It’s true — part rocks can float on water for years at a time. And now scientists know exactly how they do it, and also what causes them to eventually sink.

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X-ray studies at the department of Energy’s Lawrence Berkeley nationwide Laboratory have helped researchers to resolve this an enig by scanning inside samples that lightweight, glassyand porous volcano rocks recognized as pumice stones. The X-ray experiments to be performed in ~ Berkeley Lab’sAdvanced light Source(ALS), an X-ray resource known together a synchrotron.

The surprisingly long-lived buoyancy of this rocks — i m sorry can type miles-long debris job on the ocean known as pumice rafts that deserve to travel for hundreds of miles — can help scientists uncover underwater volcano eruptions.

In this 2006 satellite image, a big “raft” the floating pumice stones (beige) shows up following a volcanic eruption in the Tonga Islands.Credit: Jesse Alan/NASA earth Observatory, Goddard room Flight Center

And, beyond that, learning about its flotation can assist us understand exactly how it spreads types around the planet; pumice is nutrient rich and readily serves as a seafaring transport of tree life and other organisms. Floating pumice can additionally be a peril for boats, together the ashy mixture that ground-up pumice can clog engines.

“The question of floating pumice has actually been approximately the literature for a lengthy time, and it hadn’t to be resolved,” said Kristen E. Fauria, a UC Berkeley graduate student who led the study,publishedinEarth and also Planetary science Letters.

Pumice stones.Credit: Berkeley Lab

While scientists have actually known the pumice have the right to float since of pockets that gas in that pores, it was unknown exactly how those gases remain trapped within the pumice for an extensive periods. If friend soak up sufficient water in a sponge, because that example, it will certainly sink.

“It was initially thought that the pumice’s porosity is basically sealed,” Fauria said, like a corked bottle floating in the sea. However pumice’s pores are actually mostly open and connected — much more like one uncorked bottle. “If you leave the lid off and also it still floats … those going on?”

Some pumice stones have also been it was observed to “bob” in the activities — sinking during the evening and surfacing during the day.

How the works

This animation, produced from a series of X-ray microtomography images gathered at Berkeley Lab’s progressed Light Source, shows a cube-shaped sample the pumice (blue-gray) and pockets the trapped gases (other colors). The animation also shows liquid (at 18 seconds) that surrounds the gases.Credit: Berkeley Lab, UC Berkeley

To understand what’s at job-related in these rocks, the team supplied wax to coat bits of water-exposed pumice sampled from medication Lake Volcano close to Mount Shasta in north California and also Santa María Volcano in Guatemala.

They then provided an X-ray imaging technique at the ALS well-known as microtomography to research concentrations of water and also gas — in information measured in microns, or thousandths the a millimeter — within preheated and room-temperature pumice samples.

The detailed 3-D images produced by the an approach are an extremely data-intensive, which posed a difficulty in quickly identifying the concentration of gas and also water existing in the pumice samples’ pores.

To handle this problem, Zihan Wei, a visiting undergraduate researcher from Peking University, supplied a data-analysis software program tool that incorporates maker learning to automatically identify the gas and water components in the images.

Concentrations that liquid and also gas in samples the pumice stones are labeled in this images, developed by X-ray microtomography at Berkeley Lab’s progressed Light Source. The images helped researchers in identifying the mechanisms that enable pumice to float for an extensive periods. Cook pumice (shown in photos at the peak right and bottom right) samples save a smaller sized volume the trapped gas than room-temperature samples.Credit: UC Berkeley, Berkeley Lab

Researchers discovered that the gas-trapping processes that room in play in the pumice stones relates to “surface tension,” a chemistry interaction in between the water’s surface and also the air over it the acts favor a thin skin — this allows some creatures, consisting of insects and also lizards, to actually walk ~ above water.

“The process that’s managing this floating happens on the range of person hair,” Fauria said. “Many the the pores space really, really small, like thin straws every wound increase together. For this reason surface tension really dominates.”

The team likewise found the a mathematics formulation well-known as percolation theory, which helps to understand exactly how a liquid enters a porous material, offers a an excellent fit for the gas-trapping process in pumice. And gas diffusion — which explains how gas molecule seek locations of reduced concentration — explains the eventual loss of this gases that reasons the stones to sink.

Floating and also sinking

Individual gas bubbles trapped in two pumice samples (labeled “ML01” and “SM01”) space shaded with different colors. The size and connectedness that the bubbles have the right to vary commonly within a sample.Credit: UC Berkeley, Berkeley Lab

Michael Manga, a staff scientist in Berkeley Lab’s power Geosciences division and a professor in the room of Earth and Planetary scientific research at UC Berkeley that participated in the study, said, “There space two different processes: one that allows pumice float and also one that provides it sink,” and the X-ray studies helped to quantify these processes for the very first time. The study confirmed that previous estimates for flotation time were in some instances off by number of orders that magnitude.

“Kristen had the idea that in hindsight is obvious,” Manga said, “that water is pour it until it is full up only some of the spicy space.” The water surrounds and also traps gases in the pumice, developing bubbles that make the stones buoyant. Surface stress serves to save these bubbles locked inside for prolonged periods. The bobbing it was observed in laboratory experiments that pumice floatation is explained by trapped gas broadening during the warmth of day, which reasons the stones come temporarily float till the temperature drops.

The X-ray work at the ALS, combination with studies of little pieces that pumice floating in water in Manga’s UC Berkeley lab, assisted researchers to develop a formula because that predicting how long a pumice rock will generally float based on its size. Manga has also used an X-ray an approach at the ALS referred to as microdiffraction, which is valuable for examining the beginnings of crystals in volcano rocks.

Dula Parkinson, a research scientist at Berkeley Lab’s ALS who aided with the team microtomography experiments, said, “I’m always amazed at exactly how much details Michael Manga and also his partners are able come extract from the pictures they collect at ALS, and how they’re maybe to join that information with various other pieces to resolve really complex puzzles.”

These 3-D printed models present a amplified sample that pumice (black) and also a large concentration the gas (white) pour it until it is full interconnected pores within that pumice sample.Credit: Berkeley Lab

The current study triggered much more questions around floating pumice, Fauria said, such as exactly how pumice, ejected native deep underwater volcanoes, finds its way to the surface. Her research study team has also conducted X-ray experiments at the ALS to study samples from so-called “giant” pumice that measured more than a meter long.

That rock was recovered from the sea floor in the area the an active underwater volcano through a2015 research study expeditionthat Fauria and also Manga participated in. The expedition, come a site thousands of miles phibìc of brand-new Zealand, to be co-led by Rebecca Carey, a scientist formerly affiliated with the Lab’s ALS.

Underwater volcano eruptions room not as easy to track down as eruptions on land, and also floating pumice spotted through a passenger ~ above a commercial aircraft actually assisted researchers track down the source of a significant underwater eruption that emerged in 2012 and also motivated the study expedition. Pumice stones spewed from underwater volcano eruptions differ widely in size but can generally be around the dimension of an apple, while pumice stones from volcanoes top top land tend to be smaller than a golf ball.

“We’re trying come understand how this huge pumice rock to be made,” Manga said. “We don’t know well exactly how submarine eruptions work. This volcano erupted completely different than we hypothesized. Our hope is that we deserve to use this one instance to know the process.”

Fauria agreed that there is lot to find out from underwater volcano studies, and also she listed that X-ray studies at the ALS will play an ongoing role in her team’s work.

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The progressed Light source is a DOE Office of scientific research User Facility. This work-related was supported by the U.S. Nationwide Science Foundation.