&Cartridge; physics 14, 123
Laboratory experiments reveal the melting process that creates a commonly seen ice feature called a glacier table.
A glacier table – a large stone that sits precariously on a narrow ice pedestal – is a common sight on low-lying glaciers. A new study provides an explanation of how these traits arise  . Based on experiments with stones of different shapes and materials, the researchers found that the surface and heat conduction of the stone are two important properties that determine whether a table forms or not. By incorporating these properties into a model, the team estimated the minimum rock size required to form a table and found a value that was consistent with the observations. The model could allow glacier tables to provide a rough measurement of glacier melt in cases where traditional monitoring equipment is not available.
Glacier tables typically occur at altitudes below about 10,000 feet, where summer temperatures can be warm enough to melt ice. A large boulder lying on top of the glacier can slow the melting of the ice directly below it. This protected ice forms a base that is often 1 to 2 meters high and supports the rock when the surrounding ice melts. Glacier tables usually only last a few months, with the rock falling over if the base diameter becomes too small.
As a mountain sports enthusiast, Nicolas Taberlet came across glacier tablets. “I’ve come across them several times and I’ve always found them interesting,” he says. A physicist at the University of Lyon, France, asked Taberlet why large rocks often form tables while smaller rocks usually sink into the ice. So he and his colleagues designed laboratory experiments using 3 cm thick sheets of clear ice tilted at different angles to resemble miniature glaciers.
The researchers first measured ice melt in the laboratory by tracking the thickness of the plates over time. They showed that ice heating was mainly caused by radiation from the laboratory walls and convection in the surrounding air. The drainage of liquid water was less important. These results were consistent with observations the team had previously made on real glaciers.
The team then tested a variety of cylindrical stones by placing them on the ice and tracking them for several hours. Each stone is made from one of six materials that differ in their thermal conductivity, from polystyrene, the weakest conductor, to granite, the strongest. The stones were 4 to 14 cm in diameter and 0.5 to 7 cm in height.
Some of the stones formed tables, others sank under the surface of the ice. One factor was heat conduction, as shown by the fact that ice bases formed under styrofoam stones, but not under granite stones. Polystyrene is a better “blanket” that protects the ice from the warm environment.
Another factor was shape, with thinner stones forming tables more easily than thicker stones. The researchers described this behavior as a geometry-related increase in melting: a thick stone has more surface area in contact with its surroundings, i.e. it absorbs more heat, which means that the ice below melts faster than the ice under a thinner stone. Taberlet explains that a similar effect (with heat flowing in the opposite direction) allows large surface area “fins” to accelerate the release of excess heat from engines and electronic components.
The team combined the conductivity and geometric enhancement effects into a generic formula to determine whether ice covered by a rock melts faster or slower than uncovered ice. Using this formula, they estimated that the minimum size of a rock to form a slab is 10-20 cm, which is consistent with observations that most slabs on glaciers are 1 m or more wide.
With further study of the glacier tables, scientists could potentially use them as glacier benchmarks. “If you don’t have the ability to track a glacier continuously, you could just go there every month in June and measure the height of the glacier tablets,” says Taberlet. Such data could be converted into an estimate of the melt rate. An understanding of the glacier tables could also be useful on Jupiter’s moon Europa – a future mission to that icy world could worry about his lander acting like a rock on a glacier and changing the rate of melt beneath it.
The experiments and field observations provide a good explanation for the environmental conditions that lead to glacial tablets, says Bhanu Pratap, a glaciologist with the Wadia Institute of Himalayan Geology in India. However, he believes more studies are needed to understand the effects of “debris” such as rocks, debris and pollution on glacier evolution.
Michael Schirber is the corresponding editor for physics based in Lyon, France.
- M. Henot et al., “Insertion of glacier tables”, Phys. Rev. Lett.127, 108501 (2021).