Glaciers develop where the temperature are cold enough to allow snow to accumulate over periods of years. Enough snow must fall each winter to ensure that it doesn't melt in the summer. Whenever the amount of rainfall exceeds, the amount of melting snow will start to accumulate to a great depth. Newly fallen snow is very light and porous, with a density from 500 to 300 kg/m3. the snow becomes buried by subsequent snowfalls. The ice crystals making up the snowflakes particularly melt and sublimate. The individual crystals near melting point have slick liquid edges that allow them to glide along other crystal planes and readjust the space between them. Bonding between the crystals result to squeezing out of air between them toward the surface or into the bubbles. The air or vapor then condenses and over time the flakes change into granular ice crystals called FIRN which has a density greater than 500 kg/m3. After series of compression through years, a solid ice with a density approximately 900 kg/m3 is formed. This commonly known as "blue ice" or ice schist. The ice schist is approximately 50 meters thick.