Why do glaciers appear blue?

Glacier ice appears blue for the same basic reason deep water does: it selectively absorbs colours of light. When sunlight enters thick, compressed glacial ice, the ice absorbs the longer wavelengths at the red end of the spectrum more strongly, while blue light penetrates deeper and is scattered back out. The key is density: over decades, the weight of accumulating snow squeezes out air bubbles, producing dense, clear ice that transmits light far enough for this colour filtering to occur. Fresh snow and bubbly ice, by contrast, scatter all wavelengths equally and look white. The most vivid blues appear in old, deep, bubble-free ice, such as freshly exposed crevasse walls.