Jökull


Jökull - 01.12.1991, Side 32

Jökull - 01.12.1991, Side 32
Fig. 10. Grímsvötn seen from the east on October 5, 1945. The arrow points at a slight depression that is located above the eruption site of 1938 (photo Steinþór Sigurðsson, from Þórarinsson and Sigurðsson, 1947). Flugmynd af Grímsvötnum í lok hlaupsins 1945. Orin bendir á grunna sigdœld yfir gosstöðvunumfrá 1938. complicated with several ridges and mounds. Figure 9 shows six east-west profiles across the area. A 70- 200 m high ridge is located beneath the subsided area. The volume of the ridge is greatest in the north- ernmost part where the width and depth of the sub- sided area was greatest. The total volume of the ridge is of the order of 0.4 km3. The form and size of the ridge conforms reasonably well with that of the depression and its volume is sufficient to account for the ice melted by the eruption. This supports the sug- gestion that this ridge was formed in the subglacial 1938 eruption. A depression like the one formed in 1938 would be eliminated by ice flow within a few years if it were not maintained by melting by a subglacial heat source. Apparently, by 1942 the larger part of the depression had been eliminated by ice flow (Sigurðsson, 1942; 1984). However, a slight depres- sion was visible in 1945 (Fig. 10), and on vertical air photos from September 1946, the northern margin of the depression was marked by crevasses, indicating that melting of ice at the bedrock was still taking place eight years after the eruption. COMMON EFFECTS OF ERUPTIONS IN GRÍMSVÖTN Before reviewing the various reports of observations in Grímsvötn and their significance as indicators of recent eruptions we summarize the effects of the three eruptions in 1934, 1938 and 1983 which were all thoroughly described. 1. Layers of tephra were spread on the ice sur- face. 2. Openings were formed in the ice shelf or the glacier directly above the crater, often con- taining water bordered by vertical ice walls. 3. Open water or a depression in the ice at the eruption site may persist for years, indicating locally increased heat flux. 4. A pile (a mound or a ridge) of hyaloclastites is formed in the bedrock at the eruption site. 30 JÖKULL, No. 41, 1991
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