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 – follow link above). What caused this?
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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.