Hekla

Hekla, or Hecla, is an active stratovolcano in the south of Iceland with a height of. Hekla is one of Iceland's most active volcanoes; over 20 eruptions have occurred in and around the volcano since the year 1210. During the Middle Ages, the Icelandic Norse called the volcano the "Gateway to Hell" and the idea spread over much of Europe.
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. Cumulatively, the volcano has produced one of the largest volumes of lava of any in the world in the last millennium, around.

Etymology

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.

Reputation

After the eruption of 1104, stories, probably spread deliberately through Europe by Cistercian monks, told that Hekla was the gateway to Hell. The Cistercian monk Herbert of Clairvaux wrote in his De Miraculis : A poem by the monk Benedeit from 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.
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. There is still a legend that witches gather on Hekla during Easter.

Geography

Hekla is part of a volcanic ridge, long. The most active part of this ridge, a fissure about long named Heklugjá, 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.

Geology

Hekla has a morphological type between that of a fissure vent and stratovolcano 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. The Heklugjá fissure opens along its entire length during major eruptions and is fed by a magma reservoir estimated to have a top below the surface with centroid lower. The chamber extends to an unusual depth of more than, and the more silicic lavas have matured at more than.
Many of the eruptions commence with thicker more explosive rhyolite, dacite or andesite eruptives which create tephra and have the potential for pyroclastic flows. Other or the later part of eruptions come from thinner basalt tending magma which forms lava fields.
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.
It is the only Icelandic volcano to produce calc-alkaline lavas.
Phenocrysts in Hekla's lava can contain plagioclase, pyroxene, titanomagnetite, olivine, and apatite.
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. 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. Earthquakes in the volcano's vicinity are generally below magnitude 2 while it is dormant and magnitude 3 when erupting.

Eruption history

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. Some are very short whereas others can stretch into months and years. But there is a general correlation: the longer Hekla goes dormant, the larger and more catastrophic its opening eruption will be. The most recent eruption was on 26 February 2000.

Prehistoric eruptions

One of the largest Holocene eruptions in Iceland was the Hekla 3 eruption, which threw about of volcanic rock into the atmosphere, placing its Volcanic Explosivity Index 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. The dates were recently recalibrated of the major eruptions and a table is given below as the difference in dates could cause confusion.

Eruption	Year	Year
H-5	5050 BCE	± 260 BCE
H-Sv	3900 BCE	3900 BCE
H-4	2310 ± 20 BCE	2375 ± 8 BCE
H-3	950 BCE	± 140 BCE

Unless otherwise stated eruption dates in Year column are from Global Volcanism Program and Catalogue of Icelandic Volcanoes, As other sources can disagree, there has been a recent literature update. The values and range given in the Year column used IntCal20 for H-5 and H-3 and ice core data for H-4 which were not available in 2019.
Hekla 3, 4, and 5 produced huge amounts of rhyolitic ash and tephra, covering 80% of Iceland and providing useful date markers in soil profiles in other parts of Europe such as Orkney, Scandinavia, and elsewhere. H₃ and H₄ produced the largest layers of tephra in Iceland since the last ice age. During the last 7,000 years, one third of the volcanic ash deposited in Scandinavia, Germany, Ireland, and the United Kingdom originated from Hekla.

1104 to 1878

;1104
Hekla had been dormant for at least 250 years when it erupted explosively in 1104, covering which is over half of Iceland with 1.2 km³ / 2.5 km³ 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 in Þjórsárdalur valley, at Hrunamannaafréttur and at Lake Hvítárvatn were abandoned because of the damage. The eruption caused Hekla to become famous throughout Europe.
;1158
A VEI-4 eruption began on 19 January 1158 producing over of lava and of tephra. It is likely to be the source of the Efrahvolshraun lava on Hekla's west.
;1206
The VEI-3 eruption began on 4 December.
;1222
The VEI-2 eruption and the 1206 eruption distributed around of tephra mainly to the northeast.
;1300–1301
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
of land with of tephra. Over of lava was also expelled. The tephra caused significant damage to the settlements of Skagafjörður and Fljót, leading to over 500 deaths that winter.
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.
;1341
A small eruption started on 19 May and deposited around of tephra over the areas west and southwest of Hekla, leading to many cattle deaths, probably mainly from fluorosis.
;1389
In late 1389 Hekla erupted again, 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 Nordurhraun. In total around of lava and of tephra were produced.
;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.
;1510
Image:Volcanic bomb from Hekla.jpg|thumb|right|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, firing volcanic bombs as far as Vördufell, west. Tephra was deposited over Rangárvellir, Holt and Landeyjar, in total. A man in Landsveit was killed.
;1597
A VEI-4 eruption began on 3 January and lasted for over 6 months, with of tephra being deposited to the south-southeast, damaging Mýrdalur.
;1636–1637
A small eruption began on 8 May 1636 and lasted for over a year. The of tephra from the eruption damaged pasture to the northeast causing death of livestock.
;1693
Starting 13 February and lasting for over 7 months the eruption was one of Hekla's most destructive. Initially tephra was produced at 60,000 m³·s⁻¹, 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.
;1725
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.
;1766–1768
The eruption of 1766 was large and produced the second largest lava flow, covering, and third largest tephra volume,, 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 lava bombs were thrown away, and flooding was caused by the sudden melting of snow and ice on Hekla's slopes.
;1845–1846
Hekla was dormant for more than sixty years before 1845, when it suddenly burst forth on 2 September at 9 am:
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 was mainly to the east-southeast; immediately to the east of Hekla the layer was deep. Fine ash was carried to the Faroes, Shetland and Orkney. Lava flows to the west and north-west covered an area of with a volume of 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.
;1878
A small eruption occurred between 27 February 1878 and April 1878, around east of Hekla, and produced of lava from two parallel fissures covering.