Ice cracks and ridges on Lake Mendota
By John Kutzbach
Darlene Olson of the Friends emailed me a question that not only made me review my limited knowledge of lake ice processes but also sent us out to Picnic Point this sunny weekend to view the amazing ice ridges criss-crossing the Lake Mendota. Many of the visitors had questions like Darlene’s. She asked: “What causes the huge ice crack in Lake Mendota each winter? Why does it seem to occur in the same location each winter? Why is the crack not in a straight line from the end of Picnic Point to the Edgewater rather than having some curves?"
The short answer is that ice contracts and cracks when its temperature falls and expands and forms ridges when its temperature rises.
So a fairly typical winter event is that
(1) in a cold spell or on a cold night, the ice contracts and breaks apart somewhere in the lake - perhaps along some previous weak spot.
(2) Then the water in the crack refreezes.
(3) Then, if it warms up, the ice expands and often bulges up, or down, or creates broken slabs that lift and form a pressure ridge, often several feet high. This expansion often occurs near a previous contraction crack, perhaps because the newly formed ice is still thin.
If we idealize the physics to consider the magnitude of the linear or lateral expansion/contraction of ice (change in ‘length’ but not in the thickness of the ice), then a 10°C temperature drop (18°F) will cause a contraction of 0.05% (thermal expansion/contraction coefficients for iron, copper, ice and many other materials are found in physics handbooks). If you think that 0.05% is not much, consider the following hypothetical example:
Photos by Gisela Kutzbach- click to enlarge
There is some evidence that local contraction/expansion can begin in shallow areas– perhaps the temperature changes are most pronounced in these shallow areas, or perhaps the ice tends to be thinner there. But for whatever reason, there is observational evidence, accumulated from watching the lake for many years, that cracks and ridges often spread from Picnic Point or from other shallow areas.
• One frequent crack/ridge system extends from Picnic Point towards Maple Bluff.
• Another extends from Picnic Point toward the shallow areas near the Edgewater (this is the one you observed yesterday).
• Still another extends from Picnic Point past Frautschi Point and then on to the north towards Fox Bluff.
• Governors Island may be another relatively shallow area that might favor expansion/contraction joints.
In 1956, two Wisconsin scientists, Bunge and Bryson, wrote a series of papers about ice on Wisconsin lakes, including Lake Mendota. They discuss the physical processes of crack/ridge formations. They don’t reach firm conclusions about preferred locations, although they note that these features often extend outward from Picnic Point.
You ask ‘why is the crack not a straight line’? That is a very tough question. Nature doesn’t seem to like straight lines (the twisted branches of oak trees, the meandering of rivers, the outlines of cumulus clouds, curving coastlines, etc). These cracks and ridges are subject to many other forces besides contraction/expansion– shearing force and wind forces, for example. So it seems complicated.
On January 15, 1948, there was a very powerful crack of the Lake Mendota ice that was felt throughout the campus and city. A description of this “ice quake” event was published by Prof. Charles Bradley (husband of Nina Leopold Bradley) in the American Journal of Science, vol. 246, no. 6, page 390. It measured 3.8 on the Richter scale of a local campus seismograph. There is also a news release about a much smaller ice quake in 2008 (http://www.news.wisc.edu/14680).
If one walks or skates on Lake Mendota, one frequently hears the sharp sound of the ice cracking. Gisela and I have heard it. Even to the ‘informed ear' these sounds seem mysterious and powerful, and it is not hard to believe the accounts that in medieval times people gave all kinds of interpretations to these sounds (lake gods or devils or omens of various kinds).
A word of caution!
A pressure ridge is a small zone of complex topography: some ice slabs are pushed up, others may be pushed down. A downward-pushed slab may be covered by water (it's easy to see) or by a thin layer of newly formed ice (it's not so easy to judge the thickness). Enjoy viewing the power of nature at work, but always be cautious when near a pressure ridge.
John Kutzbach, Jan 19, 2015