How glaciers form
Newly fallen snow is porous and not very dense. Air easily penetrates the pore spaces, and the delicate points of each snowflake gradually evaporate. The resulting water vapor condenses, mainly in constricted places near a snowflake’s center. In this way, the fragile ice crystals slowly become smaller, rounder, and denser, and the pore spaces between them disappear.

Snow that survives a year or more gradually becomes denser and denser. The transitional phase between snow and glacier ice is a loose, porous aggregate of small ice grains called firn. When firn is no longer permeable to air, it becomes glacier ice. Eventually glacier ice will grow in grain size under increasing pressure at the base of a glacier.