Jökull


Jökull - 01.12.1991, Side 36

Jökull - 01.12.1991, Side 36
Figure 12 shows the distribution of ash in Grímsvötn according to the vertical air photos. Two months after a jökulhlaup, in September 15, 1954, patches of ash were located in the northwestem part of Grímsvötn. In August 9, 1960, seven months after a jökulhlaup, isolated patches of ash could be seen in the northwestern part, located at the same places as in 1954. No similar ash layers could be detected on air photos from August 10, 1972. However, the firn was dirty with ash near Naggur and at one location in Svartibunki (Fig. 1). In August 22,1984 the ash layer seen on earlier photos could no longer be detected but ash was spread over parts of Grímsfjall and around the 1983 crater. This ash layer is from the 1983 eruption and has not been observed after 1984. During the period from 1934 to 1960 at least, it depended on the time of the year whether tephra was observed in Grímsvötn. With the exception of 1934 and 1935, very little or no tephra was detected in Grímsvötn in winter and spring. In contrast, all obser- vations in late summer and autumn show sizable parts of the noithwestem corner of the area covered in tephra. Moreover, photos show that the surface of the ash-covered area was uneven and rough in late sum- mer, similar to ablation areas of some outlet glaciers. These observations raise the question whether the very same ash layer was being observed throughout this period (1934-1960). That would imply the whole winter accumulation in the northwest part of Grímsvötn being melted during the summer and that the area was an ablation area indeed. For such an ablation area to exist, surface melting would have to be considerably greater than typically at the elevation of 1400 m a.s.l. in Vatnajökull (Björnsson, 1988). The mean winter balance in Grímsvötn is about 4.5 m of snow, equal to 2450 mm of water (Björnsson, 1985) whereas the summer balance under normal snow conditions with albedo of 0.6 is of the order of -500 mm. Dispersal of ash over the glacier surface, decreas- ing the albedo, may explain melting of the entire win- ter accumulation. An average solar radiation flux of 200 W/m2 (as measured by Björnsson (1972) on Bægisárjökull, North Iceland during July and August; 1100 m a.s.líj could melt the whole winter accumu- lation in 70 days if the albedo were 0.2. From 1934 to the 1960’s low albedo values were maintained in the westem part of Grímsvötn during summer. Wind-blown ash and loose material were dispersed from nunataks and ash cones and ridges which became free of snow early in the summer. On these nunataks the geothermal heat flux was high and the winter accumulation was reduced by snowdrift on the uneven surfaces (see Figs. 7-8 and 13). This is e.g. confirmed by photogrammetric measurements of vertical air photos (H. Kristinsson, Hnit engineering consultants) which show that the surface of the ash covered area in the northwestern part of Grímsvötn was uneven with a relief of a few metres on September 19, 1946. Figure 12 shows a gradual decrease of the ash cov- ered areas between 1946 and 1972. Photos from the area show that a gradual decrease in the extent of the ice-free areas was also apparent by the early 1950’s. That decrease was accelerated in the late 1960’s and 1970’s (Bjömsson, 1988). Apparently increased albe- do retarded the melting of the winter accumulation and the ash layers finally became buried by snow. Further, the trend towards cooler summers in Iceland since 1946 (Einarsson, 1989) and reduced ablation may have accelerated the burying of the ash layer. On the basis of these considerations we suggest that the northwestem part of the Grímsvötn depres- sion was an ablation area, at least over the period 1934-1960 and the very same ash layer was observed by expeditions to the area throughout this period. This ash cover dates back to the eruption of 1934 but it may contain ash dispersed in earlier eruptions, especially that of 1922. According to Fig. 12 the size of this ablation area was about 20 km2 in 1945 and 1946. By 1972 the ablation area no longer existed. Effects oftephra layers on glacier mass balance The existence of an ablation area within Grímsvötn in the period 1934 to 1960 calls for an explanation of greater surface melting than at present. As the data on surface ablation within the Grímsvötn depression are limited and sporadic, estimates of total ablation have been fairly rough. Björnsson (1988) used available data to estimate the mean ablation at present as being 34 JÖKULL,No. 41, 1991
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