, with minimal assimilation of wallrock. Isotopic data (e.g., Sr and Nd) indicate a mantle-derived source modified by limited crustal contamination, consistent with the volcano's position on thickened crust. The dynamics feature a vertically elongated, finger-like extending from ~9–13 km depth at its top to 17–28 km at its base, as inferred from flank and geodetic modeling. Shallower domains (5–7 km) host differentiated, crystal-poor silicic melts prone to rapid ascent and Plinian explosivity, while deeper levels accumulate buoyant inputs, driving recharge and stratification via density contrasts. signals during eruptions, recorded via GPS and leveling, reflect volume loss from this zoned system, with short residence times (~10 years for intermediate magmas) limiting equilibration and promoting periodic destabilization through volatile exsolution and buoyancy-driven . This autocyclic behavior sustains Hekla's ~decadal eruptivity without external triggers, as modeled by coupled ascent and chamber repressurization.

Eruption History

Prehistoric and Early Activity

Hekla's volcanic activity during the early (approximately 11,700–8,200 years ) consisted primarily of intermediate-composition eruptions, evidenced by layers identified in distal sedimentary records, such as those from Løkke Sø in the . These represent the oldest documented tephras attributed to Hekla, indicating sporadic explosive events with limited volume compared to later phases, though precise eruption counts and magnitudes remain constrained by sparse proximal records. Major plinian-style eruptions emerged by the onset of the mid-Holocene, marking a shift toward more frequent and voluminous silicic activity. The Hekla 5 eruption, dated to circa 7000 years BP, produced widespread rhyolitic identifiable across , serving as a key marker in tephrochronology and reflecting shallow storage at 5–6.6 km depth. Prehistoric eruptions intensified further with events like Hekla 4, calibrated to 2310 ± 20 BC (4260 ± 20 ), a VEI 5 ejecting an estimated 11.2 km³ of (dense-rock equivalent) and contributing to regional ashfall patterns traceable to and beyond. This event exemplifies Hekla's capacity for high-silica fallout followed by potential effusive phases, though proximal deposits reveal two distinct units in some prehistoric layers. Overall, early to mid- activity at Hekla involved progressive reservoir evolution, with explosive phases dominating over effusive ones, contrasting with a noted shift around 3000 years toward less voluminous but more basaltic-influenced events in later prehistoric times. Tephrostratigraphy underscores these eruptions' role in Holocene paleoclimate proxies, though source credibility relies on calibrated radiocarbon and geochemical correlations from multiple sites.

Medieval to 19th Century Eruptions

Hekla's documented eruptive activity commenced in the medieval period with a series of explosive-effusive events characterized by initial plinian explosions producing widespread followed by fissure-fed lava flows along the summit ridge and flanks. These eruptions, occurring roughly every few decades to centuries, inflicted recurrent damage on Icelandic settlements through ash burial, livestock losses, and farm abandonment, with some events generating enough stratospheric to influence regional climate. Historical records, preserved in and sagas, detail precursors like earthquakes and provide eyewitness accounts of plume heights exceeding 30 km and audible blasts propagating hundreds of kilometers. The 1104 eruption, Hekla's first recorded, registered VEI 5 and ejected approximately 2 km³ of , blanketing southern in layers up to 1 m thick and destroying farms such as Stöng and Þjóðhildur's church, prompting evacuations and fatalities from ash suffocation and structural collapse. Subsequent medieval events included the VEI 4 eruption of 1158, which produced lava flows and damaging properties; VEI 3 in 1206 and VEI 2 in 1222 with similar localized effects; and the 1300–1301 VEI 4 outbreak, which split the summit ridge and caused fatalities amid extensive ashfall. The 1341 VEI 3 event killed and damaged infrastructure, while 1389–1390 (VEI 3) necessitated evacuations due to lava and . A lesser event southeast of the main edifice occurred around 1440, primarily affecting property. Post-medieval eruptions maintained this pattern of mixed explosivity and . The 1510 VEI 4 event involved plumes, lava flows, and fatalities; 1554 (VEI ~2) featured lava fountains southwest at Rauðubjallar; and 1597 (VEI 4) generated lahars alongside and flows. In 1636–1637 (VEI 3), lahars and fauna kills compounded property damage; 1693 (VEI 4) produced tsunamis from interactions, evacuations, and widespread destruction persisting into September. Minor activity in 1725 (VEI ~1) involved weak explosions and flows on multiple flanks. The 1766–1768 VEI 4 eruption at Bjálgagígar lasted over two years, with lahars, , and evacuations devastating lands. The 1845–1846 VEI 4 sequence, among the best-documented, initiated September 2 with explosions transitioning to flank flows, causing lahars, fauna deaths, and property losses until April. The final 19th-century event, 1878 (VEI 2) at Krákagígar east of Hekla, produced and flows from February to April amid seismic unrest.
Eruption DateVEINotable Impacts
11045Tephra ~2 km³; farm burials, fatalities
11584Property damage from lava and
1300–13014Fatalities, summit rifting
15104Fatalities, ash plumes
16934Tsunamis, evacuations
1766–17684Prolonged activity, lahars
1845–18464Documented precursors, flank flows

20th Century Eruptions

The 1947–1948 eruption of Hekla commenced on 29 March 1947 at 06:00 and continued until 1 April 1948, producing 0.8 km³ of lava and 0.18 km³ of tephra. This event generated a plume reaching 30 km in height and dispersed ash across northern Europe, causing significant agricultural disruption in Iceland due to fluorine contamination in grazing lands. The eruption featured explosive phases followed by effusive activity along fissures on the volcano's ridge. Hekla's next eruption occurred from 5 May to 5 July , marking the shortest repose interval of 10 years since 1104. This event involved Strombolian explosions ejecting incandescent bombs from vents on the northern flank, with lava flows advancing downslope. Tephra fallout was limited compared to prior events, but the activity highlighted Hekla's capacity for rapid reactivation. The 1980–1981 eruption began on 17 August 1980 with a Plinian phase producing a 15-km-high column, followed by effusive activity along a 7-km fissure system yielding 0.12 km³ of lava. This sixteenth historic eruption, again after a 10-year repose, transitioned from explosive to strombolian and effusive phases, with tephra impacting local areas and aviation. From 17 January to 11 March 1991, Hekla erupted andesitic , generating columns up to 11.5 km and approximately 0.15 km³ of combined lava and . Preceded by increased , the event featured initial venting followed by sustained lava flows, with dispersal affecting European airspace and underscoring the volcano's persistent subdecadal activity pattern in the late .

Post-2000 Activity and Monitoring

Hekla has not erupted since its last activity from February 26 to March 6, 2000, which produced a 10-12 km plume and involved opening along the southwest flank. Following this event, the volcano entered a period of characterized by intermittent unrest indicators rather than magmatic eruptions. Steady ground inflation has been observed since 2000, with rates exceeding pre-eruption levels by 2007-2009, as measured at sites like Næfurholt where monitoring began in 1985. Seismic activity post-2000 includes persistent shallow background microseismicity, with near-constant rates since January 2013 and improved detection thresholds reduced from magnitude 2.2 in 2000 to 0.9 via the Icelandic Meteorological Office's (IMO) SIL seismic network. A notable swarm occurred in March 2013, with at least seven micro-earthquakes (magnitudes 0.4-1.0) at 11-12 km depth northeast of the summit, prompting the IMO to raise the aviation color code to yellow on March 26 and the Icelandic Civil Defense to declare an uncertainty phase; both were lowered by April 4 after activity ceased, followed by an isolated magnitude 1.1 event on April 26. Small earthquakes continue, with recent detections up to magnitude 1.6, but no patterns indicate imminent eruption. The IMO maintains comprehensive surveillance of Hekla through multiple geophysical methods, including real-time seismometers at stations like Mjóaskarð and Rauðaskál, GPS and strain meters for deformation, and gas sensors using and MultiGAS techniques, which have detected no emissions since measurements began at Rauðaskál in 2014. Supplementary tools encompass web-cameras, sensors, and for rapid eruption onset detection, though warnings may be limited to under one hour based on historical precedents. Enhanced networks, such as the 2021 HERSK-TNA real-time seismic array, stream data to IMO and international centers to track microseismicity potentially preceding activity. Despite observed inflation surpassing 2000 levels and slight seismic upticks, current assessments show no immediate eruptive threat, with monitoring emphasizing the volcano's unpredictable short-notice behavior.

Environmental Impacts

Effects on Local Ecosystems

Eruptions of Hekla deposit thick layers of and lava, which physically bury vegetation, abrade foliage through fallout, and introduce chemical toxins such as , leading to widespread and in surrounding lowlands. from basaltic eruptions weathers rapidly, amplifying and nutrient imbalances that hinder plant regrowth for decades. During the 1947 eruption, tephra accumulations of 1-10 cm in nearby areas elevated fluorine concentrations in streams, contributing to vegetation stress and mortality via contaminated . Lava flows from Hekla's flanks sterilize soils and eliminate established plant communities, initiating primary succession dominated initially by pioneer lichens and mosses on cooled surfaces. Historical eruptions, such as those in 1693 and 1947, have repeatedly devastated pastures, with ash layers suppressing grass recovery and promoting erosion-prone barren landscapes exacerbated by post-eruption grazing pressure. , including grazing mammals and nesting birds, suffer acute declines due to forage scarcity and toxin , as observed in fluorosis near the following ash deposition. Ecological recovery proceeds slowly, with woodlands—once dominant but now scarce—requiring centuries to reestablish without intervention, as layers from events like Hekla 3 (circa 1000 BCE) persist as barriers to . Ongoing restoration initiatives, such as the Hekluskógar project, plant native to enhance resilience against future falls by stabilizing soils and reducing aeolian redistribution of ash. These efforts aim to mitigate cascading effects like wind-driven dust storms that further degrade habitats through abrasion and burial.

Atmospheric and Climatic Consequences

Eruptions of Hekla release substantial volumes of fine and (SO₂) into the and , enabling long-range atmospheric transport and chemical interactions that can alter regional air quality and radiation balance. The ash particles, primarily composed of rhyolitic shards, scatter and pose abrasion risks to engines, with dispersion patterns modeled to affect paths during moderate to large events. SO₂ oxidizes to form aerosols, which reflect incoming solar radiation and can induce short-term cooling, though Hekla's injections are typically insufficient for global-scale effects comparable to tropical VEI 6+ eruptions. The 1104 CE plinian eruption (VEI 5) generated a high-altitude plume depositing across , coinciding with reports of a persistent dry fog in chronicles that may have contributed to localized cooling and agricultural stress, although recent ice-core analyses attribute the primary signal to a cluster of events including possible sources rather than solely Hekla. Similarly, the 1693 eruption (VEI 5) dispersed ash to , exacerbating atmospheric haze and acid deposition. Twentieth-century events, such as 1947 and 1970, produced plumes reaching 10-15 km altitude, with ash fallout impacting visibility and acidity in ; the 1970 eruption elevated in rivers via atmospheric scavenging, indirectly linking to stress. The 2000 eruption emitted detectable SO₂ plumes tracked circumpolarly via , with in-situ measurements revealing elevated aerosols, HCl, and HF mixing ratios persisting for weeks and influencing stratospheric chemistry. These gases enhanced acidity, as evidenced by post-eruption sampling showing altered chemical compositions. Overall, while Hekla's atmospheric injections drive episodic regional perturbations— including reduced insolation and —causal attribution to broader climatic shifts remains constrained by the volcano's subpolar location and moderate eruption magnitudes, limiting stratospheric residence times compared to equatorial counterparts.

Human and Societal Impacts

Historical Societal Disruptions

The 1104 eruption of Hekla, a VEI 5 Plinian event, buried the Þjórsárdalur valley under thick layers of , leading to the rapid abandonment of multiple farms including the chieftain's hall at Stöng. This destruction preserved archaeological sites under ash but displaced local populations, contributing to regional resettlement patterns in early medieval . The 1300 Hekla eruption deposited significant ashfall across northern districts such as Skagafjörður, where layers exceeded 10 cm in places, rendering pastures unusable and precipitating a subsequent that historical link to losses and food shortages. While direct fatalities lack confirmation in records, the event exacerbated vulnerabilities in Iceland's pastoral economy, with ash-induced vegetation damage persisting for years and straining societal resilience amid concurrent climatic stresses. In 1693, Hekla's explosive eruption produced over 0.2 cubic kilometers of , with fallout exceeding 25 cm on farms leading to permanent abandonment, as documented by Sigurður Þórarinsson; such sites shifted from pastoral use to irreclaimable wasteland due to and poisoning of grazing lands. Livestock mortality from fluorosis was widespread, compounding economic hardship in an already famine-prone society and prompting migrations from affected southern highlands. Recurrent Hekla activity from the medieval period onward, including events in 1389 and 1766, inflicted cumulative disruptions through repeated veils that acidified soils and decimated hay crops, fostering cycles of and depopulation in proximity to the . These impacts underscored the precarious dependence of Icelandic society on fragile agrarian systems, where volcanic fallout often amplified baseline hardships from harsh weather and limited .

Economic and Agricultural Damages

The fallout from Hekla eruptions has repeatedly devastated Icelandic agriculture by blanketing pastures and hayfields, rendering them unusable for and forage production for periods ranging from months to years depending on ash thickness and content. Farms receiving more than 25 cm of fresh , as occurred during the 1693 eruption, were typically abandoned permanently, as the deposits inhibited regrowth and pastoral viability in Iceland's grassland-based farming system. This led to widespread relocation of farming communities and long-term shifts in land use, with affected areas like Þjórsárdalur valley seeing multiple farmsteads destroyed or deserted after the 1104 VEI 6 eruption. Fluorine enrichment in Hekla's tephra, often exceeding 1,000 ppm in fresh ash, causes acute fluorosis in , particularly sheep, through contaminated grasses and water, resulting in skeletal deformities, reduced fertility, and mass die-offs. The 1947 eruption contaminated streams with up to 8 ppm fluorine in areas receiving 1–10 cm of ash, leading to elevated stock losses despite most livestock being stalled indoors at onset. Similarly, the 1970 eruption deposited ash with up to 2,000 ppm , triggering fluorosis outbreaks in sheep and contributing to broader agricultural output declines. These events have historically halved local hay yields and forced fodder imports, straining Iceland's self-sufficient . Economically, Hekla's impacts have amplified vulnerabilities in Iceland's livestock-dependent rural sectors, where constitutes a significant portion of agricultural GDP; post-eruption losses in animal stocks and land productivity have necessitated government compensation and , though pre-20th century indicate no systematic state relief, exacerbating famines and migrations. The 1947 event caused considerable but non-catastrophic produce shortfalls, with recovery aided by mechanical ash removal on lighter deposits. Repeated disruptions, including from the 1766 and 1845 eruptions, contributed to chronic farm abandonments—over 140 sites documented in proximal regions—reducing taxable land and economic output in southern for generations. Modern monitoring has mitigated some risks through evacuation and feed supplementation, but potential future events could still impose millions in cleanup and replacement costs given current agricultural valuations.

Health Risks and Mitigation Efforts

Eruptions of Hekla have primarily posed health risks to through fluorosis, caused by -rich contaminating grazing lands and water sources. During the eruption, ash samples contained up to 2000 ppm of , leading to widespread fluorosis in sheep, with symptoms including skeletal deformities and dental damage. Similarly, the 1947–1948 eruption resulted in fluorosis among farm animals, exacerbated by ash deposition that coated vegetation and dissolved into . Human health risks have been secondary but include respiratory irritation from inhaling fine particles, which can penetrate deep into the lungs, and exposure to toxic gases such as that irritate eyes and mucous membranes during proximity to eruptions. Post-eruption, elevated levels in rivers like Galtalaekur reached 8 ppm following the event, though concentrations normalized within weeks, minimizing prolonged human exposure risks. Mitigation efforts focus on monitoring, public advisories, and environmental stabilization to reduce ash dispersal and contamination. The Icelandic Meteorological Office maintains continuous geophysical surveillance of Hekla, including seismic and gas emission tracking, to provide early warnings that enable livestock relocation and human evacuations before eruptions. During ashfall events, health authorities recommend wearing dust masks, sealing homes, and avoiding contaminated water, with air quality monitoring in populated areas to alert residents to respiratory hazards. Long-term strategies include the Hekluskógar afforestation project, initiated in the 1950s, which plants birch woodlands to bind tephra deposits, preventing wind redistribution of fluoride-laden ash and thereby reducing secondary exposure risks to both animals and ecosystems. Hazard assessments for tourists emphasize avoiding eruption zones and recognizing pyroclastic flow dangers, supported by mapped risk areas derived from historical data. These measures have proven effective in limiting fatalities, as human populations historically avoided contaminated areas unlike livestock, which bore the brunt of past events.

Scientific Monitoring and Research

Geophysical Surveillance Methods

The Icelandic Meteorological Office (IMO) employs a comprehensive geophysical monitoring network for Hekla volcano, focusing on seismicity and ground deformation to detect precursors of unrest, given the volcano's history of short pre-eruptive warning times, often less than one hour. The seismic component includes the nationwide SIL (Seismic in Iceland) system augmented by 12 broadband and intermediate-period seismometers within 50 km of the volcano, enabling detection of microearthquakes and low-frequency events associated with magma movement. Persistent shallow background microseismicity beneath the summit, recorded since the 1990s, serves as a baseline for identifying anomalies, such as increased rates prior to the 2000 eruption, though Hekla remains relatively quiet seismically between events. Geodetic monitoring detects surface deformation through continuous GPS (cGPS) stations, tiltmeters, and strainmeters, which measure inflation from magma accumulation or deflation post-eruption. The Hekla monitoring project, initiated in June 2000 following the eruption, integrated GPS technology to track radial uplift patterns, with stations revealing pre-eruptive inflation of up to several centimeters in the 1990s and rapid deflation during events like 2000. Tilt data from sites near the edifice have documented slow repose-phase deformation and abrupt changes during eruptions, such as westward tilting before the 1991 event. Complementary efforts, including the HERSK real-time seismic network deployed since 2017 with stations as close as 4 km to the summit, enhance resolution for early detection of dyke propagation or harmonic tremors. Interferometric synthetic aperture radar (InSAR) supplements ground-based data by mapping broad-scale deformation over time, as integrated in joint inversions with GPS and tilt for modeling dynamics at depths of 22-29 km. These methods collectively inform assessments, with real-time data processed in IMO's 24-hour monitoring center, though challenges persist due to Hekla's rapid onset and sparse precursor signals compared to other Icelandic systems.

Recent Studies and Predictive Models

Recent geophysical monitoring of Hekla has emphasized deformation and data to develop short-term eruption forecasts, building on the Icelandic Meteorological Office's (IMO) continuous surveillance network established post-2000, which includes seismic stations, GPS for ground deformation, and InSAR for surface changes. A 2014 study analyzed persistent shallow microseismicity beneath the summit, revealing background rates that could signal precursory unrest, as Hekla transitions rapidly from quiescence to eruption within hours, offering potential for real-time alerts if integrated with deformation metrics. Predictive models for Hekla incorporate probabilistic frameworks like the Bayesian Event Tree for Eruption Forecasting (BET_EF), which uses seismic, geodetic, and gas data to estimate eruption probabilities on timescales of days to weeks, applied retrospectively to Hekla's historical events including the eruption predicted to within minutes via observed unrest patterns. The FUTUREVOLC project (2012–2016), funded by the , advanced operational models for discharge rates and ash dispersion using VATD simulations, enhancing forecasts for Hekla's explosive phases by coupling ground-based data with satellite observations. Recent analyses, such as a 2025 geophysical assessment, indicate steady accumulation in the sub-volcanic chamber via leveling and GPS measurements, prompting refined deformation models distinguishing pressure sources from surface loads to better isolate magmatic signals. Ash transport and dispersion models, calibrated for Hekla's typical plume heights of 10–12 km, simulate fallout probabilities under varying wind regimes, as detailed in climatological studies forecasting exposure risks over Europe from short-duration explosive events. These integrate ensemble meteorological forecasts with plume rise algorithms, though limitations persist in predicting exact timing due to Hekla's subdued precursors compared to other Icelandic systems. Ongoing IMO efforts, including 24-hour data processing, aim to reduce these uncertainties through machine learning-enhanced pattern recognition in microseismic and strain data.

Cultural and Historical Perceptions

Etymology and Folklore Origins

The name Hekla originates from Old Norse hekla, denoting a "cloak" or "hooded cloak," a reference to the persistent shroud of mist, clouds, or snow that often conceals the volcano's summit, giving it a hooded appearance. This etymological link reflects early observers' empirical descriptions of the mountain's atmospheric conditions rather than abstract symbolism, as documented in Icelandic linguistic traditions. In medieval , particularly among Icelandic Norse and later European chroniclers, Hekla acquired a sinister reputation as the "Gateway to " or an entrance to , stemming directly from its cataclysmic eruption on October 16, 1104 AD. This event unleashed massive volumes of ash, (including blocks up to car-sized), and toxic gases, accompanied by earthquakes and fire fountains, which devastated southern and were interpreted through a Christian lens as manifestations of infernal activity. The perception, rooted in eyewitness accounts rather than prior pagan myths, spread via monastic writings across , associating the 's unpredictable violence with demonic realms despite its remote location. Subsequent eruptions in 1158, 1206, and 1300 reinforced this lore, though it waned with increased scientific observation by the .

Influence on Icelandic Settlement and Lore

The 1104 AD eruption of Hekla, the first since Norse around 874 AD, devastated early human habitation in the Þjórsárdalur Valley of southern Iceland. This explosive event, rated VEI-4, buried at least 22 farms under thick layers of and ash, including Stöng, a major chieftain's manor exemplifying Commonwealth-era architecture and daily life. The destruction prompted mass relocation of survivors to less hazardous areas, marking a pivotal shift in settlement patterns away from fertile volcanic lowlands toward more stable coastal and highland fringes. Subsequent Hekla eruptions, including those in 1300, 1389, and later, exacerbated this influence by repeatedly rendering nearby lands uninhabitable through ashfall, earthquakes, and jökulhlaups, leading to chronic farm abandonments and reinforcing cultural caution against permanent occupation in the volcano's shadow. Over 23 documented eruptions since settlement have collectively shaped Iceland's demographic distribution, with southern highlands remaining sparsely populated compared to western and northern regions less prone to such disruptions. In Icelandic lore, Hekla's 1104 outburst ignited persistent myths framing the volcano as the gateway to Hel, the Norse realm of the dead, or Christian , with eyewitness accounts of fire fountains and ash clouds evoking infernal imagery. Medieval Icelandic sagas and European chronicles, disseminated by Cistercian monks, described damned souls entering through its craters and witches assembling with devils on its flanks, especially at , beliefs that endured into the 19th century despite scientific scrutiny. These narratives not only reflected the terror of unpredictable eruptions but also imbued Hekla with a sacred dread, influencing oral traditions and cautionary tales that underscored the perils of defying natural forces in Iceland's harsh landscape.

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