What Exactly Is an Iceberg?

An iceberg is a piece of freshwater ice that has broken away (calved) from a glacier or an ice shelf and is now floating in the sea. Because freshwater ice is less dense than seawater, a typical iceberg rides low, with roughly 90% of its mass hidden below the surface. The visible part is called the sail, and the hidden portion is the keel.

Although they are made of frozen freshwater, icebergs often carry trace sediments and even pockets of compressed ancient air. Those inclusions can change an iceberg’s color, texture, and how it melts and rolls as it ages.

From Snowflake to Iceberg: The Long Road to Calving

Icebergs begin as snow. Over years to centuries, snowfall in polar regions compacts into firn and then into dense glacial ice. Under gravity, that ice slowly flows downslope. Where glaciers meet the ocean, they can either end in a cliff (a tidewater glacier) or spread out to form a floating ice shelf. Flexing, tides, winds, waves, and internal fractures eventually snap off pieces: the birth of an iceberg.

Two dominant calving styles

• Greenland and the Arctic (tidewater glaciers): Produce tall, irregular, non-tabular bergs. Calving is often dramatic, triggered by undercutting from warm water, surface meltwater driving cracks (hydrofracture), and tidal flexing.
• Antarctica (ice shelves): Generate vast, flat-topped tabular bergs when rifts in the shelf propagate and a slab detaches. These can be city-sized or larger.

Calving rates vary seasonally and from glacier to glacier, with peaks commonly in late spring and summer when surface melt and ocean warmth undermine ice fronts.

The Anatomy of an Iceberg

Most of an iceberg is invisible below the waterline. Because glacier ice has a density of about 0.917 g/cm³ and seawater is about 1.025 g/cm³, roughly 89–90% of the ice volume sits underwater. That means a 30 m high sail may conceal a keel several hundred meters long and on the order of 270 m deep, depending on the iceberg’s thickness and shape.

Size and shape classifications

Mariners and ice services use standard categories:

• By size (approximate):
  • Growler: less than 1 m high and under 5 m long
  • Bergy bit: 1–5 m high, 5–15 m long
  • Small: 5–15 m high, 15–60 m long
  • Medium: 16–45 m high, 61–120 m long
  • Large: 46–75 m high, 121–200 m long
  • Very large: more than 75 m high or more than 200 m long
• By shape: tabular (flat-topped), blocky, wedge, dome, pinnacle, and dry-dock (arch or U-shaped).

Colors and sounds

• Blue ice: Dense, bubble-poor ice scatters less light and appears deep blue.
• Green or “jade” bergs: Often contain marine ice (frozen seawater) and sometimes iron-rich sediments, giving a green hue.
• Black or clear faces: Freshly fractured, bubble-free surfaces can look glassy and dark.
• “Bergy seltzer”: The faint fizzing sound is air bubbles—trapped in glacier ice for centuries—popping as the ice melts.

A Short, Eventful Life: Weathering, Rolling, and Melting

Once at sea, icebergs begin to change rapidly. Waves and warm water erode their flanks; rain and sun carve channels and gullies; and undercutting makes them unstable. When the center of mass shifts, an iceberg can roll over, exposing new faces and altering melt rates. This process can repeat many times.

Melt rates vary from centimeters per day in frigid, slow-moving waters to meters per day in warmer, fast-flowing currents. Sides typically melt more slowly than the base, where turbulent water flows supply heat efficiently. As bergs shrink, they fragment into “bergy bits” and “growlers.” Small bergs might last weeks to months; colossal tabular bergs can survive for years and drift across ocean basins.

Why Icebergs Wander: The Physics of Drift and Migration

Icebergs may look like aimless drifters, but their paths are shaped by a balance of forces:

• Ocean currents: The main driver. An iceberg’s deep keel feels the flow throughout the upper ocean, often accounting for most of its motion.
• Wind: Pushes on the small exposed sail; typically contributes a smaller but important fraction of the drift, especially for low-draft bergs.
• Waves: Stokes drift and wave radiation forces can nudge bergs along and help erode their edges.
• Coriolis effect: Deflects motion to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Tides and eddies: Can trap or accelerate bergs, sometimes spinning them in place.

Global “iceberg highways”

• North Atlantic: Bergs from western Greenland fjords enter Baffin Bay and Davis Strait, then ride the cold Labrador Current toward Newfoundland, forming the famous “Iceberg Alley.” After the 1912 Titanic disaster, the International Ice Patrol began charting an annual iceberg limit to warn ships.
• Southern Ocean: Gigantic tabular bergs calved from the Ross or Filchner–Ronne Ice Shelves enter the Antarctic Coastal Current, can loop into the Weddell Gyre, and sometimes escape into the Antarctic Circumpolar Current. Some, like A-68 (calved in 2017), drifted north toward South Georgia Island, freshening and fertilizing waters along the way.

Typical drift speeds are on the order of a few tens of kilometers per week, but can be faster in strong currents. Many bergs ground on shallow banks when their keels scrape bottom, temporarily parking until they melt or currents and tides refloat them.

Grounding and Seafloor Scars

Grounded icebergs act like slow-motion bulldozers. Their keels can scour ploughmarks across continental shelves, sometimes hundreds of meters below the surface. These gouges crisscross polar seabeds and record past iceberg pathways. Grounding also controls the fate of the iceberg itself: stalled bergs can persist for seasons or years until enough melting or a strong tide sets them free.

Record-Breaking Giants

• B-15 (2000): One of the largest recorded, calved from the Ross Ice Shelf, measuring roughly 11,000 km² at birth.
• A-68 (2017): About 5,800 km² initially, it drifted out of the Weddell Sea and fragmented near South Georgia by 2020–2021.
• A-76 (2021): Roughly 4,300 km² on calving from the Filchner–Ronne Ice Shelf.

Even “small” icebergs are anything but small at human scale: a medium iceberg can rival a city block in size and tower higher than a ten-story building.

Engines of Change: Ecological and Oceanographic Roles

• Freshwater pulses: As icebergs melt, they freshen the ocean surface, which can alter stratification and mixing locally.
• Fertilization: Ice often carries iron-rich dust and sediments. As it melts, those nutrients can spark phytoplankton blooms trailing in the iceberg’s wake, supporting krill, fish, seabirds, and whales.
• Floating habitat: Penguins, seals, and seabirds rest on iceberg surfaces; algae and other organisms colonize the underside.

The net effect on regional ecosystems depends on the timing, size, and number of icebergs, as well as ambient ocean conditions.

Hazards, Management, and Myths

Because most of their mass is invisible, icebergs pose serious risks to ships and offshore infrastructure. The North Atlantic’s International Ice Patrol uses satellites, aircraft, and ocean models to map bergs and set seasonal limits for safe passage. Offshore energy operations near Newfoundland routinely conduct iceberg management—detecting, forecasting, and sometimes towing small bergs with ropes or nets to redirect them away from platforms.

Occasionally, proposals surface to tow massive Antarctic icebergs to water-stressed regions. While an intriguing idea, the practical hurdles—melting losses, storms, towing power, and logistics—are enormous, and to date such schemes remain largely conceptual.

Icebergs as Climate Messengers

Iceberg activity reflects both calving rates and ocean warmth. In Greenland, retreat of marine-terminating glaciers and warmer Atlantic waters have in many places increased calving and iceberg production. Around Antarctica, the story varies by region: some ice shelves have thinned or collapsed in response to ocean and atmospheric warming, increasing the calving of large tabular bergs; elsewhere, iceberg activity remains within historical ranges. What’s clear is that icebergs integrate signals from atmosphere, ocean, and ice dynamics, making them valuable indicators of polar change.

How We Track Icebergs Today

• Satellites: Synthetic Aperture Radar (SAR) sees through clouds and darkness to map berg outlines. Optical sensors provide color and context in clear weather.
• Aircraft and ships: Visual surveys verify positions and sizes, especially near shipping lanes.
• Models and buoys: Drift models combine winds, currents, and thermodynamics to forecast trajectories and melt. Occasionally, GPS beacons are placed on larger bergs for research.

In the Southern Ocean, U.S. and international ice centers assign identifiers to large Antarctic bergs (for example, “A-68”), where the letter denotes the quadrant of origin.

Surprising Facts About Icebergs

• Only about one-tenth of an iceberg sits above water—the rest lies hidden beneath the waves.
• Flat-topped, table-like icebergs mainly come from Antarctica’s ice shelves; jagged, towering bergs more often originate from Greenland’s tidewater glaciers.
• Iceberg keels can gouge the seafloor hundreds of meters deep along continental shelves.
• Colors tell a story: blue ice is bubble-poor and dense; green can signal marine ice or iron-rich sediments.
• Bergs often roll multiple times during their lifetimes as undercutting and melting shift their centers of mass.
• International monitoring of icebergs in the North Atlantic began after the Titanic disaster and continues each spring.

A Typical Iceberg Timeline

1. Snow accumulates and compacts into glacier ice over decades to centuries.
2. The glacier flows to the coast and begins to float or form a terminus cliff.
3. Cracks and rifts grow; tides, waves, and meltwater trigger calving.
4. The newborn iceberg stabilizes, then begins drifting with currents and winds.
5. Melting, undercutting, and waves reshape the berg; rollovers expose new faces.
6. Grounding on a bank may pause its journey; refloating sends it onward.
7. Fragmentation produces bergy bits and growlers that eventually melt away.