Q&A: Ice Tsunamis

Question: I stumbled onto this YouTube playlist that had several examples of what they called ice tsunamis. Some of the captions said they were caused by winds and tides but didn’t really explain how it works. These things are amazing to watch! What makes them happen? Thanks. — HT, Phoenix, AZ

[Disclaimer: This video can be found online in multiple formats attributed to disparate sources. I’ll use it here for educational purposes without attribution. The location is usually cited as “somewhere in Russia”. If the real videographer wants credit, please contact me and I’ll make it so or remove the video. Thanks.]

Answer: I’ve seen this phenomenon called an “ice tsunami” and also a “horizontal avalanche”. The official name is far less dramatic. Technically it’s called an ice shove, and has little in common with tsunamis or avalanches. Ice shoves can be caused by winds, currents, and tides. On inland lakes it’s mostly winds and currents. Tides, even on the Great Lakes, are rarely more than a few centimeters but could slightly amplify an ice shove.

To a lesser extent, ice shoves can also be caused by the thermal expansion of ice as it warms up, but this requires a completely frozen lake surface. Free-floating ice will expand in the direction of least resistance — toward open water.

An ice shove is at once amazing and ominous to observe — especially if you have a cabin near the shore. They advance like the Blob, seemingly alive and purposeful. But they can be explained by some fairly simple principles of mechanics.

The graphic shows a free-floating ice sheet colliding with the shore. In this case, both a current and wind are acting in tandem to drive the ice sheet aground. Air drag from the wind and viscous drag from the current force the ice sheet toward the shore. The amount of force depends on several factors including:

• wind speed and direction
• current speed and direction
• surface area of the ice sheet
• surface roughness of the ice sheet

Under the right conditions, the ice sheet can be driven with enough force to run aground on the shore where it fragments and tumbles as it’s shoved from behind. Ice can advance several meters beyond the shoreline. This sequence of events vividly unfolds in the video. By comparison with the people, that ice sheet appears close to 2 ft thick. It’s surface area is not fully visible, but it must be extensive to generate the forces involved. That large block that topples at the end of the video likely weighs 1-2 tons.

If you browse that YouTube playlist you’ll see several ice shoves damaging structures. Some videos have sound tracks that capture the cracking and rumbling of ice in motion. As with plate tectonics, this amazing phenomenon is a manifestation of invisible and irresistible mechanical forces.

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