Glacier Calving and Iceberg Formation
When a wall of ice at the front of a glacier suddenly collapses into the water below, the resulting sound — a deep, resonant crack followed by the roar of falling ice and the crash of displaced water — is one of the most dramatic events in the natural world. This process, calving, is how glaciers that terminate in ocean water or deep freshwater lakes shed ice in the form of icebergs. It is also, for many of the world's largest glaciers, the dominant mechanism of mass loss. Understanding the mechanics of calving and the factors that control calving rates is one of the central challenges in contemporary glaciology, with direct implications for projections of sea level rise.
Calving occurs where a glacier meets water deep enough to allow the glacier terminus to flex, fracture, and detach blocks. In tidewater settings — where glaciers flow directly into the ocean — calving fronts can be hundreds of metres high and produce icebergs ranging from small growlers a few metres across to tabular giants covering square kilometres. In freshwater settings, such as the proglacial lakes of Patagonia and New Zealand, calving produces smaller icebergs but is still a significant component of mass loss.
The Mechanics of Calving
At the most fundamental level, calving occurs when the ice at the glacier front develops fractures that penetrate fully through the ice thickness, detaching a block. The stress conditions that drive fracture development are complex and depend on the geometry of the terminus, the ice temperature and viscosity, the water depth, and the presence of pre-existing crevasses and damage.
A key concept is the balance between ice thickness and water depth at the terminus. When ice is thick enough relative to water depth, the base of the glacier rests on the seafloor and the glacier is said to be grounded. At the grounding line, where the glacier lifts off the bed and becomes floating, the dynamics change dramatically. Floating ice is under different stress conditions than grounded ice: the buoyancy force from the water below counteracts the weight of the ice, which reduces the normal stress on basal fractures and increases the tendency of the ice to break up.
Crevasses play a critical role. Transverse crevasses that form in the tensional stress regime near a calving front penetrate downward from the surface, and when meltwater fills them — either from surface melt pooling in the crevasse or from rainfall — the hydraulic pressure of the water can drive the crevasse deeper, a process called hydrofracture. When a surface crevasse driven by meltwater meets a basal fracture propagating upward from the base of the ice in tension, calving is imminent.
Tidewater Glacier Cycles
Tidewater glaciers — those ending in ocean water — exhibit a distinctive behaviour where extended periods of gradual retreat are interrupted by episodes of rapid calving-driven retreat that can cover kilometres in a few years. This results from a feedback between water depth, calving rate, and terminus position. As a glacier calves back into deeper water, the calving rate increases because the terminus becomes more susceptible to buoyancy-driven fracture, which drives further retreat, which exposes still deeper water, in a positive feedback that can only be arrested when the terminus reaches shallower water or a pinning point such as a submarine moraine ridge.
Columbia Glacier in Alaska is a well-documented example. Between the 1980s and the 2020s it retreated approximately 30 kilometres from its grounded terminal moraine, losing more than half its length in a calving retreat that accelerated dramatically once it began. The retreat was facilitated by the deep overdeepened basin beneath the glacier's mid-reach, which provided conditions for sustained high calving rates.
Greenland and Antarctica: Marine Ice Sheet Instability
The concern about calving in the context of sea level rise centres primarily on the marine-based sectors of the Greenland and Antarctic ice sheets, where ice is grounded below sea level on bedrock that slopes downward toward the interior of the continent. This geometry creates the conditions for marine ice sheet instability: as the grounding line retreats into deeper water, the ice column at the new grounding line is thicker and discharge increases, which drives faster retreat, in another positive feedback. Jakobshavn Isbrae in West Greenland and Pine Island Glacier and Thwaites Glacier in West Antarctica are examples of marine-based outlet glaciers where rapid calving and thinning have been documented over recent decades.
The Pine Island and Thwaites systems are of particular scientific concern because they drain a large sector of the West Antarctic Ice Sheet that is grounded below sea level. Destabilisation of the Thwaites Glacier grounding line could in principle initiate a self-sustaining retreat affecting a significant fraction of the West Antarctic Ice Sheet, with corresponding contributions to sea level.
Ice Shelf Disintegration
Some of the most spectacular calving events in the historical record have involved the sudden collapse of ice shelves — thick floating extensions of ice attached to the main ice sheet or glacier. The Larsen B Ice Shelf on the Antarctic Peninsula, which had existed for at least 12,000 years according to sediment core evidence, disintegrated in approximately three weeks in February and March of 2002. Roughly 3,250 square kilometres of ice shelf collapsed into icebergs and brash ice in one of the most rapid large-scale glaciological changes observed in the instrumental era.
The collapse was attributed to surface melt ponds that had been increasing in volume over preceding warm summers, and to the progressive weakening of the ice shelf through calving events at its margins. The loss of the ice shelf removed the buttressing force that had been slowing the flow of tributary glaciers behind it; those glaciers subsequently accelerated by factors of several times their pre-collapse flow speeds.
Icebergs: Tracking and Hazard
Icebergs calved from Greenland glaciers and Antarctic ice shelves can persist for years, drifting with ocean currents and potentially travelling thousands of kilometres from their source. The International Ice Patrol, established after the Titanic disaster of 1912, monitors iceberg drift in the North Atlantic shipping lanes, with particular attention to icebergs originating from Jakobshavn and other major West Greenland calving glaciers that enter the Labrador Current.
Large tabular icebergs from Antarctica are tracked by the National Ice Center in the United States using satellite imagery. Bergs designated by the letter codes A through D, indicating the quadrant of Antarctica from which they originate, are assigned names and tracked until they melt. Some of the largest, such as A-76 which calved from the Ronne Ice Shelf in 2021 at approximately 4,320 square kilometres, persist for years in the Southern Ocean.
Find tidewater glaciers and calving fronts around the world by exploring the map, where the locations of major calving glaciers in Greenland, Alaska, Patagonia, and Antarctica are plotted alongside their respective ice fields and drainage basins.