Stone Cold: How Rocks Become Glacial Tables

(Inside Science) -- Nicolas Taberlet is a mountaineer as well as a physicist, and in his explorations of the Alps he would often see glacial tables -- rocks sitting atop pedestals of ice, as if waiting for a party to come. They’re pretty common in low-altitude glaciers, below 10,000 feet, he said, yet scientists hadn’t really explained how these frigid floating stones came to be. For example, why did large rocks tend to create tables while smaller rocks simply sunk down into the ice below?

Taberlet said he realized that no studies had delved into the geological curiosities, and he was intrigued: “I thought there might be something interesting there.”

So Taberlet and his colleagues at the University of Lyon in France set out to do some laboratory experiments. They tested cylindrical stones made of polystyrene, PVC and granite that sat on top of large blocks of ice. As the ice melted, the plastics created ice tables while the granite sunk into the ice -- because the plastic materials were better at protecting the ice from the warm environment. They also found that shape and size mattered: Thinner stones formed tables more easily than thicker ones. The research was published in the journal Physical Review Letters earlier this month.

These tables aren’t just ice-based oddities: They can be used to track the size of glaciers and their melting rate by looking at the height of the tables at two points in the summer. This kind of low-tech monitoring could be especially useful in areas like South America or the Himalayas where glaciers aren’t monitored closely, Taberlet says. “It’s a proxy using simple measurements that gives you a very good indication of how fast that glacier melts in the summer.”

Taberlet and his team plan to publish more research soon on the overall shape of glacial tables, and how they fare over different seasons -- their research shows that some tables can survive for a couple of years while others topple over when melting ice makes their pedestal unstable.

In an era of rapidly melting glaciers around the world, uncovering the fundamental processes of unusual phenomena feels especially important to Taberlet. “It’s one thing to know by reading a paper that they saw that a glacier will lose 10 meters of thickness in one season, but it’s another thing to just go on the field and see it,” he said. “It’s absolutely a critical moment in history for the field.”