By scanning inside samples of lightweight, glassy, and porous volcanic rocks known as pumice stones, X-ray experiments have helped researchers solve the mystery of how some rocks can float on water for years at a time.
Kristen E. Fauria, a University of California, Berkeley, graduate student who led the study, which was published in Earth and Planetary Science Letters, noted that "the question of floating pumice has been around the literature for a long time, and it hadn't been resolved."
"It was originally thought that the pumice's porosity is essentially sealed," like a corked bottle floating in the sea, Fauria was quoted as saying in a news release from UC Berkeley earlier this week. But pumice's pores are in fact largely open and connected, more like an uncorked bottle.
To understand what's at work in these rocks, the team used wax to coat bits of water-exposed pumice sampled from Medicine Lake Volcano near Mount Shasta in Northern California and Santa María Volcano in Guatemala, then used an X-ray imaging technique known as microtomography to study concentrations of water and gas within preheated and room-temperature pumice samples.
The experiments were performed at Advanced Light Source (ALS), an X-ray source known as a synchrotron, of the US Department of Energy's Lawrence Berkeley National Laboratory, and the technique revealed details measured in microns, or thousandths of a millimeter.
As the 3D images produced are data-intensive, which posed a challenge in quickly identifying the concentrations of gas and water present in the pumice samples' pores, Zihan Wei, a visiting undergraduate researcher from Peking University of China, used a data-analysis software tool that incorporates machine learning to automatically identify the gas and water components in the images.
The researchers found that the gas-trapping processes in the pumice stones relates to "surface tension," a chemical interaction between the water surface and the air above it that acts like a thin skin, which allows some creatures, including insects and lizards, to actually walk on water.
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. And 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.
Michael Manga, a staff scientist in Berkeley Lab and a professor at UC Berkeley who participated in the study, said "there are two different processes: one that lets pumice float and one that makes it sink."
The researchers have developed a formula for predicting how long a pumice stone will typically float based on its size.
The long-lived buoyancy of these rocks, which can form miles-long debris patches on the ocean known as pumice rafts that can travel for thousands of miles, can help researchers discover underwater volcano eruptions; and learning about its flotation can help researchers understand how it spreads species around the planet, as pumice is nutrient rich and readily serves as a seafaring carrier of plant life and other organisms.