Sea Ice & Icebergs Explained
Icebergs are floating pieces of land-based glacier ice that break into the ocean through calving. They can tower above the water, hide most of their mass below the surface, drift for months or years, and create major hazards for ships and polar coastlines.
What Are Icebergs?
Icebergs are large floating masses of freshwater ice that have broken away from glaciers, ice shelves or ice sheets. Unlike
sea ice, which forms when ocean water freezes, icebergs begin as snow that is compressed into glacier ice on land.
Once an iceberg breaks off into the ocean, winds, waves and currents carry it away from its source. Some melt quickly near their birthplace, while others drift far into shipping lanes and open ocean.
How Do Icebergs Form?
Icebergs form through glacier calving. This happens when cracks grow through the front of a glacier or ice shelf until a block of ice breaks away and falls or slides into the sea.
Calving can release small ice chunks, building-sized blocks or enormous tabular icebergs many kilometers long. The process is influenced by glacier flow, ocean melting, surface meltwater, fractures, tides, waves and ice-shelf stability.
- Snow accumulation: snow compresses into glacier ice over time.
- Glacier flow: ice slowly moves from land toward the coast.
- Fracturing: cracks spread through the glacier front or ice shelf.
- Calving: ice breaks away into the ocean.
- Drift and melt: the iceberg moves with currents and gradually melts.
Why Most of an Iceberg Is Underwater
Icebergs float because ice is less dense than seawater. But only a small portion rises above the surface. Most of an iceberg’s mass remains hidden underwater, which is why the phrase “tip of the iceberg” exists.
This hidden underwater mass makes icebergs dangerous. A ship may see the visible top while the larger submerged body extends much farther below and around it.
Types of Icebergs
Icebergs are often described by shape. Their appearance depends on where they formed, how they fractured, how long they have been drifting and how waves and melting reshape them.
- Tabular icebergs: flat-topped slabs, often from ice shelves.
- Non-tabular icebergs: irregular blocks with domes, peaks, wedges or drydock shapes.
- Growlers: small iceberg fragments that sit low in the water.
- Bergy bits: larger than growlers but smaller than full icebergs.
- Blue icebergs: dense, old ice that absorbs red light and appears deep blue.
How Icebergs Drift
Icebergs drift with a combination of ocean currents, wind, waves and sea-ice conditions. Because much of an iceberg lies below the surface, deeper currents can strongly influence its path.
This connects icebergs directly to
ocean currents and circulation.
Icebergs can carry freshwater into the ocean, affect local stratification and sometimes transport sediment and nutrients as they melt.
Icebergs vs. Sea Ice
Icebergs and sea ice are both floating ice, but they form differently and matter in different ways.
- Icebergs: freshwater glacier ice that breaks from land-based ice.
- Sea ice: frozen seawater that forms directly on the ocean surface.
- Sea-level effect: melting floating icebergs does not directly raise sea level much, but the glacier mass loss that produces them is part of land-ice loss.
- Hazards: icebergs are major navigation hazards because of their hidden underwater mass.
Why Are Icebergs Dangerous?
Icebergs can damage or sink ships, threaten offshore platforms and complicate polar operations. Their danger comes from their size, hardness, hidden underwater volume and unpredictable drift.
Smaller fragments such as growlers and bergy bits can be especially hazardous because they are difficult to detect in rough seas, fog or darkness.
Icebergs and Climate Change
Icebergs are natural, but changing ocean and air temperatures can affect how glaciers and
ice shelves
melt, fracture and calve. More calving from an unstable glacier system can indicate rapid ice loss.
Large iceberg breakoffs do not always mean climate change by themselves, but they can be part of a larger pattern involving warming oceans, weakened ice shelves and faster glacier flow.
Related Polar Ice Topics
Icebergs connect to
Sea Ice & Icebergs Explained,
Sea Ice Explained,
Ice Shelves Explained,
Glacier Calving Explained and
Ice Mélange Explained.
FAQ: Icebergs Explained
What is an iceberg?
An iceberg is a floating piece of freshwater glacier ice that has broken away from a glacier, ice shelf or ice sheet into the ocean.
How do icebergs form?
Icebergs form when glacier ice breaks away from a glacier front or ice shelf through a process called calving.
Are icebergs made of saltwater?
No. Icebergs are made of freshwater glacier ice, unlike sea ice, which forms from freezing seawater.
Why is most of an iceberg underwater?
Ice floats because it is less dense than seawater, but most of its mass remains below the surface. Only the upper portion is visible.
Do melting icebergs raise sea level?
Melting floating icebergs do not directly raise sea level much because they are already floating, but increased iceberg discharge can signal land-ice loss from glaciers or ice sheets.
Why are icebergs dangerous to ships?
Icebergs are dangerous because most of their mass is hidden underwater, they are extremely hard, and they can drift unpredictably with currents, wind and waves.
