Ice calving is the process where chunks of ice break off the edge of a glacier or ice shelf and crash into the water. This natural phenomenon is a major mechanism for the loss of ice mass from ice sheets and glaciers. The scale and frequency of these events have increased in recent years, drawing public attention as an indicator of a changing polar environment.
Defining Ice Calving
Ice calving is the mechanical detachment of ice from the terminus of a glacier or ice shelf into a body of water. This process is distinct from mass loss due to surface melting or sublimation. It is a form of ice ablation, where the forward movement of a glacier creates instability at its leading edge, culminating in a large-scale fracture event.
The ice masses resulting from calving are classified based on size. The largest are icebergs, which are substantial pieces of ice. Smaller, car-sized fragments are known as bergy bits, while growlers are the smallest pieces, often floating low in the water.
The Physical Mechanisms of Ice Detachment
The detachment of ice is driven by a complex interplay of mechanical stresses that exceed the ice’s structural strength. The sheer weight of the glacier, pulled downhill by gravity, creates immense internal pressure and tension within the ice mass. This tension, or tensile stress, causes the formation of fissures known as crevasses, which can open both on the surface and within the glacier body.
One significant mechanism is buoyant calving, where the upward pressure of the surrounding water lifts the submerged portion of the ice front. If the ice above the waterline is thinned by surface melting or previous calving events, the reduced weight pressing down allows the buoyant force to fracture the remaining ice. This process can cause the submerged ice, sometimes referred to as an ‘ice foot,’ to break off and emerge rapidly at the surface.
Another powerful driver is hydro-fracturing, which involves surface meltwater draining into the crevasses. The water pressure inside the fissure counteracts the natural pressure of the overlying ice that would otherwise keep the crack closed. By effectively prying the ice apart, this water allows the crevasse to penetrate through the full thickness of the glacier, leading to the detachment of a block. External forces, such as the regular rise and fall of tides, can also exacerbate existing stresses, causing the ice front to flex and accelerate the opening of fractures, particularly at low tide.
Glacial Environments Prone to Calving
Calving is the dominant mass loss process for glaciers that terminate in water, which fall into distinct categories. Tidewater glaciers are those that extend directly into the ocean or a large lake, and they account for a large portion of the ice mass transferred to the sea globally. These glaciers, such as the numerous outlet glaciers in Greenland and Alaska, lose mass primarily by shedding icebergs from their terminus.
Ice shelves are floating platforms of ice that are still connected to the land-based ice sheet. They are prevalent in Antarctica, where they can be hundreds of meters thick and kilometers wide. Calving from ice shelves often occurs through the propagation of large rifts, releasing tabular icebergs.
A third environment is the ice tongue, a floating extension of a glacier that is confined within the walls of a fjord. Unlike ice shelves, ice tongues are long and relatively narrow. Calving from ice tongues produces icebergs, which can pose a hazard to shipping routes.
Impact on Global Sea Levels
The significance of ice calving for global sea levels depends on whether the detached ice was already floating or was grounded on land. Ice shelves and ice tongues are already floating in the ocean, displacing water like an ice cube in a glass. Therefore, when they calve, the resulting icebergs do not immediately raise the global sea level.
However, the loss of these floating ice features has a profound indirect effect on sea level rise. Ice shelves act as a buttress, providing a physical barrier that slows the flow of grounded ice sheets behind them. When a large portion of an shelf calves away, the buttressing force is reduced, which destabilizes the land-based ice. This loss of resistance causes the grounded glaciers to accelerate, rapidly discharging more ice into the ocean. This increased rate of grounded ice loss is the main contribution of calving to sea level rise, making the stability of ice shelves a major concern.