An iceberg is a large piece of freshwater ice that has broken away, or “calved,” from a glacier or an ice shelf and is floating freely in open water. These massive structures are composed of compressed snow that can be thousands of years old. Icebergs vary dramatically in size and shape, from small chunks known as “growlers” to enormous, flat-topped tabular icebergs. The hidden nature of the iceberg has long posed a significant hazard to maritime travel, centering curiosity on the vast, unseen portion that lies below the ocean’s surface.
The 90/10 Ratio
The widely accepted general rule is that approximately 90% of an iceberg’s total volume is submerged beneath the water line. This leaves only about 10% visible above the surface. This phenomenon is the origin of the popular phrase “the tip of the iceberg,” referring to a small, obvious part of a much larger, concealed issue.
The hidden mass makes icebergs dangerous because their underwater contours are nearly impossible to judge from the surface. An iceberg that appears small on the horizon is actually concealing a massive structure that can extend hundreds of feet below the waves. This consistent ratio holds true regardless of the iceberg’s overall size.
Principles of Buoyancy and Density
The reason for this extreme ratio lies in the physical principles of buoyancy and density. Icebergs float because freshwater ice is less dense than the surrounding seawater. Water expands when it freezes, creating an open crystalline structure that spaces the water molecules farther apart than they are in liquid form. This structure traps pockets of air, resulting in ice that has a lower density, typically around 917 kilograms per cubic meter, compared to seawater’s density of about 1,025 kilograms per cubic meter.
A floating object is governed by Archimedes’ Principle, which states that the upward buoyant force acting on it is equal to the weight of the fluid that the object displaces. For an iceberg to remain stable, the weight of the water it pushes aside must exactly balance the iceberg’s own weight. Since the iceberg is less dense than the seawater, a large volume of ice is needed to displace the necessary weight of water.
The submerged volume is calculated as a direct ratio of the two densities. Because the density of ice is approximately 90% of the density of seawater, 90% of the iceberg’s total volume must be submerged to achieve the required balance. The remaining 10% of the volume projects above the water line.
Variables That Change the Calculation
While the 90/10 ratio is a reliable average, the precise fraction of an iceberg above water is not a fixed constant. The density of the surrounding water is the primary variable that shifts the calculation. Icebergs float higher in water with greater salinity because dissolved salts increase the water’s density, boosting the buoyant force.
Conversely, an iceberg floating in less salty water, such as near a river mouth or a melting ice sheet, will sit lower. Water temperature also contributes to buoyancy, as colder water is generally denser than warmer water, slightly affecting the lift provided.
The internal structure of the iceberg introduces another variable, as the density of the ice is not uniform. Newer ice, which is more porous and contains trapped air bubbles, is less dense and floats slightly higher. In contrast, older, highly compressed glacial ice (sometimes called blue ice) is denser because pressure has squeezed out most of the air, causing it to sit lower.