An iceberg is a large, free-floating mass of freshwater ice that has separated from a glacier or an ice shelf and is adrift in open water. Its formation is not a simple freezing process but a multi-stage, years-long journey that begins with a single snowflake high on a landmass. The ultimate separation from the parent ice body, known as calving, is the final dramatic act in a cycle of accumulation, compression, and movement that shapes the planet’s vast cryosphere. This process is driven by gravity and the unique physical properties of water.
From Snow to Glacial Ice
Snow accumulates in cold, high-altitude or high-latitude regions. Freshly fallen snow has low density due to trapped air. For an ice mass to become a glacier, annual snowfall must exceed the amount lost to melting or evaporation, allowing successive layers to build up.
The weight of overlying material compresses the lower layers, initiating transformation. Snow crystals break and settle, becoming rounded and granular. After surviving a summer melt season, this compacted material is known as firn, which has a density roughly half that of water.
Continued pressure forces the firn to densify further, squeezing out trapped air. This compression, combined with sintering (ice grains bonding and recrystallizing), dramatically increases the density. When the density reaches approximately 830 kilograms per cubic meter, air spaces seal off, trapping the remaining air as isolated bubbles.
The final product is dense, blue glacial ice. This transformation can take a few years in temperate glaciers but often requires hundreds of years and a depth of 50 to 80 meters in colder polar environments.
Glacial Flow and Terminus Dynamics
Once the ice mass is thick enough, gravity causes it to flow toward the sea. Glacial motion occurs through two primary mechanisms: internal deformation and basal sliding. Internal deformation, or creep, happens as ice crystals shift and rearrange under pressure, causing the ice mass to flow like a viscous fluid.
Basal sliding occurs when the glacier slides over its bedrock. This sliding is enhanced if the base is at the pressure melting point, allowing a thin film of meltwater to act as a lubricant. This meltwater originates from surface melt or from the pressure of the overlying ice.
As the ice moves, differential flow rates cause mechanical stress that creates large cracks called crevasses. The terminus is the end of the glacier where it meets the water, and its dynamics dictate the final stage of iceberg formation.
A glacier terminus can either rest on a submerged landmass or float freely as an ice shelf. The interaction of the flowing ice with the ocean sets the conditions for separation. Stresses developed during movement precondition the ice for its break.
Calving: The Moment of Separation
Calving is the process where a chunk of ice mechanically detaches from the terminus of a glacier or ice shelf, forming an iceberg. It is triggered by mechanical and environmental stresses that exceed the ice’s structural strength.
One primary trigger is the longitudinal stretching of the ice near the terminus, which opens and deepens existing crevasses. When these fractures propagate through the full thickness of the ice, separation occurs. Buoyancy forces also play a significant role, particularly for tidewater glaciers and ice shelves.
As the terminus begins to float, the water exerts upward pressure, creating large bending forces at the grounding line. This buoyant flexure can cause massive, full-thickness fractures, leading to the detachment of large, tabular icebergs. Undercutting by melting at the waterline creates a notch that removes support for the overhanging ice.
The collapse of this unsupported ice above the waterline is known as toppling, a common, smaller-scale calving event. The loss of the upper mass reduces downward pressure, allowing the remaining submerged ice to be fractured and released by buoyant force.
Classification and Fate of Icebergs
Once separated, icebergs are classified based on size and shape, reflecting their origin and subsequent erosion. Size classifications range from small “growlers” to immense “very large” icebergs, which can stand over 75 meters tall. The largest icebergs, often called ice islands, can cover thousands of square kilometers.
Icebergs are categorized by form, generally into tabular and non-tabular shapes. Tabular icebergs, common in Antarctica, have steep sides and a flat top with a length-to-height ratio greater than five-to-one. Non-tabular icebergs, typical of Arctic glaciers, are formed by fracturing and capsize and include:
- Dome shapes
- Pinnacle shapes
- Wedge shapes
- Blocky shapes
Icebergs float because freshwater ice has a density approximately 90% that of seawater. This means roughly 90% of the ice mass is submerged below the waterline. Underwater currents often influence the iceberg’s drift path more than surface winds, as the bulk of the mass is deep beneath the waves.
The ultimate fate of every iceberg is to melt, a process that can take years or decades depending on size and water temperature. As icebergs drift into warmer waters, they continually erode, changing shape, sometimes flipping over, and eventually fragmenting into smaller pieces.