Floating Rocks at Askja EarthCache

1. Fill the measuring jug with water to about half, but preferably up to a marking. Note down the water level in the jug (in mL) - this is amount A.

2. Float the piece of pumice rock in the jug and note down the new water level (in mL) - this is amount B. Because the pumice is now floating, the difference between A and B gives you the amount of water displaced by the pumice and therefore, knowing that the density of water is 1kg/L, the mass of the pumice.

3. Subtract A from B and this gives amount C which is both the volume of water displaced, in milli-litres (mL), and the mass (in grams) of the pumice sample.

4. Next, use your finger to completely submerge the pumice rock in the water in the measuring jug. Be careful to only just submerge it - don't push it so far into the water that your finger is also in the water. Note down the new water level (in mL) - this is amount D. This allows you to calculate the total volume of the pumice sample and further allows you to calculate the density.

5. So subtract A from D and this gives amount E which is the total volume of water displaced, in millilitres (mL) by fully submerging the pumice stone, equalling the volume of the pumice rock itself. Since density is calculated as mass divided by volume, divide value C by value E to give F which is the density of the pumice rock, in kg/L.