Glaciers and icebergs are often confused, though they represent distinct stages in the life cycle of frozen freshwater. While these two forms of ice are intrinsically linked, understanding the differences between these immense structures requires looking closely at where they originate, how they are formed, and their final resting place. The relationship between them is one of parent and product.
Glaciers: Formation and Characteristics
A glacier is a persistent body of dense ice that forms on land and moves slowly under the influence of its own weight and gravity. Glaciers begin with the accumulation of snow in regions where winter snowfall exceeds summer melt, often over many decades or centuries. As new snow layers bury the old, the pressure compresses the snowflakes, squeezing out air and causing the crystals to recrystallize. This process first transforms snow into firn, a dense, granular intermediate stage. Continued compression eventually turns the firn into hard, blue glacial ice, which contains very few trapped air bubbles. Once the ice mass reaches a sufficient thickness, its immense weight causes it to deform and flow, behaving like an extremely slow-moving river of ice. Glaciers are broadly classified by their size and location, ranging from alpine glaciers confined to mountain valleys to vast continental ice sheets like those covering Greenland and Antarctica.
Icebergs: Calving and Classification
An iceberg is defined as a large piece of freshwater ice that has broken off, or “calved,” from a glacier or an ice shelf and is floating freely in open water. Calving is the process that occurs when the terminus of a glacier meets a body of water, and the stress of buoyancy and fracturing causes a block of ice to separate. This fracture is a primary method of mass loss for glaciers that terminate in the ocean, such as tidewater glaciers. Once an iceberg is waterborne, ocean currents and wind dictate its movement until it eventually melts in warmer waters. Only about one-eighth to one-tenth of its total mass is visible above the waterline. The remaining vast majority lies submerged, making the floating ice a significant hazard to navigation. Icebergs are classified based on their shape, such as tabular, which are flat-topped and sheer-sided, or non-tabular, which can be dome-shaped, pinnacled, or blocky.
Key Distinctions Between Land and Sea Ice
The fundamental difference lies in location and origin, as glaciers are exclusively land-based, while icebergs are water-based. A glacier forms in situ through the gradual compression of snow over land, moving by internal flow and basal sliding. Conversely, an iceberg is an ex situ fragment, formed by the fracturing of a glacier and then passively carried by the sea. This distinction is important when considering the effect of melting on global sea levels. Glaciers, as land ice, hold water that was previously stored on a continent, and when they melt, that water flows into the ocean, contributing directly to sea level rise. Icebergs, however, are already floating in the ocean and displacing their own weight in water. Therefore, when an iceberg melts, it does not significantly change the overall volume of the ocean, much like a melting ice cube in a glass of water.