Glacier

A glacier is a persistent mass of ice formed from accumulated snow that moves under its own weight over land. It flows slowly as ice deforms and slides over its bed.

Ice deformation: The change in shape of solid ice under stress, including the slow flow of glacier ice. Deformation lets a glacier flow even where its bed does not permit sliding.
Firn: Old snow that has survived at least one melt season but has not yet become glacier ice. Buried firn compacts into the glacier ice that drives long-term flow.
Glacier mass-balance measurement: Field and remote-sensing methods used to quantify a glacier's annual gains and losses. Repeated measurements show whether a glacier is gaining or losing ice.
Ice sheet: A continent-scale mass of glacial ice covering more than 50,000 square kilometers of land. An ice sheet is the largest land-based form of glacier ice.
Louis Agassiz: A Swiss-American naturalist who championed the theory that glaciers once covered much of northern Europe. His glacial theory helped establish ice as an agent of large-scale landscape change.
Basal sliding: Movement of glacier ice over its bed, often facilitated by meltwater or deformable sediment. Sliding can carry glacier ice faster than internal deformation alone.
Glacier ice: Dense ice formed as buried snow compacts and recrystallizes over time. It is the material that deforms and flows within a glacier.
Glacial lake outburst flood: A sudden flood caused by the failure of a natural dam holding back a glacier-fed lake. Retreating or unstable glacier environments can create lakes and flood hazards.
Ice cap: A dome-shaped mass of glacial ice covering less than 50,000 square kilometers of land. Ice caps resemble ice sheets but cover smaller areas.
John Tyndall: A nineteenth-century physicist who investigated glaciers and demonstrated the movement and deformation of ice. His Alpine studies advanced scientific explanations of glacier motion.

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