Cool ice with hot properties

The Hexagonal Mystery

The intricate symmetry of snowflakes has long symbolised the crystalline perfection of ice, reflecting the hexagonal structure of its molecules. On Earth, water freezes to blanket lakes and polar oceans, while also descending from the atmosphere as snow and hail. Surface energy reduction instigates changes in shape, while compaction diminishes the air-to-ice volume ratio. Consequently, snow metamorphoses into firn before ultimately becoming the dense ice of the glaciers and ice sheets that adorn polar regions and mountainous terrains. Although glacial ice is often described as a cold metamorphic rock, this characterization can be misleading. Ice on Earth is at temperatures close to its melting point, making it relatively ‘hot’ in terms of its physical properties.

Deforming at the Brink of Melting

Just as deformation experiments on olivine have illuminated the flow of Earth’s mantle, controlled laboratory experiments on ice—particularly near its melting point—are essential for understanding the dynamics of polar ice sheets. Under these warm conditions, ice becomes highly sensitive to stress, grain-boundary sliding, and premelting, all of which fundamentally alter its mechanical strength. Over seven decades of such experiments have driven the refinement of mechanical models that underpin predictions of accelerating polar ice flow. In parallel, advances in cryogenic and high-resolution microscopy, as exemplified here, have unveiled the microstructural origins of softening, providing a robust physical basis for linking laboratory measurements to large-scale ice-sheet dynamics. The ability to assess both mechanical and microstructural data has profoundly enriched our understanding of ice-deformation mechanisms. This is particularly significant because the mechanical behaviour of ice is intrinsically tied to its evolving grain size and crystal alignment during deformation. Yet, our understanding remains limited in fast-flowing discharge regions where ice rapidly enters the ocean—zones that control sea-level rise but experience extreme strains beyond conventional laboratory conditions. Ongoing high-pressure deformation experiments capable of reproducing such conditions offer a promising path to refining these mechanical models and improving projections of sea-level rise under a warming climate.