One-ninth showing
Why Do Icebergs Float So Low?
Ice is barely lighter than water, so almost all of an iceberg sits below the surface.
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Step 1 of 5
Water is strange when it freezes.
The same oddity bursts pipes in winter and freezes ponds from the top down. Almost everything shrinks on freezing. Water expands, because its molecules lock into an open hexagonal lattice with gaps in it.
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Step 2 of 5
So ice is lighter than water — barely.
Ice is about 92 percent as dense as seawater. Lighter, but only just, which is exactly why so little of it shows.
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Step 3 of 5
A floating object sinks until it displaces its own weight.
It settles at the depth where the water pushed aside weighs the same as the object. For ice, that means sinking most of the way in.
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Step 4 of 5
Do the sum and you get one-ninth.
Ice at 92 percent of seawater's density must push aside 92 percent of its volume in water. Only the remaining 8 or 9 percent stays above.
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Step 5 of 5
That is what makes them dangerous.
The underwater part is bigger than the visible one and often spreads out sideways. A ship can strike ice it never saw.
Icebergs are frozen fresh water — snow compressed on land — not frozen seawater. Sea ice is a different thing entirely.
The short version
Ice is only slightly less dense than seawater, so an iceberg floats with about eight-ninths hidden below.
Try it yourself
Float an ice cube in a glass and mark the waterline. Almost all of it is under — the same ratio as an iceberg.