Twenty-one years of string movements on the Liippasuo aapa mire, Finland

The movements of four 80-100 m long strings crossing a 750 m long aapa mire were measured, and two pools were drained and local snow depth and ground frost conditions were observed in order to test the role of water pressure and ground frost in string motions. Located close to the Arctic Circle, Lii ppasuo is characterized by a maximal snow depth of about 1 m, a mean annual temperature of -1 degrees C and a maximal variation in temperature of over 75 degrees C. The strings moved at the rate of 50-150 cm in the course of 21 years. The most active parts were those located in the area of steepest gradient. Periods with normal annual shifts in the range 2-5 cm were interr upted from time to time by moves of abnormally high magnitude. This resulte d in swelling of the nearby pools and increased curving of the strings, giv ing rise to a small-scale zigzag course in the strings. The principal facto rs responsible for the motion were ice/frozen peat push, solifluction and w ater pressure. Movements occurred not only during the thawing period but al so in summer-time. They were as common in the direction of the contours as when following the slope. The unexpected uphill direction of movements is e xplained by winter-time freezing and expansion of the whole mire, in the sa me way as the ice cover on a lake will expand as a result of temperature ch anges.