Jökull


Jökull - 01.12.1984, Side 4

Jökull - 01.12.1984, Side 4
Figure 1. Location of the Grímsvötn volcano in the Vatnajökull ice cap. Mynd 1. Grímsvatnaeldstöðin og næsta nágrenni. to the north they have a northerly direction (Figure 1). Possible relations between different volcanic systems are elaborated further in the discussion chapter. The volcano itself forms a definite topographic high with a central caldera subsidence. It has been suggested that the Grímsvötn caldera con- sists of three smaller, superimposed calderas (Saemundsson 1982) while ice thickness measure- ments by radio echo sounding suggest a single caldera about 10 km in diameter (Helgi Björnsson pers. comm.). The whole volcano is ice covered except the southern caldera wall where basaltic hyaloclastites are exposed in the cliffs. The caldera lake is also covered by a 200 m thick ice shelf (Björnsson 1982) The Grímsvötn volcano has the highest erup- tion frequency of Icelandic volcanoes. As it is subglacial the interaction with the ice-cover and melt water has marked effect on the eruption behaviour and the eruption products (Thórarins- son 1974). The quenching of the magma against the melt water results in hyaloclastites, mainly tuffs. Within the Grímsvötn volcano is also a geoth- ermal area, estimated at 5000 MW (Björnsson 1974, 1983), which continually supplies melt water to the ice covered caldera lake. This causes spectacular jökuihlaups (glacier bursts) every few years in the river Skeiðará which drains the Grímsvötn caldera lake. In most cases eruptions in the Grímsvötn volcano are accompanied by such jökulhlaups, the eruption under discussion being a notable exception. Possible interaction between the volcanic and geothermal activity poses many interesting questions as the ultimate source of the geothermal energy must be magma- tic and possibly from the same system that feeds the eruptions. It has also been suggested that in some cases the sudden draining of the caldera lake may trigger eruptions rather than the other way around (Thórarinsson 1974). VOLCANIC HISTORY Available knowledge of the activity of the Grímsvötn volcano and adjacent regions was summarized by Thórarinsson (1974). The earliest recorded possible eruption was in 1332 but the remoteness of the volcano and the inadequacy of the literary sources makes the early chronology very uncertain. When sources improve in the seventeenth century eruptions are mentioned at frequent intervals, often one in a decade. During this century eruptions in 1922 and 1934 were described, and a possible one in 1938. 2 JÖKULL 34. ÁR
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