What Is an Iceberg? Floating Ice Mountains Explained

An iceberg is a large chunk of freshwater ice that has broken away from a glacier, ice shelf, or ice sheet and is floating freely in the ocean. The process of ice breaking off into water is called calving. Icebergs range in size from small growlers the size of a car to enormous tabular bergs the size of small countries.

Introduction

The saying "tip of the iceberg" reflects a physical truth: because ice is less dense than seawater, approximately 90 percent of an iceberg's mass lies below the water's surface. Only about 10 percent is visible above the waterline, which is why icebergs have been a deadly hazard to navigation since humans began sailing in polar waters.

Definition & Key Characteristics

• Definition: A large piece of freshwater ice floating in the ocean after calving from a glacier or ice shelf
• Composition: Freshwater ice formed from compacted snow, not frozen seawater
• Buoyancy: About 90% of mass is underwater; only ~10% is visible above the surface
• Size Classification: From growlers (<5 m) to very large bergs (over 200 m above water)

Icebergs are fundamentally different from sea ice, which forms when ocean water freezes. Icebergs are made of freshwater ice that originated as snowfall on land, was compacted into glacial ice over centuries, and then calved into the ocean. They can be thousands of years old — the ice in an iceberg calved from the Greenland Ice Sheet may have fallen as snow during the Stone Age.

The 90 percent underwater figure comes from the density ratio between ice and seawater. Glacial ice has a density of about 917 kg/m³, while seawater has a density of about 1,025 kg/m³. The ratio means that approximately 10 percent of the ice floats above the waterline. The exact percentage varies slightly depending on the air bubble content of the ice and the salinity of the surrounding water.

How Icebergs Form

• Glacier Flow: Glaciers flow toward the coast under their own weight
• Calving: The glacier terminus breaks off where it meets the ocean, dropping ice into the water
• Ice Shelf Breakup: Large tabular icebergs can break from floating ice shelves
• Drift: Ocean currents carry icebergs far from their origin, sometimes into shipping lanes

Icebergs are born when tidewater glaciers — glaciers that terminate in the ocean — discharge ice into the sea. The glacier front extends over the water, and the buoyancy of the projecting ice, combined with wave action, tidal flexing, and internal stresses, causes chunks to break off and crash into the water in dramatic calving events.

The largest icebergs come from ice shelves — thick platforms of floating ice attached to the coastline of Antarctica. Tabular icebergs, with flat tops and steep sides, can be enormous. In 2000, iceberg B-15 calved from the Ross Ice Shelf with an area of about 11,000 square kilometers — larger than Jamaica. Such mega-bergs can persist for years as they slowly drift and break apart.

Types & Sizes

• Tabular: Flat-topped with steep sides, calved from ice shelves; can be enormous
• Pinnacle: Tall, pointed bergs with one or more spires
• Dome: Rounded top, smooth appearance
• Growler: Small iceberg less than 5 m above water — named for the noise they make
• Bergy Bit: Medium-small pieces 1-5 m above water

Tabular icebergs are the largest and are distinctive to the Southern Ocean around Antarctica, where they calve from the continent's vast ice shelves. They can have surface areas of thousands of square kilometers and stand 30 to 60 meters above the waterline. Their flat tops reflect their ice shelf origin — essentially floating sections of the continental ice sheet.

Northern Hemisphere icebergs, mostly from Greenland, tend to be smaller and more irregular in shape, with pointed pinnacles, rounded domes, and blocky forms. Despite being smaller than Antarctic tabular bergs, North Atlantic icebergs are more dangerous to shipping because they drift into busy shipping lanes. Growlers — small, dark chunks of ice barely visible above the waterline — are particularly hazardous because they are hard to detect by radar.

The Titanic & Iceberg Hazards

• RMS Titanic: Sank on April 15, 1912, after striking a North Atlantic iceberg; 1,500+ died
• International Ice Patrol: Founded in 1914 to monitor icebergs in North Atlantic shipping lanes
• Detection: Modern ships use radar, satellite imagery, and patrol reports to avoid icebergs

The sinking of the Titanic on April 15, 1912, remains history's most famous iceberg disaster. The "unsinkable" liner struck a North Atlantic iceberg on her maiden voyage, tearing a series of gashes in her hull that flooded six compartments. The ship sank in under three hours, killing over 1,500 of the roughly 2,200 people aboard.

The disaster led to the creation of the International Ice Patrol (IIP) in 1914, which monitors icebergs in the North Atlantic shipping lanes and broadcasts warnings to vessels. Today, the IIP uses satellite imagery, aerial reconnaissance, and GPS-tracked buoys to track approximately 1,000 icebergs per year that drift south of 48°N latitude. Since the IIP's founding, no vessel heeding its warnings has been lost to an iceberg.

Icebergs & Climate Change

• Increased Calving: Warming temperatures are accelerating glacier calving and ice shelf collapse
• Sea Level Rise: While floating icebergs don't raise sea levels directly, the accelerated glacier flow behind them does
• Freshwater Input: Melting icebergs add fresh water to the ocean, potentially affecting circulation

As global temperatures rise, glaciers and ice sheets are calving icebergs at accelerating rates. Greenland's ice discharge has roughly doubled since 2000, and major Antarctic ice shelves like Larsen B (which collapsed in 2002) have disintegrated, releasing armadas of icebergs. While a floating iceberg itself does not raise sea levels when it melts (it has already displaced its weight in water), the loss of ice shelves removes the "cork" that slows the flow of land-based glaciers, accelerating sea-level rise.

Melting icebergs also inject significant quantities of fresh water into the ocean, which can affect ocean circulation patterns. Fresh water is less dense than salt water, and large inputs of glacial meltwater in the North Atlantic could potentially slow the Atlantic Meridional Overturning Circulation (AMOC), which distributes heat from the tropics to northern Europe.

Key Facts

• An iceberg is a large piece of freshwater ice that has calved from a glacier and floats in the ocean.
• About 90% of an iceberg's mass is below the waterline.
• Iceberg B-15, calved from Antarctica in 2000, was larger than Jamaica.
• The sinking of the Titanic in 1912 led to the creation of the International Ice Patrol.
• Icebergs are freshwater ice from compacted snow — not frozen seawater.

Fun Facts

• Some icebergs have a striking blue color caused by glacial ice absorbing red light.
• The ice in a Greenland iceberg may have originally fallen as snow thousands of years ago.
• Antarctic icebergs can drift north to latitudes as low as 26°S — near Rio de Janeiro.
• Iceberg A-76, which calved from Antarctica in 2021, was about 170 km long — one of the largest ever recorded.

Final Thoughts

Icebergs are floating monuments to the power of glaciation — massive chunks of ancient ice set adrift in the world's oceans. They have shaped human history through maritime disasters, inspired the phrase "tip of the iceberg," and now serve as visible indicators of accelerating ice loss in a warming world. Whether a tiny growler or a tabular berg the size of a country, each iceberg tells a story of ice, time, and change.