Scientists have used X-rays to solve the mystery behind how some rocks can float on water for years at a time. A team at Lawrence Berkeley National Laboratory (Berkeley Lab) in the US scanned samples of lightweight, glassy and porous volcanic rocks known as pumice stones.
The surprisingly long-lived buoyancy of these rocks — which can form debris patches on the ocean known as pumice rafts that can travel for thousands of miles — can help scientists discover underwater volcano eruptions. While scientists have known that pumice can float because of pockets of gas in its pores, it was unknown how those gases remain trapped inside the pumice for prolonged periods.
Researchers found that the gas-trapping processes that are in play in the pumice stones are due to surface tension, a chemical interaction between the water’s surface and the air above it that acts like a thin skin. This allows some creatures, including insects and lizards, to actually walk on water.
“The question of floating pumice has been around the literature for a long time, and it had not been resolved,” said Kristen E Fauria, a graduate student at University of California (UC), Berkeley in the US.
“It was originally thought that the pumice’s porosity is essentially sealed,” like a corked bottle floating in the sea, said Fauria, who led the study published in the journal Earth and Planetary Science Letters. However pumice’s pores are actually largely open and connected – more like an uncorked bottle.
To understand what is at work in these rocks, the team used wax to coat bits of water-exposed pumice. They then used an X-ray imaging technique known as microtomography to study concentrations of water and gas – in detail measured in microns, or thousandths of a millimetre – within preheated and room-temperature pumice samples.
Researchers found that the gas-trapping processes that are in play in the pumice stones relates to surface tension. “The process that is controlling this floating happens on the scale of human hair,” Fauria said.
“Many of the pores are really, really small, like thin straws all wound up together. So surface tension really dominates,” Fauria said. The team also found that a mathematical formulation known as percolation theory, which helps to understand how a liquid enters a porous material, provides a good fit for the gas-trapping process in pumice.
Gas diffusion – which describes how gas molecules seek areas of lower concentration – explains the eventual loss of these gases that causes the stones to sink. “There are two different processes: one that lets pumice float and one that makes it sink,” and the X-ray studies helped to quantify these processes for the first time,” said Michael Manga, professor at UC Berkeley.