1. Do heavy things always sink in water?
If you put a small stone in water, it sinks immediately. However, a large wooden plank, which is much heavier than the small stone, floats. If the rule were simply "heavy things sink," this would not make sense.
What we are actually comparing is water of the same size. When an object is in water, it experiences an upward force called buoyancy, which equals the weight of the water it pushes aside [1]. When you submerge an object completely, the amount of water it pushes aside is equal to the object's volume. Therefore, if the "weight of the object" is greater than the "weight of the same volume of water," it sinks. If it is lighter, it rises to the surface and stops [1].
The weight per unit of volume is called density. Whether something sinks or floats is determined by density, not just total weight. So, how many times heavier than water is the density of an ordinary rock?
2. How much denser than water are ordinary rocks?
Let us compare using one milk carton. A cube with 10 cm sides has a volume of about 1000 cm³, which is the same as one milk carton. This volume of water weighs about 1 kg. Comparing objects of this same size, granite weighs about 2.7 kg, basalt about 3.0 kg, and calcite about 2.3 kg [2].
These figures are representative values, though they can vary slightly depending on where the rock was found. Since all of these are more than twice as heavy as water, the rocks listed here sink the moment they are placed in water.
In schools, students also measure "rock weight." A high school geology teaching material from the Iwate Prefectural Board of Education includes an activity where students measure the density of common rocks to understand the characteristics of the Earth's crust [3]. The density of rocks serves as an entry point to thinking about what is inside the Earth.
However, there are things that are in the same family as rocks but float in water. That is pumice.
3. What are the holes inside pumice?
Pumice is a rock in which many small holes are formed where gas escaped when lava cooled suddenly. The "apparent density," which includes these holes, is generally considered to be less than 1 g/cm³ [4].
The structure is like the skin of a cream puff. According to a report from the National Institute of Advanced Industrial Science and Technology (AIST), CT images of the inside of pumice show that tiny air bubbles, less than 1 mm in size, make up most of the volume. This is thought to be the reason pumice has a density lower than water [5]. Furthermore, denser parts form a shell-like layer on the outside, while larger bubbles gather in the center. Researchers describe this as a "cream puff skin structure" [5].
The pumice from Fukutoku-Okanoba that washed ashore in 2021 is described on a Chiba Prefecture page as looking like "chocolate chip cookies" with black grains inside. The Chiba Prefecture page introduces these visual features with photos [6].
Even if the rock part alone is heavier than water, if the overall density including the holes is small, it floats. So, what was the journey of this pumice across the sea?
4. How did pumice rafts from the Fukutoku-Okanoba eruption drift to Okinawa and Chiba?
In August 2021, the Fukutoku-Okanoba volcano in the Ogasawara Islands erupted. A large amount of pumice floated and spread on the sea, drifting as a pumice raft. It arrived in Okinawa in October and in Tateyama City, Chiba Prefecture, in mid-November [6].
If water enters through cracks, the air is replaced by water, and it is thought that the pumice may become harder to float. It is possible that pumice rafts do not float forever.
Pumice is a strange example of a rock that floats. There is another object made of iron that floats: a ship.
5. Does an iron ship float for the same reason as pumice?
The density of iron is about 8 times that of water, and it sinks if it is in a solid lump. However, if you shape iron into a vessel and fill the inside with light air, the mass of the iron itself does not change, but the total volume increases. The mass per unit of volume of the whole object, that is, the average density, becomes smaller. If it becomes smaller than water, it floats [7].
The air bubbles in pumice and the air inside an iron ship can both be explained by the same mechanism: "increasing the total volume to lower the average density."
What determines whether something floats is not the name of the material. It is the overall density of the material combined with air. Let us try to verify this at home.
6. How can you compare stones, pumice, and aluminum foil in a tub of water?
Put water in a washbasin or a large bowl. If you have small stones from the garden and pumice at home (pumice for scrubbing heels is fine), gently put the pumice in. Observe whether the small stone sinks and whether the pumice floats, and watch for any changes after it floats.
Next, make a small ball from aluminum foil and a dish shape from the same amount of foil, and put them in the water. Observe whether sinking or floating changes depending on the shape, even though the mass of the material does not change.
To prevent water from overflowing, use a large container and lay newspaper on the floor underneath.