Forms created by the thermal movement of lake ice in Finland in winter 1972-73

Exceptionally pronounced thermal movement of lake ice during the winter of 1972‑73 led to the formation of pressure ridges and ice‑thrust ramparts on many lakes in the central parts of Finland. The authors were able to study many such formations immediately after creation, while their form and structure was still intact, with the aim of establishing a classification of their morphology and the processes by which they were engendered. Ice formations attributable to thermal ice movements were studied and classified on 9 lakes, and shore formations studied in the case of 19 lakes and classi­fied on the basis of 32 detailed cross‑sections. Regional evaluation of the processes involved detailed examination of a total of 79 lakes. The cumulative extent of ice expansion was greatest on the largest lakes in the central part of the area studied, reaching as much as 17 dm/km, and in this same area the ice and shore formations achieved their greatest size. The thermal movements were caused by the melting of the snow cover on the ice at the beginning of December, after which fluctuations in the air temperature were able to exercise a direct effect upon temperature in the ice in the southern parts of the area up to the middle of February. In the north‑east these processes were restricted to an area which had received heavy snowfalls and where there had been little thawing. Further south the ice was relatively thin, and the larger lakes remained open up to midwinter. As a result of the exceptionally high water level in the lakes, the ice­ thrust ramparts were formed well above the mean water level, and can thus be expected to survive the effects of abrasion for some considerable time even in the case of the larger lakes, as is already apparent from data on their fate up to the summer of 1976