How do glaciers form from accumulated snow?

The transformation of snow into glacier ice is a gradual process of densification driven by the weight of overlying snow and environmental conditions like temperature and moisture.1

The process generally follows these stages:

• Snowpack Consolidation: As snow accumulates, the intricate shapes of individual snowflakes break down or evaporate and condense, causing the grains to round and fit more closely together. Wind and pressure from overlying layers further pack these grains, increasing the density of the snowpack.1
• Formation of Firn: Snow that survives a full melting season is known as firn. This is an intermediate, partially compacted granular stage between snow and glacier ice, typically having a density greater than 500 kilograms per cubic meter.12
• Glacier Ice: With continued burial and the application of stress, the material undergoes recrystallization. Once the density reaches approximately 830 to 840 kilograms per cubic meter, the air spaces between grains are sealed off, making the ice impermeable to fluids.12

This transition can take as little as a few years in warmer, wetter climates or thousands of years in cold, dry regions like the Antarctic plateau.1