Striped Icebergs

Why Icebergs are Striped

Glacier ice can have many colors. The "normal" color of glacier ice is blue, sometimes a slight greenish blue. Other colors I've seen are yellows, brown or tan, slightly pinkish, and rarely green. Occasionally ice can be colorless and clear, as an icicle. The color of the ice is an inherent one, that is, not due to reflection of a blue sea or sky, because on a very dark, overcast day (no reflections) 'bergs can still show a bluish color. However, the blue can be enhanced by such reflection.

This color has to do with how ice transmits white light by the process of scattering by atoms, molecules and impurities WITHIN the ice (NOT reflection off the surface of the ice). White light consists of many wavelengths of energy as shown by prisms and rainbows. The shorter wavelengths are absorbed, thus heating and melting the ice, but the fastest ones (blue/violet) are transmitted back to your eye.

Ice forms by accumulation of snow in piles that are white because of abundance of air between flakes. Air scatters all wavelengths, thus snow is white. It's only after the snow re-crystallizes into ice grains that compress under gravity and flow down as a glacier that all that air gets squeezed out and the ice shows blue, as in 'bergs that break off the face of a glacier. After a 'berg has floated around awhile the faces undergo microscopic cracking by thermal stresses (as under the sun). Air gets back into those tiny cracks and the 'berg eventually becomes white again (only on the outside), but if it breaks again the blue will reappear on the surface.

If you look at pictures of ice floes, you will see sea-ice floes often show blue color between the white snow at the top and that's because when the sea freezes the water doesn't have air bubbles. However, that sea ice does not have the banded structure of glacier ice.

All glacier ice has a layered structure that can be shown at the surface as banding ("striping"). The layering, however, is usually invisible to the eye. It only becomes evident if the glacier (from which the 'berg calved) had picked up silt or sand (dirt) by erosion of rocks as the ice flowed down through the mountains. Then you can see dark, dirty bands ("stripes") interlayed with clear ice. If this type of ice at the glacier front breaks off it produces a dark and light "striped" iceberg. Glacier ice is really what is called a "metamorphic" rock (as a gneiss), which is also typically banded or layered by plastic flowage of materials under pressure deep below the surface of the Earth.

The colors of the "stripes" can have many explanations and may be caused by variations in ways light is scattered by impurities in the ice. Also, dirty bands as explained above are the most common. However, as the ice floats around, diatoms (algae) can become attached to the bottom, and under special conditions ice might later crystallize from seawater. Yellowish stripes could conceivably form this way. (Diatoms are yellowish or yellow-tan in color.) Those can commonly be seen on ice floes turned over in a ship's wake; and I've also seen red or green algae on show on ice floes but those are rare.

Ice colors can also be a pinkish color due to traces of iron minerals scattered within the ice. All glacier ice has it even though not often visible.

As you can see, "striped" ice isn't a simple subject. I have skipped over matters like how ice "flows" by mechanical growth of ice crystals under pressure.

Seeing Icebergs

In the Arctic, our Greenland Semi-circumnavigation is the expedition to choose to see the largest icebergs in the North. In Antarctica, most expedition itineraries include iceberg sightings, for example Antarctic Explorer.

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