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Science Frontiers ONLINE No. 112: Jul-Aug 1997 |
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Large rotating ice discs on ice-covered rivers
The fringe literature has made much of the huge, slowly spinning discs of ice seen on some rivers in northern climes. UFOs are connected somehow. Or, more recently, they must be associated with the infamous crop circles! These ponderously rotating ice discs have also caught the attentions of scientists, who have been able to dispel some of these wild speculations.
One well-observed ice disc formed on the Pite River in northern Sweden in 1987. It was rotating in a circular hole in the ice covering the rest of the river.
"The rotating ice disc had a diameter of 49m [just over 160 feet] while the hole in the ice was 54m in diameter. The time of one full rotation was measured at 545s and 575s on 20 and 24 January respectively. Unverified measurements suggest that the time of rotation had been about 8 min a few weeks earlier. The rotation of the ice disc was anticlockwise and for most of the time the disc was in contact with the border ice. This contact point moved clockwise, i.e. the ice disc was not 'rolling' on the walls of the hole. This erosion by contact, which caused a low-frequency sound, explains why the hole in the ice was kept open for months."
The ice thickness was 0.43m. Estimated weight of the ice disc = 864 metric tons -- almost 2 million pounds! Formation of the disc began when ice floes formed upstream were captured by a big whirlpool just as the river was beginning to freeze over. As more and more floes accumulated, the disc grew. Friction with the surrounding river ice rounded off the edges until it was nearly a perfect circle
Although no deep mystery is involved here, the authors of the referenced article admitted that:
"The mechanisms that form and drive the rotating ice disc are not yet known."
(Nordell, Bo., and Westerstrom, Goran; "Large Rotating Ice Discs on Ice-Covered Rivers," Weather, 52:17, 1997.)
Reference. Ice sometimes exhibits strange phenomena, such as sinusoidal cracks, columnar jointing, and even luminosity. See Chapter ESP in Anomalies in Geology. Ordering information at here.
 | 160-foot diameter, rotating ice disc in the Pite River, Sweden, 1987. |