Glaciers are massive bodies of ice that form in areas where the accumulation of snow exceeds its ablation (melting and sublimation) over many years. [A] This process begins when snowfall builds up in layers, putting pressure on the lower layers. Under this immense weight, the light, fluffy snowflakes undergo compaction. They first transform into granular, pellet-like snow called firn. [B] As more snow accumulates, the pressure further compacts the firn, recrystallizing it into dense, blue glacial ice. [C]
Once the thickness of the ice reaches approximately 50 meters, it begins to behave plastically. Under the influence of gravity and its own weight, the glacier starts to flow down valleys or spread across landmasses. The primary mechanism of glacial movement is internal deformation, where ice crystals slide past one another. Another mechanism is basal slip, which occurs when meltwater at the bottom of the glacier acts as a lubricant, allowing the entire ice mass to slide over the bedrock. [D]
The rate of glacial movement varies dramatically, ranging from a few centimeters to tens of meters per day. Sometimes, glaciers experience sudden, rapid advances called surges, during which velocities can increase up to a hundredfold. These surges are often triggered by the buildup of pressurized water beneath the glacier's bed.