Sigurdur Thorarinsson

The Hekla eruption 1980–1981

The sixteenth eruption of Hekla since 1104 began on August 17th, 1980, after the shortest repose period on record, only ten years. The eruption started with a plinian phase and simultaneously lava issued at high rate from a fissure that runs along the Hekla volcanic ridge. The production rate declined rapidly after the first day and the eruption stopped on August 20th. A total of 120 million m3 of lava and about 60 million m3 of airborne tephra were produced during this phase of the activity. In the following seven months steam emissions were observed on the volcano. Activity was renewed on April 9th 1981, and during the following week additional 30 million m3 of lava flowed from a summit crater and crater rows on the north slope.The lavas and tephra are of uniform intermediate chemical composition similar to that of earlier Hekla lavas. Although the repose time was short the eruptions fit well into the behaviour pattern of earlier eruptions. Distance changes in a geodimeter network established after the eruptions are interpreted as due to inflation of magma reservoirs at 7–8 kilometers depth.

THE APPLICATION OF TEPHROCHRONOLOGY IN ICELAND

The situation in Iceland for establishing a tephrochronological time scale is very good. This is due to the great number of volcanic eruptions in post glacial time, a great range of variation in chemistry between tephra layers, a rapid thickening of the soil, separating layers with small age difference, and the existence of detailed accounts of many historical eruptions.

Tephra Studies and Tephrochronology: A Historical Review with special reference to Iceland

In the fourteenth century Icelandic chroniclers already discerned between three types of tephra: pumice, sand and ash. From 1625 onwards, detailed accounts of the tephra fall in all major Icelandic eruptions were written. The earliest description of tephra layers in soil sections is from 1638 and somewhat more comprehensive studies of tephra layers in soil were carried out in the middle eighteenth century.

On the Topography of the Volcanic Zones in Iceland

It has previously been pointed out by the present writer; that the Pleistocene volcanic zones of Iceland are not only a supramarine exposure of a part of the World Rift System, and as such much more accessible for geological, geophysical, and topographical research than the submarine parts of the system. The Icelandic zones are also the only supramarine part of the system where a relatively recent volcanic activity has taken place under water and thus under conditions similar to those on the submarine part. This water was meltwater from the overlying ice cover during those periods of the Pleistocene when the country was blanketed by ice. The basaltic magmas extruded in Iceland are similar enough to those of the submarine rift zones to make one expect similar shape and inner structures of the volcanic edifices built up. In both cases the formation of pillow lavas is a characteristic feature. The main difference in shape one may expect is that the subglacial edifices are likely to be somewhat more steepsided, being, at least in some cases, built up against walls of ice.

AT LEVE PÅ EN VULKAN

Thorarinsson, S.: At leve pa en vulkan. Geografisk Tidsskrift 76: 1–13. Kobenhavn, juni 1. 1977. Disasters during Icelands history caused by the four damaging factors of volcanic eruption, tephra, lava, gases and jokulhlaups are described. It is claimed that in spite of the severe threat, the volcanic character of Iceland favours the life of man.

Observations on the Drainage and Rates of Denudation in the Hoffellsjökull District

In the summer of 1936 I joined the Swedish-Icelandic Vatnajokull Expedition, led by H. W:son Ahlmann and Jon Eythorsson. When they had left Hornafjordur on 21 June, Carl Mannerfelt and myself continued the ablation measurements on Hoffellsjokull until the middle of August. Besides these ablation measurements, we took the first observations of Hoffellsjokull’s movements, and samples of the water in Hornafjardarfljot to determine its silt content.

Living on a volcano

Abstract Volcanic activity has stamped its mark on the history of the Icelanders. The damaging factors of volcanic eruptions and the range of their effects are shown in Table 1. The four factors causing the most serious damage in Iceland are tephra, lava, gases, and jokulhlaups. The most damaging tephra falls were from Hekla in 1104 and from Myrdalsjokull, a hitherto unknown eruption, in about 1375. In lowland areas, a tephra fall leads to long abandonment of farms, when the thickness of the tephra layer, as freshly fallen, exceeds 25 cm. Two lava flows were exceptionally destructive, the Lakagigar lava flow in 1783 and that on Heimaey in 1973. Most destructive among the numerous jokulhlaups were probably those from Oraefajokull in 1362 and from Katla shortly before 1179. The worst disaster in Icelands history was caused by the gases—mainly fluorine and sulphuric compounds—emitted during the Lakagigar eruption in 1783. This disaster excluded, volcanic eruptions have directly caused fewer losses of life t...