1. Why Weight Alone Doesn't Decide Floating: Compare Things of the Same Size as Water
A large wooden board floats, while many small stones sink. It seems that what decides floating or sinking is not weight itself. So, we compare things of the same size. The weight of 1 cm³ (a cube 1 cm high, wide, and deep) of water is about 1 g. We say "about" because it changes slightly with water temperature. Stones like granite weigh about 2.6 to 2.8 g for the same size, which is 2.6 to 2.8 times the weight of water. This "weight per unit of volume" is called density. Many stones like granite have a higher density than water, so they sink [3][4].
2. How Water Pushes Up on a Sunken Stone and Makes It Feel Lighter
Let us try a simple experiment. Tie a fist-sized stone with a string and put it in a washbasin of water, holding the string with your hand. Compare holding it in the air with holding it in the water. Do this with an adult in a safe place, and be careful not to drop the stone on your feet. The stone in the water feels light. When you take it out, it feels heavy again. This is because water is pushing the stone up. This upward force is called buoyancy. There is a rule that the size of the buoyancy is equal to the weight of the water the object pushed away. In other words, it is decided by the volume of the pushed-away water and the density of the water. For things floating on water, the volume of the part submerged in water is the volume of the pushed-away water. This is called "Archimedes' Principle," named after the ancient Greek Archimedes [2].
When a granite stone is put in water, the water it pushes away is 1 cm³, and its weight is about 1 g. The weight of the granite is 2.6 to 2.8 g, so 1.6 to 1.8 g worth of weight remains downward. Because this downward weight remains, the stone sinks. (Here, the density of fresh water is used as an approximation of 1 g/cm³. The density of fresh water changes slightly with temperature, so this is an "about" value.)
3. Why Pumice Floats Even Though It Is Stone: Its Holes Make It Lighter Than Water
Pumice is a rock with many small holes, created when gas blew out as lava cooled suddenly. It is said to be light and float on water [1]. The hole parts are air, and the stone material parts are thought to be heavier than water, just like rock without holes. Still, because there are many holes, the density of the whole lump becomes smaller than water. The stone material itself is thought to be heavy, but the whole thing is light. This is the interesting point about pumice.
4. How Pumice Spread by Ocean Currents and Piled Up on Okinawa's Coasts and Ports
The spewed-out pumice spread by riding on ocean currents. On October 28, researchers from the Geological Survey of Japan (GSJ) at the National Institute of Advanced Industrial Science and Technology looked from an airplane around Okinawa Island and saw many pumice lumps (rafts) floating. It is reported that the amount washed up differed even on neighboring coasts. In each port, the layer of accumulated pumice was said to be about 10 to 20 cm [6].
5. Why Pumice Can Float for Months: Two Different Explanations
Pumice is said to float on water for months, and sometimes years [5]. Regarding the reason for this, explanations differ depending on the study. Let us look at two ideas.
The first idea, shown in a 2017 paper, is that water gets into the holes and traps the air (gas) inside. The trapped gas escapes slowly over time. It is thought that when the gas has all escaped, water gets in and it sinks [5].
The second idea comes from a study in 2024 that examined pumice born in the 2021 eruption and floating in the sea. Many of the floating pumice had "closed holes" where more than 30% of the holes were not connected to the outside. Because water cannot easily enter closed holes, it is reported that even small grains of millimeter size do not sink easily [7].
In these two studies, the explanations differ. Which one applies to which kind of pumice cannot be said with just these two sources.
6. A Home Experiment: Comparing a Stone and Pumice in Water
Let us put stone and pumice in a container with the same water. If you predict "will it float or sink" before putting them in, you can learn the concept of density. You will need: a small stone (washed and dried), pumice (the kind sometimes sold at drugstores for scraping heels), and a plastic bowl with water. Most pumice floats immediately. Leave it for 1 or 2 days and observe if the situation changes. The stone should stay sunk. Sometimes no change is visible in a short time. Note: Do not put stones in your mouth. Do water play when an adult is nearby. When picking up stones in the sea or river, do not enter dangerous places.
7. What Is Still Undecided About How Long Pumice Floats, and How to Check It Yourself
Regarding why pumice floats for months, explanations differ depending on the study. Which of the two ideas applies to which kind of pumice cannot be said with just these two sources. At home, put pumice in water and record the daily situation. See if it stays floating immediately or starts to sink little by little. Does the stone stay sunk? Do this with an adult, and do not put pumice in your mouth.