What Are Icebergs?

Icebergs are large pieces of freshwater glacial ice that have broken away from a glacier, ice shelf, or ice sheet and float in the ocean or a lake. Glaciologists reserve the term for floating fragments larger than about 5 meters across, as noted in the glacier quick facts maintained by the National Snow and Ice Data Center; smaller pieces are called bergy bits, and smaller ones still are called growlers. An iceberg is distinct from sea ice, which forms by the freezing of seawater and rarely exceeds a few meters in thickness. Because glacial ice has a density near 917 kilograms per cubic meter against roughly 1,025 for seawater, about nine tenths of an iceberg's volume sits below the waterline, which is the origin of its reputation as a navigation hazard.

Icebergs from Antarctic ice shelves are commonly tabular, with flat tops and near-vertical sides, and can exceed a thousand square kilometers in area. Those calved from Greenland's tidewater outlet glaciers are usually irregular, pinnacled, or dry-dock in shape and far smaller, though they often draw several hundred meters of water.

Calving and Drift

Calving is the detachment of ice at a glacier's terminus, driven by buoyant torque, tidal flexure, crevasse propagation, and thinning near the grounding line. Large tabular bergs typically release along pre-existing rifts that may take years to propagate across an ice shelf. Once afloat, an iceberg drifts under the combined action of ocean currents, wind drag on the exposed sail, Coriolis deflection, and sea ice pressure, with currents usually dominating for deep-keeled bergs. Deterioration proceeds through wave erosion at the waterline, forced convection melting of the keel in warmer water, and the fracture of overhanging edges, and a berg that loses mass unevenly can capsize suddenly, releasing energy detectable on seismometers hundreds of kilometers away.

Detection and Tracking

Operational monitoring combines shipborne and satellite observation. The U.S. National Ice Center names and tracks Antarctic icebergs larger than about 10 nautical miles along their major axis, assigning designations such as A23A or B15 from the Antarctic quadrant of origin, and publishes position updates in its Antarctic iceberg data products. In the North Atlantic, the International Ice Patrol operated by the U.S. Coast Guard has issued iceberg limit bulletins for the Grand Banks shipping lanes since its establishment in 1914, two years after the loss of the Titanic. Synthetic aperture radar from satellites such as Sentinel-1 provides all-weather, day-and-night detection, while marine X-band radar remains the primary shipboard sensor. Growlers are the hardest targets because a small radar cross section combined with wave clutter can hide them entirely in rough seas.

Ocean and Ecosystem Effects

Melting icebergs deliver freshwater and terrigenous sediment far from the coast, stratifying the surface layer and altering local circulation. The iron and other micronutrients released from entrained rock dust stimulate phytoplankton blooms in the iron-limited waters of the Southern Ocean, so drifting bergs leave measurable chlorophyll wakes. Deep-keeled bergs also scour the continental shelf seabed, reworking benthic habitat and posing a burial risk to seafloor pipelines and cables.

Applications

Iceberg research and monitoring have applications in a range of fields, including:

• Marine navigation and shipping route planning
• Offshore oil, gas, and wind installation design and iceberg management
• Submarine cable and pipeline route surveys
• Satellite radar remote sensing and automated target detection
• Ocean and climate modeling of freshwater flux
• Polar ecosystem and biogeochemistry studies