Iceberg Calving: When Glaciers Give Way

Icebergs do not form slowly. They are created in a single, dramatic event called calving. In calving, a large block of ice breaks away from the edge of a glacier or a floating ice shelf. Calving happens because moving ice is under constant mechanical stress. As a glacier pushes forward, cracks called crevasses form and slowly grow deeper. At the ice front, buoyant forces from the water below push upward on ice that is only partly supported. This flexes the ice until cracks widen into full breaks. Where a glacier meets the ocean, the process is often sped up from below, not above. Seawater that is warmer than the ice slowly erodes and thins the submerged part of the ice front. This undercutting continues until the unsupported ice above can no longer hold its own weight, and it breaks free. One especially dramatic example happened in July 2017. A rift that had been slowly widening for years finally split completely across the Larsen C Ice Shelf in Antarctica. This released an iceberg roughly the size of the U.S. state of Delaware, weighing over a trillion tons. Calving events like this are often preceded by rifts that scientists can track for months or years using satellite images. Because of this, researchers can increasingly predict major calving events before they happen. Still, predicting the exact moment of separation remains difficult.