A close-up of pumice rock. Pumice can float in the ocean for years, and now scientists have finally found out why. James St. John / Flickr

You might think that rocks sink in water – but some can float along at the surface for years. Now scientists have finally found out why.

It's to do with the way that the rock traps bubbles of gas. Pumice is a type of volcanic rock that is filled with many tiny pores throughout. These pores are filled with air. But exactly how this air stays put and doesn't leak out over a period of years has puzzled scientists. By X-raying the stones, they've now discovered the secrets of their buoyancy.

The pores inside pumice are large and connected to one another, which you would think would make the stone easier to sink. If water flows into one of the outer pores, there's no physical barrier stopping it spreading into the rest of them and sinking the stone. To add to the riddle, some pumice stones seem to fill with water and sink, but then resurface later on in the day.

To figure out what was going on, the scientists coated the stones in wax after they had been in water. Then they X-rayed the stones to see where the water and air was inside the rock.

They found that the answer lay in the surface tension of the water inside the stone. Surface tension is the property that allows some very small insects like water-skaters to move over the surface without sinking. It's essentially a skin on the water that at very small scales resists breaking. The smaller you go, the more and more noticeable are the effects of water tension.

"The process that's controlling this floating happens on the scale of human hair," said study author Kristen Fauria of University of California, Berkeley in a statement. "Many of the pores are really, really small, like thin straws all wound up together. So surface tension really dominates."

Esteban Balseiro

This surface tension is what stops the water flowing through and filling up all of the pores. The 'bobbing' phenomenon when the stone sinks and then rises later in the day happens when the air inside the pumice expands in warmer temperatures.

Pumice forms when volcanoes erupt either on land or underwater. The eruptions can lead to vast swathes of pumice stones floating in the sea. These pumice rafts can stretch for more than a mile.

Floating rock
A pumice stone seen floating in water. Richard Grant / Flickr

The findings are hoped to shed light on how 'giant' pumice stones form. Some created during underwater volcanic eruptions measure up to a metre across. Most pumice stones are closer to the size of an apple or a golf ball.

The research is published in a paper in the journal Earth and Planetary Science Letters.

'Floating rock'
This rock in the ocean only gives the appearance of floating - but pumice stones about the size of an apple really do bob along the surface of the ocean. Giuseppe Nicotra / Flickr