The following is a graph of the density of ice:
When I first wrote the article, I said that ice expands when heated. Then I realised that ice expands when it forms, so I then reworked the article to change the time of expansion to the night and not day. Finally, I realised that actually I’d got it right first time and that the bulk ice, as it cools, it gets more dense and contracts.
In a discussion with TinyCO2 I referred to the formation of pressure ridges and refrozen cracks which form as a result of the daily change in air temperature over the ice.
This is a direct analogy to the creation of subduction zones and mid-oceanic ridges in the Caterpillar theory. So, this is quick article to put together the information on the effect of daily (or for thicker ice perhaps longer) temperature changes on ice.
First let’s see a pressure ridge:
The main feature is that it is a jumble of ice running along a line in the middle of an otherwise flat piece of ice. And here is a simple diagram of one in cross section:
All you really need to know, is that the ice of either side is subject to huge pressure bringing them together. This can occur due to the pressure of wind in places like the Arctic ocean. But in large lakes the pressure results from the expansion of the ice which pressing on the shores has no where to go until eventually the ice fractures along a line causing some ice to be pushed down, and some up. The ice pushed down, adds to the buoyancy, until the join is pushed upwards, whereupon when the ice is next pushed together it is pushed up. That is until the weight pushed the join down. The result is a chaotic mess of ice (which is not the same as a rock subduction zone – where the rock is less buoyant and tends to melt).
However, whilst the ice expands during the day (ice expands when warmed – at least until it melts), and tends to increase the size of the pressure ridge, at night it cools and contracts and exposes water that then freezes. As such cracks tend to open up as shown below.
The example shown above shows multiple ridges. These are probably from a daily temperature cycle, but it appears that the ice is generally contracting away from this crack. That may be due to a period of cooler weather. This probably illustrates the early phase of a pressure ridge. The thick ice contracts leaving open water that then freezes. But when the weather turns warmer, the ice expands, the crack closes, and because the ice in the crack is relatively thin, this is where it gives way.
Repeated hot and cold weather would leave a jumble of ice at this spot. Eventually, there would be such a tangle, that when the ice contracts, it will tend to force open another crack. At this point there can be two types of crack: one where ice tends to come together in warm weather and form a ridge, and others where it tends to contract in cold weather forming open water which then freezes. If this happens it more or less matches the Caterpillar effect:
Just a quick recap. The Caterpillar Effect is when over the much longer ice-age cycle the crust heats up and cools. During cooling, it contracts pulling apart at mid-oceanic ridges. However unlike the ice, magma is pushed up – like the ice it freezes. When the world then warms, the rock pushes against each other at subduction zones. Here one layer is pushed down under the other. But unlike ice, it melts (so does not create a jumble of rocks) giving rise to a line of volcanoes about 100km from the subduction trench. The other effect which is common to ice and crust, is that the movement is in a series of “creaks” – which in the crust are known as earthquakes.
But the general principle is the same. The pressure ridges form, because each night the ice creeps forward and each day (if cold enough) the gaps freeze. Likewise each ice-age cycle the rock pushes forward and then the mid-oceanic rock fills in the gap when it contracts.
Videos and other info
Here is a video, in which you can hear the “icequakes” as the ice pushes against itself. There is a very regular creaking plus an occasional large “boom”. These correspond to what happens to the much thicker earth over an inter-glacial:
This one shows a crack which is just restarting to freeze
And this shows a number of cracks in various stages:
I couldn’t find any decent videos of pressure ridges on lake ice which would have been nice, but eventually I found this one on sea ice.