Editing Hekla (section)

=== 1913 to 1948 ===
;1913
[[File:Image from page 30 of "Island am Beginn des 20 Jahrhunderts;" (1904).jpg|thumb|right|Hekla {{circa|1904}}]]
A small eruption (VEI-2) occurred between 25 April 1913 and 18 May 1913, around {{convert|10|km|abbr=on}} east of Hekla, and caused large fissures at [[Mundafell]] and [[Lambafit]] which produced {{convert|3.8| and|6.3|km2|abbr=on}} of lava respectively.<ref name="GVP" /><ref name="HANVp17" />

;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 {{convert|0.8|km3|abbr=on}} was produced with {{convert|0.21|km3|abbr=on}} of tephra. The height of Hekla was {{convert|1447|m|abbr=on}} before the eruption, increasing to a maximum of {{convert|1503|m|abbr=on}}, before dropping to {{convert|1491|m|abbr=on}} 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&nbsp;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 {{convert|4|km|abbr=on}} fissure on the ridge. The cloud from the eruption had ascended to a height of 30&nbsp;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 {{convert|3|-|10|cm|abbr=on}} layer. A lava bomb that landed {{convert|32|km|abbr=on}} from Hekla was {{convert|0.5|m|abbr=on}} across and weighed {{convert|20|kg|abbr=on}}. Between [[Vatnafjöll]] and Hekla, a layer of tephra up to {{convert|1|m|abbr=on}} thick was deposited, and this included bombs with a diameter larger than {{convert|0.5|m|abbr=on}}. Bombs with surface areas of {{convert|50|m2|abbr=on}} were dropped onto the slopes of Hekla, for up to {{convert|1|km|abbr=on}}. 51&nbsp;hours after the eruption had started, ash fell on [[Helsinki]], Finland, having covered {{convert|2860|km|abbr=on}} in this time.

The initial tephra production rate in the first 30&nbsp;minutes of the eruption was 75,000&nbsp;m<sup>3</sup>·s<sup>−1</sup>, dropping to 22,000&nbsp;m<sup>3</sup>·s<sup>−1</sup> for the next half-hour. The initial phase produced {{convert|0.18|km3|abbr=on}} of tephra, equating to {{convert|4.5E7|km3|abbr=on}} of [[Dense-rock equivalent]], covering {{convert|3130|km2|abbr=on}} of land and sea. 98&nbsp;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&nbsp;man-days by the end of July. The eruption produced around {{convert|3|ML|abbr=on}} of water (snowmelt and directly from the fissure) which caused flooding of the [[Ytri Rangá]] river.

In the first 20&nbsp;hours of the eruption approximately 3,500&nbsp;m<sup>3</sup>·s<sup>−1</sup> of lava was produced from the fissure, dividing into various branches and covering {{convert|12|-|15|km2|abbr=on}}. On the second day, 8 distinct [[eruption column]]s were discernible. A crater formed at {{convert|860|m|abbr=on}} 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&nbsp;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.

[[File:Hekla from Pæla 20090516.jpg|thumb|right|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 {{convert|960|m|abbr=on}} circumference at its top and the Summit Crater a {{convert|700|m|abbr=on}}circumference at its highest point, {{convert|90|m|abbr=on}} 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&nbsp;m<sup>3</sup>·s<sup>−1</sup>, dropping to 5–10&nbsp;m<sup>3</sup>·s<sup>−1</sup> in April and early May at a speed of around 20&nbsp;cm·s<sup>−1</sup> before increasing, eventually reaching 150&nbsp;m<sup>3</sup>·s<sup>−1</sup> at the end of June and at similar levels until mid-July with a peak flow speed of 2–2.5&nbsp;m·s<sup>−1</sup>. From there it gradually decreased to under 10&nbsp;m<sup>3</sup>·s<sup>−1</sup> in November. Initially the lava comprised 57–58% SiO<sub>2</sub> and 11% Fe<sub>2</sub>O<sub>3</sub>, from the time of peak flow onwards this changed to 54% SiO<sub>2</sub> and 13.5% Fe<sub>2</sub>O<sub>3</sub>.

The lava river sometimes ran through [[lava tubes]] before emerging again. The lava front had a height of up to {{convert|15|m|abbr=on}}. On 15 and 16 June, a branch of lava flow to the south of Melfell traveled over {{convert|1|km|abbr=on}} in 30&nbsp;hours before slowing and stopping by 21 June, {{convert|7.8|km|abbr=on}} from the Lava Crater. The longest lava stream produced was {{convert|8|km|abbr=on}} 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.<ref name="Morgunblaðið">{{cite web |url= http://timarit.is/view_page_init.jsp?pageId=1265411 |title=Morgunblaðið, 04.11.1947 |work=[[Timarit.is]] |year=2012 |access-date=22 July 2012}}</ref>

The lava flow stopped after 13&nbsp;months on 21 April, having covered {{convert|40|km2|abbr=on}} and with a maximum depth of {{convert|100|m|abbr=on}}. 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|CO<sub>2</sub>]] emitted from cracks in the ground pooled in hollows near to Hekla, killing 15 sheep and some wild animals and birds. In total {{convert|24000|tonne}} of CO<sub>2</sub> was emitted. Ditches were dug by farmers to drain these hollows, and the CO<sub>2</sub> emission had stopped by the end of the year.<ref name="GVP" /><ref name="HANVp19-38">Thorarinsson, pp. 19–38</ref>