Jökull


Jökull - 01.12.1953, Blaðsíða 19

Jökull - 01.12.1953, Blaðsíða 19
ípfejBi Fig. 8. The south-western- most part of the Gríms- vötn depression on 18 Aug. 1953. Suðvestasti hluti Grims- vatnalœgðarinnar séður úr lofti 18. ágúst 1953. Aerial photograph by S. Þórarinsson. height of the damming ice barrier. Besides, the glacier bursts have been irregular during the last decades. During the 1940’s we had, instead of one glacier burst of normal magnitude, three smaller ones, which, however, did approximate- ly discharge together the same quantity of water as one normal glacier burst. This irregularity is probably at least partly due to the recent thinning of the damming glacier. According to a seismic sounding carried out by A. Joset and now characterized as reliable (Joset 1952, p. 43 and Pl. XI) the height difference between the firn surface and the bottom of the SW part of the depression ab. 1 km off the Grímsfjall cliff was ab. 580 m on April 7, 1951. We know that the ice surface in the S part of the depression was ab. 400 m lower than Svíahnúkur eystri or in ab. 1300 m height after the glacier burst in 1934 (Nielsen 1937, Jonas 1948). If the seismic sounding is reliable, which I am inclined to doubt, the consequence is that the volume of the depression beneath the 1400 m level is at least 7 km3, and probably considerably more. Another consequence is that either is the ice in the depression at least 500 m thick or there is a permanent lake in the depression, dammed by a subglacial rock barrier, which seems less prob- able. The establishing of the depression being so deep further means that the craters of the 1934 eruption were situated on a steep slope more than 500 m above the bottom of the de- pression. But it is really very difficult to explain tlie plane and horizontal surface of the ice in the SW part of the depression and its regular rising between the glacier bursts, without assum- ing that the ice is floating on water, the level of which is continuously raised between the glacier bursts. Furthermore it seems impossible that more than a small amount of the total discharge of the glacier burst in 1934 can have been due to melting by the eruptions in the visible craters then active ( cf. Thorarinsson 1953). And even if it is possible that other craters were then active at lower levels within the depression, beneath the ice cover, it seems unlikely that a great part of the meltwater discharged by the glacier burst can be due to their activity. It seems thus most probable that the main part of the melt-water of a glacier burst is accumulated in the depress- ion between the eruptions and is due partly to ablation and partly to continuous subglacial melting caused by thermal activity. According to Eythórsson’s and Rist’s measure- ments in April 1951, the surface of the damming ice barrier was 105 to 125 m higher than the water level in the depression at that time. Assuming that the water level in Grímsvötn was raised 50 m from its height on 1 July 1953, 17
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Jökull

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