Glaciers begin to form where snow remains year-round and enough of it accumulates to transform into ice. New layers of snow compress the previous layers and this compression forces the icy snow to re-crystallize, forming grains similar in size and shape to cane sugar. Gradually the grains grow larger and the air pockets between the grains get smaller, meaning that the snow slowly becomes more dense. After about two winters, the snow turns into firn, an intermediate state between snow and ice. Over time the larger ice crystals become more compressed and even denser, this is known as glacial ice. Glacial ice, because of its density and ice crystals, often takes a bluish or even green hue.
