Scientists Solve Mystery Of How Pumice Stones Float

It's not the holes you notice, but the ones almost too small to see, that allow pumice to float on water. UC Berkeley, Berkeley Lab

Floating rocks sound like they belong on Pandora, not on Earth, but they are a real phenomenon, occurring when volcanoes produce rocks less dense than water. However, geologists have been puzzled how these pumice stones often continue to float for years, before eventually sinking. Now the high-powered X-rays at Berkeley's Advanced Light Source synchrotron have provided an answer.

Just as boats float because the air inside them makes their average density lower than water's, a network of gas-filled holes allows some pumice stones to rise to the surface. Underwater volcanoes can spit out so many of these stones that ships sometimes sail through pumice rafts miles wide formed from millions of stones. In 2012 scientists only discovered an underwater eruption off New Zealand because boats reported vast pumice rafts.

The phenomenon is more than a curiosity; pumice stones can carry nutrients across oceans to places where they are scarce, stimulating marine life, but the ash is a menace to ships' engines.

Although the stones' initial buoyancy is understood, water eventually gets inside, weighing the stones down until they sink. The curious thing is why this takes so long. "The question of floating pumice has been around the literature for a long time, and it hadn't been resolved," said UC Berkeley graduate student Kristen Fauria in a statement. “It was originally thought that the pumice's porosity is essentially sealed." 

However, further examination showed the holes in pumice are fairly open and often internally connected. Fauria compared this to an uncorked bottle, saying: "If you leave the cap off and it still floats ... what's going on?" Even more strangely, some stones have been seen surfacing during the day, after sinking at night.

Fauria collected pumices from the Medicine Lake Volcano, Northern California, and Guatemala's Santa Maria Volcano and coated them in wax, before exposing them to synchrotron radiation. Although some holes were large enough to let water in, she found others were around the size of a human hair, and twisted.

In Earth and Planetary Science Letters, Fauria reports the surface tension inside these small pores is high enough that gas remains trapped inside, forming bubbles that allow it to float. Fauria and her coauthors established a formula linking the length of time a stone floats to the size of the pumice, the rate at which gas and water diffuse and the extent to which the stone is saturated with water, improving on previous time estimates by a factor of more than 100.

Warmer conditions cause the gas to expand, pushing out some of the water and causing submerged stones to bob to the surface.

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