# What Is an Iceberg? > An iceberg is a piece of freshwater ice that has broken from a glacier or ice shelf and now floats in open water. Most of its mass sits below the surface because ice is only slightly less dense than seawater. NOAA's iceberg... Canonical URL: https://www.argo.net/what-is-an-iceberg/ Byline: ARGO.net Editorial Team Published: 2026-09-03T12:28:11+00:00 Categories: Earth, Explainer ![A large iceberg floating in Greenland waters](https://www.argo.net/wp-content/uploads/2026/09/argo-wave21-53584-pexels-33614456.jpg) An iceberg is a piece of freshwater ice that has broken from a glacier or ice shelf and now floats in open water. Most of its mass sits below the surface because ice is only slightly less dense than seawater. NOAA's [iceberg classification](https://oceanservice.noaa.gov/facts/iceberg.html) sets minimum dimensions for the term. A qualifying piece rises more than five meters above sea level, covers at least 500 square meters and is typically 30 to 50 meters thick. Smaller fragments receive names such as bergy bits and growlers. Their reduced size does not make them harmless, since a low piece can be difficult to detect from a ship in rough seas or poor visibility. ## Icebergs begin on land The character of newly calved ice depends on its source. A floating ice shelf commonly releases broad tabular bergs, while a glacier flowing through a narrow fjord may produce irregular blocks. Fractures already present in the parent ice influence how the berg breaks after entering the sea. Snow accumulates on a glacier and compresses into ice over time. Gravity carries the glacier toward the coast. When its floating front fractures, the detached block begins an independent journey. The break is called **calving**. It occurs along cracks opened by stress or ocean swell. Melting can widen those fractures. A single event may release a narrow tower from a glacier face or a tabular slab from an ice shelf. Because glacial ice forms from snowfall, an iceberg consists of freshwater even when it floats in the ocean. Air bubbles and compressed layers preserve clues about the ice's history before calving. ## Most of an iceberg stays underwater The familiar fraction is only an approximation because ice density varies with trapped air and seawater density changes with temperature or salinity. Mariners therefore cannot infer the underwater outline from the visible peak. **Submerged ice** may extend beyond the portion seen above the surface. Floating ice displaces a mass of seawater equal to its own mass. Since freshwater ice is less dense than seawater, enough volume remains submerged to support the smaller portion visible above. The familiar claim that only one-tenth is visible is a useful approximation, not a fixed rule. Ice density and trapped air change the fraction. Seawater salinity also contributes. Snow piled on top can also affect the profile. **Keel depth** creates the navigation hazard. A broad submerged projection can extend away from the visible outline. Wind acts mainly on the exposed sail while current pushes the much larger underwater body. The National Snow and Ice Data Center explains the physics and terminology of [floating icebergs](https://nsidc.org/learn/cryosphere-glossary/iceberg). Buoyancy keeps the berg afloat until melting and breakup reduce it to smaller fragments. ## Shape records fracture and melting A **tabular iceberg** has steep sides and a relatively flat top, often reflecting its origin in an ice shelf. Non-tabular forms include domed or wedge-shaped bergs. Pinnacles can remain after uneven melting removes weaker ice. Shape changes continuously. Waves undercut the waterline and warm air melts the exposed surface. Meltwater can enter cracks and force them wider when it freezes or when pressure builds. An iceberg may roll when melting shifts its center of mass. The new orientation can expose smooth blue ice that previously sat underwater. Rolling also sends waves outward and creates danger for nearby vessels. Color provides clues without serving as a complete diagnosis. Dense bubble-poor ice can appear blue because longer red wavelengths are absorbed. Sediment incorporated near a glacier bed may leave dark bands. The [NASA Earth Observatory](https://earthobservatory.nasa.gov/topic/ice-snow) uses satellite images to document ice shelves and major calving events. Repeated views show changes that are impossible to judge from one dramatic photograph. ## Icebergs drift with currents Wind pushes the exposed part while currents act on the much larger submerged body. The resulting track reflects both forces, along with the berg's changing shape. Grounding in shallow water can halt movement temporarily before melting or a higher tide frees the ice. Ocean current usually exerts the largest force because most of the iceberg is submerged. Wind can still alter speed or orientation through the exposed part. Waves and sea ice add short-term motion. In the North Atlantic, bergs calved from Greenland may move through Baffin Bay and the Labrador Sea. Some reach shipping lanes near Newfoundland before melting in warmer water. The [International Ice Patrol](https://www.navcen.uscg.gov/international-ice-patrol) monitors the North Atlantic iceberg limit during the season. Aircraft and satellite data support warnings for transatlantic shipping. Vessel reports add direct observations. Drift forecasts combine position with currents and wind. Expected deterioration is also considered. Uncertainty grows as a berg breaks apart because daughter pieces may follow slightly different paths. ## Antarctic icebergs receive names The U.S. National Ice Center tracks large Antarctic icebergs using visible imagery, infrared sensors and **synthetic aperture radar**. Radar can observe through cloud and during polar darkness. Names begin with a letter for the quadrant where the berg was first sighted, followed by a sequence number. A fragment that calves from a named iceberg receives an additional letter. The center's [Antarctic tracking program](https://usicecenter.gov/Resources/AntarcticIcebergs) has maintained records since 1978. Its current expansion includes icebergs of at least 20 square nautical miles as imagery allows smaller features to be followed consistently. Weekly products give locations and dimensions for named bergs. The catalog supports navigation and scientific observation, although countless smaller pieces remain outside the naming threshold. **Tracking criteria** are operational definitions. They do not change the physical status of a smaller floating block; they identify which bergs enter a particular monitoring system. ## Icebergs melt from every side Warm water can undercut the iceberg below the waterline, leaving an overhang or shifting its center of mass. Waves open fractures near the surface. When the balance changes enough, a berg may roll with little warning, exposing ice that was previously submerged. Warm water transfers heat to the submerged ice. Waves erode the waterline. Air temperature plus sunlight affect the exposed surface. Rain can speed surface melt. Melting is uneven because water temperature varies with depth. Turbulence refreshes warm water against the ice. A cold surface layer may slow loss near the top while warmer water cuts the keel. Cracks divide a large berg into smaller pieces that have more surface area relative to their volume. Their melt can accelerate and low fragments become harder to see. Icebergs add freshwater locally as they decay. They can carry mineral particles into the ocean and influence nearby mixing, but the effect depends on size and location. ## Icebergs and sea-level rise differ A floating iceberg already displaces water. Its eventual melting produces little direct change in sea level, following the same principle as ice melting in a full glass. The land ice that supplied the iceberg is different. When a glacier loses mass from land to the ocean, it adds water that was not previously part of the sea. Ice-shelf loss can also remove support that slows grounded glaciers behind it. The [NASA ice-sheet record](https://climate.nasa.gov/vital-signs/ice-sheets/) tracks changes in Greenland and Antarctica. A spectacular calving event may be part of natural glacier flow, so scientists evaluate long-term mass balance before assigning its climate significance. **Mass balance** compares ice gained through snowfall with losses from melting and discharge. It provides a stronger measure of an ice sheet's contribution to sea-level change than iceberg count alone. Icebergs are therefore both ordinary products of glacier motion and valuable signs of change. Their dimensions determine the name, their hidden keels create the maritime risk and their origins connect each drifting block to the larger frozen landscape on land. **Related reading:** [why icebergs have different colors](https://www.argo.net/why-are-icebergs-different-colors/) and [the frozen parts of Earth](https://www.argo.net/what-is-the-cryosphere/). **Related reading:** [why icebergs have different colors](https://www.argo.net/why-are-icebergs-different-colors/) and [the frozen parts of Earth](https://www.argo.net/what-is-the-cryosphere/). **Explore this topic:** [What Is the Law of the Sea?](https://www.argo.net/what-is-the-law-of-the-sea/) and [How Do Oil Spills Affect Marine Life?](https://www.argo.net/how-do-oil-spills-affect-marine-life/).