Blue ice (glacial)
Blue ice is a form of ice that occurs in a glacier. It has been highly compressed by the weight of the overlying snow such that the ice crystals become enlarged. It is this enlargement is responsible for the ice's blue color.
Appearance
Small amounts of regular ice appear to be white because of air bubbles inside and also because small quantities of water appear to be colorless. 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 colors more efficiently than blue. A large piece of compressed ice, or a glacier, similarly appears blue. Under pressure, the ice crystals increase in size and so the path that light travels through the crystal gets longer, increasing the absorption of the red end of the spectrum and intensifying the blue color.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 color will not be seen again until the ice breaks or turns over to expose ice which air could not reach. For example, 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.