What if icebergs were made of wood?

An iceberg is a giant chunk of ice floating in the sea.

We only see the top. Most of it is under the water! What if an iceberg were made of wood?

An iceberg is a huge piece of ice that has broken off a glacier or an ice shelf and floats in the sea. From a ship you only see the top. Most of the iceberg is hidden below the waves.

Wood floats too. So what if an iceberg were made of wood? Would it float the same way, sink, or sit higher? In this lab you can swap ice for wood and see.

Ice and wood both float in sea water, but not in the same way. How high a floating object rides depends on one ratio: its average density compared with the liquid's.

This lab places a solid block 1 m long, 0.5 m wide and 0.4 m tall in sea water (1,025 kg/m³). First it is ice (917 kg/m³), then wood (600 kg/m³). Same size, same water, one change: the material.

In Olly's lab, a block of ice floats in the sea.

It floats very low. Only a tiny bit pokes out of the water.

The model's block of ice floats in sea water with 89 percent of it under the surface. Only 4 centimeters of the 40-centimeter block stick out.

Real icebergs behave the same way. The U.S. Geological Survey says ice is about 90 percent as dense as water, so only about 10 percent of an ice cube or an iceberg sits above the water line. The exact amount changes a little, because real ice can trap air bubbles.

The ice block's mass is 917 × 0.2 m³ = 183.4 kg. It settles where the displaced sea water has the same mass: a draft of 183.4 ÷ (1,025 × 0.5) ≈ 0.36 m. The model reports 89% submerged and a freeboard of 0.04 m.

OpenStax shows why: since the floating object's mass equals the mass of fluid it displaces, the fraction submerged is simply ρ_object ÷ ρ_fluid, here 917 ÷ 1,025. USGS gives about 10 percent of ice above the water line, varying because ice can contain air.

Now Olly makes the block out of wood.

Boing! It floats much higher. Lots of it sticks out of the water.

Now change the material to wood, at 600 kilograms per cubic meter. The same-sized block bobs up: only 59 percent of it is under water now, and 17 centimeters stick out above the surface, about four times as much as the ice.

A wooden iceberg would be easy to spot! It is also lighter: 120 kilograms instead of 183. In the model it can even carry about 85 kilograms of cargo before its top reaches the water, while the ice block can take only about 22.

At 600 kg/m³ the block's mass falls to 120.0 kg and its weight to 1,177 N (ice: 1,799 N). Its draft is 0.23 m, so it floats 59% submerged with 0.17 m of freeboard.

The spare buoyancy is the difference between the most water the block can displace (205 kg) and its own mass: 85 kg of cargo for wood, about 22 kg for ice. On the slider's 5 kg step, ice still floats with 20 kg aboard and sinks with 25 kg.

Water pushes up on things that float. A thing sinks in until the push is as big as its weight.

Wood is lighter than ice for its size. So it needs less push, and floats higher.

A floating thing sinks into the water until it has pushed aside its own weight of water. Then the water's upward push, called buoyancy, balances its weight.

Wood is lighter for its size than ice: it is less dense. So a wooden block only needs to push aside a smaller amount of water, and most of it stays above the surface. Our model is simple: a box that stays level, never melts, and has one density all the way through.

Archimedes' principle: the buoyant force equals the weight of the fluid displaced. At equilibrium ρ_fluid·V_sub·g = ρ_object·V·g, so V_sub/V = ρ_object/ρ_fluid. Halve the density ratio and you halve the submerged fraction.

Model limits: a rectangular block that stays upright and level, one uniform density, no melting, no waves and no tipping. Real icebergs have irregular shapes, which the model cannot show. Real wood varies a lot: OpenStax lists densities from about 300 to 900 kg/m³.

Some chunks of ice are as big as a truck. But they barely poke out of the water!

Ships keep watch for hidden ice. Scientists track big icebergs.

Because so much ice hides under the water, small icebergs can be hard to see. NOAA says pieces of ice called growlers can be roughly the size of a truck or a grand piano, yet stick out less than 3 feet above the sea.

To count as an iceberg, NOAA says the ice must rise more than 16 feet above sea level. The U.S. National Ice Center watches ice conditions and names and tracks all Antarctic icebergs. Wood is different: depending on the kind, OpenStax lists its density from about 300 to 900 kilograms per cubic meter, so some woods float high and others float almost as low as ice.

NOAA classifies ice by its height above the sea: icebergs rise more than 16 ft, with thickness of 98 to 164 ft and an area of at least 5,382 ft²; bergy bits stand 3 to 16 ft; growlers, roughly truck- or grand-piano-sized, stand less than 3 ft. The U.S. National Ice Center analyzes and forecasts ice conditions and names and tracks all Antarctic icebergs.

Wood's density range, about 300 to 900 kg/m³ (OpenStax), spans submerged fractions from 29% to 88% in the model's sea water: the densest woods would float nearly as low as ice.