though rare, striped icebergs form as meltwater refreezes in crevasses atop glaciers before air bubbles can become trapped in the ice; or when supercooled seawater freezes inside cracks beneath the ice, which then becomes exposed when the iceberg flips. (flipping also explains the vertical stripes in the first photo) 

over time, the weight of accumulated snow contorts and curves these blue bands of ice, as does erosion from waves and wind. dust and volcanic ash falling on the iceberg can can darken the ice, while dissolved organic compounds entering from below can shade it towards cyan. 

accumulated snow also compresses air bubbles trapped in the iceberg, thus preventing them from otherwise interfering with the passage of light. and because water absorbs photons from the red end of the visible spectrum much better than the blue end, the icebergs take on a blue colour. 

photos by (click pic) duthion, oyvind tangen, steve nicol, john weller, youri loof, paul lee, walter huber