The phrase “tip of the iceberg” is a common metaphor used to describe a small, visible part of a much larger, hidden problem. This popular image is rooted in a fundamental scientific reality: the vast majority of an iceberg’s volume is submerged beneath the ocean surface, creating a hazard that is far more massive and dangerous than its visible peak suggests. The precise measurement of this hidden mass is determined by the laws of physics, specifically the interplay between the densities of ice and the surrounding seawater.
The Specific Ratio of Submergence
The standard rule of thumb taught in navigation and science is that approximately $90\%$ of an iceberg’s total volume is hidden underwater. In practical terms, this means that for every one unit of ice seen protruding from the sea, roughly eight to nine units of ice extend deep below the surface.
When scientists use typical density values for glacial ice and seawater, the calculation often yields a submerged volume of about $89\%$. For instance, if the density of glacial ice is $917 \text{ kg/m}^3$ and the density of seawater is $1,025 \text{ kg/m}^3$, the exact submerged fraction is $917/1025$, or $89.5\%$.
The Physics Governing Ice Flotation
The percentage of an iceberg that remains below the water is a direct consequence of Archimedes’ Principle, which governs buoyancy. This principle states that the upward buoyant force exerted on a floating object is equal to the weight of the fluid that the object displaces. For the iceberg to float in equilibrium, its total weight must be balanced by the buoyant force of the displaced seawater.
The specific ratio of submergence is determined by the density relationship between the ice and the water. The fraction of a floating object’s volume that is submerged is precisely the ratio of the object’s density to the fluid’s density. Since ice is slightly less dense than seawater, it floats low in the water, displacing a volume of water that weighs the same as the entire iceberg.
Ice is less dense than liquid water, a unique property among substances, due to its molecular structure. When water freezes, the molecules form a rigid, open, hexagonal lattice structure held together by hydrogen bonds. This crystalline arrangement maximizes the distance between molecules, creating empty space. Because the same mass of water occupies a larger volume as ice, its overall density decreases, allowing it to float.
Why the Percentage Varies in Nature
While $89-90\%$ is the accepted average, the exact percentage of an iceberg submerged can change based on the surrounding environment and the ice itself. One of the most significant variables is the salinity of the water. Saltwater is denser than freshwater, and according to Archimedes’ Principle, a denser fluid provides a greater buoyant force.
Icebergs float slightly higher in saltier, denser seawater than they would in less dense freshwater. The temperature of the water also plays a role, as water density peaks at about four degrees Celsius and decreases as it cools toward the freezing point. Furthermore, the density of the iceberg itself can vary. Glacial ice is highly compressed and contains fewer air bubbles than common freshwater ice, which can slightly increase its density and cause it to sit lower in the water.