Good Afternoon!
Having visited Katla, let’s go further north to the Eastern Volcanic Zone and take a look at Hekla.
Hekla is an active snow-covered elongated stratovolcano, lying at the southern end of the Eastern Volcanic Zone in Iceland in a rift transform junction. Following the 1104 AD eruption, Hekla was called “The Gateway to Hell” – a name that stuck until the 19th century.
The Hekla volcanic system comprises a 1490m high central volcano and a 60 km fissure swarm. The Heklugjá fissure, 5.5 km long cutting across the central volcano, is the site of many eruptions and gives Hekla its elongated shape. The Vatnafjöll fissure system, 40 km long and 9 km wide is considered part of the Hekla volcanic system. Hekla may have a small magma reservoir 4 km below the surface. She has permanent snow cover but no large glacier.
Hekla’s lavas differ from the rift zone volcanoes; her lavas are andesite, basaltic andesite, basalt / picro basalt, rhyolite and dacite. She erupts tephra and silicic to intermediate lavas from the central volcano. Eruptions tend to be a short plinian / subplinian phase followed by lava flows. Larger explosive silicic eruptions have produced enough tephra for the deposits to act as time markers in dating other eruptive activity in Iceland. She is a large fluorine producer which is hazardous to livestock. The hazards listed are tephra fallout, fluorine gas, pyroclastic flows and lava flows; the absence of a large glacier means that jökulhlaups are not a major hazard for Hekla.
The fissure system produces basaltic lavas and a small amount of tephra; its hazards are listed as lava flows and volcanic gas pollution.
Eruptive History
According to GVP, Hekla has had 65 Holocene eruptions ranging from VEI 1 to 5. Larsen and Thordarson state that there have been 100 eruptions in the past 9000 years, 23 of which occurred in the last millennium with VEIs ranging from 0 to 5. The central volcano produces eruptions of VEI 2 to 6, VEI 3 to 4 being the most frequent; the longer the repose time between eruptions, the larger the ensuing eruption. The fissure system produces less explosive eruptions (VEI 1 to 2). Hekla’s largest known eruption, a VEI 6, occurred between 3000 to 4300 years ago. Traces of ash from Hekla have been found in Scandinavia, Germany, Ireland and the UK.
Hekla’s eruptive style has changed over time: from effusive basalt 9000 to 7000 years ago; to large explosive silicic eruptions between 7000 to 3000 years ago; and, then smaller more frequent mixed silicic and basaltic eruptions from 3000 years ago to the present day.
The most recent eruption was in 2000, with a VEI 2 to 3, 0.01km3 of airborne tephra 0.01km3 and 0.12km3 of lava; it thought that magma rose through a conduit from a depth of more than 10km to 1km below before heading towards a fissure on the Hekla ridge.
Hekla has an unusually low level of seismic activity. Her largest earthquakes are in the order of 2M when dormant and 3M during an eruption. She does not give much warning of an eruption: known precursors are earthquakes 25 to 90 minutes beforehand. Monitoring has increased since the last eruption so there should be more information about any precursors to future eruptions.
Recent Seismic Activity
From the data set of earthquakes downloaded from IMO’s site for period 1 January 2016 to 31 May 2020, we extracted those for the Hekla – Vatnafjöll area: 63.7578°N, 19.4687°W to 64.0952°N, 19.9399°W. We found 1,018 earthquakes, compared to Katla’s 6,505 for the same period. The largest quake was 2.62 and the deepest 25.11km.
In our latitude v longitude scatter plot we can see that most earthquake activity is scattered along the fissure systems, with an E-W “cluster” to the south of GPS station HEK3, north of Hekla. The latitude v depth plot shows the activity near Vatnafjöll is occurring in the lithosphere, whereas there appears to be a conduit under Hekla. A close up of the cluster confirms this impression.
Our geodensity plot of the earthquakes shows a hot spot south of HEK3, north of Hekla.
We looked for swarms in the data set to see if these account for the hot spot but did not find any large ones; the hot spot appears to be an accumulation of activity over the period. Hekla seems to have a slow magma feed.
The Armchair Volcanologist
12 June 2020
© Copyright remains with the author; all rights reserved, 2020
Sources and Further Reading
“Hekla”, Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) and Thor Thordarson (Faculty of Earth Sciences, University of Iceland). In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=HEK
Fig 1: Photo: Ilynskaya,E. (2014 June 22). Hekla: photo 1 of 5. Retrieved from Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=HEK
Fig 2: Map: After Jóhannesson and Einarsson (1992), Jóhannesson and Saemundsson (1998a), Larsen et al (2013a), Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO. In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=HEK
Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu
Earthquake data: Icelandic Meteorology Office: IMO https://en.vedur.is/earthquakes-and-volcanism/earthquakes
Plots are the author’s own work.
An Armchair Volcanologist 
