The secrets of the “Zen stones” of Lake Baikal finally revealed

• "Zen stones" are stones that initially rested on a frozen surface and end up in delicate balance atop a pedestal of ice, according to our partner

  The Conversation

  .

• This phenomenon would be due to the differential ablation of the ice, that is to say a disparity in its speed of sublimation (and therefore of erosion) under the pebble and far from the pebble.

• This analysis was conducted by

  Nicolas Taberlet

  , lecturer in physics, and

  Nicolas Plihon

  , CNRS Research Director (both at the École Normale Supérieure de Lyon).

Have you ever succumbed to the guilty pleasure of throwing a stone on a frozen lake, hoping to break the ice sheet?

On the shores of Lake Baikal, located in Siberia, any attempt is doomed to failure, since the ice in the winter months reaches a thickness of one to three meters.

However, over time, the initial disappointment can turn into amazement when the weather conditions are favorable: the stone that initially rested on the frozen surface finds itself in delicate balance atop a pedestal of ice.

This rare phenomenon, called "zen stones", in reference to Japanese gardens which present stacks of pebbles in balance, remained unexplained until now.

During the winter, the thickness of the ice varies: it can grow on its underside (when the lake water freezes), but it also decreases on its upper side.

If in Western Europe this decrease is due to melting, the particular meteorological conditions of the Lake Baikal region lead the ice to "sublimate".

A vaporization with surprising effects

This change in the state of water, unusual in everyday life, causes the ice to vaporize directly into the atmosphere without passing through its liquid phase.

The ice of Lake Baikal sublimes and leads to ablation rates of a few millimeters per day of the upper side of the lake.

Our work has shown that the formation of "Zen stones" is due to the differential ablation of the ice, i.e. a disparity in its speed of sublimation (and therefore erosion) under the pebble and far from the pebble.

The upper surface of the ice is constantly shrinking all over the lake but this process, which needs daylight, is hindered by the presence of a pebble, whose shadow creates a deficit in the contribution of solar energy. .

Thus, contrary to a common belief that the foot pushes under the pebble, the latter protects the ice on which it rests, while the rest of the surface of the frozen lake lowers.

This process is analogous to the formation of hoodoos, where a solid rock protects a column of sediment less resistant to erosion due to rain and frost.

​Lake Baikal in the laboratory

We reproduced on a small scale the formation of "Zen stones" in the laboratory using a "freeze dryer".

This device contains an airtight chamber in which the low pressure and temperature lead to the sublimation of the ice.

A metal cylinder of a few centimeters initially placed on a block of ice finds itself after a few hours at the top of a delicate pedestal.

We chose to use metals because their thermal properties (conductivity, heat capacity, emissivity) are well known, but the lab experiment also works with a natural stone disk.

This device has thus enabled us to confirm the mechanism of formation of "Zen stones" in the natural environment, but also to test different configurations, quickly and reproducibly thanks to a sublimation rate almost ten times faster in the laboratory.

The presence of the depression observed under the natural pebbles is due to a subtle effect absent from our laboratory experiments.

Indeed, the pebble (but also the ice) has a temperature above absolute zero and thus emits electromagnetic radiation.

But, while the solar radiation is mainly emitted in the visible wavelengths, that coming from the pebble is maximum in the far infrared, for wavelengths around 10 micrometers.

In our experiments, the metallic disc and the ice are at the same temperature and their emissions in the infrared range compensate each other.

A rare and ephemeral phenomenon

Conversely, on Lake Baikal, the pebble can have a higher temperature than the ice, during part of the day.

Thus, this additional source of energy accelerates the sublimation in the vicinity of the pebble and leads to the depression whose contour follows that of the stone.

OUR “PHYSICAL” FILE

“Zen stones” are phenomena as rare as they are ephemeral.

Indeed, the process of sublimation, even if it is greatly slowed down under the stone, leads the foot to become thinner indefinitely until the pebble falls.

The lifespan of these structures on Lake Baikal is of the order of two to three weeks, and only a privileged few are lucky enough to witness this spectacle.

Access to this content has been blocked to respect your choice of consent

By clicking on "

", you accept the deposit of cookies by external services and will thus have access to the content of our partners

I ACCEPT

Science

Science Pictures: Why the tree/fungi “alliance” is essential for forest ecosystems

Planet

Science pictures: A new species of “lace” coral has been discovered in Brazil

This article is produced by The Conversation and hosted by 20 Minutes.

• Science

• The Conversation

• Video

• Physical

• Winter

• Experience