Science Puzzle
How Can a Rock Float?
In July 2012 an undersea volcano in the South Pacific, more than 700 metres below the surface, erupted. Within a day a raft of floating stone covered over 400 square kilometres of sea. It drifted for months, and some of its stones washed up on the beaches of eastern Australia about a year later.
Most rock is between two and three times as dense as water and sinks straight to the bottom. How can a rock float?
The Answer
The floating rock is pumice, and it's mostly holes. A lump of pumice can be well over half empty space, in the form of tiny bubbles, so the lump as a whole is lighter than the same volume of water.
Magma deep underground has gas dissolved in it, mostly water vapour, held in by the huge pressure. When the magma rises and the pressure drops, the gas comes out as bubbles, just as a fizzy drink froths when you open it. In some eruptions the frothy molten rock cools so quickly that the bubbles are frozen in place, leaving a rock that looks like a hard grey sponge.
A steel ship floats for the same reason. Steel is about eight times as dense as water, but a ship is a steel shell full of air, so on average it's less dense than the sea. A solid steel block of the same weight would sink like a stone.
Pumice doesn't float forever. Water slowly seeps into the holes and, after months or sometimes years, the stones become waterlogged and sink. Before they do, floating pumice can carry barnacles, corals and other small sea creatures for thousands of kilometres.
The principle: Density and buoyancy. An object floats if its overall density, including any trapped air, is less than that of the liquid around it.