A MICROMETER THICK COATING LAYER SLOWING THE WEATHERING RATE OF GLACIAL POLISH (Invited Presentation)

Glacial polish is the smooth, glossy bedrock surface formed under glaciers and preserves for millennia. Glacial polish has previously been thought to form by removal of material by glacier abrasion. Here we identify a micrometer-scale coating layer that suggests that the uppermost interface between ice and rock forms by accreting material to the abraded surface. This coating layer may explain the remarkable persistence of glacially polished surfaces, which can survive up to 300 million years.

Slip markers on the glacially polished surfaces, such as arcuate cracks and striations, indicate a role for abrasion during polish formation. At the small scales, bent and broken crystals in a damage zone beneath the coating layer provide evidence for abrasion at the nanoscale, which generates the fragments and amorphous matrix that ultimately comprise the coating layer. Flow and shear textures within the coating suggest that this composite material is smeared over the damage zone during ice sliding, forming the smooth surface.

The coatings are exceptionally impermeable, they shed water quickly and inhibit the colonization and growth of lichens and mosses, slowing chemical weathering. Although early work suggested that polish may perform a role in preservation, the structural origin of the impermeability had not previously been recognized.