In January I dropped some bricks into my pond, which is a metre deep. In March the pond froze over and an image of the bricks appeared like a hologram in the ice (see Photo). What caused this?
Pond water contains a certain amount of dissolved gas, including oxygen. Because of the physical properties of water, the colder it is the less gas per unit volume it can hold. Water is at its densest at a temperature of about 4掳C.
As the water temperature in the pond drops, cooled by the colder air above, the surface water sinks to the bottom by convection. Once the whole body is at 4掳C this convection stops because further cooling makes the surface water less dense. The surface starts to freeze and the coldest water begins to release its dissolved gas. Some of this would bubble upwards, but much more would diffuse down, still remaining in solution, until eventually the water surrounding the bricks becomes supersaturated.
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The rough surface of the bricks, particularly around the edges and corners, provides nucleation sites for dissolved gases. Gas molecules collect preferentially around the edges of the bricks, eventually producing bubbles. As these reach a critical size they break away and float straight upwards in the still water. Because there is a layer of ice on the surface, the bubbles become trapped and frozen into it. As the ice layer thickens and bubbles continue to rise from the brick, the 3D shape develops. The rate of bubbling was probably very slow, as was the rate of freezing, so the very detailed effect was able to form.
David Jackson, Liverpool, UK