Blue ice (glacial)

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Blue ice occurs when snow falls on a glacier, is compressed, and becomes part of the glacier. During compression, air bubbles are squeezed out, so ice crystals enlarge. This enlargement is responsible for the ice's blue colour.

Small amounts of regular ice appear to be white because of air bubbles inside and also because small quantities of water appear to be colourless. In glaciers, the pressure causes the air bubbles to be squeezed out, increasing the density of the created ice. Water is blue in large quantities, as it absorbs other colours more efficiently than blue. A large piece of compressed ice, or a glacier, similarly appears blue.

The blue color is sometimes wrongly attributed to Rayleigh scattering, which is responsible for the color of the sky. Rather, water ice is blue for the same reason that large quantities of liquid water are blue: it is a result of an overtone of an oxygen–hydrogen (O−H) bond stretch in water, which absorbs light at the red end of the visible spectrum.^[1] So, water owes its intrinsic blueness (as seen after > 3 meters of penetration) to selective absorption in the red part of its visible spectrum. The absorbed photons drive vibrational (normally infrared) transitions.

Once blue ice is exposed to warmer air, cracks and fissures appear in surface layers, and break up the large blue crystals of dense, pure ice. Within hours these air filled fissures cloud the surface making the ice appear white. The blue colour will not be seen again until the ice breaks or turns over to expose ice which air could not reach. For example, lucky tourists at Tasman Glacier, New Zealand in January 2011 saw an iceberg roll over to reveal startling blue ice, kept from air by staying underwater for months since the iceberg calved.^[2]