I remember the first time I walked out on the ice of a big lake, it was a little nerve wracking and I was acutely aware that I was walking on ice that was but a few centimeters thick and below that was a whole lot of really cold water. I also remember being out on lake ice the first time I heard the sonic boom of a pressure crack forming.
OK, maybe sonic boom is the wrong sound. It’s a little like a lightning bolt coming down the lake at you and possibly passing right by or under you. There is the movement of the sound as it races down the lake, there is the distinct and unmistakable sound of ice cracking, followed by the seismic boom of an ice quake and then an odd musical twang of the shock waves traveling through the ice.
Hearing all of this is obviously a little startling and scary, until you get use to it.
Just ask people who live on houseboats or who spend a lot of time on the ice in winter about the sounds ice makes. Some nights when you are trying to sleep it can be a whole symphony of sounds. At times a little off key and sometimes way too loud.
These pressure cracks and the ridges that follow, I find fascinating. An example of nature forces at work.
When freeze up starts, it begins from the edges of a lake and freezes toward the center or deepest part of the lake. On a big lake, under the right condition, you can get a good build up of ice along the shore.
I remember one fall I was at Colville Lake and there was a two-to three-meter wide band of clear ice close to shore. There were kids out having a great time ice skating on it and you could see the open water behind them. The ice was already several centimeters thick so safe but still a marvelous northern scene.
By the time the whole lake freezes over, the ice is well anchored to the shore. As water freezes and turn into ice it expands but it really has nowhere to expand to, so pressure builds up in it, It builds up to such a force that it cracks the ice and it usually picks the weakest point which is the narrowest sections across the lake.
If you look at the pressure crack carefully or run your fingers over it, you might notice that one side is slightly higher then the other. This is the start of a pressure ridge. As the ice thickens it will crack again, at or near the same spot.
On smaller lakes it may end up with a pressure ridge being a centimeter or so high but because they are covered with snow, few notice them. On bigger lakes like Great Slave Lake they can end up being a meter or two high. On the ocean they can be a major obstacle.
Ice is a rock. I know some people have a hard time believing this, but it is true, and it behaves as a rock.
When the pressures crack occur, it is and ice quakes and they behave the same as earth quakes, only on a small scale. The pressure crack is a fault. You even get aftershocks or what really should be called subsequent quakes, because that is what they are.
On a frozen lake with the ice anchored around its entire shore, the ice can only move up or down, so we have a thrust fault. One side of the ridge will slowly move up and the other down as the ice continues to get thicker and pressure crack after pressure crack occurs.
You can get the situation where one side of the ridge slips up onto the other side and the weight pushes it down a little. So not only do you get the obstacle of the ridge. but in front of it are pools of water and broken ice. It’s just like continental drift and plate tectonics on a much smaller scale.
So, by looking closely at pressure cracks and ridges in the ice you can better understand earth quacks, thrust faults and plate tectonics that have helped shaped our planet. So, the study of pressure cracks and ridges can give us a better understanding of how the planets crust behaves.
What I am always curious about is how these cracks and ridges affect the lake beneath the ice. There is a lot of opportunities for research into this and you could add in how human activity affects things. Considering the importance of our lakes I am surprised more research isn’t being done.
We still have lots to learn about this magnificent planet we live on.