We (the UK) have had a lot of snow and sub-zero temperatures recently, followed by brief thaws and then sub-zero temperatures, and the water in the pond has frozen white and then these large dendrite-like structures have appeared - the pond is still frozen solid. There are also what looks like bubbles frozen into the ice below the surface and odd dents in the surface..

After cutting through a pat of butter (at room temperature) with a clean knife, I found one side of the blade covered with elegant dendritic patterns (picture). What causes this?