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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.

Above the water About one-ninthBelow the water About eight-ninths
The famous ratio, and the reason for it.

  1. Step 1 of 5

    Water is strange when it freezes.

    Most liquids freeze Shrink — solid sinksWater freezes Expands — ice floats

    Almost everything shrinks on freezing. Water expands, because its molecules lock into an open hexagonal lattice with gaps in it.

  2. Step 2 of 5

    So ice is lighter than water — barely.

    Seawaterdensity 1.025Icedensity 0.917

    Ice is about 92 percent as dense as seawater. Lighter, but only just, which is exactly why so little of it shows.

  3. Step 3 of 5

    A floating object sinks until it displaces its own weight.

    A floating icebergIts weightWeight of water displaced — equal

    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.

  4. Step 4 of 5

    Do the sum and you get one-ninth.

    1 Ice is 92% asdense2=So 92% must besubmerged3 About 1/9 shows

    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.

  5. Step 5 of 5

    That is what makes them dangerous.

    What you can see A far larger mass,spreading unseen below

    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.

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