Applying long-term, constant stress to ice causes it to deform like a plastic, meaning that it won't spring back to its original shape. The deformation has typically been thought to be a smooth dynamical process similar to the laminar flow of fluids. But on page 667 of this issue, M.-C. Miguel and co-workers show that the way ice flows under stress is actually turbulent.

Miguel et al. use a technique called acoustic emission — recording of acoustic waves emitted during plastic deformation — to measure the internal dynamics of squeezed ice. They find that one-dimensional defects in the crystalline lattice, known as dislocations, move in intermittent bursts. And from the particular statistical distribution of the energies associated with each acoustic burst (it follows a power law) they conclude that plastic deformation in ice is more akin to turbulent flow than laminar flow.

Credit: M.-C. MIGUEL ET AL.

The textbook view of plastic deformation is that the dislocations glide and slip along smoothly. But this is true only of average behaviour over large length scales. A close look at artificial granular ice reveals a complex and unpredictable structure, as the image on the right illustrates. These crystals are enhanced by polarized light and the average grain size is about 1.2 micrometres.

To confirm that the effect they have identified is not specific to ice, Miguel and co-workers have done numerical simulations; what they have found is that turbulence should be considered a general feature of creep flow. These results should have implications for our understanding of plastic deformation more generally, and have practical bearing on problems in materials science and engineering.