Iceberg
The term iceberg is a partial calque from the Dutch ijsberg, meaning "ice mountain."[1] An iceberg is a large mass of freshwater ice that has broken off from the seaward edge of a glacier or ice shelf and is floating freely in open ocean waters, typically in polar regions such as the Arctic and Antarctic.[2][3] Composed primarily of compacted snow transformed into dense glacier ice, icebergs are less dense than seawater, causing approximately 90% of their volume to remain submerged below the surface, with only a small fraction visible above water.[4] This buoyancy results from the density difference between ice (around 917 kg/m³) and seawater (approximately 1025 kg/m³), making icebergs a deceptive hazard as their full extent is largely hidden.[4] Icebergs form through calving, a process where tensile stresses cause large chunks of ice to detach from the glacier front or ice shelf edge, often triggered by tidal forces, waves, or melting.[3][2] They are classified based on size and shape: a true iceberg must extend more than 5 meters (16 feet) above sea level, have a thickness of 30 to 50 meters (98 to 164 feet), and cover a surface area greater than 500 square meters (5,382 square feet).[2] Smaller fragments include bergy bits (1-5 meters high, up to 300 square meters) and growlers (less than 1 meter high, about 20 square meters), which are remnants of larger icebergs or direct calving events.[2] Shapes vary regionally; tabular icebergs, with flat tops and steep sides, predominate in Antarctica due to calving from expansive ice shelves, while non-tabular forms—such as domes, pinnacles, or irregular blocks—arise from narrower Arctic glaciers and often erode or flip during drift.[3][2] As they drift with ocean currents, sometimes traveling thousands of kilometers from their origin, icebergs influence marine ecosystems by releasing freshwater and nutrients, altering local salinity and supporting unique biodiversity like seabirds and marine mammals.[3] They also contribute to global climate dynamics by transporting cold meltwater that can modify ocean circulation patterns, such as the Atlantic Meridional Overturning Circulation, and serve as key indicators of ice sheet stability amid warming temperatures.[3] Navigationally, icebergs pose severe risks to shipping, particularly in the North Atlantic's "Iceberg Alley" near the Grand Banks, where collisions have historically caused disasters; to mitigate this, the International Ice Patrol, operated by the U.S. Coast Guard since 1914, monitors iceberg positions using satellites, aircraft, and ship reports to issue warnings and define safe limits for mariners.[5][3]Introduction
Etymology
The term "iceberg" is a partial calque from the Dutch word ijsberg, literally translating to "ice mountain," where ijs means "ice" and berg means "mountain."[1] The Dutch term itself derives from Middle Dutch ijsberch, a compound of ijs ("ice") and berch ("mountain"), reflecting its Germanic roots.[6] This word evolved in maritime contexts during the 16th and 17th centuries through Dutch and Scandinavian influences, as navigators encountered Arctic ice formations; cognates include Danish isbjerg, Norwegian isberg, and German Eisberg, all sharing the Proto-Germanic elements for "ice" and "high elevation."[7][8] The first recorded use in English appeared in 1774, describing a distant glacier resembling a humped hill, with the contemporary sense of a detached, floating mass of ice emerging around 1820.[1] By the 19th century, "iceberg" gained prominence in scientific literature and nautical terminology, particularly following increased polar exploration.[8] In other languages, adaptations followed suit, such as the French iceberg, borrowed directly from English in the early 19th century for similar maritime and descriptive purposes.[9]Definition and overview
An iceberg is a large piece of freshwater ice that originates from glaciers or ice shelves and floats in open ocean waters after calving or breaking off from its source.[2] These masses of ice form when unstable sections of land-based or floating ice detach due to natural processes like melting, cracking, or tidal forces, entering the marine environment where they drift with ocean currents.[10] By definition, an iceberg must protrude more than 5 meters (16 feet) above the sea surface, have a thickness of at least 30 meters (98 feet), and cover a surface area greater than 500 square meters (5,382 square feet) to distinguish it from smaller fragments such as bergy bits (1-5 meters high) or growlers (less than 1 meter high), which are not classified as full icebergs.[2] Due to the lower density of ice compared to seawater—approximately 917 kg/m³ for ice versus 1025 kg/m³ for seawater—about 90% of an iceberg's volume remains submerged below the waterline, with only a small fraction visible above the surface.[11] This buoyancy principle explains why the exposed portion often misrepresents the total scale and potential hazards of these floating ice features. Icebergs are predominantly distributed in polar regions, including the Arctic Ocean around Greenland and the Southern Ocean surrounding Antarctica, where they play a key role in the cryosphere-ocean system by transporting freshwater and influencing marine ecosystems.[12] Antarctic sources alone account for over 90% of the global iceberg mass in the Southern Hemisphere, far exceeding Arctic contributions due to the vast extent of the Antarctic ice sheet.[10] Occasionally, drifting icebergs reach subpolar or temperate latitudes, such as the North Atlantic shipping lanes, where they have historically posed risks to navigation.[2]Physical properties
Size and shape
Icebergs vary significantly in size, from small ones with lengths of 15–60 m and heights of 5–15 m above sea level to vast tabular forms spanning hundreds of square kilometers in surface area. Smaller floating ice fragments, such as growlers (less than 1 m high and about 20 m² in area) and bergy bits (1–5 m high and up to 300 m² in area), are not classified as icebergs.[2] Large icebergs can exceed 150 m in height above water. The largest recorded iceberg by area was B-15, which calved from Antarctica's Ross Ice Shelf in March 2000 and initially covered approximately 11,000 km², comparable to the size of the U.S. state of Connecticut. More recently, iceberg A23a, calved from the Filchner-Ronne Ice Shelf in 1986, has been the largest active iceberg as of November 2025, with a current area of approximately 3,500 km² after significant fragmentation.[13][14][15] Iceberg shapes are broadly classified into tabular and non-tabular categories, reflecting their formation origins and subsequent modifications. Tabular icebergs, derived from Antarctic ice shelves, exhibit flat tops and near-vertical sides, with a length-to-height ratio greater than 5:1, often maintaining broad, plateau-like profiles over vast areas. Non-tabular icebergs, typically calved from glaciers, display more varied morphologies, including domed (smoothly rounded summits), pinnacled (featuring tall spires or peaks), blocky (steep, vertical faces with a flat top, common in the Weddell Sea region), wedged (one sloping side and a steep edge on the other), and drydock (U- or V-shaped notches resembling a ship's dry dock).[16][17] The morphology of an iceberg is shaped by its initial calving mechanism, as well as post-formation processes like wave action and differential melting. Calvings from ice shelves produce the characteristic tabular forms, whereas glacier-derived icebergs start as irregular masses that evolve through mechanical erosion from waves, which undercut and sculpt edges, and melting patterns that preferentially remove submerged or exposed portions. Submarine melting, in particular, accelerates shape changes by creating overhangs and promoting fragmentation in tabular icebergs.[18][19] Estimating an iceberg's volume and mass relies on remote sensing techniques, including aerial photogrammetry and satellite-based methods such as interferometric synthetic aperture radar (e.g., TanDEM-X) for topographic mapping and laser altimetry (e.g., ICESat-2) for freeboard measurements, which allow inference of the submerged draft assuming ice density around 917 kg/m³. These approaches enable volume calculations by integrating surface area with height profiles; for instance, giant tabular icebergs like B-15 are estimated to contain billions to trillions of tonnes of ice, equivalent to the mass of several large cities.[20][21]Color and appearance
Icebergs predominantly exhibit white hues due to the scattering of light by numerous tiny air bubbles trapped within the ice, which reflect all wavelengths of visible light equally.[22][23] These bubbles, formed from compressed snow, create an opaque appearance that dominates the surface of most icebergs.[24] In contrast, blue tones emerge in denser sections where prolonged compression has expelled many air bubbles, allowing light to penetrate deeper into the ice; here, longer red wavelengths are absorbed, while shorter blue wavelengths are transmitted and scattered back to the observer.[25][26] "Blue icebergs," often vivid in hue, typically originate from ancient glacial ice that has undergone extensive compression over centuries, resulting in larger ice crystals and minimal bubble content.[24][27] Color variations arise from impurities and environmental interactions. Green icebergs, less common, form from marine-origin ice rich in iron oxides or dissolved organic matter, which imparts a yellowish tint that combines with the underlying blue to produce green shades; algae growth on submerged surfaces can also contribute to greenish appearances when exposed.[28][23][29] Black streaks or patches often result from embedded rock debris, sediments, or soot accumulated during the glacier's flow over land, creating dark lines that contrast sharply against lighter ice.[23] The age and degree of compression further influence these visuals, with older, more compacted ice appearing more translucent and intensely colored.[24][26] Optical effects enhance the striking appearance of icebergs. In thinner sections, the ice becomes translucent, revealing subtle blue-green shades as light passes through with less scattering.[30] Refraction within the ice can produce rainbow-like spectra when sunlight interacts with crystal edges or water interfaces, dispersing light into its component colors.[30] The presence of internal bubbles contributes to the overall opacity observed in thicker portions.[24] Perceived colors also depend on viewing conditions. The angle of sunlight alters light penetration and scattering, with low angles enhancing shadows and intensifying blues or greens, while overhead light promotes whiter appearances.[31][30] Surrounding water clarity affects the visibility of submerged portions, where clearer waters allow vibrant underwater hues to influence the overall visual impact from above.[22]Internal structure and stability
Icebergs consist primarily of freshwater ice derived from the compaction of snow over successive seasons, forming distinct layers that reflect annual accumulation cycles. This ice incorporates trapped air bubbles from the compression process, which can constitute up to 10% of the volume and contribute to internal structural variations. Sediments and minor impurities, including trace salts from atmospheric deposition or glacial entrainment, may also be embedded within the ice matrix.[32][23] The internal architecture of icebergs includes crevasses—deep fractures originating from the parent glacier—and melt channels that develop as water percolates through the ice. Density variations arise from alternating layers of firn (compacted snow) and denser glacier ice, with pure ice averaging 917 kg/m³ compared to surrounding seawater at approximately 1025 kg/m³. These heterogeneities influence how stress is distributed within the structure.[33][16] Stability is governed by the iceberg's center of gravity relative to its center of buoyancy, with roughly 90% of the volume submerged due to the density contrast between ice and seawater. Uneven melting, particularly at the base or sides, can shift the center of gravity upward or asymmetrically, increasing tipping risks, while wave action exacerbates these instabilities by inducing torque. Rollover events, where an iceberg capsizes to reorient itself, have been documented through modeling and laboratory studies, often triggered shortly after calving when the initial shape is precarious.[34] Signs of structural degradation include audible cracking from expanding fractures and the calving of smaller ice pieces, which further compromises integrity as the iceberg drifts. These processes highlight the dynamic balance between internal composition and external forces affecting longevity.[35][36]Formation and types
Sources of formation
Icebergs form primarily through calving, the mechanical breaking off of ice masses from the termini of tidewater glaciers and floating ice shelves in polar regions.[37] Tidewater glaciers, which flow directly into the ocean, and ice shelves, which are extensions of ice sheets over the sea, serve as the main sources, with calving occurring when accumulated stresses exceed the ice's tensile strength.[38] Notable examples include Greenland's Jakobshavn Glacier (now Ilulissat Isbræ), one of the fastest-flowing tidewater glaciers, which releases numerous icebergs into the North Atlantic annually.[39] In Antarctica, the Ross Ice Shelf produces massive tabular icebergs, while Pine Island Glacier in the Amundsen Sea sector frequently calves large volumes due to its rapid retreat.[32] Arctic sea ice edges contribute smaller, fragmented pieces, but these are distinct from true icebergs derived from land ice.[40] The calving process is initiated by tensile stresses at the ice front, where longitudinal extension causes crevasses and rifts to propagate, eventually leading to detachment.[37] These stresses arise from the ice's flow dynamics, buoyancy at the grounding line, and imbalances between accumulation and ablation.[38] Tidal influences play a key role by flexing the ice shelf during high and low tides, accelerating rift growth and fracture propagation, particularly on Antarctic shelves.[41] Seismic events, such as those induced by tidal bending or distant earthquakes, can also trigger calving by exploiting existing weaknesses in the ice structure.[42] Regionally, Antarctica accounts for approximately 90% of global iceberg volume, with its ice shelves and glaciers discharging vast quantities into the Southern Ocean.[43] Key contributors include the Weddell and Ross Seas, where large ice shelves dominate production, and the Amundsen Sea Embayment, home to Pine Island and Thwaites Glaciers, which together release hundreds of gigatons of ice annually.[44] In contrast, Greenland's tidewater glaciers, concentrated along the southeast and west coasts, produce the majority of Northern Hemisphere icebergs, though at a much smaller scale overall.[39] Calving rates have shown quantitative increases in recent decades compared to pre-2000 baselines. In Antarctica, studies have estimated annual calving fluxes at 1,300–2,000 Gt per year, with a highly variable 1997–2021 average of 1,600 ± 520 Gt per year and no clear pan-Antarctic trend, though with heightened activity at vulnerable sites like Pine Island Glacier.[45][44] As of 2024, total Antarctic Ice Sheet discharge (including calving) reached approximately 2,224 ± 200 Gt/yr.[46] For Greenland, iceberg discharge from tidewater glaciers rose from 462 Gt per year around 2000 to 546 Gt per year by 2012, reflecting accelerated front retreat; more recent data indicate annual ice sheet mass loss of around 250–300 Gt/yr as of 2023, with calving comprising a significant portion.[47][48] These trends result in more frequent and voluminous calving events, influencing the types of icebergs produced, such as larger tabular forms from Antarctic shelves versus irregular blocks from Greenland glaciers.[49]Classification by type
Icebergs are primarily classified by their origin, distinguishing between those calved from valley glaciers, known as glacial icebergs, which typically exhibit irregular, jagged shapes due to the dynamic flow of terrestrial ice, and those derived from floating ice shelves, called shelf or tabular icebergs, which form flat, table-like structures from the uniform breakup of extensive ice platforms. Smaller fragments originating from sea ice, rather than land-based glaciers or shelves, are generally not considered full icebergs but contribute to hazardous floating ice in polar regions. This origin-based categorization reflects the diverse processes leading to iceberg formation, with glacial types more common in the Arctic and shelf types predominant in the Antarctic.[12][2][50] Size-based classification follows the international nomenclature established by the World Meteorological Organization (WMO), which uses the iceberg's freeboard (height above sea level) and longest horizontal dimension (length) to define categories, ensuring standardized reporting for navigation and research. The system delineates progressively larger forms, starting from minor threats to major navigational hazards.[51][52] The WMO categories are summarized in the following table:| Type | Freeboard (height above sea, m) | Length (longest dimension, m) |
|---|---|---|
| Growler | < 1 | < 5 |
| Bergy bit | 1 to < 5 | 5 to < 15 |
| Small iceberg | 5 to 15 | 15 to 60 |
| Medium iceberg | 16 to 45 | 61 to 120 |
| Large iceberg | 46 to 75 | 121 to 200 |
| Very large iceberg | > 75 | > 200 |