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Nature

Why do icebergs float in the ocean?

Short answer

Water expands a little when it freezes. The same weight of water takes up more space as ice. For the same size, ice is lighter than liquid water, which means it has a lower density. That is why ice floats in seawater.

The left diagram shows weight pulling an iceberg down and buoyancy pushing it up. On the right, nine of ten equal-volume blocks show the estimate of how much iceberg volume is underwater.
The left diagram shows buoyancy balancing weight. The right uses 10 equal-volume blocks to show about nine-tenths. The iceberg sketch does not show the volume ratio or the height above water.

How it works

As an iceberg sinks into seawater, the water pushes upward on it. This upward force is called buoyancy. The iceberg floats when it has pushed aside seawater that weighs as much as the iceberg.

Based on typical ice and seawater densities, about nine-tenths of an iceberg’s volume—the space it takes up—is underwater. That does not mean nine-tenths of its height is underwater. Its shape changes the height and outline we see above the surface.

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