Rock Glaciers

Nearly 1700 individual rock glaciers have been identified on Disko Island (Humlum, 1988a). The earliest descriptions of rock glaciers were made by Steenstrup (1883, 1901) and Hammer and Steenstrup (1893), who referred to rock glaciers as 'dead glaciers'. Many rock glaciers have been named by the Inuit, for instance the large Ujaragsuit rock glacier south of Qullissat on the northeast coast. Both glacier-derived rock glaciers (Figure 2) and talus-derived rock glaciers (Figure 3) are abundant all over the island. As most of Disko Island was covered by an ice cap and descending outlet glaciers during the Wisconsin glaciation (Ingolfsson et al., 1990; Weidick and Bennike, 2007), all rock glaciers can be considered as formed during the Holocene.

Talus Glacier
Figure 2. Glacier-derived rock glacier, Qivitut, Diskofjord
Figure 3. Talus-derived rock glacier, Qivitut, Diskofjord
Figure 4. Peat-covered pingo near the junction between the outwash plain in Kuannersuit Kuussuat (Kuannersuit Valley) and the delta at the head of Diskofjord

The rock glaciers on Disko Island have been intensely studied by Humlum (1982, 1988a,b, 1996, 1997, 1998, 1999, 2000). Based upon the appearance of the frontal slope, Humlum (1988a) classifies active, inactive and relict rock glaciers, and by introducing the concepts of rock glacier appearance level and rock glacier initiation line altitude he attempts to identify the initiation mechanisms of talus-derived rock glaciers as either glacial (snow accumulation-controlled) or non-glacial (temperature-controlled), in addition to the talus accumulation rate (Frich and Brandt, 1985; Humlum 2000). The size of typical rock fragments seems to be an important control on wind ventilation and thermal regime of the active layer on rock glaciers (Humlum, 1997). The topographic and altitudinal distribution of rock glaciers (Humlum, 1998), in combination with isotopic records of rock glacier ice (Humlum, 1999), may provide palaeoclimatic information on local and regional scales that may supplement data from ice cores and radiocarbon dating. However, these approaches still need to be refined in order to represent an unequivocal method.

As opposed to the glaciers on Disko Island, many rock glaciers terminate at sea-level. Donner (1978) interpreted the lack of raised marine terraces on the frontal slopes of rock glaciers reaching the shores of Diskofjord, Mellemfjord and Nordfjord as evidence of no glacioisostatic uplift since the time of the rock glaciers reaching the current sea-level. However, as many rock glaciers are active or may reactivate after a period of inactivity, they may override existing raised marine terraces, making the argumentation of Donner (1978) untenable.

PiNGoS

Pingos are referred to as 'mud volcanoes' on the first topographic map of Disko Island, surveyed in 1931 - 1933 (GID, 1941). They seem to have escaped the interest of early explorers (e.g. Whymper, 1872; Steenstrup, 1901), although they are found near the junction between the outwash plains and the deltas of all the major fjords; Nordfjord (Donner, 1978), Mellemfjord (Christiansen, 1995, 1997), and Diskofjord (Figure 4). Pingos are also located in higher altitude passes such as in the northern part of Blresedalen and in the eastern part of Blomsterdalen. They generally form as open-system pingos on high locations in well-drained, silty or clayey deposits, and often appear as 1 - 3 m high peat-covered mounds. Cold periods during the Little Ice Age may have lead to formation and growth of many pingos (Christiansen, 1995, 1997). A detailed survey on the distribution and morphology of pingos on Disko Island is much required in order to attain further knowledge on their sensitivity to physical parameters such as climate, topography, sedimentology, permafrost hydrology and vegetation.

Aerial photographs from 1953 and 1964 indicate that a pingo with a diameter of about 50 m had formed in the inner part of Kuannersuit Kuussuat (Kuannersuit Valley) at a location, which had been deglaciated around 1920 (Jost, 1940). In 1995 - 1998, the 10.5 km surge advance of Kuannersuit Glacier overrode the site of the pingo. Thus, deglaciation of the foreland of surge-type glaciers may provide a valuable field laboratory for studies on the formation and evolution of periglacial landforms such as pingos.

Figure 5. Slightly shattered basaltic stone incorporated into the surface of an outwash plain on top of buried glacier ice, Kuannersuit Kuussuat (Kuannersuit Valley). Camera etui for scale
0 0

Post a comment