Jökull


Jökull - 01.12.1983, Page 16

Jökull - 01.12.1983, Page 16
Fig. 1. YVestern Vatnajökull. 1. mjnd. Vesturhluti Vatnajökuls. the time prior to the next jökulhlaup would be shortened and the rate of melting overestimated for that period. The result would be a low succeeded by a peak. If, on the other hand, the level fell deeper than normal the melting rate would be overestimated for that period and may be under- estimated for the next one. Such irregularities would, however, equal out in the long run and not distort calculations of average values for the melt- ing rate. VARIATIONS IN MELTING AT GRÍMSVÖTN Fig. 2 shows a base heat flux which was at about 5000 MW from year 1850 to about 1900, but has since then declined continuously. Bjömsson et al. (1982) have suggested that this base heat flux could be explained by heat extraction of water which penetrates into hot rock boundaries of magma at shallow depths. The decline in the heat flux from 5000 MW to 4000 MW in 80 years may reflect continuously deeper penetration of water into an underlying magma body and reduced heat exchange with increased depth. With a similar trend the geothermal activity would disappear in 300 years. We may, however, expect continuing intrusive activity so the geothermal system will be maintained. Four peaks rise above the base heat flux of 4000—5000 MW shown in Fig. 2. They reflect exchange of heat at a rate which is two to three times higher than for the base flux. Such peaks may be caused by eruption of magma to the glacier bed and subsequent rapid cooling through direct contact with ice or water in Grímsvötn. Observa- tions suggest that this may apply for at least two of the peaks (in 1938 and 1867). All the peaks are connected with jökulhlaups which were unusual in some respect. The highest peak on Fig. 2b is prior to the jökul- hlaup in 1938. This jökulhlaup was unusual. The hydrograph rose to a maximum in three days and receded slowly in two weeks. The typical hydro- graphs for jökulhlaups havea different shape. They rise to peak value in a fortnight and recede in a few days. Further, the jökulhlaup in 1938 was unex- pected as it occured only four years after the jökul- hlaup in 1934. A depression was formed in the glacier surface at the northern slopes of Grímsvötn in connection with this jökulhlaup. The depression was 7 km long, 2 km wide and 200 m deep, with a 400 m deep circular crater at the northemmost end (Fig. 3, Thorarinsson 1974). The depression was dis- covered from the air but no expedition went to the area. About 3.5 km3 of water must have drained from this area into Grímsvötn and triggered the jökulhlaup. An intrusion of0.3 km3 oflava up to the glacier bed could have produced the meltwater. About 20 m thick lava flow spread over 15 km2 would have cooled down from 1100°C to 0°C in a few weeks or months. The rate of accumulation of water in Grímsvötn was rather high during the first year after the jökul- hlaup in 1938. Evidence recently provided by Ragn- ar Stefánsson (personal communicadon 1982, 1983) has verified the occurrence of a previously un- authenticated jökulhlaup in Skeiðará in June of 1939. This had a sulphurous smell which disproves the former hypothesis that the flood originated from draining of a marginal lake. Furhter, he esti- mates the total volume to have been similar to that of the jökulhlaup in 1982 — that is ofabout 1 km’. The second peak on Fig. 2b is related to the jökulhlaup in 1867 which also had an abnormal 14 JÖKULL 33. ÁR
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