In late February, the winter ice may start to melt. When that happens, frozen lakes can send spectacular slow-motion waves of crushed ice cascading over the shoreline onto the land. These waves are sometimes called “ice tsunamis” but, to meteorologists, they are “ice shove” or “ice heave”.
Specific conditions are needed to produce ice heave. There must be large cracks in the lake ice, which must be separated from the shore, so that it becomes several free-floating sheets. This only occurs when a thaw starts. Then there needs to be a powerful wind blowing in the direction of the shore for at least 12 hours.
The force of the wind accelerates the massive ice sheet towards the shore; the ice moves slowly, but with unstoppable momentum. The ice sheet piles up as it hits land, and broken ice spills over. In one study, ridges of lake ice were pushed almost 400 metres over the course of several hours, although typically the incursion on to land is only a few metres.
Ice shove occurs in Canada and the northern US, and has also been filmed at Lake Baikal in Russia. The ice wave may be in the form of large slabs, smaller fragments, or freezing slush. Witnesses say it makes a roaring sound like a passing train.
Ice shove can block lakeside roads, and may damage vegetation and buildings. Homeowners have even been known to resort to dynamiting the ice sheet before it threatens, as a precautionary measure.
