Glaciers are large land-bound bodies of ice. To be called a glacier, the ice mass must be moving, or show evidence of having moved in the past. Covering about 10% of Earth's surface, glaciers store a significant amount of Earth's supply of freshwater.

Glaciers form from the buildup of snow over time. As snow accumulates, it is compressed under its own weight. Compaction, along with partial thawing and refreezing, converts the original snow to a type of granular ice called firn. As snow continues to accumulate, the firn is buried and further compacted and is eventually converted into glacial ice. Sufficient accumulation of snow and ice is critical to the ability of glacial ice to deform and flow. Pressure from above allows the solid ice to flow at depth.

Glacial ice flows outward from the center of the accumulation and/or downhill under the force of gravity. Plastic flow...