Ten people have been injured, one of them seriously, and another eleven are missing after a landslide hit the village of Gjerdrum in Viken County, southern Norway, at about 4.00 am local time on Wednesday 30 December 2020. Several houses are reported to have been buried under mud and debris in the village, with others sliding into the hole created by the landslip, and about 900 people have been evacuated amid concerns about further landslips.
The landslide is thought to have been of a type known as a 'quick clay' landslip, in which an entire bedding plain of clay loses its cohesion at once, and flows like a liquid, a type of landslide largely peculiar to areas of heavy Pleistocene glaciation, such as the northern coasts of Scandinavia, Russia, Alaska and Canada. Quick clay deposits were laid down in areas where the land was pushed down beneath sealevel by the weight of glaciers, and has subsequently been uplifted after the glaciers retreated. These clays typically contain far higher salt contents than other marine clays, which can be washed out by rainfall. Significantly, they generally contain significant levels of positively charged sodium cations, which entered the clay as the sea above froze, and help to bond the negatively charged clay particles together. However, sodium particles are particularly soluble in water, and easily washed out, with the effect that once a certain level of sodium has been lost, the entire bedding plane loses its cohesion.
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