Snow is converted into glacial ice as new layers of snow fall on top of the accumulations from previous years, adding pressure and causing the snowflakes to physically change into crystalline solids. This process can take over 100 years to complete.
In order for glaciers to form, the annual melting of snow and ice must remove less material than what is accumulated over the next winter in snowfall. New layers bury the existing snow deeper each year, eventually changing it into blue glacial ice.
The pressure from a new snowpack converts the underlying snowflakes into small crystals, similar in size and shape to sugar, by causing them to melt and refreeze. Pressure reduces the melting point of snow so that it reforms, even though the temperature remains below 32 degrees Fahrenheit. The crystals gradually get larger, and the air pockets trapped inside become smaller. In very old glaciers, the crystals reach several inches in length.
After about two years, the ice crystals turn into firn, a state somewhere between snow and glacial ice. Firn has a density approximately two-thirds that of water, and the longer it is compressed, the more its crystals resemble compressed snowflakes that have lost their fine definition.