It’s true — some rocks have the right to float top top water for years in ~ a time. And also now researchers know just how they do it, and also what reasons 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 solve this mystery by scanning within samples the lightweight, glassyand porous volcanic rocks well-known as pumice stones. The X-ray experiments were performed at Berkeley Lab’sAdvanced light Source(ALS), one X-ray source known as a synchrotron.

The how amazing long-lived buoyancy of these rocks — i beg your pardon can kind miles-long debris spot on the ocean well-known as pumice rafts that have the right to travel for countless miles — can help scientists find underwater volcano eruptions.

In this 2006 satellite image, a huge “raft” that 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 around its flotation can aid us understand exactly how it spreads types around the planet; pumice is nutrient rich and also readily serves together a seafaring transport of plant life and other organisms. Floating pumice can also be a danger for boats, together the ashy mixture the ground-up pumice deserve to clog engines.

“The question of floating pumice has been roughly the literary works 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 Planetary science Letters.

Pumice stones.Credit: Berkeley Lab

While scientists have actually known the pumice can float since of pockets the gas in the pores, it was unknown exactly how those gases stay trapped inside the pumice for an extensive periods. If you soak up enough water in a sponge, for example, it will sink.

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

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

How the works

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

To understand what’s at occupational in these rocks, the team used wax come coat bits that water-exposed pumice sampled from medication Lake Volcano near Mount Shasta in northern California and Santa María Volcano in Guatemala.

They then used an X-ray imaging technique at the ALS known as microtomography to examine concentrations the water and gas — in information measured in microns, or thousandths the a millimeter — in ~ preheated and room-temperature pumice samples.

The comprehensive 3-D images produced by the method are very data-intensive, i beg your pardon posed a an obstacle in quickly identifying the concentration of gas and also water present in the pumice samples’ pores.

To tackle this problem, Zihan Wei, a visiting undergraduate researcher indigenous Peking University, offered a data-analysis software program tool that incorporates an equipment learning to automatically identify the gas and water components in the images.

Concentrations of liquid and gas in samples of pumice stones room labeled in this images, developed by X-ray microtomography at Berkeley Lab’s progressed Light Source. The images aided researchers in identifying the instrument that permit pumice to float for lengthy periods. Heated pumice (shown in photos at the peak right and bottom right) samples save on computer a smaller sized volume the trapped gas 보다 room-temperature samples.Credit: UC Berkeley, Berkeley Lab

Researchers discovered that the gas-trapping processes that are in beat in the pumice stones relates to “surface tension,” a chemistry interaction in between the water’s surface and the air above it the acts like a thin skin — this permits some creatures, consisting of insects and lizards, to in reality walk ~ above water.

“The procedure that’s managing this floating wake up on the scale of human being hair,” Fauria said. “Many of the pores are really, yes, really small, favor thin straws all wound increase together. So surface stress really dominates.”

The team additionally found the a math formulation known as percolation theory, which help to understand exactly how a fluid enters a porous material, gives a good fit because that the gas-trapping procedure in pumice. And gas diffusion — which defines how gas molecule seek areas of reduced concentration — explains the ultimate loss of these gases that causes the stones come sink.

Floating and also sinking

Individual gas balloon trapped in 2 pumice samples (labeled “ML01” and “SM01”) space shaded with various colors. The size and connectedness of the bubbles have the right to vary extensively within a sample.Credit: UC Berkeley, Berkeley Lab

Michael Manga, a employee scientist in Berkeley Lab’s power Geosciences department and a professor in the room of Earth and Planetary science at UC Berkeley that participated in the study, said, “There room two different processes: one that lets pumice float and one that renders it sink,” and also the X-ray studies assisted to quantify these procedures for the first time. The study proved that previous estimates for flotation time to be in some situations off by number of orders of magnitude.

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

The X-ray occupational at the ALS, coupled with researches of tiny pieces of pumice floating in water in Manga’s UC Berkeley lab, aided researchers to construct a formula for predicting just how long a pumice stone will typically float based upon its size. Manga has likewise used one X-ray method at the ALS referred to as microdiffraction, i m sorry is useful for examining the origins of crystals in volcano rocks.

Dula Parkinson, a study scientist at Berkeley Lab’s ALS who aided with the team’s microtomography experiments, said, “I’m constantly amazed at just how much info Michael Manga and also his collaborators are able come extract native the images they collect at ALS, and also how they’re may be to join that details with other pieces to solve really facility puzzles.”

These 3-D printed models display a amplified sample the pumice (black) and also a large concentration that gas (white) filling interconnected pores within the pumice sample.Credit: Berkeley Lab

The recent study triggered an ext questions about floating pumice, Fauria said, such as just how pumice, ejected indigenous deep underwater volcanoes, find its means to the surface. Her study team has additionally conducted X-ray experiment at the ALS to research samples from so-called “giant” pumice that measured much more than a meter long.

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

Underwater volcano eruptions room not as straightforward to track down as eruptions on land, and also floating pumice spotted by a passenger top top a commercial aircraft actually assisted researchers track under the resource of a significant underwater eruption that developed in 2012 and motivated the study expedition. Pumice stones spewed indigenous underwater volcano eruptions vary widely in size yet can frequently be about the dimension of one apple, when pumice stones from volcanoes ~ above land often tend to be smaller sized than a golf ball.

“We’re trying to understand how this giant pumice rock was made,” Manga said. “We don’t know well how submarine eruptions work. This volcano erupted fully different 보다 we hypothesized. Our expect is the we can use this one instance to recognize the process.”

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

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