CODEX Entry 3440: Mt Hekla
Hekla had been dormant for at least 250 years when it erupted explosively in 1104, covering over half of Iceland with ash. This was the second largest eruption in the country in historical times and news of it spread throughout Europe, sparking the label the “Gateway to Hell” which lasted until the 19th century, or “The prison of Judas”. In the Flatey Book Annal it was recorded that during the 1341 eruption, people saw what looked like large birds flying in the mountain’s fire, and these were assumed to be souls. However Hekla means a short hooded cloak, due to the constant plume that cloaks the summit, created by the constant low level activity. Cumulatively, the volcano has produced one of the largest volumes of lava of any in the world in the last millennium, around 8 km3.
Hekla sits at a rift junction where the south Iceland seismic zone and eastern volcanic zone meet. Together with Callaqui in Chile they are the only two volcanoes known to be built from mixed lava and tephra eruptions. Hekla is situated on a long volcanic ridge of which the 5.5 km Heklugjá fissure opens along its entire length during major eruptions and is fed by a magma reservoir 4 km below the surface. The tephra produced by its eruptions is high in fluorine, which is poisonous to animals. It is the only Icelandic volcano to produce calc-alkaline lavas. Hekla is also unusually aseismic, with no activity at all until as little as 30 minutes before an eruption. The mountain has been known to remain active for as long as six years without pause. Eruptions in Hekla are varied and difficult to predict. One of the largest Holocene eruptions in Iceland was the Hekla eruption in 1000 BC, which threw about 7.3 km3 of volcanic rock into the atmosphere. This would have cooled temperatures in the northern parts of the globe for a few years and samples show negligible tree ring growth for a decade.
In 1693 an eruption that lasted 7 months caused a massive mud avalanche and tsunami. In 1766 and 1845 a pair of two year long eruptions threw lava bombs upto 20 km away, and flooding from the sudden melting of snow and ice on Hekla’s slopes. Rivers become so hot that the fish died. The second largest lava flow in the 20th century occurred in 1947–1948 when Mt Hekla grew from 1,447 m to 1,503 m, during the eruption. The loud roar could be heard throughout Iceland accompanied by an earthquake. A lava bomb was found 32 km from Hekla. Two days after the eruption had started, ash fell on Helsinki, Finland, having covered 2,860 km in this time. There were smaller eruptions in 1970, 1980, 1991, and 2000.
Up until this eruption, it had been assumed that Hekla was incapable of producing the most dangerous of volcanic phenomena, the pyroclastic flow. But in 2003, it was reported that traces of a pyroclastic flow, 5 km long, were found on the side of the mountain. The Hekla area was once forested. Forests are much more resilient to ash and pumice fall than low vegetation. After an eruption, almost all of the inactive areas on new lava flows are rapidly colonised by mosses expanding to a homogeneous layer up to 20 cm thick within 50 years.