Glaciers, Ice Shelves, and Calving

Glacial ice takes several forms across the polar regions: vast ice sheets and smaller ice caps that bury entire landmasses, valley or alpine glaciers confined by surrounding terrain, fast-moving outlet glaciers that drain an ice sheet's interior toward the coast, and ice shelves, the floating extensions formed where an ice sheet flows off the land and out over the ocean. The line where grounded ice lifts off its bed and begins to float is called the grounding line, a boundary whose position is central to understanding ice-sheet stability. Every glacier has a mass budget: an accumulation zone where snowfall compresses over roughly 10 to 15 years into firn and then solid glacial ice, and an ablation zone where the ice loses mass to surface melt, sublimation, and, at marine margins, calving.