An iceberg is a large mass of freshwater ice that has broken off a glacier or ice shelf and is floating in open water, a process known as calving. These ice structures vary widely in size and shape, from small chunks to massive tabular forms. The true size of an iceberg is obscured because only a small fraction of its total volume is visible above the ocean surface, posing a challenge for maritime navigation. The hidden depth below the waterline is governed by the fundamental principles of physics, specifically the contrast in density between the ice and the surrounding seawater.
The Physics of Buoyancy
The amount of an iceberg that remains submerged is determined by Archimedes’ Principle. This principle states that the upward buoyant force on an object immersed in a fluid equals the weight of the fluid it displaces. For a floating iceberg, this buoyant force must balance the iceberg’s total weight, meaning the submerged fraction is dictated by the ratio of the ice’s density to the water’s density.
Glacial ice, formed from compressed snow, is less dense than the surrounding ocean water. Glacial ice typically has a density of 900 to 917 kilograms per cubic meter, compared to the average density of saltwater, which is around 1,025 kilograms per cubic meter.
The submerged volume fraction is calculated by dividing the density of the ice by the density of the seawater. Using these average values, calculations show that 87% to 89% of the iceberg’s total volume must be underwater to displace a mass of water equal to its own weight. This means that for every part of ice seen above the water, roughly seven to eight parts of its volume are hidden below.
This physical relationship explains why the depth of an iceberg is disproportionate to its height above the water. The commonly cited ratio of roughly nine-tenths submerged is a direct consequence of the density difference between the two substances.
Variables That Change the Submerged Depth
The classic 90/10 ratio is an approximation based on average conditions, and two variables cause the actual submerged depth to fluctuate. The first is the salinity of the ocean water, which directly affects its density. Saltier water is denser and provides a greater buoyant force, causing an iceberg to float slightly higher and reducing its submerged percentage.
In areas near large glacial meltwater plumes, the ocean water is less saline and less dense. This reduction in water density decreases the buoyant force, causing the iceberg to sink slightly lower into the water column. An iceberg floating near a glacier face may thus have a greater submerged depth than one encountered in the highly saline waters of the open North Atlantic.
The second factor is the density of the ice itself, which is not uniform across all icebergs. Icebergs are formed from glacial ice, resulting from thousands of years of compression. This pressure forces out trapped air, creating ice that is denser than common freezer ice.
Older, deeply compressed ice has fewer air bubbles and a higher density, requiring a larger volume to be submerged to achieve equilibrium. Less dense, newer glacial ice with more trapped air will float higher.
Estimating the Underwater Size
Determining the exact depth and volume of the submerged portion of an iceberg is a challenge for scientists and navigators. The most basic method uses empirical formulas to estimate the total draft, or underwater depth, based on the height and shape of the visible part, known as the sail or freeboard. These estimations rely on observed relationships between the visible height and the total depth for various iceberg types.
For more accurate measurements, scientists employ technologies like sonar and multibeam mapping, especially in areas of maritime traffic. These systems use sound waves to scan the iceberg’s keel, or underwater profile, providing a detailed three-dimensional map of the submerged structure. This data helps predict an iceberg’s potential for grounding on the seabed.
On a larger scale, satellite imagery and aerial photography determine the surface area and estimate the total volume and mass of massive icebergs. By applying the known density ratio to the measured above-water dimensions, glaciologists can model the entire volume and track mass changes over time. These models are important for understanding iceberg decay and its contribution to ocean circulation and sea level dynamics.