According to The Canadian Encyclopedia:
“Most icebergs are white except along freshly calved ice cliffs, which tend to appear blue. Others may appear green, brown or black, or combinations of these colours. These icebergs have usually rolled over, exposing basal ice, or have emerged from below water level. The various colorations are caused by differences in density, air-bubble content and impurities. For example, black ice is of high density and bubble free; dark layers indicate the presence of rock materials derived from the base of the parent glacier. Occasionally, rocks may be found on the original upper surface of the iceberg. As the iceberg melts, these materials precipitate into marine or lake sediments.”
Icebergs are black or dark blue when there are no or very few air bubbles caught in the ice. This occurs when the iceberg is calfed off the parent glacier near the bottom of the glacier. This causes rock and sand sediment to mix with the iceberg during the freezing period of its formation. This forces air bubbles out of the ice and captures the sediments in it, further adding to the dark coloration.
Black icebergs also occur when the tip of a white iceberg flips over and the bottom becomes the top. The bottom part of the iceberg has been compressed by cold water and the weight of the iceberg to the point where the air bubbles are too few to make the ice color white.
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