Glaciers obey the slip law

The 21st-century rise in global sea level depends critically on when the West Antarctic ice sheet slides off the continent and melts (see Physics Today, July 2014, page 10 , and October 2019, page 14 ). The rate of travel for glaciers is affected in part by how the bed—made of sand, rocks, and other sediment at the land surface—shears when the ice above moves across it. Yet most flow models and observations have assumed that glaciers viscously drag themselves over hard, rigid beds. Now Lucas Zoet from the University of Wisconsin–Madison and Neal Iverson of Iowa State University have developed a generalized law that relates a glacier’s slip resistance to the mechanics of deformable materials.

To measure the shear of soft beds, Iverson built an experimental apparatus in which a horizontal ring of temperature-controlled ice slides at prescribed velocities and effective stresses over a bed of sandy sediment. Zoet ran two experiments: one with small grains of material and a second with a few large rocks, or clasts, added to the sediment to increase the bed’s surface roughness.

The figure shows the shear stress, normalized by the material strength, that the bed of sediment can sustain before it permanently deforms. Shear stress, and thus the resistance to slip that the ice experiences, depends on the ice’s sliding speed, as long as it’s not too high. But above some threshold speed, which varies between the experiment with clasts (blue line) and the one without (red), only the shear strength of the bed below the ice determines the slip resistance. The researchers applied the generalized equation they developed to real-world ice sheets, and the results agreed well with independent field observations for the Hofsjökull glacier in Iceland and ice sheets in Greenland . (L. K. Zoet, N. R. Iverson, Science 368, 76, 2020 .)