Hekla is a 1,491-metre stratovolcano in southern Iceland that has erupted more than twenty times since the island was settled around 874 AD, making it one of the most consistently active volcanoes in Europe. Its 1104 eruption buried farms across a wide area without warning, and the shock of that event gave Hekla a reputation across medieval Europe as an entrance to Hell — a description that appears in clerical writing for the next five hundred years.
A Ridge, Not a Cone
Hekla does not look like the textbook volcano. Instead of a symmetrical cone it forms an elongated ridge about 40 kilometres long in its broader structure, with a summit crater row — the Heklugjá fissure — running roughly 5.5 kilometres along the crest. In major eruptions the fissure opens along most of that length at once, producing a curtain of fire rather than a single vent.
The name means "hooded" or "cloaked" in Old Norse, usually taken as a reference to the cloud cap that habitually sits on the summit. Icelanders sometimes call it the "Gateway to Hell", but that phrase came from continental Europe rather than from Iceland itself.
Structurally Hekla is unusual for Iceland. It sits near the southern end of the Eastern Volcanic Zone, at a point where the rift system is propagating southwest into older crust, and it is not a straightforward rift volcano. Its magma is more evolved than the basalt that dominates Icelandic volcanism — andesite and dacite feature prominently, which is why its opening phases are far more explosive than a typical Icelandic fissure eruption.
The 1104 Eruption and the Hell That Followed
Before 1104 there is no record of Hekla erupting in the settlement period, and the farms of the Þjórsárdalur valley had been established for generations on the assumption it was a mountain like any other. The 1104 eruption was a large explosive event that dropped pumice and ash across a wide swathe of southern and western Iceland and abandoned the valley's farms outright.
One of them, Stöng, was excavated in 1939 and is now the basis of a reconstructed Viking-age longhouse nearby — the most complete picture archaeologists have of an ordinary Icelandic farmstead of the period, preserved because the ash sealed it.
The medieval reputation grew from there. Twelfth- and thirteenth-century European writers described Hekla as a prison for souls; a Cistercian monk, Herbert of Clairvaux, called it the eternal prison of the damned. Birds seen circling the summit were reported as souls. The belief was persistent enough that it survived into the eighteenth century, and no verified ascent of the mountain is recorded until 1750, when Eggert Ólafsson and Bjarni Pálsson climbed it and found no infernal machinery.
What the Ash Layers Record
Hekla's eruptions have been frequent, large and chemically distinctive, which makes them exceptionally useful to scientists. Its tephra layers are used as dating horizons across Iceland and beyond — a technique called tephrochronology, where an identified ash layer in a soil profile, lake bed or ice core gives an absolute date to everything above and below it.
Some of the recorded events:
- c. 950 BC (Hekla 3) — one of the largest Holocene eruptions in Iceland, its ash detectable in Irish bog deposits and Scottish peat, and linked by some researchers to a period of cooling recorded in tree rings
- 1104 — the first historical eruption, roughly 2.5 km³ of tephra, abandoning Þjórsárdalur
- 1300, 1341, 1389 — a cluster of damaging medieval eruptions with heavy livestock losses
- 1766–68 — a long eruption lasting around two years
- 1947–48 — the largest of the twentieth century, running thirteen months, with an initial plume reported near 30 kilometres
- 1970, 1980–81, 1991, 2000 — four eruptions in three decades, each relatively short
The 2000 eruption began on 26 February and lasted eleven days. It is the most recent to date, and the interval since is now considerably longer than the roughly ten-year rhythm of the late twentieth century, which is why the mountain is watched closely.
The Warning Problem
Hekla's defining hazard is how little notice it gives. Most volcanoes announce an eruption through weeks or months of escalating seismicity as magma forces its way upward. Hekla does not. The 1991 and 2000 eruptions were each preceded by measurable seismic activity for well under two hours — around 30 minutes of clear precursory tremor in 2000 before the fissure opened.
The explanation is thought to lie in the plumbing: a relatively open conduit and a magma storage system deep enough that ascent is fast once it begins. The practical consequence is that a hiking party on the mountain would have very little time. The Icelandic Meteorological Office maintains dedicated strain meters and seismometers around Hekla for exactly this reason, and the mountain carries standing advice that an eruption may begin with almost no warning.
The other hazard is aviation ash and the lahars that form when eruption heat melts snow on the summit, sending meltwater and debris into the Þjórsá and Rangá river systems. Fluorine contamination of grazing land is a recurring agricultural problem after Hekla eruptions — fluorine-laden ash poisons sheep that graze on it, and livestock losses have historically exceeded direct damage from lava.
The Desert the Mountain Made
Repeated tephra fall has stripped the land around Hekla in a way that outlasts any single eruption. Southern Iceland was substantially birch-wooded at settlement; centuries of grazing, felling and recurrent ash burial left much of the Hekla surroundings as black tephra desert, where the soil has no root network to hold it and wind erosion carries it away between eruptions. It is some of the most severely degraded land in the country.
Iceland has treated this as a restoration problem rather than a permanent condition. The Hekluskógar project, launched in 2007 as a partnership between the Soil Conservation Service and the Forest Service, aims to re-establish native downy birch and willow across a large area surrounding the volcano — on the order of 90,000 hectares — on the reasoning that woodland absorbs tephra fall and stabilises soil instead of letting it blow away. Planting is done in scattered nuclei intended to spread by natural seeding rather than in continuous blocks, which is slower but far cheaper at that scale.
It is a long project measured against Hekla's eruption interval, and the next eruption will bury part of it. That is anticipated in the design: the point is that recovering ground between eruptions leaves more soil in place after each one.
Hekla Beside Katla and Eyjafjallajökull
Iceland's southern volcanoes are often grouped together, but they behave very differently. Eyjafjallajökull, about 45 kilometres southwest, erupts rarely — before 2010 its previous eruption was 1821–23 — but its 2010 event closed European airspace for six days because the ice cap over the vent fragmented the magma into exceptionally fine, far-travelling ash. Hekla erupts far more often and has never caused disruption on that scale, because it lacks a substantial permanent ice cap.
Katla, under the Mýrdalsjökull ice cap, is the inverse risk profile: it erupts roughly twice a century, its last confirmed eruption was 1918, and the volume of ice above it means its main threat is the jökulhlaup — a glacial outburst flood capable of discharges rivalling large rivers. Hekla produces lahars but nothing approaching that.
Laki, on the Skaftáreldar fissure to the east, is the outlier in scale. Its 1783–84 eruption released around 14 km³ of lava and a vast sulphur dioxide load, killing roughly a fifth of Iceland's population through the resulting famine and darkening skies across Europe. Hekla has never done anything of that magnitude in historical time. The distinction matters: Hekla is frequent and locally destructive; Laki was singular and continental in effect.
Climbing It, and When
Hekla is climbed in summer, typically July to September, from a track off Route 26 in the southern highlands that requires a four-wheel-drive vehicle and is impassable outside the season. The ascent is a walk rather than a technical climb — roughly three to four hours up on loose tephra and snow patches that persist near the summit through much of the year — but the exposure is serious and the weather changes fast.
Anyone going up should check the Icelandic Met Office alert status first, and should understand that the standing advice reflects a genuine inability to forecast this particular mountain. Guided ascents are available from Hella and Hvolsvöllur.
The surrounding area rewards the trip regardless: Þjórsárdalur with the reconstructed Stöng farmstead, the Hjálparfoss and Háifoss waterfalls, and the tephra desert north of the mountain where the 1947 fall is still visible as bare ground. The broader volcanic setting — the rift, the hotspot and the other systems — is covered in the Iceland geography overview, and the species that survive on this ground are catalogued in the guide to Icelandic wildlife.